General Presentation and Evaluation

Supraventricular tachycardias (SVTs), which are arrhythmias that originate above the ventricles and involve the atria, the atrioventricular (AV) node, or the accessory bypass tracts, represent the most common tachycardias seen in infants and children, with an incidence of 1 in 250 to 1000 in the United States.¹ SVTs are classified by identifying those components of the heart that are involved in maintaining the tachycardia. SVTs include primary atrial tachycardias (those in which the AV node and the ventricles are not involved), AV reciprocating tachycardias (those that involve both the atrium and the ventricle), and AV nodal re-entrant tachycardias (those that do not involve the atria as part of the circuit). The presentation of the pediatric patient with SVT varies from palpitations to signs of marked cardiac failure. This variability is related to the patient’s age at presentation and his or her ability to communicate symptoms, the duration and rate of the tachycardia, as well as the mechanism of SVT. Parents of infants with SVT occasionally note changes in feeding habits and irritability as symptoms. Symptoms in those old enough to voice complaints include palpitations (noted as skipped or rapid heart beats, or as “heart beeping”), chest pain, shortness of breath, fatigue, dizziness, and syncope.

The evaluation of pediatric patients includes a 12- or 15-lead electrocardiogram (ECG), which may show Wolff-Parkinson-White (WPW) syndrome or other abnormalities such as an ectopic atrial rhythm. While the ECG is often normal when SVT is not present, an ECG during SVT can be very helpful in distinguishing the mechanism of the tachycardia. A recent study suggested that leads V1 and III were the most helpful in this diagnosis.² Twenty-four-hour ambulatory (Holter) monitoring is useful to screen for occult arrhythmias or to assess heart rate ranges and variability if the patient is having frequent symptoms (i.e., on a daily basis). Transtelephonic event monitors are often helpful in those with fleeting symptoms, allowing the patient or parent to record symptoms as they occur with simultaneous ECG recordings. Exercise tolerance testing is useful in those with palpitations or symptoms associated with activities. More invasive testing such as a transesophageal or intracardiac electrophysiological study (EPS) is used to induce arrhythmia and determine the particular mechanism and often as a prelude to catheter ablation.

Mechanisms

Three primary mechanisms are involved in SVT: (1) enhanced automaticity, (2) triggered automaticity, and (3) re-entry. Automaticity is the ability of cardiac myocytes to spontaneously depolarize, which leads to myocardial contractions. Sinus and AV nodes are the primary sites of automaticity in the heart. Enhanced automaticity occurs when myocytes outside the sinus or the AV node depolarize spontaneously, which leads to atrial or ventricular ectopic beats or SVT and to ventricular tachycardias (VTs) when repetitive ventricular ectopic beats occur.³

Tachycardias arising secondary to enhanced automaticity have characteristics that are distinct from those of re-entry. They are highly catecholamine sensitive, with warm-up and cool-down phases. This leads to variable rates during the tachycardia. These tachycardias are not inducible with programmed stimulation, nor are they terminated with overdrive pacing or with direct-current cardioversion. Automatic tachycardias can arise from all areas of the heart, with those from the atria referred to as ectopic atrial tachycardia, those from the AV junction as junctional ectopic tachycardia, and those from the ventricles as automatic VT.

Triggered automaticity results from spontaneous myocardial contractions that occur secondary to oscillations during repolarization reaching threshold and leading to a depolarization.^4,5 These oscillations are referred to as after-depolarizations. Arrhythmias arising from triggered automaticity have characteristics shared by both enhanced automaticity and re-entry. They are highly catecholamine sensitive and have warm-up and cool-down phases with a wide variation in heart rate, similar to other forms of automaticity. They can be induced and terminated with pacing maneuvers and respond to direct-current cardioversion, as do re-entrant arrhythmias. Triggered automaticity is thought to play a role in the arrhythmias seen in digoxin toxicity.⁶

Re-entry occurs when a wavefront of electrical activation travels through tissue for a distance and then re-enters the original tissue and propagates through the circuit again. Re-entrant SVT is the most common type of tachycardia seen in pediatrics. To have a re-entrant circuit, at least two pathways with distinct conduction properties and refractory periods must be present. The sequence of events in a re-entrant circuit is as follows. A stimulus encounters two distinct pathways, with one pathway refractory to conduction (the pathway with the longer refractory period) and one ready for conduction (the pathway with the shorter refractory period). The impulse is conducted along the pathway, with the shorter refractory period having enough conduction delay to allow the first pathway to recover and to conduct the impulse back to the original site of entry into the circuit. The impulse then re-enters the original pathway and transverses the circuit again. Re-entry can occur in the sinus node, the atrial tissue, the AV node, and the ventricular tissue or between the atrium and the ventricle. Those re-entry circuits between the atrium and the ventricle involve specialized conduction tissue referred to as accessory pathways. In AV nodal re-entrant tachycardia (AVNRT), the accessory pathway is in the AV node or in the AV nodal region. Moe and colleagues initially described the evidence of dual AV node physiology in 1956.⁷ Re-entry tachycardias have characteristics that are distinct from those seen in automatic tachycardias. They often have sudden onset and termination, with patients frequently feeling that they have been “switched” on and off. The tachycardia is regular, with little variation in rate. Re-entry tachycardias often respond to vagal maneuvers by slowing or terminating the tachycardia. These tachycardias are easily induced and terminated with pacing protocols ranging from single premature stimuli to burst pacing or by direct-current cardioversion.

