429.1 Principles of Antiarrhythmic Therapy

George F. Van Hare

When considering drug therapy in the pediatric population, it is important to recognize that there are marked differences in pharmacokinetics by age and in comparison with adults. Infants may have slower absorption, slow gastric emptying, and differing sizes of drug tissue compartments affecting the volume of distribution. Hepatic metabolism and renal excretion may vary within the pediatric age group as well as in comparison to adults. When considering antiarrhythmic therapy, it is important to recognize that the likely arrhythmia mechanism may be different for the pediatric vs. adult population.

There are many antiarrhythmic agents available for rhythm control. The majority have not been approved by the U.S. Food and Drug Administration (FDA) for use in children; their use is usually considered “off-label,” Pediatric cardiologists have experience with these drugs, and there are well-recognized standards regarding dosing.

With the availability of potentially curative ablation procedures, medical therapy has become less important. Clinicians and patients accept fewer drug side effects. Intolerable side effects, as well as the potential for a proarrhythmia induced by an antiarrhythmic drug, can seriously limit medical therapy and will lead the physician and family toward a potentially curative ablation procedure.

Antiarrhythmic drugs are commonly categorized using the Vaughan Williams classification system. This system comprises 4 classes: Class I includes agents that primarily block the sodium channel, class II includes the β-blockers, class III includes those agents that prolong repolarization, and class IV are the calcium channel blockers. Class I is further divided by the strength of the sodium channel blockade (see Table 429-1).

429.2 Sinus Arrhythmias and Extrasystoles

George F. Van Hare

Phasic sinus arrhythmia represents a normal physiologic variation in impulse discharges from the sinus node related to respirations. The heart rate slows during expiration and accelerates during inspiration. Occasionally, if the sinus rate becomes slow enough, an escape beat arises from the atrioventricular (AV) junction region (Fig. 429-1). Normal phasic sinus arrhythmia can be quite prominent in children and may mimic frequent premature contractions, but the relationship to the phases of respiration can be appreciated with careful auscultation. Drugs that increase vagal tone, such as digoxin, may exaggerate sinus arrhythmia; it is usually abolished by exercise. Other irregularities in sinus rhythm, especially bradycardia associated with periodic apnea are commonly seen in premature infants.

Figure 429-1 Phasic sinus arrhythmia with a junctional escape beat. Note the variation in P-P interval with no significant change in P wave morphology or PR interval. When the sinus rate is slow enough, the atrioventricular junction takes over and produces escape beats. This rhythm is normal.

Sinus bradycardia is due to slow discharge of impulses from the sinus node, the heart’s natural pacemaker. A sinus rate <90 beats/min in neonates and <60 beats/min in older children is considered to be sinus bradycardia. It is commonly seen in well-trained athletes; in healthy individuals, it is generally without significance. Sinus bradycardia may occur in systemic disease (hypothyroidism or anorexia nervosa), and it resolves when the disorder is under control. It may also be seen in association with conditions in which there is high vagal tone, such as gastrointestinal obstruction or intracranial processes. Low birthweight infants display great variation in sinus rate. Sinus bradycardia is common in these infants in conjunction with apnea, and may be associated with junctional escape beats. Premature atrial contractions are also frequent. These rhythm changes, especially bradycardia, appear more commonly during sleep and are not associated with symptoms. Usually, no therapy is necessary.

Wandering atrial pacemaker (Fig. 429-2) is defined as an intermittent shift in the pacemaker of the heart from the sinus node to another part of the atrium. It is not uncommon in childhood and usually represents a normal variant; it may also be seen in association with sinus bradycardia in which the shift in atrial focus is an escape phenomenon.

Figure 429-2 Wandering atrial pacemaker. Note the change in P-wave configuration in the 7th, 9th, and 10th beats. The 7th P wave may represent a fusion between the sinus P and the ectopic atrial pacemaker seen in the 10th beat.

Extrasystoles are produced by the premature discharge of an ectopic focus that may be situated in the atrium, the AV junction, or the ventricle. Usually, isolated extrasystoles are of no clinical or prognostic significance. Under certain circumstances, however, premature beats may be due to organic heart disease (inflammation, ischemia, fibrosis) or to drug toxicity, especially from digoxin.

Premature atrial contractions are common in childhood, usually in the absence of cardiac disease. Depending on the degree of prematurity of the beat (coupling interval) and the preceding R-R interval (cycle length), premature atrial complexes may result in a normal, a prolonged (aberrancy), or an absent (blocked premature atrial complex) QRS complex. The last occurs when the premature impulse cannot conduct to the ventricle due to refractoriness of the AV node or distal conducting system (Fig. 429-3). Atrial extrasystoles must be distinguished from premature ventricular contractions (PVCs). Careful scrutiny of the electrocardiogram for a premature P wave preceding the QRS will either show a premature P wave superimposed on, and deforming, the preceding T wave, or a P wave that is premature and has a different contour from that of the other sinus P waves. Atrial premature complexes usually reset the sinus node pacemaker, leading to an incomplete compensatory pause, but this feature is not regarded as a reliable means of differentiating atrial from ventricular premature complexes in children.

