Acquired Atrioventricular Block

Acquired atrioventricular (AV) block is most commonly idiopathic and related to aging, but the potential causes are many. Class I indications for permanent pacing in patients with acquired AV block include any AV block with associated symptoms, as well as AV block after AV node ablation or persistent AV block after cardiac surgery. For some patients, heart failure is a manifestation of AV block. In addition, some patients require medications that can cause symptomatic bradycardia and might need pacing. For postoperative AV block, the guidelines do not specify a period to wait postoperatively for recovery of conduction. Class I indications also include acquired AV block below the level of the AV node or associated with marked pauses, such as longer than 3 seconds, or a ventricular escape rate less than 40 beats/min. Because the cardiovascular risk is believed to be particularly high in patients with second- or third-degree block and specific underlying conditions, permanent pacing is indicated in these patients even in the absence of symptoms. These conditions include neuromuscular diseases such as myotonic dystrophy, Kearns-Sayre syndrome, peroneal muscular atrophy, and Erb limb girdle dystrophy. Myotonic dystrophy has been associated with an increased risk of sudden death believed to be caused by progressive conduction system disease.³

Exercise-related AV block is also an indication for pacing. Advanced second-degree AV block or alternating bundle branch block in the setting of bifascicular block is considered a class I indication. Asymptomatic type II second-degree AV block with a wide QRS is a class I indication. Transient, infranodal, high-grade AV block and associated bundle branch block is a class I indication.²

Class IIa indications include asymptomatic type II second-degree AV block with a narrow QRS complex, and asymptomatic second-degree AV block at intra-His or infra-His levels based on electrophysiological testing.² First-degree or second-degree AV block with hemodynamic compromise is also considered a class IIa indication, because some patients develop pacemaker-like syndrome as a result of conduction delay. In patients with bifascicular block, asymptomatic severe prolongation of the His-ventricle interval (>100 ms), asymptomatic pacing-induced infra-His block that is not physiological, or syncope when other causes such as ventricular tachycardia have been excluded are considered class IIa indications for pacing.

First-degree AV block in patients with neuromuscular disorders is considered a class IIb indication because the risk of progression to AV block is high. Another class IIb indication includes AV block that might have occurred because of drug use or toxicity, but there is a risk of recurrence of AV block. First-degree AV block in a patient with left ventricular dysfunction and congestive heart failure in whom hemodynamic improvement with AV interval optimization can be demonstrated is a class IIb indication.² Asymptomatic and persistent second- or third-degree block at the AV node level is a class IIb indication.²

Class III indications include asymptomatic first-degree AV block, asymptomatic second-degree AV block at the supra-His level or not known to be intra-His or infra-His, and AV block occurring in the setting of reversible conditions such as drug toxicity that is unlikely to recur, Lyme disease, transient increases in vagal tone, or during sleep apnea.

After a myocardial infarction, a pacemaker is generally indicated for persistent second-degree bilateral bundle branch block, third-degree AV block within or below the His bundle, or persistent symptomatic second- or third-degree AV block.

Congenital Complete Heart Block

Symptomatic congenital complete AV block remains a class I indication for pacing in pediatric patients.² In addition, in pediatric patients with congenital complete AV block, the presence of a wide QRS escape rhythm, ventricular dysfunction, or complex ventricular ectopy is also a class I indication. In pediatric patients, an average heart rate less than 50 beats/min, pauses two to three times the basic cycle length, or symptoms associated with chronotropic incompetence are considered class IIa indications. In pediatric and adult patients with adequate rate, narrow QRS complex, and normal ventricular function, pacing is a class IIb indication.

For adult patients with congenital complete AV block, the timing and indications for permanent pacing are more controversial and continue to evolve.⁴ As a result of data regarding the high incidence of unexpected syncope in adult patients with congenital complete AV block, prophylactic pacemaker implantation is often considered.^2,4

Sinus Node Dysfunction

Sinus node dysfunction may be manifested by abrupt sinus pauses or gradual sinus slowing. Symptomatic chronotropic incompetence and symptomatic sinus bradycardia occurring spontaneously or as the result of drug therapy are considered class I indications for pacing. In patients with minimal or no symptoms and chronic heart rates less than 40 beats/min, pacing is a class IIb indication.²

Class III indications for pacing include asymptomatic sinus node dysfunction. Sinus pauses occurring during sleep are generally not considered indications for pacing. Many patients, particularly trained athletes, have high levels of vagal tone and have significant pauses greater than 3 seconds and periods of sinus bradycardia. These patients do not require permanent pacing.

