■ INTRODUCTION
■ MECHANISMS OF ARRHYTHMIAS
Abnormal Impulse Formation Abnormal Impulse Conduction
■ SINUS ARRHYTHMIA
■ BRADYARRHYTHMIAS
Sinus Bradycardia
Atrioventricular Block
■ TACHYARRHYTHMIAS
Sinus Tachycardia
Premature Atrial Contractions
Supraventricular Tachycardia: Re-Entrant Atrial Flutter
Supraventricular Tachycardia: Automatic Premature Ventricular Contractions Ventricular Tachycardia
Long QT Syndrome
Catecholaminergic Polymorphic
Ventricular Tachycardia
Sudden Infant Death Syndrome and Cardiac Arrhythmias
Ventricular Fibrillation
■ EVALUATION, ASSESSMENT, AND APPROACH TO DIAGNOSIS
Evaluation of Cardiac Rhythm
Initial Assessment
Rapid Classification of Arrhythmias and General Therapeutic Approach
■ PHARMACOLOGIC THERAPY
General Considerations
Class 1A (Procainamide)
Class 1B (Lidocaine, Mexiletine, Phenytoin)
Class 1C (Flecainide)
Class II (Propranolol, Nadolol, Atenolol,
Metoprolol, Esmolol)
Class III (Amiodarone, Sotalol)
Class IV (Verapamil)
Digoxin
Adenosine
Magnesium
■ DC CARDIOVERSION
■ OVERDRIVE PACING
■ CATHETER ABLATION
■ SUGGESTED READINGS
■ INTRODUCTION
Although certain arrhythmias are more common in neonates and young infants compared to older children and adults, all types of arrhythmias can occur. Many are benign and do not cause hemodynamic compromise. Others may diminish cardiac output and cause decreased blood pressure and decreased perfusion. Sustained tachyarrhythmias may eventually cause myocardial dysfunction, which is known as tachycardia-induced cardiomyopathy. The purpose of this chapter is to review diagnosis and management of common arrhythmias in neonates and young infants.
■ MECHANISMS OF ARRHYTHMIAS
The electrical impulse normally originates in the sinoatrial (SA) node. The atrioventricular (AV) node, His bundle, and bundle branches provide the only normal pathway for transmission of impulses between the atria and ventricles. Generation of impulses from the SA node is modulated by many factors, including body temperature, blood pressure, autonomic nervous system, and circulating catecholamines. Abnormalities in any of these factors can result in bradycardia or tachycardia that are not related to any specific cardiac disorder.
Conduction through the AV node is slowed so that atrial contraction is complete before ventricular contraction occurs. If the SA node fails to depolarize, the AV node can function as an escape pacemaker.
Abnormal Impulse Formation
Abnormalities in impulse formation result in sinus bradycardia and tachycardia, premature atrial and ventricular contractions, and ectopic or automatic rhythms from the atria, AV node, or ventricles. Automatic tachycardias are usually incessant (ie, they are almost always present). Increased automaticity occurs when atrial, nodal, or ventricular cells display autonomous repetitive depolarization at a higher rate than is normal. This type of tachycardia is sometimes associated with fever, hypoxemia, electrolyte disturbances, or infusion of intravenous sympathomimetic agents. Sinus tachycardia can be considered an automatic tachycardia, but it is rarely spontaneous and characteristically resolves when the abnormal stimulus resolves. In contrast, other forms of automatic tachycardia, such as atrial ectopic tachycardia, junctional ectopic tachycardia, and the automatic form of ventricular tachycardia, may be spontaneous or triggered by the aforementioned stimuli. Regardless of site of origin, onset and termination are often gradual rather than abrupt. The rate of an automatic tachycardia is often sensitive to changes in autonomic tone. Therapies that produce only transient effects, such as direct current (DC) cardioversion and administration of adenosine, do not terminate automatic tachycardias.
Abnormal Impulse Conduction
Block within the normal conduction system is the most obvious form of abnormal impulse conduction. Block can occur at any location, but atrioventricular block is the most common site.
Re-entry, the other form of abnormal impulse conduction, is an important mechanism underlying supraventricular tachycardia in infants. The re-entrant circuit involves two functionally distinct pathways that have different conduction velocities and refractory periods (Figure 10-1). Under the right circumstances (often in response to a premature atrial contraction), an electrical impulse arrives when one of the pathways is refractory. The impulse traverses the other pathway, and conduction is delayed enough so that the impulse is able to “reenter” the blocked pathway from the other direction, thus completing the re-entrant circuit. Re-entry mechanisms usually cause paroxysmal tachycardias, which may start and stop multiple times in the course of the day. Re-entrant tachycardias start and stop abruptly, and they often terminate in response to interventions that produce only transient effects (eg, adenosine) because interruption of the re-entrant circuit usually terminates the tachycardia.
■ SINUS ARRHYTHMIA
Sinus arrhythmia is a normal phasic variation in impulse formation from the SA node that is often in cycle with respiration (Figure 10-2). This is the most common cause of an irregular heart rate, especially in older infants. The P-wave axis is usually normal. If substantial slowing occurs, junctional tissue depolarizes first, and junctional escape beats may be seen. Sinus arrhythmia is more common at slower heart rates and is therefore more frequent in sleeping infants and in any patient with increased vagal tone (Table 10-1). This rhythm is a normal variant, and no special monitoring or intervention is indicated.
■ BRADYARRHYTHMIAS
Sinus Bradycardia
The definition of sinus bradycardia depends on the method used to record the rhythm. An infant is usually stimulated by placement of the leads for a standard electrocardiogram (ECG), so bradycardia is usually defined as a heart rate <100 beats per minute. In contrast, the infant is much less stimulated during recording of a 24-hour electrocardiogram and sleeps during portions of the recording. During wakeful times, the average heart rate in young infants is 105 to 110 beats per minute, and the average minimum rate is in the low 90s. Based on these data, bradycardia in a neonate, defined as two standard deviations below the mean, is a heart rate of less than 80 beats per minute while awake and less than 60 beats per minute while asleep. This information has important implications for infants placed on apnea monitors. The alarm for low heart rate should not be set too high.
FIGURE 10-1. Re-entrant tachycardia. Two pathways, one conducting slowly and one conducting rapidly, are shown. A. Normal sinus rhythm. The impulse is traveling down both pathways but is blocked at the slow pathway. B. A premature atrial contraction is blocked in the fast pathway because this pathway remains refractory after the previous beat. The impulse is able to travel down the slow pathway and then retrograde up the fast pathway, which is no longer refractory. C. Supraventricular tachycardia. Conduction of the impulse up the fast pathway creates a re-entrant circuit.
The most common cause of sinus bradycardia in neonates is increased vagal tone (Table 10-1). The next most common, especially in premature infants, is hypoxemia related to apnea. Hypoxemia in the fetus causes apnea and bradycardia, and the apnea of prematurity is a postnatal manifestation of this response. Conversely, hypoxemia not caused by apnea stimulates tachypnea after birth, and this is associated with tachycardia rather than bradycardia. Other causes of sinus bradycardia include hypothermia, drug therapy, and hypothyroidism. Infants with long QT syndrome often have slower heart rates, so the QT interval corrected for heart rate (QTc) interval should be assessed carefully in all neonates with sinus bradycardia.
Rarely, infants with sinus bradycardia have familial bradycardia or tachycardia-bradycardia (sick sinus) syndrome. These infants may need antiarrhythmic medication and/or pacemaker placement.
Atrioventricular Block
First-Degree Atrioventricular Block
First-degree atrioventricular (AV) block is characterized by an abnormally long PR interval for age and heart rate. In neonates with normal heart rates, the upper limit of normal is 160 ms on the first day of life and 140 ms thereafter. According to these criteria, first-degree AV block is present in about 6% of normal newborn infants and most often can be considered a normal variant. First- degree AV block also results from prolonged AV nodal conduction, which is usually the result of medication (eg, digoxin) or from trauma/ischemia in patients who have had cardiac surgery. Treatment is not necessary, but further evaluation to exclude higher degrees of AV block may be required.
FIGURE 10-2. Sinus arrhythmia. The P-wave axis is normal and does not vary. The R-R interval varies from 400 to 700 ms (86 to 150 beats per minute).
Second-Degree AV Block
Second-degree AV block is defined as intermittent loss of AV conduction (failure of a normal atrial impulse to conduct to the ventricles). Mobitz type I (Wenckebach) block is characterized by gradual lengthening of the PR interval eventually followed by a P wave without a subsequent QRS complex (“dropped beat”). This results in the typical “grouped QRSs” (Figure 10-3). Mobitz type I block is typically seen during sleep and in patients who have increased vagal tone (Table 10-1). This pattern usually can be considered a normal variant. No further evaluation is necessary unless this pattern is noted during times of increased catecholamine state.
Mobitz type II block is characterized by intermittent loss of conduction of P waves to the ventricles without prolongation of the PR interval and is always considered pathologic. Every other P wave is conducted in 2:1 block. Conduction may be lost for more than one P wave (eg, 3:1 block); this is called high-grade second-degree AV block and may progress to complete AV block. This rhythm is uncommon in newborn infants and is thought to be related to block in the bundle of His. It can occur in infants born to mothers with connective tissue disease, in infants with congenital cardiovascular disease (eg, l-looped ventricles, heterotaxy syndrome [left atrial isomerism]), and in infants who have had cardiac surgery. Mobitz type II block may progress to complete AV block, and for this reason, these patients must be observed closely. Patients with Mobitz type II block and a wide QRS complex should be considered for permanent pacemaker placement.
At times, an electrocardiographic pattern of 2:1 AV block is associated with marked prolongation of the QTc interval and is caused by the very prolonged ventricular refractory period associated with the long QT interval (Figure 10-4). This can be caused by electrolyte disorders, especially hypocalcemia. Alternatively, although rare, patients may have congenital long QT syndrome and will need aggressive treatment because the risk of sudden death is high even in asymptomatic patients.
Complete (Third-Degree) AV Block
Complete, or third-degree, AV block is characterized by failure of all atrial impulses to be conducted to the ventricle. Generally, the atrial rhythm (P wave) is completely dissociated from the ventricular rhythm (QRS complex) (Figure 10-5). The atrial rate is normal for age and responds to chronotropic stimuli, such as pain and arousal, and can be used as a marker of hemodynamic stress caused by the AV block. The QRS complexes are regular, and the heart rate, which varies little, is usually 60 to 80 beats per minute in neonates. The QRS complexes may be narrow if the escape rhythm originates near the AV node and impulses flow down the normal ventricular conduction system or wide if the escape rhythm originates from below the bundle of His.