Figure 429-3 Premature atrial contraction (PAC). QRS complexes—the 8th, 10th, and final—in this strip are preceded by a P wave that is inverted, indicative of an ectopic origin of atrial depolarization. Note that the 8th and final QRS complexes resemble those of sinus origin, whereas the 10th is aberrantly conducted. This shift in origin is a function of the preceding cycle length, which influences the refractory period of the bundle branches. The fact that the pause after the PAC is longer than 2 P-P intervals implies that the premature atrial depolarization has invaded and discharged the sinus node and then reset it so that it fires later.

PVCs may arise in any region of the ventricles. They are characterized by premature, widened, bizarre QRS complexes that are not preceded by a premature P wave (Fig. 429-4). When all premature beats have identical contours, they are classified as uniform, suggesting origin from a common site. When PVCs vary in contour, they are designated as multiform, suggesting origin from more than 1 ventricular site. Ventricular extrasystoles are often, but not always, followed by a full compensatory pause. The presence of fusion beats, that is, complexes with morphologic features that are intermediate between those of normal sinus beats and those of PVCs, proves the ventricular origin of the premature beat. Extrasystoles produce a smaller stroke and pulse volume than normal and, if quite premature, may not be audible with a stethoscope or palpable at the radial pulse. When frequent, extrasystoles may assume a definite rhythm, for example, alternating with normal beats (bigeminy) or occurring after 2 normal beats (trigeminy). Most patients are unaware of single premature ventricular contractions, although some may be aware of a “skipped beat” over the precordium. This sensation is due to the increased stroke volume of the normal beat after a compensatory pause. Anxiety, a febrile illness, or ingestion of various drugs or stimulants may exacerbate PVCs.

Figure 429-4 Premature ventricular contractions (PVCs) in a bigeminal rhythm, in a patient who is hyperventilating. Note that the premature beat is wide and has a completely different morphology from that of the sinus beat. The premature beat is not preceded by a discernable premature P wave or any appreciable deformation of the preceding T wave.

It is important to distinguish PVCs that are benign from those that are likely to lead to more severe arrhythmias. The former usually disappear during the tachycardia of exercise. If they persist or become more frequent during exercise, the arrhythmia may have greater significance. The following criteria are indications for further investigation of PVCs that could require suppressive therapy: (1) 2 or more ventricular premature beats in a row, (2) multiform PVCs, (3) increased ventricular ectopic activity with exercise, (4) R on T phenomenon (premature ventricular depolarization occurs on the T wave of the preceding beat), and (5) most importantly, the presence of underlying heart disease, a history of heart surgery, or both. The best therapy for benign PVCs is reassurance that the arrhythmia is not life threatening, although very symptomatic individuals may benefit from suppressive therapy. Malignant PVCs are usually secondary to another medical problem (electrolyte imbalance, hypoxia, drug toxicity, cardiac injury, or an intraventricular catheter). Successful treatment includes correction of the underlying abnormality. An intravenous lidocaine bolus and drip is the 1st line of therapy, with more effective drugs such as amiodarone reserved for refractory cases or for patients underlying ventricular dysfunction or hemodynamic compromise.

429.3 Supraventricular Tachycardia

George F. Van Hare

Supraventricular tachycardia (SVT) is a general term that includes essentially all forms of paroxysmal or incessant tachycardia except ventricular tachycardia. The category of SVT can be divided into 3 major subcategories: re-entrant tachycardias using an accessory pathway, re-entrant tachycardias without an accessory pathway, and ectopic or automatic tachycardias. Atrioventricular reciprocating tachycardia (AVRT) involves an accessory pathway and is the most common mechanism of SVT in infants. Atrioventricular node re-entry tachycardia (AVNRT) is rare in infancy but there is an increasing incidence of AVNRT in childhood and into adolescence. Atrial flutter is rarely seen in children with normal hearts, whereas intra-atrial re-entry tachycardia (IART) is common in patients following cardiac surgery. Atrial and junctional ectopic tachycardias are more commonly associated with abnormal hearts (cardiomyopathy) and in the immediate postoperative period following surgery for congenital heart disease.