Neurocardiogenic Syncope

Neurocardiogenic syncope typically has both cardioinhibitory and vasodepressor components. Pacing during most episodes of neurocardiogenic syncope is still associated with a significant fall in blood pressure and symptoms because of the continued vasodepressor response. Therefore, even in the presence of significant bradycardia, pacing is usually not considered first-line therapy. A number of randomized trials have failed to show a substantial benefit in increasing the freedom from recurrent syncope.5,6 As a result, neurocardiogenic syncope with documented bradycardia is considered a class IIb indication for pacing; however, there are some recent studies that indicate a selective use of pacing.^7,8 In the Third International Study on Syncope of Uncertain Etiology (ISSUE-3) trial, in patients 40 years or older with at least three syncope episodes in 2 years, and with documentation of syncope with 3 seconds or more of asystole or 6 seconds or more of asystole without syncope, 77 patients were randomized to dual-chamber pacing on or off with an algorithm to increase the pacing rate with a sudden decrease in heart rate. The syncope recurrence rate was 57% in the “pacing off” group and 25% in the “pacing on” group (P = 0.039).⁷

When pacing is performed in this setting, dual-chamber pacing is necessary to preserve the atrial contribution to cardiac output. Modifications to the pacing strategy are being studied to make dual chamber pacing more effective in ameliorating neurocardiogenic syncope. The heart rate can be adjusted according to measured changes in impedance to estimate myocardial contractility, using so-called closed loop stimulation. Although retrospective data suggest that this approach may reduce the frequency of recurrence of syncope, additional prospective studies are needed.⁸

In patients with syncope of unknown origin, a randomized, multicenter study examined patients exhibiting a pause of 10 seconds or more when given an intravenous bolus of 20 mg adenosine triphosphate. These patients were randomized to atrial inhibited (AAI; 30 beats/min) or DDD pacing (70 beats/min). In the DDD pacing group, 21% of patients had recurrence of syncope compared to 66% of patients in the AAI group. Of note, the mean age in these studies was 76 years of age.⁹

Carotid Sinus Hypersensitivity

Carotid sinus hypersensitivity has been shown to be a cause of syncope, particularly in the elderly. However, because an abnormal response can occur even in asymptomatic persons, caution must be used when spontaneous bradycardia has not been demonstrated. In cases in which syncope has occurred during carotid sinus stimulation and carotid sinus pressure has resulted in pauses of 3 seconds or more, a class I indication is present. If such an abnormal carotid sinus pressure response is obtained but syncope did not occur in circumstances suggesting carotid sinus stimulation, a class IIa indication is present. In addition, there may be a role for pacing in patients with unexplained falls and evidence of carotid sinus hypersensitivity. Recent studies do not show a difference in the recurrence of events in patients pacing on treatment using ventricular inhibited (VVI), DDDR, or DDDR with sudden bradycardia response.¹⁰

Nonbradycardic Indications for Pacing

An increasing number of patients are receiving pacemakers for nonbradycardic indications. Cardiac resynchronization therapy for refractory congestive heart failure is the major indication in this category. Dual-chamber pacing for medically refractory symptoms of hypertrophic obstructive cardiomyopathy and pacing therapy for prevention of atrial fibrillation are no longer considered major nonbradycardic indications for pacing.

Pacing for Atrial Fibrillation

Numerous studies have examined various pacing algorithms to prevent atrial fibrillation. Most of these algorithms involve increasing the pacing rate to suppress atrial ectopy that is believed to trigger atrial fibrillation. Whereas most randomized studies have not demonstrated a beneficial effect, some studies have suggested a modest decrease in the occurrence of atrial fibrillation. In the Study of Atrial Fibrillation (SAFARI) trial, patients with AF recorded were randomized to atrial fibrillation pacing prevention strategies. Overall, there was a small reduction in the occurrence of atrial fibrillation. In patients with a high burden of atrial fibrillation, the reduction in atrial fibrillation was greater.¹¹ Pacing for atrial fibrillation reduction is a class III indication.

