Sinus Node Dysfunction

Historically, the most common indication for pacemaker placement was surgically induced heart block. However, this has changed with improved surgical techniques and longer patient survival after cardiac surgery. Often occurring many years after surgical repair, the most common indication for cardiac pacing in patients with congenital heart disease is now sinus node dysfunction, or sick sinus syndrome. Since 2000, this indication has accounted for 40% to 60% of new implantations, compared with 20% to 25% during the previous decade.

Most of these patients have undergone cardiac surgery many years earlier, usually involving extensive atrial procedures. The most common procedure is the atrial switch operation for transposition of the great arteries.² The likelihood of these patients needing permanent pacing increases with time since surgery.³ Even though the use of this procedure for simple dextrotransposition of the great arteries is now rare, its use is increasing in more complex disease, such as levotransposition of the great arteries combined with the arterial switch procedure (“double switch”) to place the morphologic right ventricle in the pulmonary circuit and the morphologic left ventricle in the systemic circuit.

With increased use of the Fontan procedure, or right-sided heart bypass, the incidence of sinus node dysfunction is increasing. The indications are similar to those for congenital complete heart block. In addition, the presence of tachyarrhythmias, with the subsequent risk for prolonged asystole after acute termination of the tachycardia, is also an indication for pacemaker implantation (Fig. 18-1). In patients with sinus node dysfunction after cardiac surgery, our practice is to recommend pacemaker implantation in all patients with a sleeping heart rate of less than 30 beats per minute (bpm) even in the absence of symptoms, a decreasing exercise tolerance with inadequate heart rate increase with exercise (see later discussion), or sinus pauses for longer than 3 to 4 seconds. The need for medications known to affect atrioventricular (AV) conduction for the control of tachyarrhythmias in these patients would also necessitate pacemaker placement.³

Figure 18-1 Electrocardiogram from a patient with sick sinus syndrome shows 1.5 seconds of asystole after acute termination of tachyarrhythmia. Long pauses can result in syncope and are indications for pacemaker implantation. Paper speed is 25 mm/sec.

Surgically Induced Heart Block

The second major indication for pacemaker implantation in children is surgically induced heart block, which classically has accounted for 30% to 40% of children undergoing pacemaker implantation.^4–9 MPPR data show that the indication for initial pacemaker placement was surgically induced heart block in an average of 35% of patients before 2000 (Fig. 18-2). The percentage varies from year to year and reached a low of 20% in 2001-2005. There has been no definite downward trend since 2000, with the percentage remaining near 20%, although the underlying structural cardiac diseases in patients with surgically induced heart block have changed dramatically. Since 2000, surgical heart block has accounted for 15% to 25% of initial implantations. Most recent data from our center show that the incidence is 22% over the last 5 years. Most children acquiring surgical heart block in the last 5 years have had complex disease and have undergone complex surgical repairs. The surgical procedure resulting in the greatest incidence of heart block is the repair of atrioventricular septal defect, which has accounted for 17% of patients with surgical heart block since 1988. Table 18-2 lists the other common diagnoses associated with surgical heart block in the recent era.

Figure 18-2 Percentage of all children undergoing initial pacemaker implantation in a given period whose indication was surgically induced complete heart block. There has been no significant change in this percentage during the last 10 years.

(Data from Midwest Pediatric Pacemaker Registry.)

TABLE 18-2 Most Prevalent Structural Cardiac Defects Associated with Surgically Induced Complete Heart Block*

* The most common structural cardiac lesions associated with surgically induced heart block at the time of complete repair for children undergoing initial implantation since 1988.

Data from Midwest Pediatric Pacemaker Registry.

Currently, it is unusual for a child with an isolated ventricular septal defect (VSD) to acquire heart block; previously, this was not the case. Since 1988, VSD closure has accounted for only 14% of children with surgical complete heart block; atrial switch procedures (Mustard or Senning) for the correction of dextrotransposition of the great arteries have accounted for 12%. Other common lesions associated with surgical heart block are levotransposition of the great arteries, repair of tetralogy of Fallot, and aortic valvular replacement, which usually is associated with the resection of a subaortic obstruction.

Surgical heart block can develop at the initial cardiac repair or later. In addition, the heart block acquired at repair may be temporary, with return of reliable AV conduction. For this reason, our current practice is to implant only temporary pacing electrodes at the initial surgery and to defer permanent pacemaker implantation for 10 to 14 days in the hope of a return of AV conduction. However, ventricular escape rhythms are unstable, and no child with a permanent, surgically acquired complete heart block is discharged without a permanent pacemaker. Even in the hospital, all children are supported with an external pacemaker through temporary pacing wires placed at surgery until consistent AV conduction returns or a permanent pacemaker is inserted. Monitoring should consist of both electrocardiographic (ECG) monitoring and non-ECG monitoring, such as arterial pressure measurements or pulse oximetry. Many ECG monitors detect the pacing artifact and do not recognize the lack of capture with subsequent bradycardia or asystole. This is avoided by the use of a non-ECG method of detecting cardiac ejection, such as pulse oximetry.¹⁰

Late recovery of AV conduction can be seen even years after surgery. However, rarely does such recovery result in 100% conduction. Most patients who do recover some conduction will still rely on the pacemaker 30% to 50% of the time, with intact conduction the rest of the time.

Congenital Complete Heart Block

The next most common indication for pacemaker implantation is congenital complete heart block. The cause of congenital heart block varies; an autoimmune mechanism is often implicated, with clinical or laboratory evidence of connective tissue disease in the mother.¹¹ All mothers of infants with congenital heart block should have antibody determinations performed. Even children born to such mothers but with intact conduction at birth must be monitored for the development of heart block over time. Treatment of antibody-positive mothers with steroids during pregnancy has not been shown to alter outcomes or reverse the development of heart block. Congenital heart block is also associated with specific forms of structural disease, particularly those involving abnormalities of the AV junction, such as levotransposition of the great arteries with AV discordance and atrial situs ambiguus.¹² It is common for fetal heart block to “develop” in utero with intact conduction present in the young fetus and heart block developing at 20 to 30 weeks of gestation.

Data from the MPPR indicate that 10% to 25% of patients have congenital heart block as the primary indication for permanent pacing. Most recent data from our center show that 17% of patients receiving pacemakers have congenital heart block. The age at which the pacing system is implanted varies, ranging from a few hours to more than 20 years. Most children with associated structural cardiac disease who need pacing before 1 year of age have congestive heart failure (CHF) requiring an increased heart rate for adequate therapy. The mortality rate in such children is also high, with 43% dying by 2 years of age.¹³

For children with structurally normal hearts and congenital heart block, the incidence of pacemaker implantation is lower in younger children but increases with age, associated with a gradually decreasing ventricular rate.¹⁴ The gradual and steady increase in the need for permanent pacing continues with advancing age, reaching 75% by age 20 years (Fig. 18-3). The need for permanent pacing results from the development of syncope, CHF, or increasing ventricular ectopy, often associated with prolongation of the corrected QT interval (QT_c). Death is rare in children with no structural cardiac disease (only 5% by age 20) but can occur suddenly.

Figure 18-3 Actuarial analysis of months free from pacemaker need for children with congenital complete heart block and structurally normal hearts. By age 20 years, fewer than 30% of patients are pacemaker free. Brackets represent 1 standard error around the mean.

(Data from Dorostkar P, Serwer CA, LeRoy S, Dick M II: Long-term course of children and young adults with congenital complete heart block. J Am Coll Cardial 21:295A, 1993.)

Current recommendations call for the implantation of a permanent pacemaker system whenever CHF is present. In addition, implantation is recommended if the average heart rate is less than 50 bpm in the awake child and 55 bpm in the infant, if there is a history of a syncopal or presyncopal event, if significant ventricular ectopy is present, or if there is exercise intolerance.^15,16 However, symptoms of exercise intolerance can be difficult to elicit. Many children deny such symptoms, as do their parents, when in fact their exercise tolerance would be improved with permanent pacing. Many parents return after pacemaker implantation to relate that the activity level of their child has greatly increased. They are amazed at this change, because they did not believe that the child was significantly hindered before pacemaker implantation. Exercise testing is often useful as an indicator of the child’s exercise capabilities compared with those of a normal child. The physician should also periodically assess the child for increasing cardiac size by chest radiography and for decreasing cardiac function by echocardiography. The presence of either of these conditions should be considered an indication for permanent pacemaker placement.

Some children with congenital complete heart block develop a tachydysrhythmia, specifically ventricular tachycardia (VT), which can be controlled only with permanent pacing.¹⁷ The maintenance of a minimal heart rate often suppresses the tendency toward ventricular ectopy, particularly during exercise. The development of tachyarrhythmias with the stress of exercise, even in children with otherwise asymptomatic disease, necessitates pacemaker implantation.

Controversy surrounds the need for pacing in symptom-free older children with bradycardia of less than 50 bpm while asleep. This is not an absolute indication for pacemaker implantation. However, if bradycardias lower than 50 bpm are present, a detailed history and close follow-up are required to determine the need for pacemaker implantation. Although maternal antibody is associated with congenital heart block in the absence of structural disease, there is no known correlation with the need for or the timing of pacemaker placement. In addition, congenital heart block can be associated with the development of a dilated cardiomyopathy with or without pacing. The use of biventricular pacing in this group is discussed later with the indications for cardiac resynchronization therapy in children.

Other Indications

Patients with long QT syndrome and uncontrollable VT may also benefit from pacemaker placement, as well as those with intermittent complete heart block¹⁸ (Fig. 18-4). A chronic increase in heart rate shortens the QT interval and decreases the occurrence of VT. The combined use of pacing and an ICD may be even more efficacious, particularly with the advent of the dual-chamber ICD.

Figure 18-4 Recording from 24-hour ambulatory electrocardiogram showing a period of complete heart block (CHB) with acute bradycardia (between arrowheads) in a patient with tuberous sclerosis and cardiac rhabdomyomas. Paper speed is 25 mm/sec. HR, Heart rate (bpm).

Other indications for pacemaker placement reported in the MPPR include the need for control of atrial tachyarrhythmias unresponsive to pharmacologic therapy, second-degree heart block associated with symptoms, and concern about a sudden loss of AV conduction in patients receiving certain antiarrhythmic therapies known to interfere with AV conduction. Although such indications are rare, the clinician should not restrict pacemaker use to those children with complete heart block. First-degree heart block and trifascicular block with no documented loss of AV conduction are not considered indications for pacemaker implantation.¹⁹

A relatively controversial indication for pacemaker placement is symptomatic hypertrophic obstructive cardiomyopathy with significant outflow tract obstruction. Although pacemaker placement is not effective in all children with this disorder, both hemodynamic and symptomatic improvements have been observed,²⁰ with decreases in gradient and measures of diastolic performance. Generators used for this indication must allow programming of relatively short AV intervals and rate-adaptive AV intervals to maximize the QRS width and degree of preexcitation. Younger patients with more rapid heart rates may present insurmountable difficulties, and other therapies are probably indicated initially. When pacing is employed in this setting a dual-chamber ICD should be used (see later).

Categorization of pacing indications is helpful only as a general guide. Each patient must be carefully evaluated to determine the potential benefits from permanent pacing in light of the risks of implantation and the burden placed on the family and child for subsequent care. When all patients in need of pacing are considered together, again, the indication most often present is sinus node dysfunction. The largest group requiring pacing is those with dextrotransposition of the great arteries, most of whom have undergone an atrial switch procedure (Mustard or Senning) for sinus node dysfunction.

Generator Selection

The major factors to consider are the generator features that are of significant benefit to the individual child, such as antitachycardia pacing, special pacing modes, and diagnostic capabilities, together with battery capacity and projected generator longevity and size. Because newer generators are smaller than earlier ones, size is becoming less of a factor, although not all generators are the same size. Large generators not only create unsightly protuberances that can have negative psychological consequences, but also increase the risk for skin breakdown over the generator from its erosion or trauma to the skin over the generator, especially in the active child, with a subsequent infection leading to pacing system removal.

Generator Mode Selection

The choices concerning pacing mode are related to single-chamber versus dual-chamber pacing and fixed-rate versus variable-rate pacing. In general, it has been our policy to avoid the use of fixed-rate pacing, except in situations in which sinus node and AV node function are intact most of the time, with the pacemaker serving only as a backup for those rare periods when such function is not adequate. This is often the situation when sinoatrial (SA) and AV node function is marginal and antiarrhythmic drugs are required. In addition, should a sudden rate drop occur during exercise, the lower rate of a fixed-rate generator may be inadequate to provide adequate cardiac output, and even in this patient, rate-variable programming is desired.

Although cardiac output increases with exercise, even during fixed-rate pacing (Fig. 18-9), this results from a large increase in stroke volume (Fig. 18-10), with presumed increased wall stress and potentially increased myocardial work compared with the same change in cardiac output, when a heart rate increase is possible.²⁴ However, enhanced exercise tolerance is achieved when rate-variable pacing is used.²⁵ This suggests an advantage to rate-variable pacing in the child who is expected to lead an active life. The rate-responsive mode should always be used unless the patient can demonstrate an adequate intrinsic rate response to exercise by exercise testing or ambulatory electrocardiogram.

Figure 18-9 Changes in cardiac index as measured by acetylene rebreathing from rest to maximal exercise in normal children and in children with fixed-rate ventricular pacing. The cardiac indices were identical at rest and at maximal exercise. Brackets represent 1 standard error around the mean. Pts, Patients.

(Data from Serwer G, Dick M II, Eakin B: Cardiac output response to treadmill exercise in children with fixed-rate pacemakers. Circulation 84:511-514, 1991.)

Figure 18-10 Stroke index changes from rest to maximal exercise in normal children and in children with fixed-rate ventricular pacemakers. Pre-exercise values were similar. With exercise, stroke index increased significantly in patients with pacemakers, to provide an increased output in the presence of a fixed heart rate. Brackets represent 1 standard error around the mean. Pts, Patients.