Clinical Manifestations

Re-entrant SVT is characterized by an abrupt onset and cessation; it usually occurs when the patient is at rest, although in infants it may be precipitated by an acute infection. Attacks may last only a few seconds or may persist for hours. The heart rate usually exceeds 180 beats/min and may occasionally be as rapid as 300 beats/min. The only complaint may be awareness of the rapid heart rate. Many children tolerate these episodes extremely well, and it is unlikely that short paroxysms are a danger to life. If the rate is exceptionally rapid or if the attack is prolonged, precordial discomfort and heart failure may occur. In children, SVT may be exacerbated by exposure to over-the-counter decongestants or by bronchodilators.

In young infants, the diagnosis may be more obscure because of the inability to communicate their symptoms. The heart rate at this age is normally higher than in older children and it increases greatly with crying. Infants with SVT on occasion initially present with heart failure, because the tachycardia may go unrecognized for a long time. The heart rate during episodes is frequently in the range of 240-300 beats/min. If the attack lasts 6-24 hr or more, heart failure may be recognized, and the infant will have an ashen color, and be restless and irritable, with tachypnea and hepatomegaly. When tachycardia occurs in the fetus, it can cause hydrops fetalis, which is the in utero manifestation of heart failure.

In neonates, SVT is usually manifested as a narrow QRS complex (<0.08 sec). The P wave is visible on a standard electrocardiogram in only 50-60% of neonates with SVT, but it is detectable with a transesophageal lead in most patients. Differentiation from sinus tachycardia may be difficult, but is important, as sinus tachycardia requires treatment of the underlying problem (e.g., sepsis, hypovolemia) rather than antiarrhythmic medication. If the rate is >230 beats/min with an abnormal P-wave axis (a normal P wave is positive in leads I and aVF), sinus tachycardia is not likely. The heart rate in SVT also tends to be unvarying, whereas in sinus tachycardia the heart rate varies with changes in vagal and sympathetic tone. AV reciprocating tachycardia uses a bypass tract that may either be able to conduct bidirectionally (Wolff-Parkinson-White [WPW] syndrome) or retrograde only (concealed accessory pathway). Patients with WPW syndrome have a small, but real risk of sudden death. If the accessory pathway rapidly conducts in antegrade fashion, the patient is at risk for atrial fibrillation begetting ventricular fibrillation. Risk stratification, including 24 hr Holter monitoring and exercise study, may help differentiate patients at higher risk for sudden death from WPW. Syncope is an ominous symptom in WPW and any patient with syncope and WPW syndrome should have an electrophysiology study and likely catheter ablation.

The typical electrocardiographic features of the Wolff-Parkinson-White syndrome are seen when the patient is not having tachycardia. These features include a short P-R interval and slow upstroke of the QRS (delta wave) (Fig. 429-5). Though most often present in patients with a normal heart, this syndrome may also be associated with Ebstein anomaly of the tricuspid valve, or hypertrophic cardiomyopathy. The critical anatomic structure is an accessory pathway consisting of a muscular bridge connecting atrium to ventricle on either the right or the left side of the AV ring (Fig. 429-6). During sinus rhythm, the impulse is carried over both the AV node and the accessory pathway; it produces some degree of fusion of the 2 depolarization fronts that results in an abnormal QRS. During AVRT, an impulse is carried in antegrade fashion through the AV node (orthodromic conduction), which results in a normal QRS complex, and in retrograde fashion through the accessory pathway to the atrium, thereby perpetuating the tachycardia. In these cases, only after cessation of the tachycardia are the typical ECG features of WPW syndrome recognized (see Fig. 429-5). When rapid antegrade conduction occurs through the pre-excitation pathway during tachycardia and the retrograde re-entry pathway to the atrium is via the AV node (antidromic conduction), the QRS complexes are wide and the potential for more serious arrhythmias (ventricular fibrillation) is greater, especially if atrial fibrillation occurs.

Figure 429-5 A, Supraventricular tachycardia in a child with Wolff-Parkinson-White (WPW) syndrome. Note the normal QRS complexes during the tachycardia, as well as clear retrograde P waves seen on the upstroke of the T waves. B, Later, the typical features of WPW syndrome are apparent (short P-R interval, delta wave, and wide QRS).

Figure 429-6 Schematic representation of the heart with a right-sided accessory pathway (Wolff-Parkinson-White syndrome). The asterisk indicates initiation of the sinus beat. The arrows indicate the direction and spread of excitation. The electrocardiographic complex shown represents a fusion beat that combines activation over the normal (n) and accessory (a) pathways. The latter inscribes the delta wave. NSR, normal sinus rhythm.

AV nodal re-entrant tachycardia (AVNRT) involves the use of 2 pathways within the AV node. This arrhythmia is more commonly seen in adolescence. It is one of the few SVTs that is occasionally associated with syncope. This arrhythmia is usually amenable to antiarrhythmic therapy, such as β-blockers, or to catheter ablation therapy.