Resynchronization Therapy

Numerous randomized, controlled trials have demonstrated benefit of cardiac resynchronization therapy in improving symptoms and outcomes in patients with drug-refractory heart failure. Cardiac resynchronization therapy is the pacing of the left and right ventricles to improve the hemodynamics that are impaired because of bundle branch block. These randomized studies have shown improvement in New York Heart Association classes, 6-minute walk time, oxygen consumption, brain natriuretic peptide levels, neurohormonal levels, ejection fraction, end-diastolic and end-systolic dimension, heart failure hospitalizations, and all-cause mortality.¹² The current indications for resynchronization therapy (class I) include QRS duration of 120 ms or greater, left ventricular ejection fraction of 35% or less, sinus rhythm, and class III or ambulatory class IV heart failure symptoms on optimal medical therapy. Because the largest randomized trial excluded patients with atrial fibrillation, the presence of atrial fibrillation results in classification as a class IIa indication.^2,12

Hypertrophic Cardiomyopathy

Early studies suggested that right ventricular apical pacing resulted in a significant reduction in outflow tract gradient and ameliorated symptoms. However, a subsequent randomized multicenter trial failed to demonstrate benefit in patients with pacing. A subset of elderly patients might have exhibited some improvement. An analysis from the Cochrane Database concluded that clinical trial data are inconclusive regarding an effect of pacing on outcome in this patient population.¹³ Therefore, pacing in medically refractory symptomatic patients with significant resting or provoked gradient owing to hypertrophic cardiomyopathy is a class IIb indication.² These patients should be considered for a dual chamber ICD because of the risk of sudden death.

Ventricular Inhibited Pacing

In the ventricular inhibited (VVI) pacing mode, pacemaker output is inhibited by a sensed ventricular event (Figure 117-1). The lower rate interval determines the longest interval between any sensed or paced ventricular events. The interval from the previous sensed or paced ventricular event to the subsequent paced ventricular event is the lower rate interval. If the time interval from the last sensed ventricular event to the first paced ventricular beat is longer than the time interval between ventricular paced beats, ventricular hysteresis is present. Any ventricular event occurring after a paced or sensed ventricular event within the ventricular refractory period is not sensed and does not reset the timing cycle.

Atrial Inhibited Pacing

Atrial inhibited (AAI) pacing incorporates the same timing cycles as VVI pacing, with the obvious difference that pacing and sensing occur in the atrium, and pacemaker output is inhibited by a sensed atrial event (Figure 117-2). An atrial paced or sensed event initiates a refractory period during which atrial events do not reset the timing cycle. Confusion can arise when multiple ventricular events occur during atrial pacing. For example, when the atrial timing cycle ends, an atrial pacing stimulus is delivered regardless of ventricular events, because an AAI pacemaker does not sense in the ventricle. If the ventricular signal is inappropriately sensed by the atrial lead (far-field sensing), the atrial timing cycle is reset. This abnormality can sometimes be corrected by making the atrial channel less sensitive or by lengthening the atrial refractory period so that a conducted ventricular complex is not sensed.

Single-chamber, rate-adaptive pacing modes (AAIR, VVIR) have timing cycles that are not markedly different from those of their counterparts that are not rate adaptive. The difference lies in the potential variability of the paced rate. Depending on the sensor incorporated and the patient’s level of exertion, the basic interval shortens from the programmed lower rate limit. Shortening requires that an upper rate limit be programmed to define the absolute shortest cycle length allowable.

DDD Pacing

Four different rhythms can be seen as a result of normal DDD function: normal sinus rhythm, atrial pacing, AV sequential pacing, and P-synchronous pacing. In the DDD mode, the lower rate limit determines the lowest rate at which the atrial and ventricular events will occur (Figure 117-3). If the intrinsic atrial rate is too slow, an atrial paced event will occur. After a paced or intrinsic atrial event, if there is no ventricular event, a paced ventricular event occurs, at the programmed interval called the AV delay or interval. If an intrinsic atrial event occurs before the programmed lower rate, the atrial event will be followed by a ventricular paced event, a function called atrial tracking. This function permits the ventricular pacing to follow the atrial rate as the metabolic demands change. There is a maximal tracking rate to prevent the tracking of atrial arrhythmias and extraneous signals at excessively rapid rates. In addition, if the P wave occurs too early after the previous ventricular event, it will occur in the postventricular atrial refractory period (PVARP) and will not be tracked. The atrial rate at which two atrial sensed events will be followed by one ventricular paced event is equal to the total atrial refractory period, which is the sum of the programmed AV delay and the PVARP.