FIGURE 10-3. Second-degree atrioventricular block, Mobitz type I (Wenckebach block). Progressive lengthening of the PR interval is present before the nonconducted or dropped beats occur (arrows).
The onset of complete AV block in neonates is usually during fetal life and the condition is called congenital complete AV block (CCAVB). If the heart is structurally normal, CCAVB is often associated with maternal collagen vascular disease, such as systemic lupus erythematosus or Sjogren syndrome. Maternal autoantibodies to SSA/ Ro and SSB/La proteins cross the placenta and interact with the developing conduction system. The exact mechanism for antibody-mediated AV block remains to be defined, but it appears that a series of immune-mediated inflammatory events results in fibrosis of the AV node and distal conduction system. In addition to CCAVB, affected infants may show signs of neonatal lupus, including discoid lesions, leukopenia, thrombocytopenia, and hemolytic anemia. Many mothers have no signs or symptoms, so all mothers who have offspring with CCAVB without structural cardiovascular abnormalities should be evaluated for connective tissue diseases. The incidence of CCAVB is 1% to 2% in offspring of mothers who have anti-Ro and anti-La antibodies. The risk of CCAVB is 15% to 20% in subsequent pregnancies after the birth of one child with CCAVB.
CCAVB diagnosed during fetal life that is associated with hydrops fetalis and cardiac enlargement carries a poor prognosis. Therapy for this condition is discussed in Chapter 4. Newborns with CCAVB are often asymptomatic because stroke volume increases to compensate for the decreased ventricular rate, and thus cardiac output is maintained. Infants with structurally normal hearts who are born with or develop congestive heart failure often respond well to supportive therapy. Inotropic agents, such as isoproterenol, and pacemaker placement are often necessary. Although rare, bradycardia may be severe at birth and produce signs and symptoms of inadequate cardiac output. In those cases, emergency pacing is necessary in the delivery room. This can be accomplished by placement of a temporary transvenous pacemaker or pacing by use of transcutaneous pacing electrodes until a permanent pacemaker is placed.
Criteria for pacemaker placement in neonates and young infants with CCAVB include congestive heart failure, cardiomegaly and/or ventricular dysfunction, premature ventricular contractions or ventricular tachycardia, prolonged pauses, prolonged QTc interval, and a wide complex (ventricular) instead of narrow complex (junctional) escape rhythm. Controversy exists as to whether a pacemaker should be placed in neonates solely because of a slow ventricular rate (<55 beats per minute). As many as 20% of those with CCAVB diagnosed during fetal life or shortly after birth may be at risk of developing dilated cardiomyopathy during childhood; the presence of maternal antibodies is likely a risk factor. Serial followup of ventricular function is important in these patients.
FIGURE 10-4. Electrocardiographic pattern of 2:1 AV block caused by prolonged ventricular refractoriness associated with a prolonged QT interval. Every other P wave is conducted to the ventricles. The corrected QT interval is 550 ms.
FIGURE 10-5. Complete atrioventricular block. Rhythm strip recorded in a neonate shows that P waves and QRS complexes are independent of each other. The atrial rate is 145 beats per minute, and the ventricular rate is 62 beats per minute. The QRS complex is narrow.
Congenital cardiovascular defects associated with CCAVB include l-looped ventricles (eg, corrected transposition of the great arteries), heterotaxy syndrome (left atrial isomerism), and atrioventricular septal defects. Such defects are present in about 50% of infants with CCAVB. These infants occasionally develop nonim- mune hydrops. Despite aggressive treatment, the prognosis is poor in infants with complex heart disease and CCAVB.
Complete AV block may also occur after cardiac surgery, especially in those patients with l-loop or in those who have had surgery involving the ventricular septum (eg, tetralogy of Fallot, ventricular septal defect, or atrioventricular septal defect). Permanent pacemaker placement is always indicated for postoperative patients in whom complete AV heart block related to cardiac surgery does not resolve within 10 to 14 days. Temporary pacing (transvenous, transcutaneous, temporary pacing wires) may be indicated, and, rarely, isoproterenol infusion is necessary before a permanent pacemaker is placed. The threshold for the temporary pacer lead must be checked daily; a patient who has an inadequate underlying rhythm and high-pacing threshold should be considered for immediate permanent pacemaker placement.
■ TACHYARRHYTHMIAS
Sinus Tachycardia
Sinus tachycardia is characterized by a normal P-wave axis (upright in lead II) and a rate up to 240 beats per minute in neonates (Figure 10-6). Variability in the rate is common. Sinus tachycardia is usually caused by some other problem in neonates, such as hypovolemia, fever, hypoxemia, sympathomimetic medications, anemia, pain, and inadequate sedation. When the heart rate is >170 to 180 beats per minute, P waves may be difficult to see because they are superimposed on the preceding T wave. Sometimes, vagal maneuvers (see following text) will transiently decrease the heart rate enough that sinus rhythm can be more easily identified. Neither vagal maneuvers, administration of adenosine, nor DC cardioversion will terminate sinus tachycardia. Instead, treatment should address the underlying cause of tachycardia. For example, administration of analgesia to a postoperative patient in pain will decrease the heart rate and confirm sinus rhythm.
FIGURE 10-6. Sinus tachycardia. The electrocardiogram recorded in a 2-month-old infant with septic shock shows P waves in almost every lead. The heart rate is 230 beats per minute. The P-wave axis is normal.
Premature Atrial Contractions
Premature atrial contractions (PACs; also known as supraventricular premature contractions) are caused by premature heart beats originating in the atria; early P waves are seen on electrocardiographic recordings (Figure 10-7). They are most commonly an incidental finding in infants who have been placed on cardiac monitors for other reasons. The morphology of the P wave may be different than that of the normal sinus P wave and reflects the ectopic origin of the impulse within the atrium. At times, the P wave may be superimposed on the preceding T wave. Most often, PACs are conducted normally to the ventricles, and the QRS complex is normal. Occasionally, there is conduction with aberrancy; a bundle branch pattern with a wide QRS complex is seen because the bundle branch is still refractory from the previous depolarization. If the premature P wave is very early, it will not be conducted to the ventricles because the AV node or proximal His bundle is refractory; this is called a blocked PAC. This will tend to slow the heart rate, and frequent blocked PACs can cause bradycardia (Figure 10-8). Iatrogenic causes include endocardial irritation from an intracardiac catheter or extracorporeal membrane oxygenation cannula and effects of pharmacologic agents, such as caffeine, theophylline, dopamine, epinephrine, and isoproterenol. Rarely, electrolyte or metabolic abnormalities, cardiac tumors, myocarditis, or structural heart disease are present. Possible predisposing conditions should be treated, but a specific etiology is not determined in most cases. Even if frequent, PACs do not cause hemodynamic compromise and do not need to be treated. A healthy newborn infant who does not have any risk factors and who has a normal physical examination does not require further work-up. PACs may occur in up to one-third of neonates and usually disappear within the first 3 months of life.
FIGURE 10-7. Premature atrial contraction (arrow).
FIGURE 10-8. Premature atrial contractions. Lead II rhythm strip showing frequent blocked premature atrial contractions. P waves falling at the end of the T wave (arrow) are conducted to ventricles with aberration. The premature P waves falling within the ST segment or on the upstroke of the T wave are not conducted to the ventricles or blocked (*). The frequent blocked premature beats caused asymptomatic bradycardia in this neonate.
Supraventricular Tachycardia: Re-Entrant
Supraventricular tachycardia (SVT) is an abnormal tachycardia that requires atrial or AV nodal tissue for initiation and maintenance. Excluding sinus tachycardia, re-entrant SVT is the most common arrhythmia in infants and children, and the incidence has been estimated to be as low as 1 in 25 000 and as high as 1 in 250 infants.
AV Reciprocating (Accessory Pathway-Mediated) Tachycardia
More than 75% of SVT in infants is related to an accessory AV pathway. Accessory pathways are anomalous bands of tissue that form an extra electrical conduction pathway between the atrium and the ventricle. Many accessory pathways will conduct from the atrium to the ventricle (antegrade) and from the ventricle to the atrium (retrograde). Patients in whom conduction occurs antegrade across the accessory pathway have ventricular pre-excitation with a short PR interval and a delta wave. SVT and pre-excitation is known as Wolff-Parkinson- White syndrome (WPW; Figure 10-9). Pre-excitation may be difficult to see on the electrocardiogram in infants because of the rapid conduction through the AV node. In addition, up to one-third of patients with WPW show intermittent pre-excitation, and thus some of their tracings may appear normal. The electrical impulse from the SA node passes through both the AV node and the accessory AV pathway. Impulses passing through the AV node are delayed as in normal conduction; the impulse passing through the accessory pathway is not delayed, and thus the early (“pre-excited”) ventricular activation is reflected by the short PR interval and the slurred early QRS delta wave (Figure 10-10B). In other patients, the accessory pathway does not conduct in an antegrade manner. These patients have a normal ECG and what is known as a “concealed” accessory pathway (Figure 10-10C). This mechanism is responsible for more than half the SVT that occurs in infants. Most infants (60% to 90%) will not have recurrent SVT beyond 1 year of age.
FIGURE 10-9. Wolff-Parkinson-White syndrome. The short PR interval (80 ms) and a delta wave are present in multiple leads, consistent with pre-excitation.
During an episode of abnormal tachycardia, a re-entry circuit forms between the AV node and the accessory pathway (Figure 10-10D). This is often triggered by a premature atrial contraction that travels normally through the AV node but is blocked in the accessory pathway. The impulse from the ventricle is then conducted retrograde in the accessory pathway back to the atrium, thus completing the re-entrant circuit. The atrium is reactivated by the retrograde impulse, and in this manner, the re-entrant circuit becomes self-perpetuating and thus sustains the abnormal tachycardia. The QRS complex is normal because the normal pathway is used for antegrade conduction, producing the normal sequence of ventricular activation. The accessory pathway is used for retrograde conduction to maintain the re-entry loop. This is the most common form of re-entrant SVT and is called orthodromic reciprocating tachycardia (Figure 10-10D). When the impulse travels in the other direction (ie, forward through the accessory pathway and retrograde through the AV node), the QRS is wide because of the abnormal sequence of ventricular activation. This is called antidromic reciprocating tachycardia and can be mistaken for ventricular tachycardia because of the wide QRS complex (Figure 10-10E).
AV Nodal Re-Entrant Tachycardia
The re-entrant circuit in AV nodal re-entrant tachycardia also involves two pathways, but in this case, one pathway is within the AV node, and the other is a distinctly different pathway that may be within the AV node or a few millimeters outside the AV node (Figure 10-10F). The effective refractory period of one pathway is longer than that of the other pathway. This allows initiation of re-entrant tachycardia when a premature atrial contraction is blocked in the pathway with the longer refractory period. This occurs rarely in infants but is the most common mechanism for SVT in adult patients.