(Data from Serwer G, Dick M II, Eakin B: Cardiac output response to treadmill exercise in children with fixed-rate pacemakers. Circulation 84:511-514, 1991.)

Single-chamber pacing in either the atrium or the ventricle has been advocated for the treatment of sick sinus syndrome.¹⁵ When atrial pacing is chosen, the presence of normal AV node function must be established by provocative electrophysiologic testing before implantation, especially in the postsurgical patient, because AV nodal disease can accompany SA nodal disease and may not be apparent in the resting, nonprovoked state. AAI(R) pacing has the advantage of preserving the normal ventricular activation sequence with potentially better cardiovascular function. In addition, some evidence in animals points toward the long-term development of myocardial changes when an abnormal pattern of myocardial activation is present.²⁶ A comparison of cardiac myocyte changes in ventricular free wall pacing versus high septal pacing near the bundle of His, which has a narrower QRS morphology, is striking; the clinical implication of these changes is unknown.²⁷

A newer pacing mode, managed ventricular pacing (MVP; Medtronic) has great utility in a patient with predominantly sinus node dysfunction but also with intermittent lack of AV conduction.²⁸ In this mode, pacing is AAIR, but the device monitors for ventricular activity between two consecutive P waves without regard to timing. If two consecutive P waves occur without an intervening R wave, a ventricular pace is generated. If this occurs twice within four P waves, the device shifts mode to DDD(R). After a predetermined time, the ventricular output is inhibited, and the device searches for an R wave between two P waves. If one occurs, the mode reverts to AAIR; if not, DDDR mode remains in force (Fig. 18-11).

Figure 18-11 Example of managed ventricular pacing function (MVP; Medtronic). Note the intact AV conduction (AP) for the first 2 beats, then the loss of AV conduction for the third beat, with the ventricular-paced beat after the fourth paced P wave (VP). After this occurs again, the pacemaker converts to DDD pacing.

Atrial electrodes are somewhat less reliable than ventricular electrodes, even when they are placed endocardially. Therefore, conditions associated with potential early atrial electrode failure should serve as a contraindication to AAI pacing. Such conditions include prior extensive atrial surgery in which extensive atrial fibrosis is likely, small atrial size, and prior placement of a large intra-atrial baffle, limiting venous access to viable atrial tissue. After a Fontan procedure (right atrial–to–pulmonary artery connection), patients may have a low flow velocity within the atrium, increasing the risk for venous thrombosis when endocardial electrodes are placed; long-term anticoagulation may be indicated in these patients. In addition, the amount of excitable tissue that can be accessed from a transvenous approach may be limited. Most current Fontan procedures use either a lateral tunnel created along the lateral right atrial wall, limiting this area for pacemaker placement, or an extracardiac conduit, in which case no atrial tissue is accessible.

Single-chamber ventricular pacing, or VVI(R), allows the use of more stable electrode systems and, in the rate-responsive mode, still allows rate variability to be maintained in the ambulatory child. The importance of atrial systole in the maintenance of cardiac output is debatable and varies from child to child. Because most children have good myocardial function, the atrial contribution is probably minimal. The cardiac output increase with exercise is improved in children with DDD versus VVI pacing. It is unclear, however, whether this increase is the result of atrial synchrony or rate variability. Pacemaker syndrome from VVIR pacing is uncommon but can occur, especially over time. In one series, 19 of 33 patients developed symptoms suggestive of pacemaker syndrome over time (median, 11 years) that resolved with upgrading to dual-chamber pacing.²⁹ The major factor to be considered in such a choice is the difficulty in placing an adequate atrial electrode. If prior surgery or underlying structural disease precludes atrial electrode placement, single-chamber pacing is an acceptable alternative. However, dual-chamber pacing should be considered for all patients, with single-chamber pacing used only if contraindications to dual-chamber pacing exist, as discussed later. Even in patients with sinus node dysfunction, dual-chamber pacing should be considered, particularly if AV nodal function is suspect.

We now consider dual-chamber pacing to be the mode of choice in children. We use single-chamber pacing only if a contraindication to dual-chamber pacing exists, as well as in some small infants who will need cardiac surgery in the immediate future, with an upgrade to dual-chamber pacing performed at that time. The major contraindications to dual-chamber pacing are (1) persistent atrial tachyarrhythmias; (2) changing AV nodal status, making numerous programming changes necessary; and (3) inability to place reliable atrial and ventricular electrodes. An example of the third contraindication is the small child in whom endocardial pacing is preferred, but the presence of two electrodes in the superior vena cava might present a high risk for thrombosis. Another example is the child requiring epicardial electrode placement in whom atrial electrode placement would necessitate a greatly enhanced surgical procedure. One must remember that rate sensors do not function in nonambulatory infants. Therefore, in young infants, single-chamber pacing becomes fixed-rate pacing. The generator’s size and functionality are no longer considerations, because dual- and single-chamber pacemakers are comparable in both regards. Table 18-3 lists the most common contraindications to dual-chamber pacing.

TABLE 18-3 Major Contraindications to Dual-Chamber Pacing in Children

General Characteristics

The most important consideration is related to the range of energy output available, which includes both the pulse width and the pulse amplitude programmability. Although most pacemakers are programmed to have 2.5 or 5 V of amplitude, the presence of other amplitudes is of key importance. Specifically, when a generator is used with an epicardial electrode, high-output features are mandatory. Although few children require long-term pacing at output greater than 5 V, many have an initial threshold rise and temporarily require such high output. Even with endocardial implants, acute increases in threshold can occur, and the ability to increase the pacemaker amplitude to values greater than 5 V may avert the necessity for emergency electrode replacement. In addition, threshold testing at multiple low-pulse amplitudes allows a more accurate determination of the characteristics of the strength-duration curve. This testing is mandatory to determine the lowest, but still safe, pulse amplitude and width settings. The strength-duration curve characteristics are not constant, varying not only with time but also in relation to activity and time of day.³⁰ Such changes are discussed more fully in the consideration of appropriate follow-up. Knowing where the steep part of the strength-duration curve begins is crucial for appropriate programming; the clinician wants to ensure an adequate safety margin while minimizing the energy output, to maximize generator longevity. The ability to determine thresholds at a multitude of pulse amplitudes is a necessity.

The same argument also applies to the ability to vary the duration of the pulse width. Again, although the pacemakers in most children are programmed to a relatively small number of pulse durations, the ability to choose from a much larger number of such settings increases the accuracy with which the clinician can characterize the strength-duration curve.

Newer pacemakers now can automatically determine the voltage threshold, either on a beat-to-beat basis or at predetermined intervals throughout the day, and then adjust the pulse amplitude within a predetermined range to minimize energy drain and potentially increase generator longevity. This is accomplished by looking for an evoked potential after the test pulse, within a predetermined window of time indicating myocardial depolarization. Initially, these pacemakers required special low-polarization electrodes to distinguish true evoked potentials from electrode polarization. Newer designs have improved this discrimination, and now this feature has been expanded to function with most types of electrodes. Some devices still require bipolar electrode systems; some do not. Both endocardial electrodes³¹ and epicardial electrodes^32,33 have been employed with newer devices. Once the voltage threshold has been determined, the amplitude is adjusted to a predetermined value above the threshold. Threshold data are saved in the pacemaker, and later interrogation can provide the clinician with the threshold trend over time. Such a feature has been extended to the atrium as well as the ventricle in some devices. Even if the clinician is reticent to allow automatic changes to the output parameters, use of the feature in a “monitor only” mode can provide information as to the long-term changes in threshold and the potential need for programming changes. This feature has the potential to increase generator longevity and decrease the number of pacemaker replacements needed.

The third parameter of key importance to children is the rate. Although the use of fixed-rate pacemakers is becoming less common, the availability of a wide range of both lower and upper pacing rate limits is important in meeting the varying metabolic demands of the patient with congenital heart disease. Programming the upper rate limit (URL) of dual-chamber or rate-responsive pacemakers to less than 150 bpm is inadequate, particularly in a small child. Even older patients can raise their heart rates well above this value when exercising maximally in the absence of heart block; therefore, pulse generators must provide URL of at least 180 bpm, and preferably higher. Newer devices now provide a maximal URL of 210 bpm. The lower rate limit (LRL) needs are also variable. Immediately after surgery, greater LRLs are often necessary to maintain an adequate cardiac output. This is especially important after atrial surgery for patients in whom sinus node function may be impaired. In our opinion, lower rates must be programmable from at least 50 to 120 bpm. Higher LRLs may also be needed to decrease the incidence of tachyarrhythmias.

Another parameter often overlooked is the refractory period. In single-chamber pacing, this is often fixed to an arbitrary value of 325 msec, without much thought about whether this value is appropriate. For the ventricular channel, the refractory period must be of sufficient duration to prevent inappropriate T-wave sensing but not prevent sensing of spontaneous ventricular depolarizations. Measurement of the pace or sensing point to well beyond the T wave using the intracardiac electrogram (EGM) is straightforward (Fig. 18-13). In healthy children, the QT interval decreases with an increasing heart rate. When rate-variable pacing is used, the ventricular refractory period may be appropriate at rest but too long during exercise. Ideally, the period should vary with the pacing rate. Therefore, this value must be long enough to prevent T-wave sensing at the resting heart rate and short enough not to limit appropriate sensing at the upper pacing rate. Children with complete heart block may have spontaneous ventricular beats during the stress of exercise, which must be appropriately sensed. Appropriate programming is discussed later. Again, the wider the range of available refractory periods, the more universally applicable the pacemaker generator is to the entire pediatric population.

Figure 18-13 Intracardiac electrogram from a patient with VVI pacing shows duration of ventricular refractory period (“dashed” line) after either ventricular pacing (VP) or ventricular sensing (VS). In this example, the ventricular refractory period extends well beyond the termination of electric activity seen by the electrode, to prevent inappropriate T-wave sensing.

For AAIR pacing, the refractory period must be long enough to prevent sensing of ventricular events, but again, must not be too limiting in terms of the upper rate. Recording of the intracardiac EGM shows the extent to which ventricular events are sensed by the pacemaker and the minimum value to which the atrial refractory period may be safely programmed (Fig. 18-14).

Figure 18-14 Intracardiac atrial electrogram recorded from a patient with AAI pacing. The refractory period (“dashed” line) must be long enough to prevent sensing of ventricular events by the atrial sensing (AS) electrode. Note the marked increase in refractory period required compared with VVI pacing (see Fig. 18-13).

Rate-Responsive Pacing

For the single-chamber rate-responsive pacemaker, appropriate settings to mimic the pediatric response to exercise are mandatory. During exercise, the healthy child’s heart rate increases linearly with the increasing intensity of the exercise³⁴ (Fig. 18-15). When healthy children are exercised using the Bruce treadmill protocol, the heart rate increases an average of 20 bpm with each increase in exercise stage. This continues throughout the course of the exercise. After an abrupt increase in exercise intensity (from stage I to II), the heart rate shows a sudden rapid increase, reaching a plateau value. The child’s pacemaker should increase its rate in an appropriate manner with increasing exercise intensity—and this must occur quickly, reaching a plateau value rapidly for that level of intensity. Therefore, not only must the heart rate increase to an appropriate degree, but also must increase in an appropriate time frame to mimic the normal physiologic response to exercise in the pediatric patient.

Figure 18-15 Normal heart rate increases from the resting value in normal children exercised using the Bruce treadmill protocol. Brackets represent 1 standard error around the estimate.

(Data from Serwer GA, Uzark K, Beckman R, Dick M II: Optimal programming of rate altering parameters in children with rate-responsive pacemakers using graded treadmill exercise testing. Pacing Clin Electrophysiol 13:542, 1990.)

After termination of exercise, the heart rate decreases exponentially (Fig. 18-16). Although an initial rapid drop occurs, the heart rate does not reach resting levels for at least 10 minutes. Inappropriate rapid declines in heart rate after exercise termination may not meet the body’s metabolic demands and may result in inadequate cardiac output and a syncopal episode.

Figure 18-16 Normal heart rate decreases after exercise compared with pre-exercise value in normal children. Even at 10 minutes after exercise, the heart rate has not yet reached the resting value. Brackets represent 1 standard error around the estimate.

(Data from Serwer GA, Uzark K, Beckman R, Dick M II: Optimal programming of rate altering parameters in children with rate-responsive pacemakers using graded treadmill exercise testing. Pacing Clin Electrophysiol 13:542, 1990.)

With normal heart rate responses to exercise in children taken into account, the ideal rate-responsive pediatric pacemaker must have the ability to offer a variety of linear increases in heart rate with increasing exercise intensity (rate-response curves). In addition, it should offer a range of acceleration times (rate of heart rate increase with increased exercise intensity), with more rapid times preferred. Such increases in heart rate should be independent of resting and maximal rates. After the termination of exercise, the heart rate decline should be exponential but slow enough that the LRL is not reached for at least 10 minutes. Also, the pacemaker must be able to tailor its detection of increasing exercise levels to the individual patient. Different approaches address this problem; manufacturers realize that not all patients produce the same characteristics detectable by the pacemaker in response to the same degrees of exercise. Although simplicity in programming is desirable, the clinician must weigh against it the ability to tailor the pacemaker’s settings and optimize its performance for a given patient.

Numerous types of sensors have been used, the most common being activity (as a function of body vibration), blood temperature, and minute ventilation. Body vibration sensing is the most useful in children because it does not require special electrodes and is much the same in the child as in the adult. Blood temperature and minute-ventilation sensing³⁵ have been used in children, but to a lesser degree.

Dual-Chamber Pacing

The previous considerations also apply to dual-chamber pacing. Additional programmable settings must also be considered, however, primarily the ability to program an appropriate AV interval and decrease it with increasing atrial rate. Such shortening of the AV interval is clearly desirable in children and should occur with changes in the sensed atrial rate as well as with increases in the paced atrial rate during DDDR pacing. Because this decrease mimics the physiologic response more closely and provides a shorter total atrial refractory period (TARP) at higher rates, the multiblock rate is higher. This is probably the most important feature of dual-chamber generator selection, because children often reach much higher atrial rates than adults. If TARP is inappropriately long, multiblock occurs during the course of normal exercise, with a subsequent sudden decline in ventricular rate and the potential for syncope. Children typically reach atrial rates in excess of 180 bpm during routine exercise. If TARP, of which the AV delay is a major part, is abnormally long, problems will occur.