Specific Mechanisms in Children

The most common specific mechanism of SVT in children, representing 75% to 90% of arrhythmias in children without other cardiac conditions, is re-entry through an accessory pathway between the atrium and the ventricle or within the AV node.^8,9 Over two thirds of these pathways are concealed, and one third are manifested as WPW or other forms of pre-excitation. AV nodal re-entry represents around 15% of pediatric SVTs, most commonly seen in adolescents, with up to one third of SVTs presenting in adolescence, the cause being AVNRT.^9,10 Automatic atrial ectopic tachycardia represents 10% to 18% of SVTs in children. Atrial flutter and atrial fibrillation (AF) are responsible for 10% and 40% in non-postoperative and postoperative children, respectively, and junctional ectopic tachycardia responsible for less than 1% in non-postoperative patients.

Presentation and Treatment

Some variability exists in the presentation of AV reciprocating tachycardia in childhood. This may relate more to the age of the patient than to the specific type of tachycardia. For this reason, the presentation and treatment of pediatric patients with AV nodal re-entry, concealed bypass tract, and WPW re-entrant tachycardia are combined in the following overview, with emphasis on the different age-related presentations and treatment strategies.

Tachycardia may be seen as early as during fetal development. The diagnosis of fetal SVT may be made by auscultation of the baby’s heart rate during maternal examination and is confirmed by fetal ultrasonography or fetal magnetocardiography.¹¹ See Chapter 76 for detailed information on these arrhythmias.

It has been reported that 50% to 60% of SVTs in children present in the first year of life, with accessory pathway–mediated re-entrant tachycardia being the most common mechanism.^12,13 The heart rate in infants with SVT is usually 220 to 320 beats/min. Neonates and infants with tachycardia are often very ill, presenting with congestive heart failure, which occurs if the tachycardia has been present for more than 48 to 72 hours before the patient receives medical care. This occurs because of the infant’s inability to communicate and the family’s lack of awareness that the child could have a significant medical condition. Parents may note that the infant is acting somewhat different from normal, is more irritable, or is not eating well. This is often interpreted as colic or some other “normal” childhood problem. At presentation, these babies often are acidotic from decreased cardiac output and may need aggressive resuscitation, including artificial ventilation and rapid termination of the tachycardia. Intravenous (IV) adenosine is effective in the acute termination of SVT in this population, but IV access is often difficult in a 3- to 4-kg baby with congestive heart failure. It has been suggested that in the treatment of infants, the starting doses of adenosine should be higher than those generally recommended as the average effective dose in infants, which is around 200 µg/kg.¹⁴ If IV access is not possible, transesophageal overdrive pacing has proven to be effective. Once the rhythm is restored to normal and the cardiac function has begun to recover, IV access becomes easier, and IV medications can be administered. If the infant is hemodynamically compromised and the rhythm cannot be medically converted, electrical synchronized cardioversion is indicated.

Medications for the acute treatment of SVT are shown in Table 75-1. Digoxin is a first-line medication used in the treatment of SVT in infants with decreased myocardial performance. It is contraindicated as a chronic treatment in patients with WPW syndrome but can be used acutely in those with decreased ventricular function with careful monitoring. The authors of this chapters do not discharge patients with WPW syndrome on digoxin because of its potential effect to enhance conduction of atrial impulses to the ventricle but switch them to another medication once the cardiac function has normalized. Other IV medications used acutely to treat SVT include β-blockers such as esmolol, amiodarone, and procainamide. These medications must be used with caution because of their negative inotropic effects, which can lead to worsening of cardiac function. The use of IV calcium channel blockers is contraindicated in infants and children younger than 1 year because as sudden death has been reported in some cases.^15,16 In a long-term study from Sweden, the SVT was managed with a single drug in 62% of patients.¹⁷

Table 75-1 Pharmacologic Agents for Acute Treatment of Supraventricular Tachycardia

AGENT	INITIAL TREATMENT (IV)
Adenosine	50–100 µg/kg, increase by 50-µg/kg increments
	Every 2 min to 400 µg/kg or 12 mg maximal dose
Amiodarone	IV: 5 mg/kg over 1 hour, followed by IV bolus of 2.5 mg/kg q4–6h
Digoxin	Dose is age dependent
	Give in 3 doses ( TDD, TDD, TDD)
	Preterm infant: 10–20 µg/kg TDD
	Term newborn to adolescent: 30–40 µg/kg TDD oral to maximal TDD of 1–1.5 mg (IV 3/4 PO)
	Oral maintenance: 10 µg/kg/day q12h
Esmolol	IV Load: 200–500 µg/kg/min over 2–4 min. May increase in 50–100 µg/kg/min increments (maximum dose = 1000 µg/kg/min)
	Maintenance infusion: 50–200 µg/kg/min
Phenylephrine	100 µg/kg bolus
	10 µg/kg/min infusion
Procainamide	5 mg/g over 5–10 min or 10–15 mg/kg over 30–45 min
	20–100 µg/kg/min infusion
Propranolol	0.05–0.1 mg/kg over 5 min q6h
Verapamil	0.05–0.30 mg/kg over 3–5 min
	Maximal dose: 10 mg

IV, Intravenous; TDD, total digitalizing dose.