FIGURE 10-10. Mechanisms for conduction. A. Normal. Conduction in sinus rhythm. B. Wolff-Parkinson- White syndrome. The impulse from the sinoatrial node passes through both the AV node and the accessory pathway. There is no delay within the accessory pathway, so the early, or “pre-excited,” ventricular activation produces a short PR interval, and the delta wave as seen on the ECG. C. “Concealed” accessory AV pathway. If the accessory pathway is blocked during sinus rhythm, the ECG is normal because conduction is antegrade through the normal conduction system. D. Orthodromic reciprocating tachycardia. Normal antegrade conduction through the AV node results in a normal QRS complex. The re-entrant circuit is completed by retrograde conduction through the accessory AV pathway, resulting in atrial activation shortly after ventricular depolarization (note the abnormal P waves just after the QRS complexes). E. Antidromic reciprocating tachycardia. Conduction is antegrade through the accessory pathway and retrograde through the AV node. The abnormal sequence of ventricular depolarization causes a wide QRS complex, and P waves are often difficult to see on the surface ECG. This rhythm can be mistaken for ventricular tachycardia. F. AV node re-entry tachycardia. Typically, slow antegrade conduction occurs through a posterior “pathway” of atrial tissue (wavy line), and retrograde conduction travels via a “fast” pathway involving more anterior aspects of the AV node. The P waves are not seen; they are buried in the QRS complex because atrial and ventricular activation occur simultaneously. Abbreviations: AVN, atrioventricular node; His, His bundle; LBB, left bundle branch; RBB, right bundle branch; SN, sinoatrial node.
Permanent Junctional Reciprocating Tachycardia
In the permanent form of junctional reciprocating tachycardia (PJRT), the accessory pathway is concealed and conducts slowly in the retrograde direction. The rate in neonates is usually slower than typical SVT with rates of 160 to 200 beats per minute. PJRT may be present as an incessant tachycardia during fetal life. The rate is more variable than most re-entrant tachycardias because both limbs of the circuit are influenced by autonomic tone. Initially, this tachycardia is fairly well tolerated because of the slower heart rate and may not be detected in normal neonates and young infants. Eventually, a tachycardia- induced dilated cardiomyopathy may develop. The ECG shows an abnormal superior P-wave axis, usually a negative P wave in lead II and an upright P wave in lead aVL. During tachycardia, the interval from the R wave to the next P wave is relatively long; the P wave is located closer to the subsequent QRS complex than the previous one (Figure 10-11). This form of re-entrant SVT responds less well to the usual medications (see following text). Flecainide and other medications may be efficacious in some patients, but others are refractory to medical management and will need to undergo ablation by a pediatric electrophysiologist.
Clinical Features
Typically, the onset of re-entrant tachycardia is sudden. If the tachycardia converts to sinus rhythm (either spontaneously or in response to therapy), there is an abrupt cessation of tachycardia rather than a gradual slowing in heart rate. In infants, the heart rate is usually >250 beats per minute and is often about 300 beats per minute but can be as low as 150 beats per minute. Infants will generally tolerate these rapid heart rates initially, but after 36 to 72 hours, signs and symptoms of heart failure develop. Cardiac output becomes compromised in part because the decreased duration of diastole interferes with coronary arterial flow to the myocardium. Infants with SVT rarely present in cardiogenic shock.
Patients with accessory bypass pathways typically have structurally normal hearts. However, 8% to 25% have structural heart disease, most commonly Ebstein malformation of the tricuspid valve or levo- or corrected transposition of the great arteries.
ECG Findings and Differential Diagnosis
The typical ECG shows a regular and narrow QRS tachycardia (Figure 10-12). P waves are often not visible. If visible, the P waves located just before the QRS complex are usually less prominent than normal and often are negative in leads II, III, and aVF (typical findings in PJRT). Retrograde P waves (seen within or just after the QRS complex) are also sometimes present (Figure 10-13). Sometimes, the first few beats of SVT are wide because of aberrant conduction, and then the QRS complex becomes narrow.
The most important differential diagnosis is sinus tachycardia. The maximal heart rate is usually <240 beats per minute in patients with sinus tachycardia. A normal P wave (upright in lead II) is suggestive of sinus tachycardia (Figure 10-6). Any patient with an increased heart rate should be evaluated for remediable causes of sinus tachycardia, such as fever, anemia, pain, sympathomimetic medications, and so on. Variability in the heart rate, such as during crying or blood drawing, is consistent with sinus tachycardia. Sometimes, administration of adenosine will transiently decrease the heart rate enough that sinus rhythm can be more easily identified. Other causes of narrow QRS tachycardia are discussed in the following text (Table 10-5).
FIGURE 10-11. Persistent junctional reciprocating tachycardia. Rhythm strip shows intermittent narrow-complex tachycardia initiated by a premature ectopic beat. The PR interval is long (see text).
FIGURE 10-12. Atrioventricular reciprocating tachycardia (SVT). The QRS complex is normal. The heart rate is 315 beats per minute.
Treatment
The AV node is an essential part of the re-entry circuit in many patients with SVT, and thus therapy directed toward slowing conduction through the AV node is often effective in breaking the tachycardia. Even if the AV node is not involved, slowing conduction through the AV node may decrease the ventricular rate, thereby revealing the mechanism of the tachycardia. In either case, it is very important to record a rhythm strip during all attempts to terminate an episode of tachycardia.
Vagal maneuvers are the simplest, quickest, and safest way to terminate an episode of re-entrant tachycardia. For patients <12 months of age, application of ice slurry to the forehead induces strong vagal stimulation (diving reflex). Ice and water are put in a glove or washcloth that is placed over the patient’s forehead, eyes, and bridge of the nose. It is important not to cover the nose or mouth and not to press on the eyes. Contact between the ice slurry and upper face is maintained for 10 to 20 seconds. This may be repeated several times, but if this maneuver is performed correctly, it usually is effective on the first or second attempt. Repeated ice applications may cause skin damage in neonates, so care should be taken. Nasogastric stimulation or stimulation of a gag reflex may also be effective, but carotid massage is a much less effective vagal maneuver in young infants. Pressure should never be applied to the eyes because of the risk of retinal detachment.
FIGURE 10-13. Atrioventricular reciprocating tachycardia.
The rhythm strip shows retrograde P waves (arrow).
Synchronized DC cardioversion is the treatment of choice for SVT in critically ill infants (very low or not measurable blood pressure, nonpalpable pulses, poor perfusion, and altered level of consciousness). The initial energy is 0.5 J/kg and should be delivered synchronized to the QRS complex. This may be increased to 1 to 2 J/kg if there is no response to the first attempt. If however, this higher discharge energy is not effective or tachycardia recurs immediately, DC cardioversion should not be repeated. Instead, pharmacologic therapy or overdrive pacing should be considered.
For infants in whom vagal maneuvers, adenosine, and/or cardioversion have resulted in only transient or no termination of the arrhythmia, drugs such as β-adrenergic blocking agents (eg, esmolol or propranolol), class I antiarrhythmic agents (eg, procainamide or flecainide), or class III agents (eg, sotalol or amiodarone) may be useful. Administration of intravenous verapamil is contraindicated in patients younger than 1 year of age (see following text).
Placing an electrode in the esophagus to record an atrial electrogram can be helpful in defining the precise mechanism of SVT and can also be used by a pediatric cardiologist to terminate SVT by overdrive pacing (pacing the atria at a rate somewhat higher than the rate of the SVT for a brief period of time). This method of terminating SVT can be very effective while waiting for plasma concentrations of antiarrhythmic medications to become high enough to prevent reinitiation of tachycardia and can be performed repetitively with low risk of adverse effect.
Prophylactic antiarrhythmic therapy is indicated for most infants with SVT because 20% to 30% will have more than one episode of SVT. Digoxin is commonly used in patients without evidence of pre-excitation. In adults and older children with WPW (SVT and preexcitation), digoxin is contraindicated because it may increase conduction through the accessory pathway, thereby permitting a very rapid ventricular rate during atrial fibrillation. Although atrial fibrillation is rare in newborns, some cardiologists prefer to use ^-adrenergic blocking agents, such as propranolol or atenolol, as initial therapy for newborns with pre-excitation. Some patients may require treatment with more than one agent. Administration of both digoxin and atenolol is very useful in patients who do not have pre-excitation. Patient refractory to therapy may be treated with sotalol, flecainide, or amiodarone. All of these agents have important toxicities (see following text).
The recurrence rate of neonatal SVT after 6 to 12 months of age is less than 50%. Therefore, antiarrhythmic therapy may be discontinued after this time period in patients without signs of recurrence. Patients who show persistent ventricular pre-excitation on the electrocardiogram should be followed prospectively by a cardiologist even if they do not have recurrent SVT, as evaluation of the conduction characteristics of the accessory pathway is indicated when they are 8 to 12 years of age. In patients with recurrent SVT, oral medication is continued until they are older and the risks of ablation therapy are lower.
Ablation therapy is indicated only for infants with symptomatic SVT refractory to medical management.
Atrial Flutter
Atrial flutter is a re-entrant rhythm that is confined to the atrium. Atrial flutter is relatively rare in young infants, and if it occurs in neonates, the heart is usually structurally normal. The flutter rate depends on the rate of conduction within the re-entrant circuit and the length of the circuit but is usually quite rapid (375 to 500 beats per minute) in neonates and infants. The ventricular response is variable, but 2:1 and 3:1 AV block are common. If the atrial rate and degree of block are such that the ventricular rate is >250 beats per minute, congestive heart failure may occur. The typical ECG shows flutter waves that are P waves, which give a sawtooth appearance to the baseline (Figure 10-14A). This may be difficult to discern in the presence of 2:1 AV block. A definite diagnosis can be made either by increasing AV block transiently, such as by administering adenosine, or by performing transesophageal ECG recordings to facilitate visualization of the flutter waves (Figure 10-14B). The R-R interval is constant except when the degree of AV block changes.
Although adenosine is sometimes helpful in establishing the diagnosis, this agent does not convert atrial flutter to sinus rhythm because the re-entrant circuit does not involve the AV node. Neonates with structurally normal hearts and atrial flutter represent a special group of patients in that once flutter has been terminated, it rarely recurs, and long-term therapy is almost never needed. Atrial flutter in these neonates can be converted to sinus rhythm by use of overdrive pacing or DC cardioversion, and no pharmacologic therapy is necessary. If pharmacologic therapy is preferred, digoxin is the first drug of choice because this agent decreases conduction through the AV node, thus increasing the degree of AV block and decreasing the ventricular rate. Digoxin often does not convert atrial flutter to sinus rhythm. To achieve this, overdrive pacing or DC cardioversion may be necessary. Newborns without structural cardiovascular disease who have been successfully cardioverted do not require follow-up.