In addition, multiple settings for the postventricular atrial refractory period (PVARP) also are considered desirable, because of its contribution to TARP and ultimately to the multiblock rate. This parameter must have enough programmability to prevent inappropriate ventricular sensing by the atrial electrode while allowing a multiblock rate of at least 200 bpm (preferably 220 bpm), particularly in younger children. Many newer devices automatically decrease the PVARP with increasing rate to raise the multiblock rate. One should always check the minimal value to be sure it is short enough to provide an adequate multiblock rate. Also, one must be aware of the PVARP value at rest, to ensure it is not inappropriately long.

One closely allied feature that is mandatory is the ability to control the degree of PVARP extension after a spontaneous ventricular depolarization. An automatic extension of the PVARP after spontaneous ventricular depolarization is often used to prevent sensing of retrograde atrial activation, thus avoiding pacemaker-mediated tachycardia. This is not necessarily desirable in children, because the presence of retrograde-only ventriculoatrial conduction is rare, and therefore the risk for pacemaker-mediated tachycardia is rare. With exercise, spontaneous ventricular depolarizations do occur, and if an inappropriate PVARP extension occurs, normal atrial depolarizations may not be sensed, resulting in a sudden overall decline in the heart rate. The ability to disable this feature must be present for the generator to be appropriate for use in children. This concern is discussed later in regard to the use of exercise testing in follow-up.

Other dual-pacing features that must be considered include the ability to lower the URL in the presence of atrial tachycardia. Such rhythms, especially atrial flutter, may occur, particularly in the postoperative patient. Another potentially useful feature is the ability to decrease the LRL based on time of day. Children, who tend to have a much more predictable schedule than adults, can benefit from having their pacemakers programmed to a lower pacing rate during sleep than during the daytime hours, when a higher heart rate may be needed. This is particularly useful for the child with sinus node disease, because the intrinsic atrial rate cannot be relied on to govern the paced ventricular rate. With an intact sinus node, the pacing rate can simply be set at an appropriately low level for sleep and rest, knowing that it will be at an appropriate rate during waking hours. If sinus node disease is present, however, this may not be the case, and the ability to vary the lower pacing rate with the time of day may be helpful, because this feature can lower the average daily rate and prolong generator life. This feature may also be helpful in the single-chamber rate-responsive pacemaker. Another feature recently introduced is the ability to extend temporarily the AV interval searching for AV conduction. If conduction is found, the AV delay can be lengthened. This is an attempt to promote AV conduction and limit unnecessary ventricular pacing.

Summary

Traditional factors in pediatric pacing, such as generator longevity and size, are now less important in the selection of a generator. All generators are much smaller than previous models, and yet longevity has not been sacrificed, a result of improved circuit efficiency. The difference in size among single-chamber, dual-chamber, and rate-responsive pacemakers is often undetectable. Pediatric patients generally have long life expectancy, and the ability to have a highly programmable pacemaker implanted to meet changing metabolic demands of growth, age, or patient choices is indispensable. The difference in cost between a highly programmable unit and one with fewer features is minimal, particularly when the cost is spread over the life of the pacemaker. Box 18-1 summarizes the features that must be considered for the appropriate selection of a generator. The choice of a pacemaker should be based solely on the features it possesses and its ability to meet the demands of the patient.

Endocardial versus Epicardial Pacing

Initially, almost all electrode systems implanted in children were epicardial, because of the large size of the endocardial electrodes and pacing generators. The development of smaller electrodes and generators has changed this, although children still undergo epicardial lead placement as a result of small patient size or other factors that do not allow placement of an endocardial electrode system. MPPR data show a gradual increase in endocardial electrode use, but almost one half of all patients still receive epicardial electrodes (Fig. 18-17). Our basic approach is to assume that all children should undergo the placement of an endocardial system; we then evaluate the child for factors that do not allow endocardial electrode use. The major factors to be considered, in addition to patient size, are (1) venous access to the ventricle, (2) presence of intracardiac right-to-left shunting, (3) increased pulmonary vascular resistance, (4) right-sided prosthetic valves, and (5) severe right ventricular (RV) dysfunction or fibrosis causing either increased risk of thrombus formation or an inability to stimulate the endocardium.

Figure 18-17 Change in endocardial versus epicardial electrode use in children. Note the gradually increasing use of endocardial electrodes, although almost half of all electrodes placed are epicardial.

(Data from Midwest Pediatric Pacemaker Registry.)

Initially, it was believed that children weighing less than 15 kg (33 lb) and those younger than 4 years should not undergo placement of endocardial electrodes.³⁶ This was based on the belief that the subclavian vein and superior vena cava (SVC) were too small, leading to a high risk for thrombosis with vessel occlusion, and that the large size of the generators made implantation in the subclavicular area impractical. With increased experience and smaller generator and electrode sizes, however, many centers now routinely implant endocardial lead systems in children weighing less than 15 kg.^9,37–39 The lower range for weight is not yet known. From a technical standpoint, children as small as 3 kg (6.6 lb) can undergo endocardial electrode placement, but the follow-up of such children is too limited to know whether this is in their best interest. A study of 39 patients weighing 2.3 to 10 kg showed that 23% had some pacing system problem related to endocardial electrode placement.³⁹ Such problems included skin necrosis over the generator, subclavian vein thrombosis, and endocarditis on the electrode. Another 23% required electrode extraction. All except one patient had received a single-chamber device.

The risk for vessel thrombosis appears to be less than once believed, at least in the short term.⁴⁰ Although SVC thrombosis has been reported,^41,42 the true incidence is unknown, because noninvasive methods of detecting thrombosis are not sensitive and angiography is not routinely done unless thrombosis is suspected clinically (Fig. 18-18). Lead displacement secondary to growth remains a concern, although techniques have been proposed to deal with this problem.^9,38 The placement of large electrode loops within the atrium was proposed to allow for growth, but these loops may not be as effective as originally believed because they can fibrose to the cardiac wall, precluding uncoiling with growth.

Figure 18-18 Venous angiogram in a patient after endocardial pacemaker implantation through the left subclavian vein. Note the complete obstruction of the subclavian vein at the site of electrode entry (arrow), with collateral flow around the obstruction.

The major objection to endocardial electrode use in the small child is related to long-term problems. Because young children can be expected to require numerous electrodes over their lifetime, many more than in adult patients, the clinician must consider how many electrodes can be left in place before problems with vessel obstruction or tricuspid valve dysfunction occur, as well as the difficulty of extraction of old electrodes. Although now more widely used,^43,44 lead extraction still represents a significant problem in children, with the potential for damage to the cardiac structures, mainly the tricuspid valve. To commit a child to potentially numerous lead extractions is still a concern. In our institution, current guidelines call for the placement of dual-chamber endocardial systems in children weighing 15 kg or more and the placement of single-chamber endocardial systems, if single-chamber pacing is appropriate, in children heavier than 8 kg (17.5 lb). These guidelines may change as electrode development continues and as data on the long-term follow-up of children with endocardially placed electrodes become available.

The next factor that must be considered is the presence of intracardiac shunting. Electrodes are potential sources of small particulate matter, with the risk for subsequent embolization until endothelialization occurs.⁴⁵ This does not tend to be a problem when particulate matter goes only to the lungs, where it is filtered out of the circulation and eventually absorbed, except in the presence of preexisting elevated pulmonary vascular resistance (PVR) or after a Fontan procedure. In the presence of right-to-left shunting, however, the potential for systemic embolization is great. The general recommendation is to avoid such electrodes in patients with documented right-to-left shunting.³⁶ This also must be considered in patients with the potential for right-to-left shunting, even if their net intracardiac shunt is left to right. Children with atrial and ventricular septal defects can show right-to-left shunting in the setting of elevated RV pressure, even with a net left-to-right shunt.^46,47 The specific hemodynamic situation of the individual child must be considered before endocardial electrode implantation is performed.

The same concerns apply to the child with elevated PVR or Fontan physiology, in whom pulmonary embolization of even small matter may further elevate PVR. Whether short-term anticoagulation of such patients until lead endothelialization can occur would preclude such concerns and permit transvenous pacemaker placement has yet to be investigated. If epicardial pacing is not possible, this may be an acceptable alternative, but given the current lack of knowledge on the benefit of anticoagulation in this setting, it should not be general practice.

The presence of a mechanical tricuspid valve prosthesis negates the ability to use an endocardial pacing system. There have been isolated reports of endocardial electrode placements at open-heart surgery through the perivalvular area.⁴⁸ This requires cardiopulmonary bypass and can be done only at valve placement. This technique cannot be used in the usual transvenous implantation; and it prevents lead extraction should that become necessary, except during repeat open-heart surgery. In cases of a heterograft valve rather than a mechanical one, the new, smaller transvenous leads have been placed without interfering with valve function (Fig. 18-19). Placement of a ventricular lead through the coronary sinus in the patient with an artifical tricuspid valve has been reported.⁴⁹

Figure 18-19 Echocardiogram from a patient after placement of pacing electrode across bioprosthetic tricuspid valve. A, Electrode is coursing between the valve struts. B, Color Doppler tracing shows only trivial regurgitation (arrow).

The physician also must consider the state of the right ventricle. Severe RV dysfunction and endocardial fibrosis can occur in children with congenital cardiac disease and may prevent adequate pacing of the right ventricle. This tends to be more prevalent in the older child with long-standing disease. In such children, left ventricular (LV) pacing and therefore epicardial pacing may become necessary. In the patient with severe RV dysfunction and dilatation, an appropriate endocardial site that permits both adequate sensing and pacing may not be achievable. In addition, severe RV dysfunction can lead to an increased risk of thrombus formation.

The patient who has undergone a Fontan procedure presents a somewhat unique situation. As mentioned previously, controversy surrounds the best approach for electrode placement in these children. If ventricular pacing is required, the epicardial approach is the only one available. However, many of these patients have intact AV conduction, require pacing only for sinus node dysfunction, and are best served by atrial-only pacing. The original Fontan procedure connected the right atrial appendage to the pulmonary artery, leaving the entire right atrial chamber available for electrode placement. The more common techniques at present either create a tunnel along the lateral right atrial wall from the inferior vena cava to the pulmonary artery or use an extracardiac prosthetic conduit. Often, a communication exists from the system to the pulmonary atrial chamber, resulting in a right-to-left shunt. Transvenous pacing has been performed in such patients despite anticoagulation,⁵⁰ but electrode placement can be difficult because of the small-system venous chamber, and right-to-left embolization with neurologic sequelae has been reported.⁵⁰ For these reasons, our approach has been to avoid the transvenous approach unless the epicardial approach is not possible because of extensive fibrosis. Pacing in this group of patients can be extremely challenging regardless of the approach chosen.

In summary, endocardial pacing is generally preferable because of the ease of implantation and the improved longevity of the electrode. Long-term thresholds are as stable as epicardial electrodes and tend to be lower (Fig. 18-20). This permits lower-output programming of the pacing generator, enhancing its longevity. However, endocardial electrode use is contraindicated in many situations (Box 18-2). As such, epicardial electrodes still play a significant role in pediatric pacing.

Figure 18-20 Comparison of long-term pulse-width thresholds for endocardial versus epicardial electrodes measured at a pulse amplitude of 2.5 V. Thresholds for both groups show no significant rise, and do not differ significantly from each other. Brackets represent 1 standard deviation around the mean.

Unipolar versus Bipolar Pacing

The choice between a unipolar and a bipolar electrode configuration was formerly an issue only for endocardial implantation. Since the development of bipolar epicardial electrodes, bipolar epicardial pacing is an available option. MPPR data show an increasing trend toward bipolar epicardial pacing. In our institution, more than 95% of all epicardial implants now use bipolar electrodes. The epicardial electrode used is the Model 4968 (Medtronic, Minneapolis), which has two electrode heads that are sutured to the epicardial surface. This electrode has thresholds comparable to unipolar models but much higher impedance, often 600 to 1200 Ω, comparable to endocardial electrodes.

For endocardial pacing systems, the choice between unipolar and bipolar pacing becomes more controversial. Again, MPPR data show that most endocardial systems are bipolar (Fig. 18-21). It was initially believed that unipolar pacing was preferable because of the smaller size of the unipolar pacing electrode.⁵¹ With improved electrode design, however, this size difference has become negligible. Electrode body diameter is important in determining the risk for venous thrombosis, currently with minimal difference in size. For example, the Model 4057M unipolar screw-in electrode (Medtronic) has a body diameter of 2.2 mm and the 4058M bipolar version, 2.4 mm. Such minimal difference is also seen in passive-fixation electrodes, such as the Model 4023 unipolar steroid-eluting electrode, which has a body diameter of 1.2 mm, compared with 1.9 mm for the bipolar version (Model 4024). Newer electrodes have continued this trend toward small bipolar electrodes; the Medtronic SelectSecure has a lead body diameter of 1.4 mm (4.1 French). In all cases, diameter of the electrode head is smaller for a unipolar electrode, which may make insertion easier, although once implanted, size of the head is of little consequence.

Figure 18-21 More than two thirds of endocardial electrode systems use bipolar electrodes.

(Data from Midwest Pediatric Pacemaker Registry.)

An MPPR comparison of acute implantation characteristics between unipolar and bipolar electrodes of similar design showed no significant difference for threshold values of voltage, current, or resistance (Fig. 18-22). However, some believe that long-term thresholds are improved in unipolar pacing.⁵¹ This may result from the smaller size of the head and its lower weight, which creates less tension on the endocardial surface, particularly with active-fixation electrodes, and therefore results in less tip fibrosis. However, this issue appears to affect only active-fixation leads and has not been reported with passive-fixation tined electrodes.