The timing of the last ventricular beat to the next atrial paced beat is determined by the ventriculoatrial (VA) interval. This interval is determined by subtracting the AV interval from the lower rate interval in milliseconds. Thus, the lower rate interval minus the AV interval equals the VA interval. The AV interval can be adjusted based on the ventricular rate, a parameter called rate-responsive AV interval. In patients with AV block and rapid sinus rates that might exceed the programmed upper rate limit and 2 : 1 atrial tracking rate, the rate-responsive AV interval might permit much higher upper rate limit tracking rates and maximal 2 : 1 atrial tracking rates. The AV interval might also be extended to promote intrinsic AV conduction, a function called positive AV hysteresis (discussed later under Intrinsic Conduction Preference). Negative AV hysteresis refers to a function in which the AV interval is shortened to promote ventricular pacing, which is desirable in biventricular pacing.

Selection Criteria

In selecting the optimal pacing mode, the patient’s overall physical condition, associated medical problems, exercise capacity, and chronotropic response to exercise must be considered, along with the underlying rhythm disturbance.¹⁴

VVIR pacing is indicated for patients with chronic atrial fibrillation and slow ventricular response. Although VVI pacing protects the patient from lethal bradycardia, its limitations include its inability to restore or to maintain AV synchrony and to provide rate responsiveness in the chronotropically incompetent patient. In addition, some patients with VVI pacing experience symptomatic hemodynamic deterioration with ventricular pacing. Adverse hemodynamics resulting from pacing are known as pacemaker syndrome if the normal coordination of atrial filling and ventricular emptying is absent because of VA conduction or long AV conduction times. The systolic blood pressure might decline during ventricular pacing with VA conduction (Figure 117-4). The incidence of pacemaker syndrome depends on the definition being used, and its incidence has been estimated to be 7% to 10% of patients with VVI or VVIR pacing. The most common symptoms reported were shortness of breath, dizziness, fatigue, pulsations in the neck or abdomen, cough, and apprehension.

AAIR pacing is appropriate for patients with sinus node dysfunction. The obvious disadvantage of atrial pacing is the lack of ventricular support if AV block occurs. If the patient with sinus node dysfunction is carefully assessed for the presence of AV node disease at the time of pacemaker implant, the occurrence of clinically significant AV node disease is low, at less than 2% per year. The concern of progression to AV block when patients are carefully selected for AAIR pacing has been dispelled by long-term follow-up of patients. Assessment before AAI pacemaker implantation should include incremental atrial pacing with an adequate response defined as 1 : 1 AV nodal conduction to atrial pacing rates of 120 to 140 beats/min. Newer pacing algorithms (discussed later) to avoid the deleterious effects of ventricular pacing allow atrial pacing to occur, and they switch to a mode that supports ventricular pacing only when necessary,

The DDD pacing mode is most appropriate for patients with normal sinus node function and AV block. DDDR pacemakers are capable of all the variations described for DDD pacemakers. In addition to use of P-synchronous pacing as a method for increasing the heart rate, the sensor incorporated in the pacemaker might also drive the increase in heart rate. The resulting rhythm may be sinus driven (alternatively called atrial driven or P-synchronous) or sensor driven.

DDDR pacing is indicated in patients with sinus node dysfunction and AV conduction disease. DDDR pacing is also appropriate for patients with pure sinus node dysfunction and when backup ventricular pacing is desired.