Intra-atrial re-entrant tachycardia (IART) is very similar to atrial flutter and occurs primarily in patients who have had cardiac surgery. The P waves tend to be lower in amplitude than in atrial flutter. Digoxin or β-adrenergic blocking agents are administered to slow conduction through the AV node. Overdrive atrial pacing from the esophagus usually terminates IART. DC cardioversion (initial dose of 0.5 to 1 J/kg) is also therapeutic. Sotalol or amiodarone are used if necessary to prevent recurrence.
FIGURE 10-14. Atrial flutter. A. Negative flutter waves are present in lead II. The flutter rate is 400 beats per minute, and there is 2:1 AV block. B. Administration of adenosine increases AV block and facilitates visualization of the flutter waves. The flutter rate in this patient is 460 beats per minute.
Supraventricular Tachycardia: Automatic
Atrial Ectopic Tachycardia
In infants, cells in the atrium rarely undergo diastolic depolarization faster than the cells in the SA node. When this occurs, an incessant atrial ectopic tachycardia (AET) can develop. This rhythm is very sensitive to catecholamines. The P-wave morphology is often different from the normal sinus P wave because of the ectopic origin of the atrial impulse (Figure 10-15). In contrast to re-entrant SVT, the onset and termination of AET are often gradual, and the rate is variable. AET may occur during fetal life and may cause hydrops fetalis. Neonates are not usually symptomatic immediately, but this rhythm may cause a tachycardia-induced dilated cardiomyopathy and congestive heart failure after several months.
AV block, such as that induced by adenosine, does not terminate this tachycardia but can transiently slow the ectopic rhythm, and this may facilitate identification of the mechanism of the tachycardia. Similarly, overdrive pacing and cardioversion will slow the heart rate for a very short period of time but will not result in sinus rhythm. Intravenous infusion of the short-acting β-adrenergic blocking agent esmolol is often useful in controlling the heart rate acutely. It is usually impossible to eradicate this rhythm; a more reasonable goal of therapy is to decrease the ventricular rate so that ventricular dysfunction associated with chronic tachycardia does not occur. A variety of oral agents, including propranolol, atenolol, and sotalol, are administered to patients with AET. Amiodarone is used in refractory cases. AET resolves spontaneously in many young patients. Definitive therapy involves catheter ablation of the ectopic focus, but because important long-term adverse effects may occur in small infants, this is reserved for those who do not respond to medical therapy. Surgical excision of the ectopic focus has been reported.
FIGURE 10-15. Atrial ectopic tachycardia. Negative P waves are present in this lead II rhythm strip.
Chaotic or Multifocal Atrial Tachycardia
Chaotic atrial tachycardia is a very rare tachycardia associated with three or more distinct P-wave morphologies (Figure 10-16). AV conduction is variable, so the QRS complexes are irregularly irregular. The onset and termination are gradual, and the rate is variable. The exact mechanism of this arrhythmia is unknown; it may involve multiple ectopic foci or re-entrant circuits. This arrhythmia may resolve spontaneously in 50% to 80% of infants, and asymptomatic infants may not need therapy. Control of this arrhythmia is difficult, and antiarrhythmic agents are not consistently beneficial, but rate control with digoxin, propranolol, or other agents can be useful if the infant develops ventricular dysfunction or is compromised by rapid ventricular response.
Junctional Ectopic Tachycardia
Junctional ectopic tachycardia (JET) is an automatic rhythm arising from the AV node or bundle of His. The QRS complex is narrow, and JET is the only narrow QRS tachycardia in which the ventricular rate is higher than the atrial rate. The presence of AV dissociation confirms the diagnosis of JET (Figure 10-17).
JET is an incessant tachycardia and similar to AET. JET may cause tachycardia-induced cardiomyopathy. When present during fetal life, JET may be associated with hydrops fetalis. Many cases are familial. Some patients eventually develop complete AV block. Congenital JET is very difficult to treat and does not respond to DC cardioversion, overdrive pacing, or routine medical therapy. Amiodarone is the most effective agent, although β-adrenergic blocking drugs or flecainide may control the heart rate adequately in some patients. Catheter ablation of the ectopic junctional focus is the best treatment available at the present time for patients who do not respond to pharmacologic therapy. Cryoablation is the preferred approach to minimize the risk of complete heart block (see following text).
Although JET is a rare rhythm in patients with structurally normal hearts, it may be seen in 5% to 15% of infants during the first few days after cardiac surgery. In patients who already have decreased cardiac reserve after surgery, the tachycardia, combined with loss of the normal AV activation sequence, often causes important hemodynamic compromise. Since this is an automatic tachycardia, treatment of it is often difficult. Unfortunately, decreased blood pressure and perfusion, which frequently occur in the postoperative patient, often cause caregivers to increase the doses of sympathomimetic medications, further predisposing to JET.
FIGURE 10-16. Chaotic atrial rhythm. Rhythm strip shows irregular P waves with differing morphologies consistent with chaotic atrial rhythm. The R-R interval is irregular because of variable AV conduction.
FIGURE 10-17. Junctional ectopic tachycardia. AV dissociation is present. P waves are indicated by the arrows. The atrial rate is 135 beats per minute, and the ventricular rate is 185 beats per minute.
FIGURE 10-18. Premature ventricular contractions (*).
For postoperative patients with JET, decreasing the doses of sympathomimetic agents, correcting acidosis, treating fever if present, normalizing electrolytes (including magnesium), and eliminating vagolytic agents (depolarizing muscle relaxants) may be beneficial. If the ventricular rate is not too high, pacing the atrium at a slightly higher rate (overdrive pacing) will restore AV synchrony and improve cardiac output. Intravenous amiodarone is efficacious in patients who are hemody- namically unstable. Cooling the core temperature to 35°C may be effective in decreasing the heart rate, but this is required only if the interventions listed above are not successful. JET in postoperative patients is usually transient, so control is necessary for only a limited period of time.
Premature Ventricular Contractions
A premature ventricular contraction (PVC) (or ventricular premature beat) is a premature ventricular complex of different morphology from that of the sinus beat (Figure 10-18). It is not preceded by a P wave. In newborn infants, the QRS complex associated with a PVC may not be much wider than the normal QRS complex. PVCs with the same morphology are called uniform, and PVCs with different morphologies are called multiform. It is sometimes difficult to distinguish a PAC with aberrant conduction from a PVC. A fusion beat can result from near simultaneous activation of the ventricles from a supraventricular site (often the SA node) and an ectopic site in the ventricle; the morphology of the fusion beat is intermediate between the normal and the wide QRS. The presence of fusion beats confirms the ventricular origin of the premature beats. PVCs are relatively uncommon in newborn infants; PACs are much more common. Sometimes, PVCs are related to administration of drugs such as caffeine, sympathomimetic agents, digoxin, or antiar- rhythmic agents.
An infant with PVCs should be evaluated for metabolic abnormalities (hyperkalemia, hypokalemia, hypoxia, acidosis, hypoglycemia), the congenital long QT syndrome, myocarditis, cardiac tumors, and structural cardiovascular disease. Most often, these studies are normal, and no specific etiology is identified. No specific therapy is indicated for isolated uniform PVCs. There are no data suggesting that the morphology or frequency of PVCs influences outcome, and they usually resolve over the first 2 months of life. Persistence of PVCs beyond 2 months of age should prompt additional investigation but may still have a benign prognosis. Multiform PVCs at any age require further investigation.
Ventricular Tachycardia
Ventricular tachycardia is a wide QRS complex rhythm usually resulting from re-entry with the ventricle. In neonates, the QRS complex may not appear as wide as in older children and adults. The QRS duration is between 60 and 110 ms in patients who do not show much widening. Ventricular tachycardia should be recognized by altered QRS morphology during tachycardia when compared to the QRS complex during sinus rhythm. The rate should be at least 20% greater than the normal sinus rate; this distinguishes ventricular tachycardia from accelerated idioventricular rhythm. Additionally, abnormal repolarization causes the polarity of the T waves to be opposite that of the QRS complex (Figure 10-19). Nonsustained ventricular tachycardia is defined as a run of ventricular complexes lasting from three beats to 30 seconds, and sustained ventricular tachycardia is a run lasting more than 30 seconds. Ventricular tachycardia with a uniform QRS complex morphology is described as “monomorphic.” Polymorphic ventricular tachycardia is characterized by varying morphologies of the QRS complexes.
FIGURE 10-19. Ventricular tachycardia. The heart rate is 195 beats per minute.
Ventricular tachycardia is sometimes difficult to differentiate from some supraventricular rhythms, such as SVT conducted with aberrancy or SVT in a postoperative patient with right bundle branch block. The presence of P waves that are dissociated from the QRS complexes usually confirms the ventricular origin of the tachycardia. However, retrograde ventricular-to-atrial conduction is often present in infants with ventricular tachycardia. The presence of intermittent sinus capture beats (dissociated sinus beats occurring at a time when conduction through the AV node to the ventricles is possible) or fusion beats with tachycardia is highly suggestive that the tachycardia is of ventricular origin.
Ventricular tachycardia is relatively uncommon in neonates. Causes include electrolyte and metabolic abnormalities, myocarditis, drug toxicity, long QT syndrome, maternal cocaine and heroin abuse, CNS lesions, myocardial tumors, scarring from previous surgery, and ischemia, usually related to congenital cardiovascular defects, such as anomalous origin of the left coronary artery from the pulmonary artery or severe aortic stenosis. Infants with ventricular tachycardia who have a structurally normal heart and no hemodynamic compromise usually have a benign course with resolution of the tachycardia before 1 year of age.
Incessant ventricular tachycardia (present 10% to 90% of the day) in infancy is often difficult to treat. Most commonly, pharmacologic therapy does not decrease the amount of time that the patient is in tachycardia but may decrease the tachycardia rate. Tachycardia-induced cardiomyopathy can develop if the rate is not at least partially controlled. These infants often have associated myocarditis or myocardial fibrosis. Microscopic Purkinje cell tumors have been found in some patients. These and other myocardial tumors may not be seen on an echocardiographic examination. Magnetic resonance imaging is sometimes helpful in visualizing tumors, but often the diagnosis is confirmed only at the time of surgery. Some of these patients have undergone mapping and then excision and/or cryoablation of the tachycardia site.