Figure 18-22 Acute implantation thresholds for bipolar versus unipolar endocardial electrodes are not significantly different for voltage, current, or electrode impedance. All were measured at a 0.5-msec pulse width. Brackets represent 1 standard deviation around the mean. MA, milliamperes (current).

(Data from Midwest Pediatric Pacemaker Registry.)

Although sensing using a bipolar electrode was initially thought to be inferior to unipolar sensing because of the proximity of the two electrodes, MPPR data show this is not the case. Acute-implantation R-wave amplitudes and slew rates show no significant difference between unipolar and bipolar electrodes (Fig. 18-23). However, unipolar sensing is more affected by myopotentials and is more prone to oversensing and inappropriate pacemaker inhibition, affecting an estimated 31% to 93% of patients, as discussed later. Bipolar sensing is rarely affected by such myopotential inhibition because of the closer proximity of the electrodes. The degree to which such inappropriate inhibition is seen depends on the location of the generator, the provocative tests used, and the generator model implanted. All series, however, report a significant incidence of this problem, which can be a particular concern in active children.

Figure 18-23 Acute implantation electrode sensing for bipolar versus unipolar endocardial electrodes demonstrates no significant difference in either R-wave amplitude (millivolts) or slew rate (volts per second). Brackets indicate 1 standard deviation around the mean.

(Data from Midwest Pediatric Pacemaker Registry.)

In most active children, bipolar pacing is preferable, particularly with the smaller electrode sizes now available. Sensing does not appear to be a problem, and myopotential inhibition is seen less often than in the past. In addition, with bipolar pacing, the risk for extracardiac pacing of the surrounding muscle is minimized, particularly if it is difficult to position the generator in a location where all such stimulation of surrounding muscles can be avoided. This is particularly relevant to implantations with the pocket placed in the subclavicular region. In many children, the lack of significant subcutaneous tissue requires placement of the generator in the subpectoral position, where the risk for extracardiac pacing with a unipolar system increases.

Epicardial Electrode Types

Compared with endocardial electrodes, relatively few epicardial electrode types are available. Previously, all electrodes were intramyocardial, with this portion either a corkscrew coil or a single wire with barbed end (fishhook).^52–54 A true epicardial electrode with steroid-eluting capabilities is sutured to the epicardial surface and requires direct contact of the electrode with excitable myocardium^55–58 (see later discussion). Based on MPPR data, since 2000 the most widely used electrode types are the bipolar true epicardial (>90% of implants), unipolar true epicardial, screw-in corkscrew, and barbed fishhook, used exclusively in the atrium (Fig. 18-24). The choice of electrode depends on the chamber to be paced and the preference of the implanting physician.

Figure 18-24 Distribution of epicardial electrode types used since 2000 (Medtronic models). The most common epicardial electrode is the true epicardial bipolar (4968), followed by true epicardial unipolar (4965), screw-in (5017/6917/5069/5071), and fishhook (4951M) electrode. The Medtronic 4968 has become the most widely used epicardial electrode in both the atrium and the ventricle (>90%).

Atrial epicardial implantation requires an electrode that sits on the epicardial surface or has only shallow penetration into the atrial myocardium. Should the electrode extend through the chamber wall into the atrial cavity, a low-resistance circuit is established through the blood pool, with an inability to pace reliably. Few data can be found on the longevity or thresholds of atrial epicardial electrodes, partly because of the constant change in electrode design, making long-term comparisons difficult. The most widely used atrial electrode is the fishhook or stab-in electrode, because it does not penetrate deeply into the myocardium, compared with the screw-in or corkscrew type. The Medtronic Model 4951M(P), which has a platinized coating, has resulted in slightly improved thresholds at acute implantation (Fig. 18-25). The average threshold at implantation is about 1.05 V and 3.3 mA, measured at a pulse width of 0.5 msec. The average acute electrode impedance is 320 Ω.

Figure 18-25 Acute implantation voltage and current threshold values measured at a 0.5-msec pulse width for the original fishhook electrode (Medtronic Model 4951) compared with the newer platinized version (4951M[P]) and the steroid-eluting electrode (4965) when implanted on the atrium. Acute thresholds are not statistically different. Current at threshold for the steroid electrode is slightly higher, implying a lower impedance. Brackets represent 1 standard deviation around the mean. MA, milliamperes.

(Data from Midwest Pediatric Pacemaker Registry.)

The steroid-eluting true epicardial or epimyocardial electrode is increasingly used as an atrial epicardial electrode and is becoming the dominant electrode for atrial epicardial pacing. Although implantation may be slightly more difficult, acute thresholds are comparable. In our institution, this type of electrode is currently used in more than 95% of all atrial epicardial implants. The acute threshold average is 1.0 V at 0.50-msec pulse width, and somewhat higher for the bipolar version. The average impedance is 301 Ω for the unipolar Medtronic Model 4965 and 705 Ω for the bipolar 4968 electrode. We prefer the bipolar version because of the decreased far-field R-wave (FFRW) oversensing and increased impedance.

More diversity exists in the choice of the electrode for ventricular pacing. A comparison of the original nonplatinized fishhook electrode (Medtronic Model 4951) with the screw-in electrode (Model 6917 or 5069) reveals essentially no difference in acute implantation thresholds. However, implantation thresholds for the currently available platinized fishhook electrode, 4951M(P), are slightly improved (Fig. 18-26). Both steroid-eluting electrodes tend to have slightly higher thresholds. Lead survival appears to be longer for the screw-in electrode than for the fishhook type, with 5-year lead survival rates of about 84% and 65%, respectively.⁵⁵ Most failures were caused by exit block. Other studies, however, have found little difference in longevity between these two types of electrodes.⁵³ Some of this interstudy discrepancy is believed to be related to different surgical modes of implantation for the fishhook electrode. Whether or not the lead was stabilized by being sutured to the myocardium was variable. Lack of such stabilization may cause more electrode movement in the myocardium and therefore increased fibrosis and risk for exit block develpoment⁵⁴ (see Implantation Techniques).

Figure 18-26 Comparison of thresholds for Medtronic Model 6917 (now 5069) screw-in electrode, Model 4951 fishhook electrode, Model 4951M(P) platinized fishhook electrode, and steroid-eluting electrode (Model 4965), when implanted on the ventricle, shows no significant difference in voltage threshold, but impedance is slightly lower for steroid-eluting electrode, similar to findings for atrial use. Brackets represent 1 standard deviation around the mean. Current on Y axis is in milliamperes.

(Data from Midwest Pediatric Pacemaker Registry.)

A key advance in epicardial pacing, as mentioned earlier, is the steroid-eluting electrode.^55–58 This electrode consists of a platinized flat electrode that sits atop the epicardial surface, with a dexamethasone-impregnated silicone plug to allow elution of the drug onto the area of myocardium being stimulated. About 1 mg of dexamethasone is present within the electrode. Both a unipolar version (Model 4965) and a bipolar version (4968) of this electrode are available (Medtronic). This electrode is affixed to the epicardial surface by sutures, with the active portion of the electrode not extending into the myocardium as with other epicardial electrodes. This is both advantageous and disadvantageous, depending on the patient. For patients whose epicardial surfaces are relatively nonfibrotic, this appears to be an advantage, because there is less myocardial injury to provoke subsequent fibrosis. However, for patients with extremely fibrotic epicardial surfaces, as often occurs after multiple open-heart surgical procedures, these electrodes are difficult to use. In such patients, the surgeon must either find an area of myocardium with limited epicardial fibrotic reaction or strip away a fibrotic layer to expose the myocardium, which simply leads to further fibrosis.

Initial experience with this steroid-eluting electrode is encouraging. Although acute thresholds appear to be comparable with those of other epicardial electrodes (see Fig. 18-26), the threshold rise is less over the first several months after implantation (Fig. 18-27).⁵⁷ Midterm survival is also encouraging. For the Model 4968 lead placed on either the ventricle or the atrium, longevity is 85% at 75 months (Fig. 18-28). For the unipolar version (Model 4965), longevity is 75% at 75 months and 55% at 115 months for ventricular usage (Fig. 18-29).

Figure 18-27 Comparative thresholds of Medtronic Model 4951-P platinized fishhook electrode versus the epicardial steroid-eluting electrode (Model 10295A, now 4965) show a lack of significant threshold rise in the first 3 months after implantation for the steroid-eluting epicardial electrode, with threshold improvement at 1 year after implantation.

(From Karpawich PP, Harkini M, Arciniegas E, Cavitt DL: Improved chronic epicardial pacing in children: steroid contribution to porous platinized electrodes. Pacing Clin Electrophysiol 15:1152, 1992.)

Figure 18-28 Kaplan-Meier survival curve for Medtronic Model 4968 bipolar electrode shows 85% survival at 75 months for electrodes placed in both the atrium (top) and the ventricle (bottom).

Figure 18-29 Kaplan-Meier survival curve for Medtronic Model 4965 unipolar electrode placed in the ventricle shows 75% survival at 75 months and 55% at 115 months.

In select patients with limited myocardial fibrosis, epicardial electrodes that do not penetrate the myocardium appear to be beneficial; however, care must be taken to choose the most appropriate form of ventricular epicardial electrode to fit the individual patient. All three types of ventricular electrodes in current use have advantages and disadvantages, and no one type is ideally suited to all patients.

Endocardial Electrode Types

As in the adult population, several general types of endocardial electrodes are in pediatric use: passive-fixation non-steroid-eluting electrodes, passive-fixation steroid-eluting electrodes, and active-fixation electrodes—both non-steroid-eluting and developed steroid-eluting models. However, the vast majority of electrodes used currently are steroid eluting. The most common endocardial electrode type in children continues to be the active-fixation screw-in electrode (Fig. 18-30). The use of this electrode has been advocated because of its better fixation qualities, given the tremendous range of anatomic variations in children with congenital heart disease. For implantation within the morphologic left ventricle in a child who has undergone an atrial switch repair for transposition of the great arteries, or in a child with ventricular inversion, the active-fixation electrode is almost universally used. Also, in a child whose right ventricle is greatly dilated, such that it is difficult to wedge the electrode into the trabecular recesses of the right ventricle, active-fixation electrodes are preferable. Alternate site RV pacing also requires an active-fixation electrode.

Figure 18-30 Distribution of endocardial electrode types used: Active, active-fixation (screw-in) electrode; Passive, passive-fixation (tined) non-steroid-eluting electrode; Steroid, passive-fixation steroid-eluting electrode.

(Data from Midwest Pediatric Pacemaker Registry.)

A comparison of acute threshold data shows that active-fixation and passive-fixation electrodes have similar thresholds (Fig. 18-31). Both non-steroid-eluting passive-fixation electrodes and steroid-eluting passive-fixation electrodes have lower acute thresholds and do not differ from each other. The presence of the steroid does not influence the acute implantation threshold, although with increasing electrode age, this is no longer the case. Follow-up data indicate significantly lower electrode thresholds for the steroid-eluting electrode compared with other types.⁵⁸ A comparison of steroid-eluting and non-steroid-eluting electrodes showed no difference in chronic thresholds at 5-V pulse amplitude but did show differences at 2.5 and 1.6 V (Fig. 18-32). Therefore, it appears that the strength-duration curve is shifted leftward for the steroid-eluting electrodes.

Figure 18-31 Acute thresholds for endocardial electrode types show a tendency toward lower thresholds in the passive-fixation and steroid-eluting electrodes compared with the active-fixation electrodes. However, the difference in voltage threshold is minimal. The changes become more evident in current and impedance values, with steroid-eluting electrodes having the lowest impedance and the highest current flow. Brackets represent 1 standard deviation around the mean. Current on Y axis is in milliamperes.

(Data from Midwest Pediatric Pacemaker Registry.)

Figure 18-32 Chronic long-term electrode thresholds are significantly higher for active-fixation electrodes than for either passive-fixation or steroid-eluting electrodes. At low pulse amplitudes, steroid-eluting electrodes show significantly lower thresholds than the other types. Brackets represent 1 standard deviation around the mean.

(Data from Serwer GA, Dorostkar PC, LeRoy S, Dick M II: Comparison of chronic thresholds between differing endocardial electrode types in children. Circulation 86:1-43, 1992.)

Such changes affect generator programming. In one study, only 33% of generators that used non-steroid-eluting electrodes could be programmed to 2.5 V of pulse amplitude, compared with 77% of generators that used steroid-eluting electrodes.⁵⁸ The remainder required a pulse amplitude of 5 V or greater. Therefore, the use of steroid-eluting passive-fixation electrodes allowed chronically lower output settings for the pulse generator, thereby increasing generator longevity. The follow-up averaged 3.3 years (median, 3.6 years). There are insufficient data to compare the steroid-eluting active-fixation electrodes with the older types.

A smaller bipolar active-fixation electrode introduced in 2005 (Medtronic Model 3830) differs in many ways from other leads. It has an isodiametric design with diameter of 4.1 French and a fixed helix. With no central lumen, the electrode must be introduced through a guiding catheter (see Implantation Techniques). This makes the lead much smaller and thus more advantageous in children. It is steroid eluting, but instead of a steroid plug behind the helix, the steroid actually coats the helix. This electrode is particularly useful in patients with venous narrowing and should decrease the incidence of postimplant venous occlusion. Early data show excellent acute thresholds and low dislodgement rates comparable to stylet-placed leads.⁵⁹ The catheter delivery system makes this an excellent choice for children with complex anatomy and alternate-site pacing.

Single-lead VDD systems have been employed in children. These systems obviate the need for two electrodes yet maintain AV synchrony.⁶⁰ However, a major problem is lack of atrial pacing capabilities when sinus node function is inadequate. Also, the spacing of the atrial electrodes from the electrode tip often is too great for use in children. The electrode must be buckled so that the atrial dipole is within the ventricle (Fig. 18-33), which could lead to ventricular electrode dislodgement. These problems, together with the increased complexity of the electrode construction, with a concomitant potential for increased electrode failure, combine to limit the usefulness of this approach in children, especially smaller children. Also, these are rather large leads, and with the development of smaller leads, utility of these leads is minimal.