Effect of Pacing Mode on Morbidity and Mortality

Several large, randomized, controlled trials have demonstrated benefits of physiological AAIR or DDDR versus VVIR. Andersen et al. published prospective data on pacing mode and survival with the random assignment of 225 patients with sinus node dysfunction to AAI or VVI pacing.^14a The study demonstrated a lower incidence of atrial fibrillation and thromboembolism in the AAI group than in the VVI group. The Canadian Trial of Physiologic Pacing (CTOPP) randomly assigned 2568 patients to physiologic pacing (dual-chamber or AAIR) or ventricular pacing (VVI). No difference in mortality rate or quality of life was demonstrated; however, there was an 18% relative risk reduction of atrial fibrillation.

Two additional trials have examined the outcomes of physiological pacing: Pacing Mode Selection in the Elderly and the Mode Selection Trial. The Pacing Mode Selection in the Elderly study failed to demonstrate any statistically significant benefit of physiological pacing over ventricular pacing. When the patients were separated by underlying rhythm disturbance—that is, sinus node dysfunction versus AV block—the sinus node dysfunction group had some quality-of-life improvements. In addition, there was a crossover of 26% of patients assigned to ventricular pacing because they were unable to tolerate ventricular pacing. The Mode Selection Trial, like the CTOPP, did not demonstrate any difference in mortality rates, but it did demonstrate a lower incidence of atrial fibrillation with physiological pacing, reduced signs and symptoms of heart failure, and a slightly improved quality of life. The study concluded that overall dual-chamber pacing offered significant improvement compared with ventricular pacing. Most recently, the DANPACE trial randomized 1415 patients to AAIR or DDR pacing. There was no significant difference in stroke, heart failure, death, and chronic atrial fibrillation, but there was a lower incidence of paroxysmal atrial fibrillation—28.4% in the AAIR group compared to 23.0% in the DDDR group.¹⁵

These studies demonstrate the benefit of physiological pacing with the maintenance of AV synchrony compared to ventricular only pacing in terms of atrial fibrillation and possibly quality of life.

Activity and Accelerometer Sensors

Accelerometer sensors have replaced basic vibration-based activity sensors. Both of these sensors use a piezoelectric crystal, which produces a minute electric current in response to motion. The piezoelectric vibration activity sensor senses vibration from up-and-down motion, and the accelerometer senses anterior and posterior motion. The accelerometer is relatively free from detection of motion leading to inappropriate increases in heart rate, and responds rapidly to the onset of exertion. The accelerometer is the most widely used form of rate-adaptation sensor because it is simple, easy to apply clinically, and rapid in onset of rate response.

Minute-Ventilation Sensor

Minute volume (respiratory rate × tidal volume) has an excellent correlation with metabolic demand. The transthoracic impedance, which varies with respiration, can be used to determine minute volume. Because minute ventilation maintains a predictable relation with metabolic demand, minute-ventilation values can be used to modulate the heart rate during various levels of activity for the patient. Long-term reliability of the minute-volume sensor has been excellent.

Stimulus-T or QT-Sensing Sensor

The interval from the onset of a paced QRS complex to the end of the T wave has been used for rate adaptation for many years. This stimulus-T interval is affected by autonomic activity and heart rate. The relation allows measurement of the stimulus-T interval to be used for rate adaptation. The QT-sensing, rate-adaptive pacing system has been successful clinically and responds well to mental and emotional stress.

Peak Endocardial Acceleration Sensor

Using a miniature accelerometer within a pacing lead, an index called peak endocardial acceleration has been introduced. This sensor, placed in the right ventricular lead, provides a surrogate measure of myocardial contractility and sympathetic activation. The sensor is also being tested as a means to optimize the AV interval and right ventricle–left ventricle timing in dual-chamber and biventricular pacing devices.

Right Ventricular Impedance-Based Sensor

Closed-loop stimulation measures impedance from the right ventricular unipolar pacing. The impedance measurements correlate with dP/dt_max, which is a surrogate for ventricular contractility and in turn is a reflection of autonomic activity. As a reflection of autonomic activity, this sensor has the potential to respond to nonexertional stimuli such as mental stress.¹⁷

Dual-Sensor Combinations

The perfect sensor would be resistant to all nonphysiological stimuli. A multisensor, rate-adaptive pacing system could improve specificity by having one sensor verify or crosscheck the other. The most widely used sensor combination is the accelerometer–minute ventilation combination or blended sensor. Using this combination, the accelerometer provides a rapid response to exertion, and the minute ventilation provides an excellent physiologic response, achieving a more normal physiological heart rate response.¹⁸ A dual sensor using a closed-loop sensor and an accelerometer could have the advantage of being able to result in increased heart rates with mental stress.¹⁹

Pacemaker Complications

Complications can occur related to the implantation procedure or because of failure of a component of the pacing system. Box 117-2 lists the most commonly encountered complications and whether they are more likely to be seen immediately or over the long term.