Another form of ventricular tachycardia is an ectopic focus tachycardia often seen in infants called accelerated idioventricular rhythm. This is distinguished from more typical ventricular tachycardia by a slower rate that is about 10% above the sinus rate. Idioventricular rhythm is associated with sinus slowing and frequent transitions between sinus and ventricular rhythms are seen (Figure 10-20). This likely reflects increased automaticity of a ventricular focus. AV dissociation is typically seen. Although there is loss of the normal AV activation sequence, the rate is low enough that hemodynamic compromise does not occur. These infants are asymptomatic, and specific therapy is not required. This arrhythmia usually resolves within a few months.
FIGURE 10-20. The rhythm strip shows sinus rhythm gradually transitioning to accelerated idioventricular rhythm. The rate is about the same as the sinus rate. Fusion beats are evident (arrows).
Torsades de pointes (“twisting of the points”) is a particular form of ventricular tachycardia in which the morphology of the QRS complex is constantly changing such that it appears to spiral around the baseline (Figure 10-21). This rhythm is usually associated with underlying prolongation of the QT interval.
Treatment
Sustained ventricular tachycardia is a medical emergency. A brief history, vital signs, assessment of peripheral pulses and perfusion, laboratory studies, and a complete ECG should be obtained if the clinical situation permits. Reversible causes of ventricular tachycardia, such as electrolyte abnormalities, should be treated immediately. Medications that may cause ventricular tachycardia should be discontinued if possible.
FIGURE 10-21. Torsades de pointes.
Any patient who is hypotensive and unconscious should undergo immediate DC cardioversion with 1 to 2 J/kg. If the first shock is not successful, the energy should be doubled and repeated once or twice but not more.
If the patient is conscious and has reasonably stable vital signs or if DC cardioversion is unsuccessful, intravenous amiodarone at 5 mg/kg should be administered as an infusion over 20 to 60 minutes. Esmolol may also be administered as necessary. Verapamil and digoxin are absolutely contraindicated for emergency treatment of ventricular tachycardia. Extracorporeal membrane oxygenation should be considered for ventricular tachycardia resistant to medical management. Magnesium sulfate infusion is the drug of choice for recurrent torsades de pointes. Administration of esmolol and overdrive pacing have reportedly also been effective.
A variety of agents are available for long-term suppression of ventricular tachycardia. β-adrenergic blocking agents are the drugs of choice for patients with long QT syndrome. For other causes of ventricular tachycardia, β-adrenergic blocking agents are often used in combination with other agents, such as sotalol, mexiletine, amiodarone, or procainamide. Some asymptomatic infants, particularly those with accelerated idioventricular rhythm, do not need therapy.
Long QT Syndrome
The long QT syndrome is an abnormality of ventricular repolarization that is often inherited and is characterized by a prolonged QT interval on the electrocardiogram, polymorphic ventricular tachycardia (torsades de pointes), and sudden death. This syndrome, which has a prevalence of 1 in 2000 to 2500 persons, is genetically heterogeneous and is caused most commonly by mutations in genes that encode various cardiac ion channels that regulate potassium, sodium, or calcium currents. These mutations can result in synthesis of nonfunctional channels or alter channel characteristics such that ion current is either increased or decreased. Most commonly, the repolarizing potassium current is decreased, or the depolarizing inward sodium or calcium currents are increased. Hundreds of mutations in 15 genes have been described, but mutations in just three genes (KCNQ1 [LQT1], KCNH2 [LQT2], SCN5A [LQT3]) are present in the majority of cases (Table 10-2). Homozygous or compound heterozygous mutations in KCNQ1 and KCNE1 have a high lethality and are associated with congenital deafness. About 20% of patients with long QT syndrome do not have one of the known mutations and may have either novel mutations or other conditions that manifest in a similar manner to long QT syndrome. In addition, phenotype does not always follow genotype. Some patients with gene mutations have neither symptoms nor abnormal electrocardiograms. The syndrome may become evident if these patients are found not to shorten the QT interval appropriately with increased heart rate during exertion or develop excessive prolongation of the QT in response to medications that are known to lengthen the QT interval. Additionally, up to one-third of persons who carry an abnormal mutation do not have prolonged QTc on the ECG, and about 5% of family members experience life-threatening arrhythmias or syncope despite having a normal QTc interval. Thus, certain mutations exhibit variable clinical expression that is mediated by genetic background, modifier genes, sympathetic nerve function, and environmental factors.
Clinical manifestations of long QT syndrome in neonates result from episodes of ventricular tachycardia and include syncope, seizures, and sudden death. A small percentage of infants whose deaths are attributed to sudden infant death syndrome also have mutations in ion channels associated with prolonged QT interval (see following text). An important observation is that neonates with long QT syndrome often show relative bradycardia. Syncope and seizures correlate with episodes of nonsustained torsades de pointes, whereas sudden death likely results from a prolonged episode. An electrocardiogram with careful manual measurement of the QT interval should be obtained on any patient with unexplained seizures or syncope. In addition, electrocardiograms should also be done on relatives of patients in whom congenital long QT syndrome is diagnosed and on relatives of patients who experience sudden, unexplained death.
Although genetic testing is commercially available, identifying patients with long QT syndrome may be difficult. The finding of a “variant of unknown significance” may make results difficult to interpret. Within a specific family it is important to determine if the variant cosegregates with QT prolongation or symptoms. Diagnosis is generally based on the clinical characteristics of the individual patient and the family. Diagnostic criteria have been established (Table 10-3). This algorithm assigns points to diagnostic criteria based on clinical significance. Genetic testing is indicated for any patient with an affected first- degree family member and for any patient with a strong likelihood of LQTS based on clinical and family history and electrocardiographic finding of QT prolongation (>480 ms at age 4 to 5 days up until puberty) that is not explained by other conditions that might prolong the QT interval.
Electrocardiographic abnormalities present in long QT syndrome include prolongation of the QTc interval, sinus bradycardia, T-wave abnormalities, and episodic torsades de pointes. Automatic computer-generated measurements of the QT interval are often not accurate. The QT interval is measured manually in lead II, V5, or V6 and is corrected for heart rate according to the Bazett formula, QTc = QT/ (R-R)1/2 (Figure 10-22). This measurement is often difficult even in adult patients because the measured QTc interval varies among the ECG leads, a prominent U wave may be present, and sinus arrhythmia can affect the QTc interval. The U wave is included when measuring the QTc interval if the amplitude of the U wave is at least 50% of the amplitude of the T wave. Measurement of the QTc interval in young infants is often additionally challenging because the more rapid heart rates make separating the T wave from the P wave more difficult. Findings consistent with long QT syndrome include relative sinus bradycardia, 2:1 AV block (related to prolonged ventricular refractory time), and T-wave alternans. Nonsustained ventricular arrhythmias may be present.
(Continued)
When evaluating patients, it is important that acquired causes of prolonged QTc interval be considered. Table 10-4 lists various medications and conditions associated with prolonged QTc interval.
The length of the QT interval is the most robust predictor of risk; QTc interval >500 ms confers an increased risk of cardiac events. Other factors associated with increased risk include aborted cardiac arrest in the first year of life, male gender in the first decade of life, female gender in adolescence and adulthood, and recent syncope. Interestingly, a positive family history of sudden cardiac death does not appear to be a predictor of outcome in childhood.
Therapy with β-adrenergic blocking drugs is considered standard of care for most patients in whom congenital long QT syndrome is diagnosed. Propranolol and naldolol are thought to be more efficacious than metoprolol, though the more frequent dosing necessary for propranolol in infants and young children may adversely affect compliance. Data regarding efficacy or lack thereof for atenolol are incomplete. Mexiletine is often considered in patients known to have a sodium channel defect (SCN5A, LQT3). The importance of compliance must be stressed to parents, and the medication dose must be increased as the infant grows. Affected individuals must avoid all medications known to prolong the QTc interval (http://www.azcert. org/medical-pros/drug-lists/list-03.cfm).
Pacemaker placement is indicated for those patients who have second-degree AV block. Pacemakers are also often needed in patients in whom baseline sinus bradycardia is exacerbated by treatment with β-adrenergic blocking drugs. Pacing may decrease pause-dependent ventricular arrhythmias.
High-risk patients are potential candidates for placement of an implantable cardioverter-defibrillator (ICD). Implantation of these devices clearly benefits high-risk adolescent and adult patients, but indications for placement are incompletely defined. The relatively large size of the device and associated complications make placement even more problematic in very young infants. Most commonly, ICDs should be considered only for those infants who fail medical therapy (not related to poor compliance). An exception is patients with prolonged QT associated with congenital deafness (Jervell and Lange-Nielsen syndrome); ICD placement should be considered early because β-adrenergic blocking drugs have limited efficacy in this group.
Given the known adrenergic dependence of the arrhythmias, left cervicothoracic sympathetic ganglionectomy can be useful especially for patients who do not tolerate medical therapy or who have breakthrough events despite adequate medical therapy. In the future, more precise genotype-phenotype correlation may improve diagnosis and management of these patients.
Catecholaminergic Polymorphic Ventricular Tachycardia
Catecholaminergic polymorphic ventricular tachycardia (CPVT) is characterized by syncope or sudden death associated with adrenergic activation (exercise or emotional stress) in patients with normal cardiac structure and function and a normal QT interval. Children and adolescents are most frequently affected, but young infants with this condition have been reported. Polymorphic or bidirectional ventricular tachycardia is seen with exercise testing or during acute emotion. CPVT is linked to mutations in the cardiac ryanodine receptor (RYR2, sarcoplasmic reticulum calcium release channel) or in calsequestrin, the most important calcium-binding protein within the sarcoplasmic reticulum. These mutations result in calcium leak from the sarcoplasmic reticulum during sympathetic stimulation. β-blockers are always administered to patients with this condition, but mortality is high despite various modes of therapy.