Prognosis

While the majority of SVTs will resolve around 1 to 2 years of age, one third will recur by adolescence. Recurrence is only 10% for AVRT secondary to concealed accessory pathway but more than 31% if a WPW pattern persists.¹⁷

Presentation in Older Children

After the peak of presentation in the first year of life, the other two common ages of presentation are early childhood (6 to 8 years of age) and adolescence.^12,18 SVT in children is predominantly caused by concealed or manifested accessory pathways (WPW) throughout early childhood, with the proportion of patients with AVNRT increasing with age.⁸ The heart rate in older children is generally in the range of 160 to 280 beats/min. Older children often present with palpitations or dizziness. It is not uncommon to hear a 3- or 4-year-old complain that the heart is “beeping” too fast. These patients are generally not in SVT long enough to develop congestive heart failure, as seen in infants, because they can communicate to their caregivers that they are experiencing something abnormal or unusual. With the exception of those with WPW syndrome with rapid conduction down their accessory pathway during an atrial tachycardia, patients rarely present with syncope during SVT.

Treatment

The long-term treatment of infants with AV reciprocating tachycardia includes the use of oral preparations of the medications listed in Table 75-2, with digoxin or propranolol/β-blocker medications being the most commonly used agents. During initiation of therapy with oral antiarrhythmics in infants, the authors of this chapter monitor the patients in the hospital for at least five half-lives of the medication, allowing a steady state to be reached, watching for side effects, and educating the family about administering the medication and recognizing the tachycardia. The family can also be taught about simple vagal mechanisms to interrupt the tachycardia. When starting therapy with oral β-blockers in infants and young children, the patient must be monitored for hypoglycemia and the caregivers must know how to recognize the symptoms of hypoglycemia. The authors of this chapter ask the family to notify medical personnel if their child is not tolerating the medication or if side effects occur. It is not the authors’ practice to discharge patients with heart rate monitors. Families can assess their children for recurrent SVT without resorting to continuous monitoring. Most infants are treated with oral medications for 10 to 12 months, with the dosage adjusted on the basis of weight gain. At 10 months to 1 year of age, if no recurrences of the SVT have occurred, the patient will be weaned from the medication unless the WPW pattern is still present on the ECG. Approximately one third of all patients who develop SVT in the first 3 months of life will outgrow it by 1 year of age.^12,18,19,20

The short- and long-term medical management of children and adolescents is similar to that of infants. The exception is that catheter ablation becomes more of a therapeutic option around 4 to 5 years of age and is more commonly used after 8 years of age. Ablation can and has been performed in younger children, but the overall consensus is that it should only be used for patients 4 years of age or younger who have medically refractory arrhythmias, as the risks of the procedure are higher under 4 years of age.^21,22 See Chapter 78 for detailed information on catheter ablation in children.

Parents should be reassured regarding the generally benign nature of these arrhythmias. Parents should be taught about vagal maneuvers that can be used to interrupt the tachycardia, including applying ice or cold to the face, inducing gagging, breath holding, Valsalva maneuver, and performing a headstand or handstand. Schools should have emergency plans in place regarding whom to contact and what symptoms should prompt a call for emergency help, such as when a student faints and is unresponsive or seems to be experiencing a cardiac arrest.

Concealed Bypass Tachycardia

Concealed bypass tachycardias are tachycardias in which unidirectional conduction in an accessory pathway is present. The re-entrant circuit uses the AV node for antegrade conduction and the accessory pathway for retrograde conduction. In these pathways, the conduction is only from the ventricle to the atrium (retrograde), with no antegrade conduction. No indication of the accessory pathway is seen on a resting ECG. These accessory pathways can occur anywhere along the AV groove, either on the right side or on the left side of the heart or in the septal region. These tachycardias are seen throughout all phases of childhood. Patients can present with concealed bypass tract tachycardias in the neonatal period as well as throughout adolescence. The tachycardia may resolve spontaneously in children when it results from this mechanism in the first year, whereas it is not likely in those with presentation after 2 years of age.

Persistent Junctional Reciprocating Tachycardia

Persistent junctional reciprocating tachycardia (PJRT) is a specific form of AV re-entrant tachycardia using a concealed accessory pathway that is generally located in the posterior septal region. These pathways have long refractory periods with slow retrograde conduction, resulting in a long P-R interval with negative P waves in leads II, III, and aVF. PJRT tends to be an incessant arrhythmia and can be refractory to single-drug therapy. It is frequently confused with ectopic atrial tachycardia (especially those arising from the low right atrium) secondary to ECG findings and is resistant to treatment.