Treatment

Vagal stimulation by placing of the face in ice water (in older children) or by placing an ice bag over the face (in infants) may abort the attack. To terminate the attack, older children may be taught vagal maneuvers such as the Valsalva maneuver, straining, breath holding, or standing on their head. Ocular pressure must never be performed, and carotid sinus massage is very rarely effective. When these measures fail, several pharmacologic alternatives are available (see Table 429-1). In stable patients, adenosine by rapid intravenous push is the treatment of choice because of its rapid onset of action and minimal effects on cardiac contractility. The dose may need to be increased if no effect on the tachycardia is seen. Because of the potential for adenosine to initiate atrial fibrillation, it should not be administered without a means for DC cardioversion near at hand. Calcium channel blockers such as verapamil have also been used in the initial treatment of SVT in older children. Verapamil may reduce cardiac output and produce hypotension and cardiac arrest in infants younger than 1 yr; it is therefore contraindicated in this age group. In urgent situations when symptoms of severe heart failure have already occurred, synchronized DC cardioversion (0.5-2 J/kg) is recommended as the initial management (Chapter 62).

Once the patient has been converted to sinus rhythm, a longer acting agent may is selected for maintenance therapy. In patients without an antegrade accessory pathway (non-WPW), the β-blockers are the mainstay of therapy. Digoxin is also popular, and is effective in infants, but less so in older children. In children with WPW, digoxin or calcium channel blockers may increase the rate of antegrade conduction of impulses through the bypass tract, with the possibility of ventricular fibrillation, and are therefore contraindicated. These patients are usually managed in the long term with β-blockers. In patients with resistant tachycardias, procainamide, quinidine, flecainide, propafenone, sotalol, and amiodarone have all been used. Most antiarrhythmic agents have the potential of causing new dangerous arrhythmias (proarrhythmia) and decreasing heart function. Flecainide and propafenone in particular should be limited to use in patients with otherwise normal hearts.

If cardiac failure occurs because of prolonged tachycardia in an infant with a normal heart, cardiac function usually returns to normal after sinus rhythm is reinstituted, although it may take days to weeks. Infants with SVT diagnosed within the 1st 3-4 mo of life have a lower incidence of recurrence than do those in whom it is initially diagnosed at a later age. These patients have an 80% chance of resolution by the 1st yr of life, although about 30% will have recurrences later in childhood; if medical therapy is required; it can be tapered within a year and the patient watched for signs of recurrence. Parents should be taught to measure the heart rate in their infants, so that prolonged unapparent episodes of SVT may be detected before heart failure occurs.

Twenty-four hour electrocardiographic (Holter) recordings are useful in monitoring the course of therapy and in detecting brief runs of asymptomatic tachycardia, particularly in younger children and infants. Some centers use transesophageal pacing to evaluate the effects of therapy in infants. More detailed electrophysiologic studies performed in the cardiac catheterization laboratory are often indicated in patients with refractory SVTs who are candidates for catheter ablation. During an electrophysiologic study, multiple electrode catheters are placed transvenously in different locations in the heart. Pacing is performed to evaluate the conduction characteristics of the accessory pathway and to initiate the tachyarrhythmia, and mapping is performed to locate the accessory pathway. Catheter ablation of an accessory pathway is often used electively in children and teenagers, as well as in patients who require multiple agents or find drug side effects intolerable or for whom arrhythmia control is poor. Ablation may be performed either by radiofrequency ablation, which creates tissue heating, or cryoablation, in which tissue is frozen. The overall initial success rate for catheter ablation ranges from approximately 80% to 95%, depending on the location of the accessory pathway. Surgical ablation of bypass tracts may also be successful in selected patients.

The management of SVT due to atrioventricular node re-entry tachycardia (AVNRT) is nearly identical to that for AVRT. Children with AVNRT are not at increased risk of sudden death, as they do not have a manifest accessory pathway. In practice, their episodes are more likely to be brought on by exercise or other forms of stress, and the heart rates can be quite fast, leading to chest pain, dizziness, and occasionally syncope. The choice of chronic antiarrhythmic medications is with β-blockers being the drugs of choice; AVNRT does respond to adenosine. Less is known about the natural history, but patients with AVNRT are seen quite commonly in adulthood, so spontaneous resolution seems unlikely. Patients are quite amenable to catheter ablation, either using radiofrequency energy or cryoablation, with high success rates and low complication rates.

Atrial ectopic tachycardia is an uncommon tachycardia in childhood. It is characterized by a variable rate (seldom >200 beats/min), identifiable P waves with an abnormal axis, and either a sustained or incessant nonsustained tachycardia. This form of atrial tachycardia has a single automatic focus. Identification of this mechanism is aided by monitoring the electrocardiogram while initiating vagal or pharmacologic therapy. Re-entry tachycardias “break” suddenly, whereas automatic tachycardias gradually slow down and then gradually speed up again. Atrial ectopic tachycardias are usually more difficult to control pharmacologically than are the more common re-entrant tachycardias. If pharmacologic therapy with a single agent is unsuccessful, catheter ablation is suggested and has a success rate >90%.