Sudden Infant Death Syndrome and Cardiac Arrhythmias
Sudden infant death syndrome (SIDS) is defined as unexpected death within the first year of life for which no cause is identified on postmortem examination. The peak incidence is between 2 and 4 months of age. The cause of SIDS is likely heterogeneous. In addition to abnormalities in brain stem respiratory control, developmental neurologic defects, dysautonomia, environmental factors, metabolic abnormalities, immune dysfunction, and cardiac arrhythmias (especially long QTc syndrome) have been proposed to explain at least some cases of SIDS. Some investigators have identified important genetic variants in long QT syndrome genes in nearly 10% of children diagnosed with SIDS. Interestingly, half of the identified mutations occurred in the sodium channel gene SCN5A (long QT 3). Defects in this gene, which are typically found in <8% of patients with congenital long QT syndrome, are most often associated with ventricular arrhythmias occurring during rest or sleep. Despite these provocative findings, the vast majority of children with SIDS have normal QT intervals, no history of symptoms suggestive of an arrhythmia, and no family history of sudden death or prolonged QT interval. Although patients with congenital long QT syndrome have multiple episodes of ventricular arrhythmias, ventricular arrhythmias have been documented in only a very few patients at risk for SIDS despite extensive monitoring of these patients in the hospital and at home. Nevertheless, the finding of potentially disease-causing mutations has raised the question of whether routine neonatal ECG screening might identify infants at risk for SIDS, but this approach is problematic. The QTc interval is variable and often prolonged in the first few days of life. This is likely related to transient electrolyte abnormalities and/or to disturbances in autonomic control. The vast majority of infants with a prolonged QTc interval (>440 ms) will not die from SIDS. In addition, a single QTc interval measurement on an electrocardiogram will not identify all infants with long QT syndrome. The low incidence of SIDS directly related to congenital long QT syndrome results in an extremely low positive predictive value (<1%), which thus decreases the power of the ECG in identifying infants at risk for SIDS.
Recommendations from the American Academy of Pediatrics and the National Institutes of Health promoting supine or side positioning for sleeping have markedly decreased the incidence of SIDS in compliant populations. Further investigation is needed to more precisely define the role of primary cardiac arrhythmias in sudden infant death.
FIGURE 10-22. Long QT syndrome. Electrocardiogram from a 1-day-old infant with congenital long QT syndrome shows markedly prolonged QTc interval (460/[670]1/2 = 562 ms) measured in lead III and bradycardia with a heart rate of 90 beats per minute.
Ventricular Fibrillation
Ventricular fibrillation is characterized by uncoordinated and ineffective ventricular depolarizations and inadequate cardiac output. The ECG shows low-amplitude oscillations instead of recognizable QRS complexes. The presence of ventricular fibrillation in an infant should raise the possibility of long QT syndrome, myocarditis, drug toxicity, or electrolyte abnormalities. Immediate DC cardioversion is indicated for all patients with ventricular fibrillation.
■ EVALUATION, ASSESSMENT, AND APPROACH TO DIAGNOSIS
Evaluation of Cardiac Rhythm
A 15-lead ECG (standard 12 leads and V3R, V4R, and V6R) is invaluable for diagnosing arrhythmias. Caution should be used when looking at bedside monitors or even paper recordings from these monitors because wave morphology is highly dependent on lead placement. If P waves are difficult to discern on the ECG, an atrial electrogram can be recorded by positioning a flexible electrode catheter in the esophagus behind the left atrium. The amplitude of the atrial electrogram recorded transesophageally is much greater than the P wave recorded on the surface ECG. Recording the atrial electrogram and the surface ECG simultaneously often facilitates diagnosis of arrhythmias. For infants with intermittent arrhythmias, continuous monitoring on a telemetry unit or with 24-hour electrocardiography (Holter monitor) allows evaluation of the beginning and ending of arrhythmias as well as determination of minimum, maximum, and average heart rates.
Patients who have had cardiac surgery may have temporary epicardial wires in place. These are usually used for pacing if necessary. Electrograms recorded from the pacing wires can also be helpful in defining cardiac rhythms in postoperative patients.
Initial Assessment
A careful history, including prenatal and perinatal events, must be obtained for any infant with a suspected cardiac arrhythmia. The medication history is especially relevant. A careful family history, including history of heart disease, arrhythmias, syncope, seizures, stillbirths, and sudden unexpected deaths (including drowning and motor vehicle accidents), is also important.
TABLE 10-4. Causes of Acquired Prolonged QT Interval
Drugsa
Antiarrhythmic agents (ibutilide, quinidine, procainamide, disopyramide, sotalol, dofetilide, flecainide, amiodarone [rare])
Antimicrobials (erythromycin, clarithromycin, azithromycin, moxifloxacin, chloroquine, pentamidine)
Neuroleptics (phenothiazines, haloperidol, pimozide)
Opiate agonist (methadone)
Oral hypoglycemics
Organophosphate insecticides
Sedative (droperidol)
Electrolyte abnormalities (acute hypokalemia, chronic hypocalcemia, chronic hypokalemia, chronic hypomagnesemia)
Medical conditions
Arrhythmias (complete AV block, severe bradycardia, sick sinus syndrome)
Cardiac (myocarditis, tumors, cardiomyopathy, infarction)
Endocrine (hyperparathyroidism, hypothyroidism, pheochromocytoma)
Neurologic (dysautonomia, cerebrovascular accident, encephalitis, head trauma, subarachnoid hemorrhage)
aThe Advisory Board at QTdrugs.org classifies drugs associated with prolonged QT interval as follows: (1) those with known risk of torsade de pointes, (2) those with possible risk of torsade de pointes, (3) those with conditional risk of torsade de pointes, and (4) those that must be avoided by congenital long QT syndrome patients. Selected drugs in the first group that are available in the United States are listed here. The complete list of drugs associated with prolonged QT interval is available at http://www.qtdrugs.org.
The hemodynamic status must be assessed quickly and the infant classified as critically ill (very low or not measurable blood pressure, nonpalpable pulses, poor perfusion, and altered level of consciousness), seriously ill, or minimally ill to asymptomatic. If the clinical situation permits, laboratory evaluation (electrolytes, calcium, magnesium, phosphate, glucose, lactate, and arterial blood gas), chest radiograph, and 15-lead ECG should be obtained.
Although these data are important, there should be no delay in administering therapy to a critically ill infant.
Rapid Classification of Arrhythmias and General Therapeutic Approach
All critically ill infants should be treated immediately. Infants with symptomatic bradycardia should be paced by use of a transvenous pacing catheter, epicardial leads, or transcutaneous electrodes if the bradycardia is not associated with another condition, such as hypoxia. Transesophageal pacing may be useful for treating infants with symptomatic sinus bradycardia but is unlikely to be effective in those with atrioventricular block. Intravenous atropine and isoproterenol should be administered if pacing is not immediately available. Once pacing is established, the etiology of the bradycardia can be determined.
Critically ill infants who have tachycardia should undergo DC cardioversion. For infants who are not critically ill, the tachycardia should be classified as narrow or wide QRS complex, and a complete assessment should be done (see earlier discussion). The differential diagnosis of narrow QRS tachycardia is shown in Table 10-5. Induction of AV block by vagal maneuvers or by administration of adenosine will restore sinus rhythm in most re-entrant tachycardias and may assist in diagnosis of atrial flutter and automatic tachycardias. Careful examination of the ECG is essential.
A wide QRS tachycardia should always be treated as ventricular tachycardia until a definite diagnosis is made. An important caveat is that a conscious patient should never undergo emergent DC cardioversion. Time must be taken to completely assess a patient who is reasonably stable because these patients may not have ventricular tachycardia or may have the much more benign idioventricular rhythm (Table 10-6). Elective cardioversion may be performed after the patient is deeply sedated. Amiodarone is efficacious for both SVT and ventricular tachycardia, so this agent is a reasonable choice if the diagnosis is uncertain. Esmolol is also a good choice in these situations for infants who are only minimally symptomatic. This agent should be avoided in patients with poor ventricular function or hypotension.
■ PHARMACOLOGIC THERAPY
General Considerations
In contrast to older children and adults in whom catheter ablation (see following text) has decreased the need for pharmacologic management of arrhythmias, antiar- rhythmic drug therapy remains very important in young infants because of the technical limitations and increased risks of ablation in this age-group. Pharmacokinetics differ in infants compared to older children (Chapter 11), and developmental changes in ion channels and the autonomic nervous system (Chapters 2 and 3) affect the responses of young infants to these agents. Despite these considerations, no controlled trials have been performed in young infants. Thus, therapy must be extrapolated from studies in adults.
TABLE 10-5. Classification of Narrow-QRS Complex Tachycardias
|
Diagnosis |
P waves |
Onset and termination |
Response to vagal maneuvers and to adenosine |
Response to cardioversion |
Comments |
|
Re-entrant tachycardias Accessory pathway-mediated SVT |
P waves have abnormal axis and are not seen or follow QRS complex typically on upstroke of T wave |
Abrupt |
Terminate |
Terminate |
After termination, those with WPW syndrome have pre-excitation |
|
AV nodal re-entry SVT |
P waves usually not visible, superimposed on QRS complex |
Abrupt |
Terminate |
Terminate |
|
|
Permanent form of junctional reciprocating tachycardia |
P-wave axis abnormal and P waves precede QRS complex, long RP interval |
Incessant |
Terminate |
Terminate |
|
|
Atrial flutter |
“Sawtooth” flutter waves |
Abrupt |
Continues in presence of AV block |
Terminate |
Rate up to 400 to 500 beats per minute in newborn infants, variable block common |
|
Atrial fibrillation (likely re-entry) Automatic tachycardias |
Irregular and low amplitude |
Abrupt |
Continues in presence of AV block |
Terminate |
Irregularly irregular QRS complexes |
|
Sinus tachycardia |
Normal P-wave axis, P wave before each QRS complex |
Gradual |
Continues in presence of AV block |
None |
Rate varies with autonomic tone |
|
Atrial ectopic tachycardia |
Abnormal P-wave axis, P wave before each QRS complex |
Gradual |
Continues in presence of AV block |
None |
No AV dissociation |
|
Chaotic (multifocal) atrial tachycardia |
Multiple P-wave morphologies |
Gradual |
Continues in presence of AV block |
None |
No AV dissociation |
|
Junctional ectopic tachycardia |
Normal P-wave axis with slower atrial than ventricular rate |
Gradual |
Continues in presence of AV block |
None |
May see AV dissociation and capture beats but no fusion beats |
|
SVT, supraventricular tachycardia; WPW, Wolff-Parkinson-White. |
|||||
TABLE 10-6. Classification of Wide QRS Complex Tachycardias
|
Diagnosis |
Comments |
|
Ventricular tachycardia |
AV dissociation is usually diagnostic of ventricular tachycardia, but ventricular-atrial conduction common in young infants; fusion beats diagnostic of ventricular tachycardia |
|
SVT with aberration (rate-dependent bundle branch block) SVT with pre-existing bundle branch block |
AV dissociation not present; aberration often resolves after first few beats of tachycardia AV dissociation not present; RBBB most common; seen in patients who have had cardiac surgery; QRS complex morphology same as that in sinus rhythm |
|
WPW syndrome with antidromic SVT |
P waves seen before QRS complexes; QRS morphology similar to that of pre-excited sinus rhythm |
|
WPW syndrome with atrial fibrillation |
Irregularly irregular QRS complexes |
SVT, supraventricular tachycardia; WPW, Wolff-Parkinson-White.