Figure 18-33 Chest radiograph showing placement of single-pass VDD system. Note the lead buckling in the atrium, which is necessary to place the atrial sensing electrodes (arrow) within the atrium.

Summary

We believe that the ideal pacing system in children is a dual-chamber system with the capacity for rate-responsive pacing, using endocardial atrial and ventricular bipolar electrodes, with active-fixation electrodes in the atrium and steroid-eluting active-fixation electrodes in the ventricle. The small, Medtronic Model 3830 electrode has become our first-line endocardial pacing lead, although it does require a relearning of implantation techniques. When the child’s size is marginal or endocardial pacing is contraindicated, we use epicardial pacing. In these patients, the preferred electrode is the steroid-eluting bipolar electrode for both the atrium and the ventricle or, in the presence of significant epicardial fibrosis, the platinized version of the fishhook electrode for the atrium and the helix (corkscrew) electrode for the ventricle.

Epicardial Implantation Techniques

The approach used for the placement of epicardial electrodes is highly variable, depending on the patient’s circumstances. Approaches to the epicardial surface have included thoracotomy, sternotomy, and subxiphoid incision. Regardless of the approach, an attempt should be made to place the electrode on the left ventricle rather than the right ventricle to minimize dyssynchrony and to preserve ventricular function.

The subxiphoid approach requires the smallest incision. Through the same incision, both electrode implantation on the epicardial surface of the heart and pacemaker implantation in the abdomen can be accomplished.^61–63 This approach has been used for both ventricular and atrial epicardial electrode implantation. The disadvantage of the subxiphoid approach is that it exposes only a limited ventricular and atrial surface, and in a patient who has extensive epicardial fibrosis from prior cardiac surgery, finding a suitable location for implantation of the electrode can be challenging. Such an approach requires a 6- to 7-cm skin incision from the xiphoid tip to a point superior to the umbilicus. The dissection is continued as deep as believed necessary to provide adequate tissue between the pacemaker and the skin surface. Depending on the depth of subcutaneous tissue, the pacemaker can be implanted above the rectus muscle, below the rectus muscle but above the peritoneum, or in some circumstances, intraperitoneally, housed in a Silastic pouch sutured to the underside of the peritoneum and anchored to the rectus fascia.⁶² The depth to which the incision is carried should be governed solely by the tissue available to cover the pacemaker. When only minimal tissue is present over the pacemaker, besides an unsightly bulge, the skin is more susceptible to trauma, with a resultant risk of pacemaker erosion and infection.

A left thoracotomy approach is often used in the child who has had cardiac surgery and thus is at high risk for significant epicardial fibrosis.⁶⁴ This approach affords an increased myocardial surface from which to choose an appropriate pacing site. The generator can be implanted in the abdomen, in the subclavicular region, or in rare settings, intrathoracically. However, for implantation of a dual-chamber system, the left thoracotomy approach affords less exposure of the atrium and requires left atrial pacing. Although this has been accomplished, it introduces complexity into the programming, because the postpace AV interval must be prolonged sufficiently to afford adequate time for RV filling, because of the time necessary for left-to-right atrial excitation spread. The postsense AV interval must be short to avoid an excessive AV interval. For this reason, we believe that right atrial pacing is preferable, but we use left atrial pacing if right atrial pacing is not possible. When the thoracotomy approach is used with abdominal placement of the generator, the electrode must be passed subcostally to a pocket created in the abdomen through a separate incision. The electrode must not be passed over a rib and must be tunneled from the thoracic cavity to the abdomen as medially as possible, to minimize the risk for traumatic electrode fracture.

A median sternotomy approach can also be used, but this creates the largest and most obvious scar. However, it affords the best exposure of the epicardial surface of both the ventricle and the right atrium. In a patient with significant epicardial scarring and fibrosis, this provides the highest likelihood of finding an appropriate pacing site. After the electrode has been implanted, the device can be pulled through the subxiphoid region to the abdominal wall, where, through a separate incision or an extension of the median sternotomy incision, an appropriate pocket can be created.

Regardless of the electrode type chosen, appropriate anchoring of the electrode to the epicardial surface is crucial. Our approach is to insert the electrode into the myocardium and determine the threshold strength-duration curve. If it is acceptable, the electrode is sutured to the epicardial surface, and the thresholds are rechecked. Suturing the electrode to the surface theoretically reduces movement of the electrode tip within the myocardium, with less formation of fibrotic tissue and better long-term performance of the electrode. When the fishhook electrode is used and there is significant epicardial fibrosis, unbending of the barb to permit deeper penetration is often beneficial. Several types of myocardial electrodes should be available to the physician during implantation, to address any situation encountered.

When the steroid-eluting electrode is used, a suitable position is found by holding the electrode in contact with the epicardial surface and quickly determining the voltage threshold at a single pulse width. If it is acceptable, the electrode is sutured to the epicardial surface, and a complete strength-duration curve is determined. Often, several sites must be tested before a suitable site is found. When using the bipolar version of this electrode, proper orientation of the two heads to each other is mandatory to achieve optimal sensing. For atrial implantation, the interelectrode axis should be perpendicular to the long axis of the ventricle, to minimize FFRW sensing. For ventricular use, however, the interelectrode axis should be parallel to the ventricular long axis, to maximize FFRW sensing.

A gentle loop of the electrode should be left in the thoracic cavity to allow for some growth, but excessive loops should be avoided. Cases of entrapment of vascular structures by pacemaker electrodes have been reported.^65,66 The extra length of the electrode is easily coiled and placed within the generator pocket; even this may allow for some growth, because the electrode wire slowly uncoils with growth of the patient.

The exact placement of the abdominal pocket is generally left to the discretion of the surgeon. However, it should be placed away from the belt line and not in the right upper quadrant; placement there interferes with subsequent assessment of hepatic size, which may be important in children with structural cardiac disease. The electrodes should be tunneled from the thorax to the abdomen as near the midline as possible, to minimize the risk for electrode fracture.

The placement of unused electrodes is not recommended. It was once believed that the placement of an unused electrode would provide a “spare” electrode that could be used if the primary electrode failed, thus precluding the need for a second thoracotomy. It was found, however, that if the primary electrode failed, the redundant electrode likely was also unusable.⁶⁷ Therefore, the extra electrode provided no benefit to the child.

Endocardial Implantation Techniques

Before endocardial implantation, echocardiography should be performed to make certain no contraindications to endocardial implantation exist—specifically, tricuspid valve dysfunction, right-to-left shunting, interatrial communications, or SVC obstruction. Whether the pacemaker is placed under the left or right clavicle is somewhat arbitrary but may be important for some patients, and the location should be discussed with them. Our general policy is to place the pacemaker on the side opposite the patient’s handedness (right-handed, pacemaker on left side).

The endocardial approach used for implantation in children is similar to that in adults. Before beginning, transcutaneous pacing electrodes are placed to provide emergency pacing ability. A sudden decrease in intrinsic heart rate caused by the stress of the procedure or the general anesthesia, if administered, is not unusual. Although placement of a temporary transvenous pacing electrode can be done, the stress of this procedure alone can cause bradycardia. In addition, the risk for infection of the permanent system by the temporary electrode must be considered. Transcutaneous pacing electrodes for all sizes of children are now available and function well.

Next, venous angiography is performed to assess the anatomy and patency of the axillary and subclavian vein, as well as SVC patency. With a left-sided implant, presence of a left-sided SVC should be excluded. While introduction of electrodes through a left SVC can be done, we prefer to avoid this unless there is no option.

Whether the electrode is introduced into the vascular system by a direct subclavian vein puncture,^37,68,69 a cephalic vein cutdown procedure,^9,38 or axillary vein puncture should be guided by the experience of the implanting physician. In our institution, direct vein puncture is used exclusively. Our approach is to enter the subclavian or axillary vein percutaneously and introduce a guide wire. An incision is then made laterally either along the deltopectoral groove or in the axilla,⁷⁰ if the axillary vein approach is chosen. For the more common subclavian vein approach, the dissection is then carried down to the pectoral muscle. At this point, the depth of subcutaneous tissue is examined to determine whether tissue is sufficient to cover the generator. In many children, this tissue is inadequate, and placement of the generator above the pectoral muscle results in a significant pacemaker bulge as well as an increased risk for pacemaker erosion and trauma to the tissue covering the pacemaker.⁶⁹ This is also psychologically important, because many children are self-conscious about a prominent bulge. If the tissue is believed to be inadequate, the dissection is carried through the pectoral, using blunt dissection to separate the muscle fibers, and a pocket is created in the subpectoral region.

For the axillary approach, the pocket is always created under the pectoral muscle.⁷¹ After a pocket adequate to accommodate the pacemaker is created and the guidewire is incorporated into the pocket, a sheath and dilator are introduced into the subclavian vein over the previously positioned guidewire. The electrode is then passed into the right atrium, together with a new guidewire. The sheath is then removed. Retention of the guidewire allows for the introduction of a second electrode, in the case of a dual-chamber implant, or the option to remove the prior electrode and replace it without having to repuncture the subclavian vein. Some prefer to perform two separate vein punctures for introduction of two guidewires. This has the advantage of separating the entry sites of the electrodes such that manipulation of one is less likely to affect the other. Also, having two smaller holes in the vein rather than one larger hole may decrease bleeding. After the electrodes have been positioned and tested, any retained guidewire can be removed. The electrodes are then connected to the generator and placed in the pocket.

For the lumenless Medtronic Model 3830 electrode, a slightly different technique is required. Rather than introduction of a sheath and dilator over the guidewire, a splittable guiding catheter and dilator are introduced and advanced over the guidewire into the desired chamber. The two styles of guiding catheters are deflectable and fixed curve. The deflectable catheter allows for fine control over tip position; however, current models are 9 French (9F) in diameter, requiring a hole in the vein much larger than the pacing electrode. The fixed-curve 7F catheter requires a smaller hole, but the curve style must be chosen before introduction. Several curve styles and sizes are available. Once the catheter tip is positioned at the point of electrode attachment, the wire and dilator are removed. The pacing electrode is then introduced through the catheter, emerging at the point of attachment. The pacing electrode is rotated 3 to 4 turns to affix the electrode tip in the endocardium. Electrode testing is then performed, and if adequate, a large loop of catheter is introduced. This is critical, because once the guiding catheter is removed, advancement of the electrode is difficult if not impossible. Once the catheter is removed, final position is achieved by carefully pulling back on the electrode. If two electrodes are to be placed, we often keep both guiding catheters in place until both electrodes are in place and tested. This reduces the chance of moving the first catheter with the second, necessitating repuncture of the vein; electrode repositioning is not possible once the guiding catheter has been removed. After removal of all guiding catheters, electrode sensing should be retested because small changes in electrode orientation can affect the measured values.

Another implantation approach is a hybrid of the epicardial and endocardial methods previously described. In some patients, transvenous implantation would be preferable but there is no venous access; the electrode can be inserted into the atrium through an incision in the atrial wall, then advanced into the ventricle across the AV valve.⁷⁰ The atrial incision is closed with a purse-string suture after the electrode has been positioned in the ventricle. This provides the advantages of endocardial pacing with improved thresholds when venous access to the ventricle is not available. We have employed this approach in children with unacceptable epicardial electrode thresholds but with no venous access (Fig. 18-34). This can be especially useful after a right-sided heart bypass (Fontan) procedure, even though anticoagulation is necessary because of implantation within the systemic ventricle.⁴⁵

Figure 18-34 Chest radiograph showing placement of endocardial electrode, which has been introduced through atrial wall, passed through atrioventricular valve, and secured to ventricular endocardial surface. The electrode is tunneled to the generator pocket in the abdominal wall.

The optimal site within the right ventricle for electrode placement has been questioned. Initially, the RV apex was used because it affords a stable site. However, the alteration of excitation sequence may lead to altered hemodynamics.^72,73 Studies suggest that apical pacing may induce long-term changes in both RV and LV performance. Midseptal pacing may be more hemodynamically beneficial, but it is more difficult to achieve a stable electrode position with this approach.⁷³ Use of the Medtronic 3830 using the guiding catheter makes midseptal positioning easier to obtain.

Acute Electrode Evaluation

After placement of the electrode, its electrical characteristics must be determined. For active-fixation or intramyocardial electrodes, 15 minutes should be allowed after placement of the electrode before threshold testing, to permit acute myocardial changes caused by electrode entry into the myocardium to subside. All changes do not subside in this period, but this short delay is warranted. Our initial approach is to measure the electrode’s impedance and, if possible, the intrinsic EGM amplitudes. The impedance should be between 200 and 600 Ω for an epicardially placed electrode (values up to 1200 Ω can be acceptable with bipolar Medtronic 4968 electrode) and between 300 and 700 Ω for an endocardially placed electrode. The amplitude of the EGM should be sufficient to allow appropriate sensing by the generator being implanted. Care must be taken to avoid measurement of injury current rather than true signal amplitude (Fig. 18-35). Also, pacing thresholds can be adversely affected in the presence of injury current, and all measurements should be postponed until it resolves. The minimally acceptable spontaneous signal amplitude varies, depending on the specific generator.

Figure 18-35 Electrograms after acute endocardial placement of active-fixation electrode.

A, Note the injury current present, making it impossible to determine waveform amplitude accurately. B, After 15 minutes, injury current (bar) has decreased, allowing accurate measurement of EGM amplitude. The true amplitude is much lower than the magnitude of the injury current in A. Also, thresholds may be adversely affected in the presence of injury current. All measurements should be postponed until it has resolved.