Atrial Arrhythmia Detection and Automatic Mode Switching

The onset of atrial arrhythmias in the DDD or DDDR mode results in tracking of the atrial activity and rapid ventricular pacing, typically at the upper rate limit. Modern algorithms detect the rapid atrial rate on the atrial channel and result in automatic switching of the mode to a nontracking mode such as DDI, DDIR, VVI, or VVIR (Figure 117-9). In any of these modes, tracking of atrial activity ceases. Once the atrial arrhythmia stops, the mode switches back to the DDD or DDDR mode. Atrial mode switching is generally considered accurate. Simulation studies have demonstrated that rate-based algorithms are most effective in detecting atrial arrhythmias.²¹

Algorithms might also detect pacemaker-mediated tachycardia and intervene by prolonging the PVARP transiently or by avoiding atrial tracking for a single cycle. Pacemaker-mediated tachycardia occurs when VA conduction is longer than the programmed PVARP and is typically induced by a premature ventricular beat or atrial failure to capture.

There is some variability in the ability to detect atrial fibrillation by different atrial tachyarrhythmia detection algorithms. Undersensing can lead to decreased detection of episodes and in the incorrect determination that an episode has terminated. In such a situation, a greater number of shorter episodes might be recorded rather than fewer longer episodes. Longer detection times during onset can decrease inappropriate far-field oversensing. Atrial electrodes with shorter tip-to-ring distances, particularly shorter than 10 mm, can also reduce far-field R wave sensing. Postventricular atrial blanking periods can also contribute to undersensing.²²

Intrinsic Conduction Preference

There has been an increasing recognition that hemodynamics are improved when intrinsic ventricular conduction is permitted in the absence of bundle branch block. New algorithms have been designed to permit the automatic restoration of intrinsic conduction. These algorithms can represent a form of AV search hysteresis or mode switching from DDD or DDDR to AAI or AAIR. When ventricular pacing occurs, AV search hysteresis periodically lengthens the AV interval so that intrinsic AV conduction occurs. If AV conduction does not occur, AV pacing resumes. In mode-switching algorithms, the pacing mode is AAI or AAIR until AV block occurs and the mode switches to DDD or DDDR (Figure 117-10). Periodically, the mode switches back to AAI or AAIR to check that AV conduction has resumed (Figure 117-11). These algorithms are extremely successful in decreasing the incidence of ventricular pacing in patients with intact AV conduction and can decrease the incidence of atrial fibrillation.

Automatic Testing and Data Storage

Many pacemakers can perform reliable automatic threshold testing of the leads. These data can be used to adjust the pacing output automatically. Automatic capture verification appears to be reliable in long-term follow-up and provides the benefit of maintaining an adequate safety margin, even when the threshold has changed. In addition, the use of automatic capture verification can prolong pacemaker battery life.²³ Such automatic features can be effectively programmed with a need for reprogramming in 6% of patients in 12 months.²⁴ Pediatric patients with constant intrinsic high heart rates might not be able to have effective automatic threshold testing.²⁵ In addition, many pacemakers keep long-term data regarding lead thresholds, lead impedances, and R and P wave amplitudes. Such trends are extremely helpful in detecting early abnormalities in lead function and in identifying pacing abnormalities. Many pacemakers can store events that are classified as pacemaker-mediated tachycardia, atrial tachyarrhythmia, or rapid ventricular rate.

Routine Follow-Up and Remote Monitoring

Pacemakers should be checked routinely to confirm normal device and lead function and to assess the battery status. Traditionally, these functions have been performed at office visits or using home transtelephonic monitoring. New technologies permit some devices to be analyzed at home using Internet-based services. These systems can assess battery status and create alerts notifying the physician of device or lead abnormalities.

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