Any rhythm disturbance should be documented as thoroughly as possible, preferably with a complete ECG, before therapy is begun. The rhythm should also be recorded continuously during any acute interventions, and another complete ECG should be recorded after any change in the rhythm.
All patients receiving antiarrhythmic agents must be monitored carefully because some of these agents have the potential to produce arrhythmias other than those being treated (proarrhythmia). Serial ECG examinations are helpful in evaluating responses to various agents that may be proarrhythmic. Serum drug concentrations can be measured for most agents, and therapeutic drug monitoring should be considered during initiation of therapy, with dose changes, and with administration of drugs that may affect metabolism of these agents. In addition, it may be helpful in nonresponders to determine if the lack of response is due to inadequate drug exposure (poor compliance, limited bioavailability, or rapid metabolism). A steady-state concentration is usually reached after five times the drug’s half-life. Proarrhythmia is most likely to occur soon after initiation of treatment, but late proar- rhythmic effects have been reported.
The Vaughan Williams classification of antiarrhyth- mic medications describes antiarrhythmic actions and is used traditionally. However, the usefulness of this scheme is somewhat limited from a clinical standpoint because several drugs have more than one effect, antiarrhythmic actions do not predict efficacy, and some useful agents, such as adenosine, do not fit into this classification. Recommended doses, pharmacokinetic details, and general indications are shown in Table 10-7.
Class 1A (Procainamide)
The 1A drugs decrease the upstroke velocity of the action potential by blocking sodium channels. This slows conduction time in the atrial and ventricular muscle cells, His-Purkinje cells, and accessory AV pathways. Auto- maticity is decreased. These agents also block potassium channels. The PR interval is prolonged, the QRS duration is increased, and the QTc interval is prolonged. These agents are contraindicated in patients with long QT syndrome and should not be used with other drugs, such as amiodarone, that prolong the QTc interval. Because class 1A drugs have anticholinergic activity and so tend to increase AV node conduction, they should not be administered concomitantly with digoxin or β-adrenergic blocking agents.
Procainamide is administered intravenously, but the patient must be monitored carefully for hypotension during the infusion. It is metabolized to N-acetylprocainamide (NAPA), which has class III actions. The risk of proarrhythmia, especially torsades de pointes, is moderate and seems unrelated to serum drug concentration. Procainamide causes mild depression of myocardial function. This drug is used less commonly than in the past because of the availability of newer drugs, such as amiodarone.
Class 1B (Lidocaine, Mexiletine, Phenytoin)
These drugs block fast sodium channels, thereby shortening action potential duration and the refractory period primarily in Purkinje fibers and in ventricular myocytes. Automaticity is decreased. Cells in the SA and AV nodes and autonomic tone are minimally affected. The ECG may show a slight decrease in QTc interval. Proarrhythmic effects are relatively uncommon with these agents. Lidocaine is given intravenously. High plasma concentrations depress myocardial function, and toxicity often causes drowsiness, disorientation, muscle twitching, and seizures. Mexiletine is available for oral administration and is used in some forms of congenital long QT syndrome because of its effects on sodium channels. Phenytoin is used rarely and is generally restricted to the treatment of ventricular arrhythmias induced by digoxin toxicity.
Class 1C (Flecainide)
These agents markedly decrease the upstroke velocity of the action potential and decrease conduction in fast response cells. They do not affect autonomic tone. The PR interval is prolonged, and the QRS duration increases. Flecainide is a particularly effective inhibitor of abnormal automaticity and re-entry within atrial and ventricular muscle and in accessory AV pathways. It has been used successfully to treat many arrhythmias, including SVT, persistent junctional reciprocating tachycardia, and ventricular tachycardia. The relatively high incidence of proarrhythmia, especially torsades de pointes, limits the use of flecainide in patients with structural heart disease, but flecainide is useful for treating patients with SVT and structurally normal hearts who do not respond to β-adrenergic blocking agents and digoxin.
Class II (Propranolol, Nadolol, Atenolol, Metoprolol, Esmolol)
These β-adrenergic blocking agents block binding of catecholamines to β -adrenergic receptors, decreasing automaticity and slowing AV conduction. Other effects include prolongation of the action potential duration and effective refractory period. Additionally, the threshold for ventricular fibrillation is increased. The slowing of AV conduction and suppression of premature beats that may initiate a re-entrant circuit explain the efficacy of these agents in treating re-entrant tachycardias. These agents are negative inotropes and must be used cautiously in patients who are hypotensive or who have decreased ventricular function.
Propranolol and nadolol are “nonselective” β-blocking agents. Oral administration of propanolol is necessary every 6 to 8 hours. Nadolol is given only once daily, but pediatric dosing is not well defined. Esmolol is used when an intravenous β-adrenergic blocker is needed. Esmolol is a β1-selective agent, so bronchial constriction is less of a problem. Onset is rapid, and the short duration of effect makes this agent relatively safe for therapeutic trials. Atenolol has minimal β2 effects and has the advantage of requiring only twice-a-day oral administration in young infants. Fewer central nervous system effects occur with atenolol than with propranolol because atenolol does not cross the blood-brain barrier. Metoprolol is also a β1-selective agent that can be given once daily, but pediatric dosing is not well defined. These agents must be used with caution in patients with reactive airways disease.
Class III (Amiodarone, Sotalol)
These potassium channel-blocking drugs prolong action potential duration by prolonging the plateau of the action potential. The upstroke velocity is not affected.
The pharmacologic effects of amiodarone are complex. Sodium, calcium, and outward potassium channels are inhibited. The action potential duration is increased, and the effective refractory period is prolonged in atrial and ventricular muscle, Purkinje fibers, and accessory AV pathways. The rate of automatic discharge for the SA and AV nodes is decreased. Amiodarone also has a- and β-blocking properties but does not depress myocardial function. Automaticity is decreased. Marked changes occur on the ECG, including sinus slowing, prolongation of the PR interval, minimal widening of the QRS complex, and prolongation of the QTc interval. Proarrhythmic responses occur infrequently. This drug is highly efficacious, but it has toxicity in multiple systems; corneal microdeposits, hyper- or hypothyroidism, pulmonary interstitial fibrosis, hepatitis, peripheral neuropathy, and a slate-blue discoloration of the skin have been reported. These adverse effects are less common in pediatric patients than in adult patients. Baseline liver, renal, and thyroid function tests; ophthalmologic examination; and pulmonary function tests should be obtained before starting any patient on long-term therapy and then repeated every 6 months as long as the patient is taking this drug. Postoperative patients who have JET do not need these evaluations, as the duration of amiodarone therapy will be short (see earlier text). Intravenous administration may cause hypotension. Amiodarone interacts with digoxin, phenytoin, and warfarin, so the doses of these medications should be decreased and closely monitored when amiodarone is administered.
Sotalol is a nonselective β-adrenergic blocking agent at low doses but shows class III activity at higher doses. The QTc interval increases in a dose-dependent manner. Torsades de pointes occurs in up to 10% of pediatric patients, usually within a few days of starting therapy. Close monitoring of the QTc interval on the ECG is recommended. Sotalol should not be administered with other drugs, such as procainamide, that also prolong the QTc interval.
Class IV (Verapamil)
Verapamil acts on the slow calcium current in SA and AV node cells, thereby decreasing the rate of phase 4 automaticity and phase 0 depolarization and prolonging refractoriness and conduction time. Administration of intravenous verapamil is contraindicated in patients younger than 1 year of age because of the risk of precipitating cardiovascular collapse. This is likely the result of the increased dependence of the immature myocardium on extracellular rather than intracellular calcium for contraction. This agent is also contraindicated in patients with WPW because it increases conduction down the accessory pathway.
Digoxin
Digoxin inhibits sodium-potassium ATPase activity and exerts complex effects on the cardiovascular system. Digoxin decreases the action potential duration and effective refractory period. Additionally, digoxin increases vagal tone, causing slowing of SA node discharge and decreased AV node conduction. The ECG shows sinus slowing, prolongation of the PR interval, mild depression of the ST segment, and mild flattening of T waves.
Digoxin is used primarily for its effect on AV conduction and is therefore used to decrease the rate of ventricular response in atrial fibrillation, ectopic atrial tachycardia, and other abnormal supraventricular rhythms. Digoxin is also effective in all re-entrant arrhythmias in which the AV node is involved in the re-entrant circuit. However, digoxin should not be used in patients with WPW syndrome because it may shorten the effective refractory period of the accessory AV pathway and thus allow very rapid ventricular response rates in patients with atrial fibrillation or flutter.
Cardiac toxicity caused by digoxin is characterized by sinus bradycardia, AV block, and ventricular ectopy. These effects are potentiated by hypokalemia and hypercalcemia. Life-threatening toxicity should be treated by intravenous administration of digoxin-immune Fab antibodies. Temporary pacing is used for symptomatic AV block, and phenytoin is recommended for treatment of digoxin-induced ventricular ectopy.
Adenosine
Adenosine is an endogenous nucleoside that slows or entirely blocks conduction through the AV node. Sinus node slowing is also noted. This drug, which is the drug of choice to terminate SVT, has a very short half-life, and when administered as a bolus dose, it causes AV conduction block. This will convert most re-entrant tachycardias to sinus rhythm. Interestingly, these effects are not seen when adenosine is administered as a continuous intravenous infusion; this is likely the result of reflex sympathetic stimulation. Thus, this agent must be given as a rapid intravenous bolus and then flushed in immediately.
It is very important to obtain an electrocardiogram during administration of adenosine, as this may help to determine the etiology of the tachycardia. The most common reason for failure of adenosine to convert a patient to sinus rhythm is poor administration technique. If adenosine is not effective after two or three doses or it is effective but with rapid or frequent reinitiation of tachycardia, there is no reason to continue repeating the dose. If adenosine does not terminate tachycardia, other modalities should be considered because the tachycardia may not involve the AV node. At times, induction of AV block by adenosine allows flutter waves or P waves consistent with ectopic atrial tachycardia to be seen more easily on the ECG.
Magnesium
Magnesium, a cofactor in many enzymatic reactions, inhibits calcium channels. The resulting decreased intracellular calcium concentration likely explains the antiarrhythmic effects. In the past, magnesium was used for many arrhythmias, but current data support administration only for torsades de pointes and documented hypomagnesemia.
■ DC CARDIOVERSION
DC cardioversion is indicated for any patient with severe hemodynamic compromise. The defibrillator should always be synchronized to the QRS complex. This, of course, is not possible if the patient has ventricular fibrillation and may be difficult in patients with a rapid low-voltage QRS complex polymorphic ventricular tachycardia. Elective cardioversion should be performed under general anesthesia or deep sedation. Unsuccessful cardioversion must be distinguished from immediate arrhythmia reinitiation after successful conversion. If reinitiation occurs, medication should be administered to prevent reinitiation before further shocks are delivered. Failure of DC cardioversion to convert a tachycardia is highly suggestive of an automatic mechanism, and other types of therapy, such as overdrive pacing or pharmacologic agents, should be considered. Repeated DC cardioversion or excessive energy discharge may damage the myocardium.