Chaotic or multifocal atrial tachycardia is defined as atrial tachycardia with ≥3 ectopic P waves, frequent blocked P waves, and varying P-R intervals of conducted beats. This arrhythmia occurs most often in infants younger than 1 yr, usually without cardiac disease, although some evidence suggests an association with viral myocarditis or pulmonary disease. The goal of drug treatment is slowing of the ventricular rate, as conversion to sinus may not be possible, and multiple agents are often required. When this arrhythmia occurs in infancy, it usually terminates spontaneously by 3 yr of age.

Accelerated junctional ectopic tachycardia (JET) is an automatic (non–re-entry) arrhythmia in which the junctional rate exceeds that of the sinus node and AV dissociation results. This arrhythmia is most often recognized in the early postoperative period after cardiac surgery and may be extremely difficult to control. Reduction of the infusion rate of catecholamines and control of fever are important adjuncts to management. Congenital JET may be seen in the absence of surgery. It is incessant, and can lead to dilated cardiomyopathy. Intravenous amiodarone is effective in the treatment of postoperative JET. Patients who require chronic therapy may respond to amiodarone or sotalol. Congenital JET can be cured by catheter ablation, but long-term AV block requiring a pacemaker is a prominent complication.

Atrial flutter, also known as intra-atrial re-entrant tachycardia, is an atrial tachycardia characterized by atrial activity at a rate of 250-300 beats/min in children and adolescents, and 400-600 in neonates. The mechanism of common atrial flutter consists of a re-entrant or rhythm originating in the right atrium circling the tricuspid valve annulus. Because the AV node cannot transmit such rapid impulses, some degree of AV block is virtually always present, and the ventricles respond to every 2nd-4th atrial beat (Fig. 429-7). Occasionally, the response is variable and the rhythm appears irregular.

Figure 429-7 Neonatal atrial flutter. Note that the flutter waves are not obvious in the first tracing (A) but once a dose of adenosine is given, flutter waves at a rate of about 450 beats/min emerge (B).

In older children, atrial flutter usually occurs in the setting of congenital heart disease; neonates with atrial flutter frequently have normal hearts. Atrial flutter may occur during acute infectious illnesses but is most often seen in patients with large stretched atria, such as those associated with long-standing mitral or tricuspid insufficiency, tricuspid atresia, Ebstein anomaly, or rheumatic mitral stenosis. Atrial flutter can also occur after palliative or corrective intra-atrial surgery. Uncontrolled atrial flutter may precipitate heart failure. Vagal maneuvers (such as carotid sinus pressure or iced saline submersion) or adenosine generally produce a temporary slowing of the heart rate due to increased AV block. The diagnosis is confirmed by electrocardiography, which demonstrates the rapid and regular atrial saw-toothed flutter waves. Atrial flutter usually converts immediately to sinus rhythm by synchronized DC cardioversion, which is most often the treatment of choice. Patients with chronic atrial flutter in the setting of congenital heart disease may be at increased risk for thromboembolism and stroke and should thus undergo anticoagulation before elective cardioversion. Digoxin, β-blockers, or calcium channel blockers may be used to slow the ventricular response in atrial flutter by prolonging the AV node refractory period. Other agents may be used to maintain sinus rhythm, and choices include type I agents such as procainamide or propafenone, type III agents such as amiodarone and sotalol. Other modalities, including catheter and surgical ablation, have been used in older patients with congenital heart disease with moderate success. Following cardioversion, neonates with normal hearts may be treated with digoxin for 6-12 mo, after which the medication can usually be discontinued, as neonatal atrial flutter generally does not recur.

Atrial fibrillation is uncommon in children and is rare in infants. The atrial excitation is chaotic and more rapid (400-700 beats/min) and produces an irregularly irregular ventricular response and pulse (Fig. 429-8). This rhythm disorder is often associated with atrial enlargement or disease. Atrial fibrillation may be seen in older children with rheumatic mitral valve stenosis. It is also seen rarely as a complication of atrial surgery, in patients with left atrial enlargement secondary to left AV valve insufficiency, and in patients with WPW syndrome. Thyrotoxicosis, pulmonary embolism, pericarditis, or cardiomyopathy may be suspected in a previously normal older child or adolescent with atrial fibrillation. Very rarely, atrial fibrillation may be familial. The best initial treatment is rate control, most effectively with calcium channel blockers, to limit the ventricular rate during atrial fibrillation. Digoxin is not given if WPW syndrome is present. Normal sinus rhythm may be restored with intravenous procainamide or amiodarone, or by DC cardioversion, and DC cardioversion is the first choice in hemodynamically unstable patients. Patients with chronic atrial fibrillation are at risk for the development of thromboembolism and stroke and should undergo anticoagulation with warfarin. Patients being treated by elective cardioversion should also undergo anticoagulation.