Presentation

WPW syndrome in children is very similar to that in adults. This syndrome involves an accessory pathway between the atrium and the ventricle that usually has bi-directional conduction properties. The reported incidence of WPW syndrome ranges from 1 to 4 per 1000 live births.²³ Of all patients who present with WPW syndrome in childhood, one fifth to one third will have associated cardiac abnormalities. The most common congenital lesions include Ebstein’s anomaly of the tricuspid valve and L-transposition of the great arteries.¹⁸ Patients with WPW syndrome present any time from fetal life through adolescence. The patients who present during fetal life and early infancy present with re-entrant SVT. Children and adolescents also present with re-entrant SVT but can occasionally present with atrial fibrillation (AF) with rapid conduction down the accessory pathway. Approximately 10% of children will present with an antidromic tachycardia that uses the accessory pathway as the antegrade limb, with approximately half of these patients having multiple accessory pathways.²⁴ A number of asymptomatic patients are noted to have the pattern of WPW syndrome on ECGs obtained for other reasons. Sudden death can be the first manifestation of WPW, leading some authors to suggest that risk stratification, including an EPS, should be done for all patients with WPW beyond infancy and early childhood.²⁵ The presence of a short refractory period or multiple pathways increases the risk of a subsequent life-threatening event, and catheter ablation is advised.²⁶ Occasionally, an infant presents with a narrow-complex tachycardia with no evidence of a WPW pattern on a baseline ECG, but pre-excitation becomes obvious when the patient is treated with a medication (e.g., digoxin) that slows AV nodal conduction.

Treatment

In the young adolescent or older child who presents with WPW syndrome, the authors of this chapter offer catheter ablation as a first-line therapeutic option. In their practice, in children with known WPW syndrome older than 6 to 8 years of age, the accessory pathway conduction or the refractory period of the accessory pathway is routinely assessed by using exercise testing, esophageal pacing, intracardiac EPS, or combinations of these methods. When given the option, many patients who are old enough to decide or their caregivers agree to an intracardiac EPS so that a catheter ablation procedure can be performed and the WPW addressed definitively. The treatment strategies for patients with WPW syndrome vary on the basis of the patient’s age and symptoms at the time of presentation. The mainstay of treatment in infants and young children with WPW has been the use of antiarrhythmic medications. The authors refrain from the use of digoxin or calcium channel blockers in patients with WPW because conduction down the accessory pathway may be enhanced, along with blockade of the conduction through the AV node. This is thought to lead to an increased risk of rapid conduction of AF or premature atrial contractions through the accessory pathway. The authors recommend β-blockers as a first-line therapy unless a contraindication such as severe reactive airway disease is present. When β-blocker therapy is contraindicated or fails to control the tachycardia, the authors use other medications such as flecainide, amiodarone, or sotalol. Occasionally, a neonate with WPW syndrome presents with significant signs of cardiac decompensation and cardiogenic shock. In those circumstances, to improve ventricular function as well as to suppress the tachycardia, the authors use digoxin while the patient is still in the hospital. The patient is always converted from digoxin to another antiarrhythmic medication before being discharged.

A special consideration in the pediatric population is the patient who presents with asymptomatic WPW syndrome following an ECG obtained for some other indication such as chest pain or for a school physical examination. The authors recommend that patients older than 6 to 8 years of age have the evaluation mentioned above to assess whether they have rapid conduction down their accessory pathways. Generally, these patients are taken to the electrophysiology laboratory to measure their minimum cycle length of pre-excitation both with atrial pacing and during AF. A minimum cycle length of pre-excitation of less than 220 ms is used during AF as a marker that the patient may have a significant risk of sudden death. This is based on studies performed by Bromberg et al and Paul et al, which demonstrated that these values are helpful, though not fully predictive of cardiac arrest and syncope.^27,28 Additional studies by Pappone and Santinelli show an increased risk for sudden death with WPW with short accessory pathway effective refractory periods and with multiple pathways and recommend that catheter ablation be performed.^26,29,30

Activity Recommendations

Activity restriction is generally not required in individuals with controlled SVT. If WPW is present, risk stratification with EPS and possible catheter ablation are recommended before participation in any activity, according to the Bethesda Guidelines.³¹

Atrioventricular Nodal Re-entrant Tachycardia

In AVNRT the re-entry circuit is the region of the AV node. AVNRT occurs more commonly in young adults and adolescents than in younger children. It is infrequently seen in patients during the neonatal period. The classic finding on an ECG is a tachycardia with very short R-P interval or no visible P wave. This is secondary to the fact that the retrograde limb of the pathway conducts very rapidly from the ventricle to the atrium. Treatment is aimed at the AV node, with the most commonly used drugs in older children being digoxin, β-blockers, and calcium channel blockers.