■ OVERDRIVE PACING
Pacing at a rate greater than that of the intrinsic rhythm creates an area of refractory tissue within a re-entrant circuit, thus interrupting the tachycardia. This will not be effective for automatic tachycardias. In young infants, overdrive pacing can be performed with a transesophageal pacing catheter. The esophageal catheter is positioned so that the maximum amplitude on the recorded atrial deflection is obtained. Pacing is begun at a rate 10% higher than the tachycardia rate and continued for 10 seconds. In contrast to DC cardioversion, overdrive pacing can be performed repeatedly without risk of myocardial damage.
■ CATHETER ABLATION
The development of radio-frequency ablation as a potentially curative therapy for many tachycardias has revolutionized care of older children and adults with these disorders. However, data from the Pediatric Electrophysiology Society Radiofrequency Catheter Ablation Registry show a higher complication rate in patients younger than 3 years of age compared to that in older patients. These complications involve sudden death within weeks of the procedure, coronary arterial ischemia, and pericardial effusion. Nevertheless, radio-frequency ablation is indicated for the rare infant with life-threatening arrhythmias refractory to medical management.
Cryothermal ablation is a useful addition to catheterbased ablation, especially in smaller patients. As compared to radio-frequency energy, cryotherapy takes longer to permanently destroy tissue. This allows creation of a reversible lesion during which the planned site of ablation can be evaluated for success as well as for adverse effects before completing the procedure. If adverse effects are noted, conduction will usually return when cryoablation is stopped. This has become the ablation method of choice for many when ablating tissue in close proximity to the AV node (eg, junctional tachycardia) to avoid the complication of complete heart block. Catheter stability due to tip adherence at cold temperatures is an added benefit. Despite the safety profile of cryothermal ablation, the indication to pursue an ablation procedure in lieu of antiarrhythmic therapy has not changed for infants.
SUGGESTED READINGS
General
Dick M. Clinical Cardiac Electrophysiology in the Young. New York, NY: Springer; 2006.
Southall DP, Richards J, Mitchell P, et al. Study of cardiac rhythm in healthy newborn infants. Br Heart J. 1980;43(1):14-20.
Walsh EP, Saul JP, Triedman JK. Cardiac Arrhythmias in Children and Young Adults with Congenital Heart Disease. Philadelphia, PA: Lippincott Williams & Wilkins; 2001.
Atrioventricular Block
Ambrosi A, Wahren-Herlenius M. Congenital heart block: evidence for a pathogenic role of maternal autoantibodies. Arthritis Res Ther. 2012;26:14(2):208.
Buyon JP, Clancy RM, Friedman DM. Cardiac manifestations of neonatal lupus erythematosus: guidelines to management, integrating clues from the bench and bedside. Nat Clin PractRheumatol. 2009;5(3):139-148.
Glatz AC, Gaynor JW, Rhodes LA, et al. Outcome of high- risk neonates with congenital complete heart block paced in the first 24 hours after birth. J Thorac Cardiovasc Surg. 2008;136(3):767-773.
Llanos C, Friedman DM, Saxena A, et al. Anatomical and pathological findings in hearts from fetuses and infants with cardiac manifestations of neonatal lupus. Rheumatology (Oxford). 2012;51(6):1086-1092.
Lopes LM, Tavares GM, Damiano AP, et al. Perinatal outcome of fetal atrioventricular block: one-hundred-sixteen cases from a single institution. Circulation. 2008;118(12): 1268-1275.
Supraventricular Arrhythmias
Adamson PC, Rhodes LA, Saul JP, et al. The pharmacokinetics of esmolol in pediatric subjects with supraventricular arrhythmias. Pediatr Cardiol. 2006;27(4):420-427.
Cain N, Irving C, Webber S, Beerman L, Arora G. Natural history of Wolff-Parkinson-White syndrome diagnosed in childhood. Am J Cardiol. 2013;112(7):961-965.
Collins KK, Van Hare GF, Kertesz NJ, et al. Pediatric nonpostoperative junctional ectopic tachycardia medical management and interventional therapies. J Am Coll Cardiol. 2009;53(8):690-697.
Fish FA, Mehta AV, Johns JA. Characteristics and management of chaotic atrial tachycardia of infancy. Am J Cardiol. 1996;78(9):1052-1055.
Salerno JC, Kertesz NJ, Friedman RA, Fenrich AL Jr. Clinical course of atrial ectopic tachycardia is age-dependent: results and treatment in children <3 or >3 years of age. J Am Coll Cardiol. 2004;43(3):438-444.
Sanatani S, Potts JE, Reed JH, et al. The study of antiar- rhythmic medications in infancy (SAMIS): a multicenter, randomized controlled trial comparing the efficacy and safety of digoxin versus propranolol for prophylaxis of supraventricular tachycardia in infants. Circ Arrhythm Electrophysiol. 2012;5(5):984-991.
Shamszad P, Cabrera AG, Kim JJ, et al. Perioperative atrial tachycardia is associated with increased mortality in infants undergoing cardiac surgery. J Thorac Cardiovasc Surg. 2012;144(2):396-401.
Texter KM, Kertesz NJ, Friedman RA, Fenrich AL Jr. Atrial flutter in infants. J Am Coll Cardiol. 2006;48(5): 1040-1046.
Zampi JD, Hirsch JC, Gurney JG, et al. Junctional ectopic tachycardia after infant heart surgery: incidence and outcomes. Pediatr Cardiol. 2012;33(8):1362-1369.
Long QT Syndrome/Catecholaminergic Polymorphic Ventricular Tachycardia/Sudden Infant Death Syndrome
Arnestad M, Crotti L, Rognum TO, et al. Prevalence of long-QT syndrome gene variants in sudden infant death syndrome. Circulation. 2007;115(3):361-367.
Crotti L, Tester DJ, White WM, et al. Long QT syndrome- associated mutations in intrauterine fetal death. JAMA. 2013;309(14):1472-1482.
Goldenberg I, Moss AJ, Peterson DR, et al. Risk factors for aborted cardiac arrest and sudden cardiac death in children with the congenital long-QT syndrome. Circulation. 2008;117(17):2184-2191.
Guidicessia JR, Ackeman MJ. Genetic testing in heritable cardiac arrhythmia syndromes: differentiating pathogenic mutations from background genetic noise. Curr Opin Cardiol. 2013;28:63-71.
Leenhardt A, Denjoy I, Guicheney P. Catecholaminergic polymorphic ventricular tachycardia. Circ Arrhythm Electrophysiol. 2012;5:1044-1052.
Levin MD, Stephens P, Tanel RE, Vetter VL, Rhodes LA. Ventricular tachycardia in infants with structurally normal heart: a benign disorder. Cardiol Young. 2010;20: 641-647.
Priori SG, Wilde AA, Horie M, et al. JRS/EHRA/APHRS expert consensus statement on the diagnosis and management of patients with inherited primary arrhythmia syndromes. Heart Rhythm. 2013;10(12):1933-1963.
Richards JM, Alexander JR, Shinebourne EA, et al. Sequential 22-hour profiles of breathing patterns and heart rate in 110 full-term infants during their first 6 months of life. Pediatrics. 1984;74(5):763-777.
Schwartz PG, Crotti L, Insolia R. Long-QT syndrome, from genetics to management. Circ Arrhythm Electophysiol. 2012;5:868-877.
Southall DP, Arrowsmith WA, Stebbens V, Alexander JR. QT interval measurements before sudden infant death syndrome. Arch Dis Child. 1986;61(4):327-333.
Spazzolini C, Mullally J, Moss AJ, et al. Clinical implications for patients with long QT syndrome who experience a cardiac event during infancy. J Am Coll Cardiol. 2009;54(9):832-837.
Triedman J. The meaning of lethal events in infants with long QT syndrome. J Am Coll Cardiol. 2009;54(9):838-839.
Van Hare GF, Perry J, Berul CI, Triedman JK. Cost effectiveness of neonatal ECG screening for the long QT syndrome. Eur Heart J. 2007;28(1):137-139.
Vincent GM, Schwartz PJ, Denjoy I, et al. High efficacy of beta-blockers in long-QT syndrome type 1: contribution of noncompliance and QT-prolonging drugs to the occurrence of beta-blocker treatment “failures.” Circulation. 2009;119(2):215-221.
Webster G, Berul CI. An update on channelopa- thies, from mechanisms to management. Circulation. 2013;127(1):126-140.
Pharmacologic Therapy
Saul JP, Scott WA, Brown S, et al. Intravenous amiodarone for incessant tachyarrhythmias in children: a randomized, double-blind, antiarrhythmic drug trial. Circulation. 2005;112(22):3470-3477.
Catheter Ablation
Bar-Cohen Y, Cecchin F, Alexander ME, et al. Cryoablation for accessory pathways located near normal conduction tissues or within the coronary venous system in children and young adults. Heart Rhythm. 2006;3(3):253-258.
Chanani NK, Chiesa NA, Dubin AM, et al. Cryoablation for atrioventricular nodal reentrant tachycardia in young patients: predictors of recurrence. Pacing Clin Electrophysiol. 2008;31(9):1152-1159.
McDaniel GM, Van Hare GF. Catheter ablation in children and adolescents. Heart Rhythm. 2006;3(1):95-101.
Morwood JG, Triedman JK, Berul CI, et al. Radiofrequency catheter ablation of ventricular tachycardia in children and young adults with congenital heart disease. Heart Rhythm. 2004;1(3):301-308.
Van Hare GF, Javitz H, Carmelli D, et al. Pediatric Electrophysiology Society. Prospective assessment after pediatric cardiac ablation: demographics, medical profiles, and initial outcomes. JCardiovascElectrophysiol. 2004;15(7):759-770.
Devices
Berul CI, Van Hare GF, Kertesz NJ, et al. Results of a multicenter retrospective implantable cardioverter-defibrillator registry of pediatric and congenital heart disease patients. J Am Coll Cardiol. 2008;51(17):1685-1691.
Burns KM, Evans F, Kaltman JR. Pediatric ICD utilization in the United States from 1997 to 2006. Heart Rhythm. 2011;8(1):23-28.
Walsh EP. Practical aspects of implantable defibrillator therapy in patients with congenital heart disease. Pacing Clin Electrophysiol. 2008;31(suppl 1):S38-S40.