Figure 429-8 Atrial fibrillation, characterized by the absence of clear P waves and an irregularly irregular ventricular response. One can appreciate the irregular, rapid undulations (F waves). Fibrillatory waves may not be visible in all leads and should be carefully sought in every tracing with irregular R-R intervals. Note that no two R-R intervals are the same.

429.4 Ventricular Tachyarrhythmias

George F. Van Hare

Ventricular tachycardia (VT) is less common than SVT in pediatric patients. VT is defined as at least three PVCs at >120 beats/min (Fig. 429-9). It may be paroxysmal or incessant. VT may be associated with myocarditis, anomalous origin of a coronary artery, arrhythmogenic right ventricular dysplasia, mitral valve prolapse, primary cardiac tumors, or cardiomyopathy. It has been seen with prolonged Q-T interval of either congenital or acquired (proarrhythmic drugs) causation, WPW syndrome, and drug use (cocaine, amphetamines). It may develop years after intraventricular surgery (especially tetralogy of Fallot and related defects) or occur without obvious organic heart disease. VT must be distinguished from SVT with aberrancy or rapid conduction over an accessory pathway (Table 429-2). The presence of clear capture and fusion beats confirms the diagnosis of VT. Although some children tolerate rapid ventricular rates for many hours, this arrhythmia should be promptly treated because hypotension and degeneration into ventricular fibrillation may result. For patients who are hemodynamically stable, intravenous amiodarone, lidocaine, or procainamide are the initial drugs of choice. If treatment is to be successful, it is critical to search for and correct any underlying abnormalities such as electrolyte imbalance, hypoxia, or drug toxicity. Amiodarone is the treatment of choice during cardiac arrest (Chapter 62). Hemodynamically unstable patients with VT should be immediately treated with DC cardioversion. Overdrive ventricular pacing, through temporary pacing wires or a permanent pacemaker, may also be effective, although it may cause the arrhythmia to deteriorate into ventricular fibrillation. In the neonatal period, ventricular tachycardia may be associated with an anomalous left coronary artery (Chapter 426.2) or a myocardial tumor.

Figure 429-9 Ventricular arrhythmias.

(From Park MY: Pediatric cardiology for practitioners, ed 5, Philadelphia, 2008, Mosby/Elsevier, p 429, Fig 24-6.)

Unless a clearly reversible cause is identified, electrophysiologic study is usually indicated for patients in whom VT has developed, and depending on the findings, catheter ablation and/or ICD implantation may be indicated.

A related arrhythmia, ventricular accelerated rhythm, is occasionally seen in infants. It is defined the same way as VT, but the rate is only slightly faster than the coexisting sinus rate (within 10%). It is generally benign.

Ventricular fibrillation is a chaotic rhythm that results in death unless an effective ventricular beat is rapidly re-established (see Fig. 429-9). A thump on the chest may occasionally restore sinus rhythm. Usually, cardiopulmonary resuscitation and DC defibrillation is necessary. If defibrillation is ineffective or fibrillation recurs, amiodarone or lidocaine may be given intravenously and defibrillation repeated (Chapter 62). After recovery from ventricular fibrillation, a search should be made for the underlying cause. Electrophysiologic study is indicated for patients who have survived ventricular fibrillation unless a clearly reversible cause is identified. If WPW syndrome is noted, catheter ablation should be performed. For patients in whom no correctable abnormality can be found, an ICD is nearly always indicated, because of the high risk of sudden death.

429.5 Long Q-T Syndromes

George F. Van Hare

Long Q-T syndromes (LQTS) are genetic abnormalities of ventricular repolarization, with an estimated incidence of about 1 per 10,000 births (Table 429-3). They present as a long Q-T interval on the surface ECG and are associated with malignant ventricular arrhythmias (torsades de pointes and ventricular fibrillation). They are a cause of syncope and sudden death and may be associated with sudden infant death syndrome or drowning. At least 50% of cases are familial, but due to variable penetrance, this may be an underestimate. The old distinction between dominant and recessive forms of the disease (Romano-Ward syndrome [RWS] vs Jervell and Lange-Nielsen syndrome [JLNS]) is no longer commonly made, as the latter “recessive” condition is known to be due to the homozygous state. JLNS is associated with congenital sensorineural deafness. Asymptomatic patients carrying the gene mutation may not all have a prolonged Q-T duration. Q-T interval prolongation may become apparent with exercise or during catecholamine infusions.