If these measurements are found to be acceptable, threshold testing is performed. It is our general practice to set a given pulse width and then determine the minimum pulse amplitude necessary to maintain capture. Multiple threshold determinations at differing pulse widths are performed to define the strength-duration curve adequately. It is important to determine the shape of the strength-duration curve to ensure that the minimum pulse width necessary to pace at any pulse amplitude and the minimum amplitude necessary to pace at any pulse width are sufficiently removed from the proposed generator settings, to allow for some movement in the strength-duration curve without risking loss of capture (Fig. 18-36). Data from the MPPR suggest that the ventricular threshold of a new electrode at a 0.5-msec pulse width is less than 1 V, and for the atrium, less than 1.5 V. Such threshold guidelines apply to both endocardial and epicardial electrodes.

Figure 18-36 Example of strength-duration curve determined at implantation. From this curve, the minimum voltage needed to pace at any pulse width and the minimum pulse width needed to pace at any voltage can be determined.

Before the pulse generator is connected to the electrodes, it is advisable to pace the ventricle at the maximal output of the pacing system analyzer. In children, because the diaphragm is close to the electrode, diaphragmatic pacing can occur. If diaphragmatic pacing occurs at the maximal pacing system analyzer output, the electrode must be moved. This is particularly relevant for children with transposition of the great arteries, in whom a ventricular electrode is positioned within the left ventricle. Positioning of the electrode tip at the LV apex puts it close to the diaphragm, and there is a high incidence of diaphragmatic pacing. To prevent this, we position the electrode on the midposterior free wall at the approximate level of the papillary muscles (Fig. 18-37). This affords acceptable thresholds with a minimal risk for diaphragmatic pacing. This problem is more often seen in endocardial pacing than in epicardial pacing, but it can occur in either setting. Diaphragmatic stimulation can be seen with atrial pacing as a result of phrenic nerve stimulation. Thus, high-output atrial pacing must also be performed.

Figure 18-37 Posteroanterior (A) and lateral (B) chest radiographs from a patient who underwent endocardial electrode implantation after performance of an atrial switch (Senning) procedure for dextrotransposition of the great arteries. Note the electrode position in the left ventricle, away from the ventricular apex and the diaphragm, to avoid diaphragmatic stimulation.

Pacemaker Clinic Visit

During a pacemaker clinic visit, all patients undergo a complete history and physical examination, pacemaker interrogation, threshold testing, evaluation of electrode sensing, and routine ECG testing. When indicated, other tests, such as chest radiography and echocardiography, are also performed. Specific attention must be paid to obtaining a complete history as it relates to potential pacemaker malfunction. Symptoms suggesting pacemaker malfunction include sudden exercise intolerance, dizziness, nausea, and loss of consciousness. In particular, parents often note that their child has “less energy.” A common symptom in patients with intermittent atrial sensing malfunction is a sudden lack of energy or transient dizziness, related to loss of atrial sensing with a resultant acute drop in the pacing rate to the LRL of the pacemaker. This can often be a subtle finding and must be carefully sought, not only in the older child from whom a history can be obtained directly, but also from the parents of the younger child, who must be asked whether they have ever noted sudden changes in the child’s activity state or temperament, sudden interruptions of playtime, interruptions of eating, or sudden staring episodes suggestive of acute decrease in cardiac output.

The physical examination should relate specifically to complications produced by the presence of pacemaker electrodes, such as AV valve incompetence, acute appearance of pericardial rubs (suggesting lead perforation), irregular heart rates, pocket infections, or in unusual situations, stenotic lesions, suggesting extracardiac compression of vessels by the epicardially implanted pacemaker electrodes.

Pacemaker interrogation must include programmed settings and pacemaker performance data, comprising the electrode’s impedance, the generator battery’s voltage and cell impedance, the generator’s measured pulse amplitude, the electrode’s current flow, the energy delivered, the pulse width, and the measured magnet rate. Such parameters can often reveal early signs of electrode dysfunction or approaching generator end-of-service life. Electrode impedance changes are of particular usefulness in indicating early exit block (manifested as increasing impedance) and insulation fracture or erosion of the electrode tip (manifested as a decline in electrode impedance).

Interrogation of the pacemaker’s diagnostic counters is then performed to assess the extent of pacemaker use and the rate variability the patient is experiencing. Information provided by diagnostic counters is variable, depending on the pacemaker model. The most useful data are those collected over many months to assess degree of the pacemaker’s rate variability and the patient’s dependence on the pacemaker. This information is useful in assessing the appropriateness of the LRL in DDD pacing, the degree of AV conduction, and sinus node function (Fig. 18-38).

Figure 18-38 Examples of data obtained from generator diagnostic counters showing the number of beats occurring in each rate range. A, Patient had acceptable heart rate variability with predominantly atrial pacing and ventricular sensing (intact AV conduction). B, Patient had minimal heart rate variability; reprogramming to a rate-responsive mode was required. C, Information about the types of atrial and ventricular beats occurring. This patient had mostly atrial and ventricular sensing. There was less than 20% atrial pacing, indicating an appropriate lower rate limit. The MVP feature has been enabled to promote AV conduction and minimize ventricular pacing (see Fig 18-11).

Intracardiac EGMs are then recorded. The usefulness of such tracings is discussed later. Sensing thresholds are also performed to determine the least sensitive settings that still maintain appropriate sensing.

Lastly, threshold determinations from both atrial and ventricular leads are obtained. Voltage thresholds both at a fixed pulse width and pulse width thresholds at a fix voltage are performed. All threshold determinations are performed in duplicate. Thresholds are reported as the minimum value that maintains 100% capture at each fixed setting. From these data, strength-duration curves are constructed and compared with previously obtained curves to detect shifts. After review of the data, appropriate programming decisions can be made.

Intracardiac Electrogram Determinations

Intracardiac EGMs should be obtained whenever possible. Electrograms are obtained both simultaneously with a surface electrocardiogram and with telemetered annotation markers that indicate the point during the EGM at which sensing or pacing occurs and the beginning and length of each refractory period. The relationship of programmed refractory periods to the waveform is shown. This is particularly useful for the atrial channel, to ascertain the appropriateness of the programmed settings. For example, if the atrial electrode has been implanted in the left atrium, a determination of the appropriate AV interval can be difficult because the spontaneous P wave on the body surface significantly precedes the time at which atrial sensing occurs (Fig. 18-39). This results in a prolonged PR interval, as determined from the surface ECG recording, which may raise concerns about appropriate generator function.

Figure 18-39 Intracardiac electrogram from patient with left atrial pacing.

In both panels, the top recording is the atrial EGM, and the bottom recording is the body surface electrocardiogram (ECG). A, During atrial pacing, the atrial EGM and the surface P wave occur close to each other. B, During atrial sensing, surface P wave now precedes the atrial EGM by a considerable time.

Recording the annotation markers with the intracardiac EGM also permits minimization of refractory periods, to maximize the multiblock rate but not risk oversensing of ventricular depolarization or repolarization by the atrial channel (Fig. 18-40). This is an even greater concern in young children who experience high atrial rates and are at risk for multiblock, with a subsequent sudden decrease in the ventricular rate and a concomitant decrease in cardiac output if TARP is too long (see later discussion on results of exercise testing).

Figure 18-40 Intracardiac electrogram recorded in DDD mode.

Annotation markers show the times of onset and termination of the various refractory periods. The dashed line after a denotation of atrial pacing (AP) or atrial sensing (AS) shows the duration of the total atrial refractory period (TARP), indicating it is of significant duration to prevent inappropriate sensing by the atrial amplifier of ventricular events. The dashed line after a ventricular pacing (VP) shows the duration of the ventricular refractory period but is of little use in this recording.

We also find the intracardiac EGM to be helpful in determining decreasing atrial amplitudes over time, with the potential for loss of atrial sensing. As the electrode ages and the tip fibroses, recorded EGM amplitudes may decline, and appropriate changes in atrial sensitivity often need to be made. This is particularly relevant to exercise, during which atrial amplitudes may further decline.

The intracardiac EGM is also useful in the diagnosis of atrial dysrhythmias. In the active child, presence of a high pace rate may not be unusual or may suggest an atrial tachycardia. An example of such a situation is in the presence of atrial flutter (Fig. 18-41). In the first example in the figure, the rapid atrial rate was apparent on the body surface ECG recording, and the ventricular pacing was at the URL. In the second example, the body surface ECG recording showed an apparently regular rhythm at a rate below URL; atrial EGM showed atrial flutter, with every other atrial beat in the refractory period.

Figure 18-41 Atrial electrograms from two patients in atrial flutter.

A, The time of atrial sensing (AS) is variable, as is the atrioventricular interval, because the pacemaker is at the upper rate limit (URL), or maximal paced rate (MPR). Many atrial beats are ignored as they fall in the atrial refractory period (AR). The changing total atrial refractory period (TARP) is a result of the Wenckebach behavior imposed by URL. B, There is a definite 2 : 1 block, and the pacemaker is not at the URL. The only suggestion of atrial flutter on the surface electrocardiogram was that the heart rate was completely regular. VP, Ventricular pacing.

Exercise Testing

Exercise testing should be an integral part of all pacemaker follow-up in children old enough to undergo treadmill testing. Such testing often pinpoints inappropriate programming, manifested by inappropriate performance. In one series, 43% of patients had clinically significant inappropriate programming while exercising that was not apparent on routine pacemaker testing.^74–75 Of these patients, 75% showed an inadequate heart rate increase with the stress of exercising. Although most were paced in the VVIR mode, one patient in the DDD mode was shown to have an inadequate exercise response indicative of chronotropic incompetence. All such patients underwent reprogramming with subsequent appropriate heart rate increases. Other patients displayed spontaneous beats at maximal exercise, causing an automatic extension of the PVARP and acute multiblock, with an acute decline in the heart rate (Fig. 18-42). The development of multiblock was also seen with an acute decline in the heart rate from an inappropriately long TARP. The multiblock rate was only slightly above the URL, causing an abrupt decline in heart rate (Fig. 18-43). If decreasing the AV interval or the PVARP cannot adequately shorten TARP, use of the DDDR or VDDR mode may be helpful. When the multiblock rate is reached during either of these pacing modes, the ventricular rate falls to the sensor rate rather than the inappropriately sensed atrial rate, providing a smaller decline in the heart rate. Loss of capture at the maximal heart rate was also observed, even though there had been 100% capture at rest and programming of the pacemaker was believed to be appropriate, based on resting threshold testing (Fig. 18-44).

Figure 18-42 Continuous ECG recording during exercise treadmill testing shows the development of spontaneous beats at maximal exercise. This resulted in automatic extension of the postventricular atrial refractory period (PVARP), with an overall decline in the heart rate from 167 to 141 bpm. This was associated with sudden fatigue and near-syncope and was corrected by elimination of automatic PVARP extension. Paper speed is 12.5 mm/sec, with each row representing 20 seconds. The time since onset of exercise is noted by the numbers in the left margin.

Figure 18-43 Continuous ECG recording during exercise testing shows the development of multiblock with an acute decline in the heart rate, resulting in syncope. Patient’s upper rate limit was 180 bpm, with a multiblock rate of 187 bpm. This lack of difference between URL and multiblock rate provided no gradual heart rate decline but rather an acute decline in heart rate and cardiac output. The multiblock rate was increased by shortening the postventricular atrial refractory period. Paper speed is 12.5 mm/sec, with each row representing 20 seconds. The time since onset of exercise is noted by the numbers in the left margin.

Figure 18-44 Continuous ECG recording during exercise testing in a patient who developed acute intermittent loss of capture at maximal exercise. Threshold testing at rest indicated excellent thresholds, with the minimal pulse width necessary to pace being 0.15 msec at 5.4-V amplitude. Programmed settings were pulse width of 0.5 msec and amplitude of 5.4 V. At rest and during early exercise, there was 100% capture; loss of capture occurred only at maximal exercise. Pulse width was subsequently increased to 1.0 msec, with no loss of capture on repeat testing. Paper speed is 12.5 mm/sec, with each row representing 20 seconds. The time since onset of exercise is noted by the numbers in the left margin.

Changes in pacing thresholds do occur and can worsen, as the previous example shows, or can improve.^26,76 For fixed-rate pacing, thresholds decline after exercise, so resting thresholds are not necessarily indicative of those present during the stress of exercise (Fig. 18-45). However, these findings may not be applicable to the patient whose pacemaker is in the rate-responsive mode, as the example in the patient who developed loss of capture at maximal exercise showed. This is a potentially serious problem that would not have been apparent had evaluation been performed only while the patient was at rest.

Figure 18-45 Data from a patient with fixed-rate VVI pacing who demonstrated a decrease in thresholds after termination of exercise compared with pre-exercise values. At higher pulse amplitudes, there was a minimal change, but marked changes were noted at lower pulse amplitudes. This denotes a shift of the strength-duration curve leftward, implying increased excitability of the heart with exercise.

(Data from Serwer GA, Kodali R, Eakin B, et al: Changes in pacemaker threshold with exercise in children and young adults. J Am Coll Cardiol 17:207A, 1991.)

Changes in P-wave amplitude were also documented at maximal exercise.^77–79 A decrease in P-wave amplitude can result in loss of atrial sensing; in the DDD mode this can cause an immediate drop to the LRL of the pacemaker. The result can be syncope caused by a sudden decrease in cardiac output. Although DDDR pacing can minimize the magnitude of this heart rate drop, the physician should not rely on this mechanism, because the sensor rate may not be the same as the atrial rate, especially during activities such as swimming or cycling. Myopotential sensing by either channel can have similar effects.^80,81 This is more evident in unipolar systems, but can occur in bipolar systems.

Ambulatory Electrocardiographic Monitoring

Ambulatory ECG monitoring is also essential in the appropriate follow-up of pediatric patients with pacemakers. This is particularly true for patients who are unable to exercise, because it may be the only method to evaluate high-rate performance. In many series, 24-hour ambulatory ECG monitoring was the only means by which pacemaker malfunction was detected.^82–84 Such monitoring is particularly valuable in evaluating atrial and ventricular sensing problems and intermittent lack of capture. This is important because myocardial characteristics and thus appropriate pacemaker programming may vary, depending on the activity state and time of day.³⁰ Interactions between the patient’s intrinsic rhythm and the pacemaker are also more completely evaluated by ambulatory monitoring, potentially resulting in more optimal programming.