Ectopic Atrial Tachycardia

Ectopic atrial tachycardia (EAT), or automatic atrial tachycardia, is an arrhythmia arising from both atria with inappropriately fast atrial rates. This tachycardia represents approximately 10% of the SVT seen in the overall population.³² The heart rates seen in EAT vary on the basis of the patient’s age and catecholamine state during the tachycardia. The heart rates in automatic atrial tachycardias will be inappropriately fast for the patient’s level of activity and are generally in the 130 to 250 beats/min range. In general, the rates are not as fast as those seen in re-entrant tachycardias. The pulse rate may not be reflective of the atrial rate because of variable atrial conduction through the AV node.

Tachycardias arising from foci of increased automaticity can be found in all areas of the atria. The automatic foci are more likely to be located in certain areas than in others. In the right atrium, the foci are frequently found in the right atrial appendage, and in the left atrium, they are often mapped to the orifices of the pulmonary veins.^33–35

Evaluation

ECGs in patients with an EAT generally show a P-wave axis distinct from sinus rhythm. When the focus arises from the left atrium, the P wave is negative in lead I; those with the focus in the low right atrium show a negative P-wave axis in lead aVF and a positive P wave in lead I. Occasionally, the focus is in an area close to the sinus node or in the high right atrium with the P-wave axis similar to sinus tachycardia (0 to 90 degrees in the frontal plane). This can lead to a delay in diagnosis and institution of therapy, when the rhythm is thought to be sinus tachycardia. EAT, as well as other primary atrial tachycardias, can show variable degrees of AV conduction on ECG and Holter monitoring, which is often helpful in establishing the diagnosis. Figure 75-1 represents an ECG of a patient with EAT.

FIGURE 75-1 Electrocardiogram of ectopic atrial tachycardia in a 10-year-old. Note atrial rhythm originating from a low right atrial focus.

Holter monitoring is helpful in establishing the diagnosis and in determining the frequency of EAT. A careful review of the patient’s diary of activities along with the corresponding heart rates allows the determination of inappropriate elevation of heart rate. The overall average heart rate over 24 hours provides an indication of the amount of time the patient is in SVT. Special attention should be given to the heart rate during sleep. These noninvasive evaluations may be the only way to assess the patient with EAT. Exercise testing is frequently not useful in evaluating EAT because as the sinus heart rate increases, the automatic focus is suppressed. In most cases, EAT cannot be induced in the cardiac catheterization laboratory using conventional pacing protocols but may be induced by rapid atrial pacing of the heart (if triggered automaticity is the mechanism) or by isoproterenol infusion.

A patient with EAT should have a complete echocardiographic evaluation to rule out congenital heart disease, especially the types that could lead to an abnormal sinus node (and therefore abnormal P-wave axis), such as heterotaxy syndrome. A careful assessment of ventricular function, as well as the presence of AV valve regurgitation, should be performed. This evaluation should be done as a baseline study, before medical intervention, to document the progression of the disease or the possible deterioration of ventricular function secondary to the negative inotropic properties of the medical management or to inadequate control of the tachycardia.

Multifocal Atrial Tachycardia or Chaotic Atrial Tachycardia

Multifocal atrial tachycardia (MAT), or chaotic atrial tachycardia (CAT), is a primary atrial tachycardia arising from multiple areas of enhanced automaticity in the atria. By definition, the tachycardia must have at least three distinct P-wave morphologies to be considered a MAT. These tachycardias are similar to other primary atrial tachycardias in that there may be variable conduction from the atrium to the ventricle. MAT is occasionally confused with AF in that multiple P-wave morphologies and variable P-R and R-R intervals are present. The most common presentation occurs in the newborn, with a large percentage resolving spontaneously over time.⁴⁸

Junctional Ectopic Tachycardia

Junctional ectopic tachycardia (JET) is an automatic tachycardia that arises from the AV junction. Two distinct types of JET are seen in childhood. The first is a familial form that occurs in early infancy and may be associated with congenital heart defects in up to 50% of patients.⁵⁷ The second type is seen in the early postoperative period following repair of congenital heart disease (see Chapter 80).⁵⁸ In both forms, the tachycardia appears to be secondary to enhanced automaticity. In those patients who present with the familial type of JET, the heart rates will range from 180 to 240 beats/min. The ECG findings in JET show a tachycardia with a ventricular rate that is faster than the atrial rate, with a narrow QRS complex similar to that seen in the patient’s normal sinus rhythm. The ECG findings of JET are shown in Figure 75-2. Rarely, patients with JET develop rate-related aberrancy, and some postoperative patients have a pre-existing bundle branch block that will lead to a wide-complex tachycardia. If the QRS is wide, the diagnosis of VT must be considered and ruled out by comparison with the QRS in normal sinus rhythm or by pacing the atrium faster than the ventricular rate to demonstrate conduction through the His-Purkinje system with persistent wide-complex QRS morphology.

FIGURE 75-2 Electrocardiogram of junctional ectopic tachycardia. Note narrow-complex tachycardia with atrioventricular dissociation. The ventricular rate is 240 beats/min, and the atrial rate is 120 to 140 beats/min.