Genetic studies have identified mutations in cardiac potassium and sodium channels (see Table 429-3). Additional forms of LQTS have been described, but these are much more uncommon. JLNS has been seen in patients who have homozygous mutations of KVLQT1 and minK, whereas the heterozygous state is manifested as RWS. Genotype may predict clinical manifestations; for example, LQT1 events are usually stress induced, whereas events in LQT3 often occur during sleep. LQT2 events have an intermediate pattern. LQT3 has the highest probability for sudden death, followed by LQT2 and then LQT1. Drugs may prolong the Q-T interval directly but more often do so when drugs such as erythromycin or ketoconazole inhibit their metabolism (Table 429-4).

The clinical manifestation of LQTS in children is most often a syncopal episode brought on by exercise, fright, or a sudden startle; some events occur during sleep (LQT3). Patients can initially be seen with seizures, presyncope, or palpitations; about 10% are initially in cardiac arrest. The diagnosis is based on electrocardiographic and clinical criteria. Not all patients with long Q-T intervals have LQTS, and patients with normal Q-T intervals on a resting electrocardiogram may have LQTS. A heart rate–corrected Q-T interval of >0.47 sec is highly indicative, whereas a Q-T interval of >0.44 sec is suggestive. Other features include notched T waves, T wave alternans, a low heart rate for age, a history of syncope (especially with stress), and a familial history of either LQTS or unexplained sudden death. Twenty-four hour Holter monitoring and exercise testing are adjuncts to the diagnosis. Genotyping is available and can identify the mutation is about 75% of patients known to have LQTS by clinical criteria. Genotyping is not useful in ruling out the diagnosis in individuals suspected of having the disease, but when positive is very useful in identifying asymptomatic affected relatives of the index case.

Short Q-T syndromes (see Table 429-3) manifest with atrial or ventricular fibrillation and are associated with syncope and sudden death.

Treatment of LQTS includes the use of β-blocking agents at doses that blunt the heart rate response to exercise. Some patients require a pacemaker because of drug-induced profound bradycardia. In patients with continued syncope despite treatment, an implantable cardiac defibrillator is indicated for those who do not respond to β-blocking drugs and those who have experienced cardiac arrest. Genotype-phenotype correlative studies have suggested that β-blocking agents are not effective in patients with LQT3, and in those individuals, an ICD is usually indicated.

429.6 Sinus Node Dysfunction

George F. Van Hare

Sinus arrest and sinoatrial block may cause a sudden pause in the heartbeat. The former is presumably caused by failure of impulse formation within the sinus node and the latter by a block between the sinus pacemaker complex and the surrounding atrium. These arrhythmias are rare in childhood except as manifestations of digoxin toxicity or in patients who have had extensive atrial surgery.

Sick sinus syndrome is the result of abnormalities in the sinus node or atrial conduction pathways, or both. This syndrome may occur in the absence of congenital heart disease and has been reported in siblings, but it is most commonly seen after surgical correction of congenital heart defects, especially the Fontan procedure and the atrial switch (Mustard or Senning) operation for transposition of the great arteries. Clinical manifestations depend on the heart rate. Most patients remain asymptomatic without treatment, but dizziness and syncope can occur during periods of marked sinus slowing with failure of junctional escape (Fig. 429-10). Pacemaker therapy is indicated in patients who experience symptoms such as exercise intolerance or syncope.

Figure 429-10 The “tachy-brady” syndrome with sinus node dysfunction. Note the bursts of supraventricular tachycardia, probably multifocal atrial in origin, followed by long periods of sinus arrest and by sinus bradycardia. Often, symptoms are due to the long sinus pauses following termination of tachycardia, rather than to the tachycardia itself.

Patients with sinus node dysfunction may also have episodes of SVT (“tachy-brady syndrome”) with symptoms of palpitations, exercise intolerance, or dizziness. Treatment must be individualized. Drug therapy to control tachyarrhythmias (propranolol, sotalol, amiodarone) may suppress sinus and AV node function to such a degree that further symptomatic bradycardia may be produced. Therefore, insertion of a pacemaker in conjunction with drug therapy is usually necessary for such patients, even in the absence of symptoms ascribable to low heart rate.