Remote and Transtelephonic Pacemaker Monitoring

Transtelephonic pacemaker monitoring also plays an important role in pacemaker follow-up and has undergone major changes. Previously, transtelephonic monitoring consisted of only a real-time electrocardiogram with and without magnet application. This did provide some information, but was often difficult to perform in the uncooperative child. No stored data or trend data were available. Newer remote systems permit complete pacemaker interrogation of current and stored information, together with real-time EGMs and electrode threshold data performed automatically by the pacemaker in the ventricle,⁸⁴ and more recently in the atrium.⁸⁵ If interrogation is interrupted during data retrieval, it is halted until reception is again established, and retrieval resumes. This is particularly helpful in the young, uncooperative patient. When interrogation is complete, the data are transmitted in digital form to a central computer, eliminating the need for simultaneous data retrieval and telephone transmission. An example of such a system is the Medtronic CareLink system. Similar remote systems include Merlin.net (St. Jude Medical) and Biotronik Home Monitoring Service and also allow pacemaker follow-up. Benefits of such a system for early determination of potential problems have recently been evaluated.⁸⁶ Other systems, such as Latitude (Boston Scientific), allow remote follow-up of ICD devices but not pacemakers.

In the protocol used at our follow-up clinic, the patient sends a transmission according to a predetermined schedule at a convenient time. If this is only a routine transmission, nothing further is required from the patient. If there are concerns, the patient calls the clinic and relates the concerns to the pacemaker nurse or technician. Each morning, the system is interrogated and new transmissions are retrieved. If problems are found, the patient is contacted. If the patient has concerns during nonbusiness hours, the on-call physician can be contacted and can retrieve the transmission from any computer. The data are reviewed, and the information is relayed to the patient and the responsible cardiologist. The information provided is summarized on the first page of the report. It indicates battery status, electrode thresholds and long-term trends, electrode impedances and long-term trends, counter data (percent paced and sensed in each chamber and number of high-rate episodes), and current programming. Subsequent pages provide a real-time EGM, magnet response, and stored EGMs from high-rate episodes. This information is equivalent to that obtained in a clinic visit. Although the system does not permit reprogramming or simultaneous recording of a surface ECG, it may otherwise obviate the need for a clinic visit if no problems are seen.

For the standard transtelephonic system, longer rhythm strips must be run to assess pacemaker function if a motion artifact is present. Routinely, 60-second strips are run to ensure that adequate data are obtained. Both nonmagnet and magnet strips are obtained, as in adult patients. We allow the parents to choose the best time of day to call; they often vary the time they call, based on nap or school schedules.

Transtelephonic monitoring is cost-effective, decreasing the number of outpatient visits needed, and also provides a method to address parent/child concerns about proper pacemaker function.⁸⁷ The capability of the child or the parent to detect pacemaker malfunction is very limited.⁸⁸ Use of transtelephonic ECG monitoring not only can confirm pacemaker malfunction but also can reassure the parent and child of proper function.

Early Follow-Up Period

Follow-up during the crucial early period must be frequent and thorough. This is especially true after a new electrode has been implanted. Our current protocol includes both pacemaker interrogation and noninvasive threshold determination at multiple pulse amplitudes performed at 1 day, 6 weeks, and 12 weeks after implantation. During this period, thresholds often vary, and the generator’s output may also need to be reprogrammed to maintain reliable pacing. Exit block, in particular, is common during this period, whereas little to no exit block is noted beyond 3 months.⁸⁷ In another series, there appeared to be a slight increase in threshold after 3 months, but this occurred in only 24% of patients.⁸⁸ It is important to determine thresholds at multiple pulse amplitudes, because the strength-duration curve can move in a horizontal direction, with thresholds at the higher outputs unchanging and thresholds at lower outputs showing marked increases (Fig. 18-46).

Figure 18-46 Example of change with time in strength-duration curve. In this patient, the strength-duration curve shows changes that would not have been apparent if thresholds had been determined only at pulse amplitudes greater than 2.5 V.

In addition, 24-hour ambulatory ECG monitoring is performed within the first week after implantation to assess proper pacemaker function throughout the day, not only for a brief period during pacemaker interrogation. For children old enough to undergo exercise testing, this is performed the first month after implantation. Remote or transtelephonic monitoring is done monthly during this period.

Intermediate Follow-Up

During the intermediate follow-up period, defined as 3 months to 5 years after implantation, there tends to be only a small incidence of electrode or generator malfunction. Pacing systems tend to be more stable during this time and do not require as frequent reevaluation. Our current protocol is to see patients annually for pacemaker interrogation and threshold testing and to obtain 24-hour ambulatory ECG recordings if indicated. Remote or transtelephonic monitoring is done bimonthly unless complicating features are believed to require more frequent monitoring. In addition, patients and parents are encouraged to send transmissions whenever there is a question about proper pacemaker function. With the enhanced remote follow-up systems, fewer in-office follow-up visits may be needed.

Late Follow-Up

During the period from 5 years after implantation to pacemaker replacement, generator failure is more likely than electrode failure. Although electrode failure does occur, the thresholds, in general, tend to be stable, and most electrode failures during this period are a consequence of acute lead fracture and not exit block. There usually is no warning of lead fracture, and no current follow-up method has been useful in predicting such events. However, depletion of the generator’s battery can be adequately detected early through the use of monthly transtelephonic ECG monitoring. Ambulatory ECG recordings are still obtained yearly.

Implantable Defibrillator Use

The use of ICDs in children had been limited by many factors, including uncertainty of the indications for use, device size, and the difficulty initially of implanting the epicardial electrodes. Randomized studies are lacking in patients with structural heart disease to determine those at highest risk who would receive the greatest benefit from ICD placement. With additional experience in the adult population and smaller devices and electrodes, both epicardial and endocardial, the pediatric use of ICDs has increased. Studies include a large multicenter experience with initial use of ICDs in children age 1.9 to 20 years,⁸⁹ a single-center experience with 11 children age 4 to 16 years,⁹⁰ use of nonthoracotomy electrode systems in 17 children age 12 to 20 years,⁹¹ and a larger single-center experience with 27 patients.⁹¹

The issues surrounding the use of ICDs in children involve appropriate patient selection, selection of epicardial versus endocardial implantation and the techniques involved, appropriate tachyarrhythmia detection and therapy programming, and appropriate follow-up.

Patient Selection

The only current class I indication for ICD placement is for secondary prevention in patients who have sustained an aborted sudden cardiac death (aSCD) episode not amenable to surgery or ablation. In all the series mentioned, all patients had experienced a syncopal or sudden cardiac death (SCD) episode documented to be a ventricular arrhythmia or had an inducible ventricular arrhythmia at electrophysiologic study after their syncopal or SCD event. Silka et al.⁸⁹ found that 76% of children were survivors of an SCD episode, whereas 10% had experienced a syncopal episode with inducible sustained ventricular tachycardia; the remainder had drug-refractory VT. The underlying diagnoses were hypertrophic cardiomyopathy, long QT syndrome (LQTS), dilated cardiomyopathy, or repaired congenital heart disease. Fewer than one-half the children had depressed ventricular function. In general, any child who has survived an SCD episode or has a condition that can result in SCD should be considered for ICD implantation. Medical therapy should always be employed, but one must decide whether medical therapy alone will completely eliminate the likelihood of a repeat episode. By example, the patient with LQTS who has experienced an aSCD episode should be treated medically and should also undergo ICD placement. Conversely, a child with congenital complete heart block and a documented episode of ventricular fibrillation (VF) may require only permanent pacing.

Although cardiomyopathies and primary electrical diseases constitute the most common diagnoses requiring ICD placement, children who have undergone repair of congenital heart disease are beginning to present with malignant ventricular arrhythmias. Patients with tetralogy of Fallot, aortic stenosis, and transposition of the great arteries after atrial switch repair have been identified as higher-risk patients.⁸⁹ In our experience, 15% of all patients receiving ICDs had undergone prior repair of tetralogy of Fallot. Children with this diagnosis accounted for 22% of patients in one report.⁹² Any child who has undergone a ventriculotomy or repair of a coronary artery abnormality is at risk for ventricular arrhythmias.

There are fewer data to support guidelines for which pediatric patients would benefit from an ICD for primary prevention. The most recent guidelines list implantation for high-risk patients without syncope as a class IIb indication (class IIb defined as probably useful but less well established by evidence).¹⁹ Patients with inherited arrhythmias such as the malignant types of LQTS, short QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia (CPVT) are often considered for ICD placement for primary prevention. Minimal data support ICD use in patients based solely on QRS duration. In tetralogy of Fallot, QRS duration is caused mostly by RV enlargement, and risk of VF is unknown. Our practice has been to place an ICD for episodes of documented VT or VF, syncope with an unknown etiology, or inducible VT at electrophysiologic testing. Placement for primary prevention is rare in our hands.

Implantation Approaches

Two decisions affect ICD implantation: epicardial versus endocardial electrode placement and subclavicular versus abdominal device placement. Each can be made independently of the other. Initially, only epicardial electrode placement was used in children because of the large size of the transvenous electrode and the inappropriate shocking coil spacing, or the need to place a second electrode. Even with transvenous electrode improvements (discussed later), epicardial placement remains the preferred method for smaller children. The smallest child to receive an endocardial system in the study of Kron et al.⁹¹ weighed 32 kg, and the smallest in Hamilton et al.⁹⁰ weighed 27 kg. In our center, the smallest child weighed 15 kg. Historically, we used epicardial patches in children who weigh less than 15 kg or in whom there is no venous access to the ventricle.

When used, patches are usually placed to maximize the distance between them and to maximize exposure of the myocardium to the defibrillation energy. Patches are sutured to the pericardium rather than the epicardial surface, to minimize growth effects and patch distortion (Fig. 18-47). With time, distortion can develop, but it does not necessarily have an adverse effect on the defibrillatory thresholds. The leads are then tunneled to the abdomen, and the pocket is created, as for implantation of an epicardial pacemaker system. In one case of a child with absent abdominal musculature, the device was placed intrathoracically.

Figure 18-47 A, Chest radiograph taken after acute placement of epicardial defibrillation patches in a child. B, Chest radiograph in same child 4 years after implantation shows buckling of the patches, although the defibrillatory thresholds remain low. Note the placement of a transvenous atrial electrode, which was tunneled to the device in the abdomen using a lead extender, because the device had been upgraded to a dual-chamber pacemaker-defibrillator. The epicardial ventricular pacing and sensing electrodes placed originally continue to be used.

In some patients, even the small patch electrodes may be too large. Recently, we have begun to use coils placed in the posterior mediastinal space medially and along the lateral LV wall laterally or posteriorly behind the ventricular mass. Defibrillatory energy is delivered between these electrodes, traversing the heart in an anterolateral direction (Fig. 18-48). Such an arrangement has resulted in acceptable defibrillatory thresholds, using a multiple coil or a single coil and an active can.^93–96 Such systems have shorter longevity than traditional transvenous systems but still afford adequate defibrillation.⁹⁶

Figure 18-48 Anteroposterior (left) and lateral (right) chest radiographs after placement of dual-coil intrathoracic defibrillator system; one coil is in the inferoposterior mediastinum (arrow) and the other in the left lateral chest. Star indicates the pace-sense epicardial electrodes.

With the development of smaller-diameter transvenous electrodes and use of the device can as the second defibrillation electrode, a transvenous system can be used in most children. The currently available electrodes, both single and dual coil, can be introduced with a 7F to 9F introducer depending on the type of electrode used. However, the intercoil spacing in the dual-coil electrode is usually too large to be used in children. Positioning the second coil properly requires buckling of the electrode within the right atrium, potentially dislodging the electrode tip (Fig. 18-49). Use of the single-coil electrode eliminates this problem. Even with electrode buckling, the SVC coil remains close to the clavicle, placing it at risk for fracture. In addition, the second coil may make extraction more difficult. The second coil may also increase the risk of an extraction procedure later, because it may cause more fibrosis around the SVC coil in an area that is narrow and potentially at the innominate vein–SVC junction, where the lead makes a sharp bend, unlike in the larger child or adult, where the coil seats entirely in the SVC.

Figure 18-49 Chest radiograph shows placement of dual-coil single-lead transvenous defibrillator system, with the lead tunneled to the abdomen. Note the buckling of the lead within the atrium, which was necessary to place the distal coil properly within the innominate vein. This could potentially dislodge the right ventricular coil and still leave the distal coil near the clavicle/first rib space.

Use of a single-coil electrode requires that the device can serves as the second electrode; alternatively, a second intravascular coil or a subcutaneous array can be used as the second electrode, if initial defibrillatory thresholds are not satisfactory. Initially, use of the active can was restricted to implantation of the device under the left clavicle. However, we have implanted the device in a left upper quadrant abdominal pocket, with the single-coil, bipolar-pacing, steroid-eluting electrode introduced into the left subclavian vein and tunneled subcutaneously to the device pocket⁹⁷ (Fig. 18-50). This technique is similar to the method proposed by Molina et al.⁹⁸ for transvenous pacemaker implantation in small patients. Defibrillatory thresholds have been less than 10 J in all cases, even though the electrode area is reduced.⁹⁹ No patient has required an additional electrode (subcutaneous array or second transvenous electrode).

Figure 18-50 Chest radiograph shows placement of a single-coil transvenous defibrillator system, with the lead tunneled down the lateral chest wall to the device. This also serves as the second defibrillation electrode and is placed in a left upper quadrant abdominal pocket. Because of small patient size, defibrillation threshold was good, even though the defibrillatory vector is less than ideal.