JET tends to be faster and more incessant when it manifests before 6 months of age.⁵⁹ Patients with JET can present with signs and symptoms of congestive heart failure secondary to the persistently elevated heart rate. Sudden death has been reported in this patient population.^59,60

Treatment

Treatment strategies for the familial form of JET include digoxin to slow the rhythm and provide inotropic support. Digoxin alone may not be sufficient to manage this arrhythmia, and the addition of a class IA, class IC, class II, or class III antiarrhythmic agent may be required. It has been the chapter authors’ experience that a combination of these medications is required to control the tachycardia. Class III agents such as IV amiodarone are quickly added to treat individuals who have rapid rates and poor ventricular function, while carefully monitoring blood pressure and cardiac output. The amount of antiarrhythmic medication needed to control the rate to prevent decompensation of cardiac function may suppress the sinus node considerably, necessitating a pacemaker. Amiodarone appears to be the most effective agent in the largest group reported with a 60% success rate. Patients with JET have been treated with catheter ablation with a 80% to 85% success rate but with a risk of AV block, which has led to the more recent recommendations of the use of cryothermal energy (see Chapter 78).^61,62 Because of the proximity of the AV node and His bundle to the area of enhanced automaticity that is responsible for JET, catheter ablation in this setting carries a relatively high risk of causing complete heart block. It is possible that, over time, the junctional rate will slow to a point that the patient could be weaned from the chronic medications. Refer to Chapter 80 for additional information on postoperative JET.

Evaluation

An echocardiogram should be obtained to look at associated factors and conditions, including structural abnormalities such as hypertrophic cardiomyopathy and abnormalities in cardiac function that might accompany myocarditis or dilated cardiomyopathy. Rarely, cardiac tumors such as rhabdomyomas are identified, or noncompaction of the ventricular myocardium is seen. The evaluation should include a standard ECG on which the QTc (corrected Q-T interval) is carefully measured manually. A 24-hour Holter monitor will determine the amount and complexity of the ectopy. In the presence of a normal heart, less than 20% ectopy usually does not interfere with cardiac function and can be monitored. More than 30% ectopy may result in ventricular dysfunction over time. Patients with this degree of ectopy may have underlying cardiac disease that was not initially diagnosed. Magnetic resonance imaging (MRI) may be indicated if arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC) is suspected. In patients with more frequent PVCs, an exercise stress test should be performed. Generally, suppression of PVCs during exercise is a positive finding, whereas an increase in ventricular arrhythmia with exercise is not. Prolongation of the corrected Q-T interval, especially in the recovery phase, may be seen in patients with long QT syndrome (LQTS). An EPS would rarely be indicated, unless symptoms suggest more complex arrhythmias or the PVCs are associated with conditions that might predispose the patient to VT or VF.

Prognosis

Long-term follow-up suggests that PVCs and VT disappear over time in 37% to 65% of patients with normal hearts.⁶⁴ Sudden death is rare in children with PVCs with otherwise normal health, but a few cases have been reported in the literature. In children with abnormal hearts, PVCs may be precursors of more serious arrhythmias, especially if they are complex-multiform, coupled, or associated with VT.

Clinical Signs and Symptoms

Children with PVCs are frequently asymptomatic and unaware of their arrhythmias, especially if they are younger than 5 years. Older children may complain of a skipped or hard beat or a fluttering in their chest. Some children perceive PVCs as painful.

Treatment

PVCs do not need intervention unless they are frequent enough to interfere with cardiac output, are closely coupled, or frequently fall on the T wave in patients judged to be vulnerable to such occurrences (e.g., those with LQTS). PVCs associated with heart disease such as myocarditis, cardiomyopathy, or congenital heart disease may require further investigation and treatment, especially if they are frequent or occur in runs resulting in hemodynamic instability. Treatment for PVCs is discussed along with treatment for VT below.

Electrocardiographic Manifestations

The electrocardiographic diagnosis of VT is made most easily in the presence of a wide QRS tachycardia with AV dissociation. Many children have ventriculoatrial (VA) conduction with relatively rapid 1 : 1 retrograde VA conduction, and AV dissociation may not occur. VT must be differentiated from all other forms of wide QRS tachycardias, including SVT with bundle branch aberrancy; antidromic SVT using an accessory AV connection; SVT using a nodoventricular, nodofascicular, or atriofascicular connection; or atrial flutter with aberrant conduction. Although other mechanisms of wide QRS tachycardia have been described, until proven otherwise, a wide QRS tachycardia in a child must be considered to be VT. It must be remembered that the normal QRS duration in infants and young children is 40 to 80 ms, so a wide QRS in an infant might only be slightly longer than 80 ms. The rates of VT in pediatrics vary from 120 to 300 beats/min. The T-wave vector is divergent from the QRS vector, but opposite polarity will not occur in every lead. Left bundle branch block (LBBB) is the most common morphology, but right bundle branch block (RBBB) or alternating RBBB and LBBB may occur. The presence of PVCs during sinus rhythm with the same configuration as VT is a suggestive sign of the ventricular origin of the arrhythmia. AV dissociation is suggestive of VT, but 1 : 1 VA conduction is common, especially in young children. Fusion beats are commonly noted at the onset or termination of the VT, which may be sustained (>30 consecutive complexes) or nonsustained (3 to 30 consecutive complexes).