429.7 AV Block

George F. Van Hare

AV block may be divided into 3 forms. In 1st-degree AV block, the PR interval is prolonged, but all the atrial impulses are conducted to the ventricle (Fig. 429-11). In 2nd-degree AV block, not every atrial impulse is conducted to the ventricle. In 1 variant of 2nd-degree block known as the Wenckebach type (also called Mobitz type I), classically the PR interval increases progressively until a P wave is not conducted. In the cycle following the dropped beat, the PR interval normalizes (see Fig. 429-11). In Mobitz type II, there is no progressive conduction delay and subsequent shortening of the PR interval after a blocked beat. This conduction defect is less common but has more potential to cause syncope and may be progressive. A related condition is high-grade 2nd-degree AV block, in which more than 1 P wave in a row fails to conduct. This is even more worrisome. In 3rd-degree AV block (complete heart block), no impulses from the atria reach the ventricles (see Fig. 429-11). Generally, an independent escape rhythm is present, but may not be reliable, leading to symptoms such as syncope.

Figure 429-11 Atrioventricular (AV) block.

(From Park MY: Pediatric cardiology for practitioners, ed 5, Philadelphia, 2008, Mosby/Elsevier, p 446, Fig. 25-1.)

Congenital complete AV block in children is presumed to be caused by autoimmune injury of the fetal conduction system by maternally derived IgG antibodies (anti-SSA/Ro, anti-SSB/La) in a mother with overt or, more often, asymptomatic SLE or Sjögren syndrome. Autoimmune disease accounts for 60-70% of all cases of congenital complete heart block and ≈80% of cases in which the heart is structurally normal (Fig. 429-12). A homeobox gene mutation, NKX2-5, is described in which congenital AV block is seen most commonly in association with atrial septal defects. Complete AV block is also seen in patients with complex congenital heart disease and abnormal embryonic development of the conduction system. It has been associated with myocardial tumors and myocarditis. It is a known complication of myocardial abscess secondary to endocarditis. It is also seen in genetic abnormalities including LQTS and Kearn-Sayre syndrome. It is also a complication of congenital heart disease repair and, in particular, repairs involving VSD closure.

Figure 429-12 Congenital complete atrioventricular (AV) block. The ventricular rate is regular at 53 beats/min. The atrial rate is somewhat variable, from 65 to 95 beats/min, and completely dissociated from the ventricle. The QRS morphology is normal, which is common in congenital complete AV block.

The incidence of congenital complete heart block is 1/20,000-25,000 live births; a high fetal loss rate may cause an underestimation of its true incidence. In some infants of mothers with SLE, complete heart block is not present at birth but develops within the 1st 3-6 mo after birth. The arrhythmia is often diagnosed in the fetus (secondary to the dissociation between atrial and ventricular contractions seen on fetal echocardiography) and may produce hydrops fetalis. Infants with associated congenital heart disease and heart failure have a high mortality rate.

In older children with otherwise normal hearts, the condition is commonly asymptomatic, although syncope and sudden death may occur. Infants and toddlers may have night terrors, tiredness with frequent naps, and irritability. The peripheral pulse is prominent as a result of the compensatory large ventricular stroke volume and peripheral vasodilatation; systolic blood pressure is elevated. Jugular venous pulsations occur irregularly and may be large when the atrium contracts against a closed tricuspid valve (cannon wave). Exercise and atropine may produce an acceleration of ≥10-20 beats/min. Systolic murmurs are frequently audible along the left sternal border, and apical mid-diastolic murmurs are not unusual. The first heart sound is variable, due to variable ventricular filing with AV dissociation. AV block results in enlargement of the heart on the basis of increased diastolic ventricular filling.

The diagnosis is confirmed by electrocardiography; the P waves and QRS complexes have no constant relationship (see Fig. 429-12). The QRS duration may be prolonged, or it may be normal if the heartbeat is initiated high in the AV node or bundle of His.

The prognosis for congenital complete heart block is usually favorable; patients who have been observed to the age of 30-40 yr have lived normal, active lives. Some patients have episodes of exercise intolerance, dizziness, and syncope (Stokes-Adams attacks); this symptom requires the implantation of a permanent cardiac pacemaker. Pacemaker implantation should be considered for patients who develop symptoms such as progressive cardiac enlargement, prolonged pauses, or daytime average heart rates of ≤50 beats/min. In addition, prophylactic pacemaker implantation in adolescents is reasonable considering the low risk of the implant procedure and the difficulty in predicting who will develop sudden severe symptoms.

Cardiac pacing is recommended in neonates with low ventricular rates (≤50 beats/min), evidence of heart failure, wide complex rhythms, or congenital heart disease. Isoproterenol, atropine, or epinephrine may be used to try to increase the heart rate temporarily until pacemaker placement can be arranged. Transthoracic epicardial pacemaker implants have traditionally been used in infants; transvenous placement of pacemaker leads is available for young children.

Postsurgical complete AV block can occur after any open heart procedure requiring suturing near the AV valves or crest of the ventricular septum. Postoperative heart block is initially managed with temporary pacing wires. The likelihood of a return to sinus rhythm after 10-14 days is low; a permanent pacemaker is recommended after that time.