Programming Considerations

The issues of appropriate pediatric programming are similar to those in the adult patient. The major concern is the higher sinus rates that can occur in a child. Initially, β-blockers were employed to reduce the maximal sinus rate. However, this approach often resulted in significant exercise intolerance. Although β-blockers may be indicated to treat the underlying disease (e.g., LQTS), their use solely to reduce the maximal sinus rate is no longer necessary. Current devices can now be programmed to very high rates as a criterion of VT and can also employ other markers, such as increased QRS duration or altered QRS morphology and sudden onset. For most children, however, use of high rate alone is usually sufficient to detect fibrillation. Also, many children have wide QRS complexes as a result of prior cardiac surgery, making the QRS morphology a difficult criterion to use. In practice, we program the device to a detection cycle interval that is 50 msec greater than the documented fibrillation cycle interval, consistent with the approach of Hamilton et al.⁹⁰ QRS duration is used only if cycle interval alone has been unable to prevent inappropriate therapy delivery.

In patients with LQTS, the physician must consider the prolonged T-wave interval and possible T-wave oversensing that can result in double counting of the ventricular rate and inappropriate therapy delivery. Careful testing for T-wave oversensing must be performed at implant and at each interrogation. One may also want to consider evaluation of this during exercise testing.

Follow-Up Procedures

At our center, all children initially underwent induction of VF at implant and 1 week after implantation to ensure that detection and therapy were appropriate. However, if no problems were suspected clinically, defibrillatory thresholds proved to be acceptable in all cases. Determination of thresholds beyond implantation is now reserved for patients in whom there is a suspicion of system malfunction.

At each clinic visit, the device is interrogated, and each recorded event is inspected for cycle length and QRS morphology. Pacing thresholds are also determined, as for pacemaker patients. Transtelephonic systems have been developed to permit interrogation of ICD status remotely. Such systems allow for determination of battery voltage, charge time, electrode impedance, and viewing of event counters, recent events, and real-time EGMs. These systems allow for better and more frequent ICD follow-up without the need for repeated clinic visits in otherwise asymptomatic patients.

After each therapy delivery, the child is seen in the clinic and the device interrogated, or a remote transmission is sent to evaluate the event and to ensure the therapy was appropriate. This is particularly crucial because inappropriate therapies are not rare. Up to 50% of patients have been found to receive at least one inappropriate therapy within 4 years.¹⁰⁰ Also, the medical therapy is reevaluated to ensure that it does not require alteration. Finally, the psychological aspects of the child’s reaction to the therapy are evaluated, and support is offered as needed. Assessing the need for long-term support is extremely important, as discussed later.

Antitachycardia Pacing

Pace termination of atrial arrhythmias is often used in patients with congenital heart disease.¹⁰¹ This has become possible with the development of a dual-chamber antitachycardia pacemaker with a rate response suitable for use in some patients. Although it does not have defibrillator capabilities and therefore is not suitable for pace termination of VTs, the device has proved useful for treatment of atrial flutter, particularly in postoperative patients, in whom atrial flutter is often seen but is difficult to treat with medication or ablative techniques. The Medtronic AT500 has been used in a series of patients with congenital heart disease and atrial tachycardias.¹⁰¹ The most recent version (Medtronic EnRhythm) has improved functionality and allows enhanced remote follow-up; it successfully terminated the tachycardia in 54% of episodes. Success is influenced by many factors, including proximity of the electrode to the reentrant circuit, atrial size, and the potential for multiple reentrant circuits. It does require placement of both atrial and ventricular electrodes, even if only atrial pacing is anticipated, because ventricular sensing is needed to prevent pacing in 1 : 1 AV rhythms. In addition, these electrodes must be bipolar in configuration, and close electrode spacing is required to minimize far-field oversensing, especially of the atrial electrode.

Proper electrode placement and selection are more critical for antitachycardia pacing (ATP) than for routine pacemaker placement. Proper programming of the tachycardia sensing and termination parameters increases its complexity, but these devices provide an important adjunct for patients with drug-resistant atrial tachyarrhythmias, especially when there are indications for bradycardia pacing. However, inherent limitations in this device restrict its use in children (e.g., maximum URL of 150/min, lack of automated threshold determination).

Children with Pacemakers

Comprehensive care of the child with a pacemaker requires awareness of the psychosocial issues faced by children and families. Although there are issues specific to having a pacemaker, such as dependence on a mechanical device and the certainty of periodic surgeries throughout life, the overall challenges are similar to those faced by children with other types of chronic illness. Treatment goals include facilitation of positive child and family adjustments and prevention of avoidable negative psychosocial outcomes. Important illness-related tasks for parents of children with pacemakers include participating in the treatment plan, preserving emotional well-being for themselves and their children, and preparing for an uncertain future.¹¹⁴ Children who are at high risk for emotional or behavioral adjustment problems may benefit from early identification and intervention.¹¹⁵

Because they require lifelong device therapy, these children, by definition, have a chronic illness.¹¹⁶ Because of the underlying disease and permanent need for a device, the children and their families are chronically subjected to stressful experiences, including repeated surgeries, body image changes, and exposure to the possibility of death.¹¹⁷ Although disease severity varies, most patients can anticipate many fruitful adult years, and many are anticipated to have a normal or nearly normal life span.

When examining psychosocial outcomes in this population, no significant differences were found between children with pacemakers and comparison children on standardized measures of trait anxiety, self-competence, and self-esteem.¹¹⁸ Children with pacemakers were more external in their locus of control orientation than healthy controls, but not significantly different from children with comparable heart disease who did not have a pacemaker.¹¹⁹ Interview data suggested that comparison children were likely to report negative stereotypes about children with pacemakers. However, most chronically ill children make positive adjustments, although an increased risk for psychosocial problems suggests that up to 30% of children with chronic illness demonstrate secondary psychosocial impairment by 15 years of age.^120,121

The risk for psychosocial problems in the pediatric pacemaker population can be understood as a response to chronic stress that challenges the adaptive capacities of children and families.^122,123 Consistent with this model, negative adjustments are likely to occur when stress exceeds the child’s adaptive capacities. Certainly, there is wide variation in children’s adaptations, and some children are able to make positive adaptations despite extremely stressful circumstances.^124,125

Many of the factors linked with children’s adjustments reflect a continuum, with one end representing a protective factor and the other a risk factor. Families who are cohesive and supportive are associated with positive adaptations in children, whereas family functioning that is disengaged and rigid is associated with emotional and behavioral problems in the children.¹²⁶

Disease-related biomedical factors anticipated to affect children’s adjustments include diagnosis (disease severity, prognosis, treatment), functional impairment, cognitive abilities, and visibility (physical changes).^127,128 For children with a chronic illness, diagnosis and disease severity are less important predictors of adjustment than other factors.¹²⁷ However, the diagnosis of heart disease during childhood may pose particular challenges, for reasons not well understood.¹²⁹ Biomedical factors that may negatively influence psychosocial outcomes for children with heart disease include early mother-infant attachment problems, neuropsychological changes, learning disabilities, and exclusion from competitive sports.^130–132

Inherent childhood factors that may affect adjustment include temperament, developmental stage, and competency.¹²⁷ For example, an academically gifted child may experience opportunities for self-esteem, socialization, and social recognition that ameliorate the negative effects of having a chronic illness, whereas the child with a learning disability who has a chronic illness is at increased risk for serious adjustment problems.¹³³

Social-environmental factors linked with children’s adjustments include family environment, maternal psychological functioning, family economic resources, and children’s peer relationships. Adaptive, cohesive families who communicate effectively support positive adjustments for children.^134,135 Maternal adjustment may play a particularly important role in children’s adjustment,^136,137 and persistent maternal distress (e.g., symptoms of anxiety or depression) warrants intervention.¹²⁷ Economic resources appear to be powerful moderators of children’s adjustment to stressful life circumstances.^125,138,139 The quality of children’s friendships is also important, because positive peer relationships are linked with prosocial behavior, academic achievement, stress resistance, and later, adolescent competence.^140–142 Conversely, negative child peer relationships are associated with academic failure, aggressive behavior,^139,143 and general psychopathology, which persists during adulthood.¹⁴⁴ Social isolation and feelings of loneliness are potential sources of emotional distress for chronically ill children and family members.¹³⁵

Coping methods are the processes used by children and families to accomplish disease-related tasks, maintain normal function, and minimize family disruption. Adaptive parental coping strategies include maintaining family integrity through cooperation and maintenance of an optimistic outlook; maintaining social support, self-esteem, and psychological stability; and understanding the medical situation through communication with other parents and health care providers.¹⁴⁵ Coping is a dynamic process that may promote or hinder positive adjustments. Coping methods can be categorized as problem focused or emotion focused; in general, both methods are used by individuals responding to stress.¹⁴⁶ Problem-focused coping methods involve actions taken to address a stressful situation, whereas emotion-focused methods are those coping methods that change the emotional reaction to an event, such as altering the perception of the experience, blunting, or denial. The different coping strategies employed by fathers and mothers may be a source of distress in the marriage.¹⁴⁷ Children’s coping strategies reflect their developmental stage, and problem-focused strategies precede emotion-focused strategies during childhood.¹⁴⁸

Children with Implantable Cardioverter-Defibrillators

Minimal data are available regarding the adjustments of children and adolescents after ICD implantation. In nine patients age 13 to 49 years, Vitale and Funk¹⁵⁰ reported difficulty sleeping, low energy, and interruptions in planned activities. Problems reported for older patients after ICD implantation include increased anxiety levels, excessive anger, psychological disturbances, sexual abstinence, and restricted physical and social activities.^151,152 Some studies emphasize that most older adults appear to make positive adjustments after ICD implantation.¹⁵³

The pediatric ICD population would also appear to be at risk for the development of psychosocial problems from disease-related factors, including disease severity, functional impairment, and body image changes. Disease severity is significant, because these children have survived, or are at high risk for, a lethal arrhythmia. Many are survivors of out-of-hospital cardiac arrests, with the accompanying fear, drama, and exposure to death. The incidence of anoxic encephalopathy after cardiac arrest is not insignificant; if it does occur, it can be anticipated to affect later adjustment. Visibility is becoming less of an issue; devices are becoming smaller, yet remain large in relation to a child’s body mass. The treatment delivered by the device is another source of distress for patients, with ramifications exceeding the pain and potential embarrassment at therapy (see later discussion). Also, many children who require ICD implantation have inherited diseases, such as LQTS, familial dilated cardiomyopathy, or hypertrophic cardiomyopathy. Therefore, health-related concern extends beyond that child to other family members who may be affected. Because of the hereditary nature and severity of these diagnoses, many families have experienced the death of one or more family members, which adds to feelings of grief, anxiety, and guilt.

In our experience, the potential for a life-threatening arrhythmia and receiving a shock are sources of considerable anxiety, particularly for adolescents. Conscious shocks, particularly repeated conscious shocks, can be extremely traumatic. Three adolescent patients with ICDs experienced conscious (appropriate) shocks, and two experienced repeated conscious shocks. After these events, all three patients experienced severe anxiety that persisted longer than 1 month. In two patients, school phobias developed. The other patient was home for summer vacation and, uncharacteristically, would not participate in usual summer activities, preferring to stay at home. One young patient demonstrated classic symptoms of posttraumatic stress disorder, including intrusive reexperiencing of the traumatic event, suicidal thoughts, feelings of detachment, and high levels of anxiety. All three patients required professional counseling, and one was treated with antidepressant medication and biofeedback. All have since returned to school; two are now in college and one in high school. It is important to note that none of these adolescents had a history of psychiatric problems before the ICD.

It is known that repeated unpredictable aversive stimuli, in addition to producing fear and avoidance behavior, can have long-term debilitating effects.¹⁵⁴ Many of the experiments done on the effects of aversive conditioning actually used unpredictable electrical shocks as the source of the stimuli. Maier and Seligman¹⁵⁵ proposed that, in such circumstances, people may develop the expectation that their behavior has little effect on their environment, and this expectation may generalize to a wide range of situations. This behavior mode, referred to as learned helplessness, has been linked with human depression.

Complicating the situation for patients with tachyarrhythmias is the catecholamine release that occurs secondary to anxiety. The increased heart rate is often perceived as the possible onset of the arrhythmia, leading to increased anxiety and feelings of loss of control. Relaxation under these conditions can present a challenge for young people. Potential therapies, in addition to counseling, include relaxation methods and biofeedback.¹⁵⁶

Support groups have been helpful in promoting positive adjustments to chronic illness in family members.¹⁵⁷ Factors associated with support group participation that may be therapeutic for patients and family members include sharing of information, instilling of hope, universality, altruism, and interpersonal learning. Although this is an effective intervention, the use of support groups by this population thus far appears limited. Professionals working with these patients report that many younger patients do not participate in ongoing ICD patient support groups, because the groups do not address issues of personal concern, such as work, intimate relations, childbirth, school, friends, and dating. Also, the relatively small numbers of young defibrillator patients in any one geographic area have limited the ability to initiate support groups specifically for this population.

In an attempt to address the needs of this unique population, we have collaborated with the adult electrophysiology service at the University of Michigan to have an annual “Youth and Young Adult Support Seminar” for ICD patients in their 30s and younger. In addition to educational workshops on topics of interest to young patients, professionally facilitated support groups were provided, according to age, gender, and role (patient, spouse, or parent). It is our belief that the children and adolescents benefited from their interactions with young adult ICD patients, because they could see firsthand that a meaningful future is possible for them. Evaluations revealed that many of the patients had never met another young person with a defibrillator before the conference.

School reentry after ICD implantation has been a source of considerable concern for parents and school professionals. Education of school personnel by clinic staff regarding the child’s arrhythmia, how the device works, what to do if therapy is given, and development of an emergency plan for school personnel is routine after implantation. Parental permission is always obtained before the school is contacted, and parents are included as important participants in all school-based meetings. During the meetings, it has been helpful to emphasize the protection afforded by the device and that the purpose of undergoing device implantation is to permit the child to live, as much as possible, a normal life.

The long-term outlook of children requiring either ICD or pacemaker implantation continues to improve. Therefore, a positive attitude must be conveyed to the child, the parents, and the caregivers to allow the child to lead as normal a life as possible.