Further differentiation is made according to the morphology, with VT being described as monomorphic or polymorphic. Two types of polymorphic VT have been described, torsades de pointes and bi-directional VT (Figure 75-3). Torsades de pointes is associated with LQTS and so named because of its twisting, undulating nature. Bi-directional VT, with beat-to-beat variation in the QRS axis on ECG, has been associated with digoxin toxicity, familial hyperkalemic paralysis, or CPVT.⁶⁶

FIGURE 75-3 Electrocardiogram of bi-directional ventricular tachycardia. Note the two distinctly different wide QRS morphologies.

Etiology of Ventricular Tachycardia

Causes of acute VT not associated with congenital heart defects are shown in Box 75-1. These most common causes include metabolic and electrolyte abnormalities; infectious processes such as myocarditis, which may cause LV microaneurysms⁶⁷; human immunodeficiency virus (HIV) infections⁶⁸; blunt cardiac trauma, including commotio cordis^69–71; coronary ischemia, especially in association with Kawasaki disease; and drugs such as caffeine, inhalation anesthetics, and recreational drugs, including amphetamines and cocaine.

The causes of chronic or recurrent VT include congenital heart disease, both preoperatively and postoperatively; acquired heart diseases; metabolic disorders, including disorders of fatty acid metabolism⁷²; neuromuscular disorders such as Duchenne’s muscular dystrophy; cardiomyopathies, including ARVC; hypertrophic cardiomyopathy (HCM); tumors and infiltrates; left ventricular noncompaction (LVNC), VT originating in both the right ventricle and the left ventricle, associated with structurally normal hearts; VT associated with LQTS; and other primary electrophysiological abnormalities such as Brugada syndrome and CPVT. A detailed list is provided in Table 75-3. While failure to identify a specific cause for VT in children is not unusual, persistence in evaluations often identifies the pathology in patients initially thought to have an unidentifiable etiology.^73–77

Table 75-3 Etiology of Chronic Ventricular Tachycardia

AI, Aortic insufficiency; AS, aortic stenosis; AV, atrioventricular; CHD, congenital heart disease; DORV, double outlet left ventricle; D-TGA, D-transposition of the great arteries; IHSS, idiopathic hypertrophic subaortic stenosis; LQTS, long QT syndrome; LV, left ventricular; PV, pulmonary valve; RV, right ventricular; SLE, systemic lupus erythematosus; VT, ventricular tachycardia.

Mechanism of Ventricular Tachycardia

VT has been reported to result from re-entry, triggered automaticity, and abnormal automaticity. An EPS is helpful in differentiating these mechanisms. The mechanisms of VT in children include re-entry in 60% and abnormal automaticity in 40%.⁷⁸ Re-entry is most often the mechanism in patients with congenital heart disease after surgical repairs, related to re-entry circuits that develop around suture lines and ventriculotomy scars. Triggered automaticity is thought to be the mechanism in CPVT.

Clinical Correlations

Presentation of patients with VT varies and depends, to a large extent, on the underlying etiology and clinical status with regard to myocardial function and structure. In one study of patients with VT with structurally normal hearts, presentation was most common in infancy (48%), with 58% being younger than 6 months.⁷³ Associated findings were heart failure in 30%, hemodynamic compromise or collapse in 23%, and in utero diagnosis in 18%. Diagnosis was incidental in 30%. No specific etiology was found in 50%, with cardiomyopathy or myocarditis (20%) being the most common etiology identified.

Clinical Signs and Symptoms

The type and degree of symptoms appear to be rate related, with symptoms being most common in patients with rates greater than 150 beats/min. Except for those patients with underlying cardiac disease or ventricular dysfunction, patients with VT have symptoms similar to those of SVT, with the severity of symptoms relating more to the rate than to the mechanism of tachycardia. Symptoms include dyspnea, shortness of breath, chest or abdominal pain, palpitations, dizziness, syncope, and cardiac arrest or sudden death. Older children may exhibit exercise intolerance or easy fatigability. Infants may feed poorly and be irritable or lethargic. Patients with VT and heart disease usually have symptoms, whereas only one third with normal hearts and VT have symptoms. The type of symptom relates to both the tachycardia rate and the underlying state of the myocardium. Sudden death occurs most commonly in the presence of an abnormal heart but has also been reported in patients with normal hearts.^74,79,80 Children younger than 5 years or those in incessant tachycardia may not have a perception of a fast heart rate or be able to accurately express what they are feeling. Signs include palpitations, sensation of a rapid heart rate, tachypnea, or hypotension with accompanying pallor and diaphoresis as well as signs of congestive heart failure. Although VT usually has a sudden onset, it may occur during exercise and be difficult to perceive. It may gradually “warm up” or increase in rate.

Specific Associated Conditions

Long QT Syndrome

Congenital LQTS is an inherited condition characterized by syncope, seizures, and sudden death, associated in most individuals with a prolongation of the Q-T interval on the ECG.¹⁰² An example of the ECG in LQTS is shown in Figure 75-4. In addition to the prolongation of the QTc, these patients often have bizarre or notched T-wave morphology with prominent U waves or T-wave alternans. They develop life-threatening VT, known as torsades de pointes, or VF. This syndrome includes Jervell and Lange-Nielson syndrome (JLNS), described in 1957, associated with congenital deafness caused by an autosomal recessive inheritance and Romano-Ward syndrome, described in 1963 and 1964, demonstrating autosomal dominant inheritance, without hearing deficit.^103–105

FIGURE 75-4 Electrocardiogram of patient with long QT syndrome. The QTc measures 490 ms. Note the long, notched T waves.

The prevalence of the disorder is estimated to be 1 : 2000 to 1 : 10,000.^106–108 This condition has variable expression and penetrance within families, leading to a spectrum of severely affected members with repeated cardiac events and arrest to those with the identical mutation but who are totally asymptomatic.¹⁰⁹

In 1993, statistics from a group of 287 children were compiled from a number of medical centers.¹¹⁰ The initial presentation was cardiac arrest (9%), syncope (26%), seizures (10%), presyncope, or palpitations (6%). Sixty-seven percent with symptoms had exercise-related symptoms. Thirty-nine percent were identified because of family history or the identification of other family members with the syndrome. Thirty-nine percent of the patients were asymptomatic at presentation; of these, 4% experienced sudden death compared with 8% overall. The strongest predictors of sudden death were QTc longer than 0.60 seconds and noncompliance with taking recommended medication.

Bradycardia is commonly seen in these patients, and some may develop or present with second-degree AV block. This is more common in neonates who may have second-degree or third-degree AV block, but it may be seen in older children, especially during exercise.¹¹¹

One series reported sudden death occurring in 73% without treatment, and others have reported sudden death in 21% of symptomatic patients in the first year after presenting with syncope.^112,113

Diagnosis and Evaluation

The diagnosis of this syndrome is made from a variety of criteria. Schwartz et al provided some criteria and suggested a scale for identifying these patients by categorizing them into high-risk, intermediate-risk, or low-risk groups.¹¹⁴ All criteria involve measurement of the Q-T interval and taking a careful history of syncope, seizures, and arrhythmias in the patients and their families. Commonly, a complete history may reveal syncope or seizures associated with exercise or emotional stress or a family history of sudden death in young relatives, including unexplained automobile accidents or drowning.

Additional studies such as 24-hour monitoring and exercise stress testing may provide helpful information in the form of significantly prolonged Q-T intervals, especially during recovery from exercise, or the occurrence of polymorphic ventricular arrhythmias during or after exercise. The use of provocative tests such as isoproterenol or epinephrine infusions can identify some subclinical cases, especially LQT1 with a prolongation or paradoxical response in these individuals.^115,116

A high level of suspicion is needed to diagnose LQTS. Any patient who presents with syncope during or immediately after exercise or with VT, especially of the polymorphic or torsades de pointes type, or in association with physical or emotional stress should have an ECG, with determination of QTc intervals. Evaluation of a resting ECG may not be sufficient as more than 10% to 15% of gene carriers may have normal ECGs.¹¹⁷ A more worrisome study by Priori showed an even lower penetrance of the gene in Italian families, with probands initially thought to have sporadic occurrences of LQTS. Genetic studies revealed multiple family members who were genetic carriers but with normal ECGs.¹¹⁸ ECGs should be obtained in all family members of identified individuals with LQTS. When genetic mutations are identified, this can be used to screen family members more specifically.

Since a variety of drugs can prolong the QTc, a careful history of medication use should be obtained during the evaluation. Those with prolonged QTc with medication often are subclinical LQT cases or have single nucleotide polymorphisms for LQTS genes.

Clinical Implications of Molecular Genetics

The molecular genetic understanding of LQTS began in 1991.¹¹⁹ Many other genetic discoveries have resulted in the current and rapidly expanding knowledge of the etiology of LQTS.^120–125 Genetic studies have identified repeated mutations in at least 12 genes that encode for proteins that modulate the ion channels, primarily potassium or sodium channels, causing LQTS by altering cardiac repolarization and increasing the risk for ventricular arrhythmias. Mutations in a few other genes have been found in only isolated individuals. It is estimated that 25% of the gene mutations have not yet been identified. The mutations are primarily in the coding regions of the genes. These mutations result in a decrease in repolarizing potassium currents prolonging repolarization or in late entry of sodium or calcium into the cardiac cell prolonging depolarization or repolarization. Both of these mechanisms prolong the Q-T interval. These genes that involve sodium and potassium currents include KCNQ1(I_Ks), KCNH2(I_Kr), minK, and MiRP1, and SCN5A

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