SUPRAVENTRICULAR ARRHYTHMIAS

Single premature atrial contractions (PACs) are not uncommon in infants and children. Rarely is interven-tion required. Closely coupled PACs may block in the AV node. If a neonate has frequent blocked PACs, then bradycardia may result. Frequent blocked PACs are one of the most common causes of bradycardia in the new-born. If the infant is otherwise well and without hemo-dynamic compromise, therapy is often not needed.

Sinus arrhythmia is not an abnormal rhythm but deserves mention as it is a common finding in pediatrics.

Sinus arrhythmia refers to the normal reflex-derived changes in heart rate in response to the respiratory cycle.

During inspiration, the heart rate accelerates, and, during expiration, the heart rate decelerates. Sinus arrhythmia is typically more pronounced in school-aged children and adolescents. It should be considered in the differen-tial diagnosis of an otherwise well child with an irregular heart rhythm.

Reentrant Supraventricular Tachycardia SVT is the most common tachycardia seen in the pediatric population, with a reported incidence of 1 in 250–1000.² Figure 10-3 ECG showing supraventricular tachycardia in an infant.

Mechanisms of SVT include reentry and enhanced auto-maticity. The most common mechanism of childhood SVT is reentry.³

Presentation

Reentrant SVT may present at any age in infancy and childhood and even in the fetus. Accessory pathway-mediated tachycardias are more common in infants and young children, whereas AVNRT is more common in ado-lescence. Clinical presentation is often dependent on age. Fetal tachycardia may be detected by auscultation of the fetal heart rate or by fetal echocardiography.

Prolonged fetal tachycardia is one cause of nonimmune fetal hydrops. Infants frequently will have symptoms of irritability and poor feeding. If the SVT is longstanding (i.e., over 24–48 hours), congestive heart failure may develop. Manifestations of heart failure in the infant include tachypnea, diaphoresis, pallor, and lethargy.

Older children will complain of palpitations or

flutter-ing in their chest, chest pain, abdominal pain, and dizzi-ness. Syncope may occur if the tachycardia rate is suffi-ciently fast to impede diastolic filling, resulting in hypotension and cerebral hypoperfusion. Children old enough to describe their symptoms may relate a sudden onset or termination of the tachycardia as well as an abil-ity to self-terminate the arrhythmia with a Valsalva maneuver or other vagal maneuvers such as standing on their head.

Types of Reentrant Supraventricular Tachycardia Wolff–Parkinson–White Syndrome

WPW anomaly is diagnosed when an ECG in sinus rhythm has a short P-R interval and a delta wave, which represents ventricular pre-excitation (Fig. 10-6).

Ventricular pre-excitation is defined as activation of ventricular muscle earlier than would be expected for normal conduction through the AV node and down the His-Purkinje system. WPW syndrome requires an Figure 10-4 ECG showing idiopathic ventricular tachycardia in a 2-year-old.

Figure 10-5 ECG showing antidromic supraventricular tachycardia in a 10-year-old.

accessory pathway that supports both antegrade and retrograde conduction. The SVT seen in WPW may be either orthodromic tachycardia (down the AV node) or antidromic tachycardia (down the accessory pathway) with the significant majority being orthodromic. Patients with WPW have a high risk of having atrial flutter or atrial fibrillation. If the accessory pathway supports rapid con-duction of the atrial arrhythmia, sudden death may result.

In patients with WPW, the risk of sudden death is 1%

every 10 years.⁴

The incidence of WPW in childhood is approximately 1 in 1000.⁵Congenital heart disease occurs in approxi-mately 20% of patients with WPW, with Ebstein’s anomaly and L-transposition of the great arteries being the most common. Although the SVT seen in WPW frequently presents in early infancy, presentation may occur at any age. Of those who present in infancy,only about one third continue to experience episodes of SVT after 1 year of age.⁶Conversely, later age of SVT presentation indicates a greater chance for recurrence. A subgroup of patients with WPW is asymptomatic, and their diagnoses are made incidentally. This group of patients provides a clinical dilemma, because their risk of sudden death is unknown.

Some studies suggest that assessment of the conduction properties of the accessory pathway may be the best means for risk-stratifying this group of patients. This has not been clearly defined in the pediatric population.

Concealed Bypass-Tract Tachycardia

Concealed bypass tracts only support retrograde conduc-tion and orthodromic tachycardia. The orthodromic tachycardias found with WPW and concealed bypass

tracts are virtually indistinguishable during the tachycar-dia. The only way to differentiate between these arrhyth-mias is by ECG when the patient is no longer having SVT.

In normal sinus rhythm, the ECG of a patient with a con-cealed bypass tract appears normal, with a normal PR interval and no evidence of pre-excitation. Although the differences between these two entities may seem subtle, they in fact have significant ramifications when choosing between therapeutic options. Like WPW, patients with concealed bypass tracts can present with SVT at any age from infancy through adolescence and into adulthood.

Caution should be advised in definitively ruling out WPW by the resting ECG in the infant because some pathways that can conduct antegrade may be subtle or latent and difficult to see on the initial ECG. As the child grows and the heart rate slows, pre-excitation may become manifest on the resting ECG.

An unusual type of SVT involving a concealed bypass tract is the permanent junctional reciprocating tachy-cardia ( PJRT ). PJRT has a concealed bypass tract that has a slow retrograde conduction. Therefore, the RP interval during tachycardia is quite long. The descrip-tor “permanent” is used because the arrhythmia is often quite difficult to convert, medically or electrically, to sinus rhythm. Many of these bypass tracts insert into the atria at the posterior septum, which is near the AV node, also called the AV junction. More recently, elec-trophysiology studies have suggested that these tracts may insert anywhere along either the left or the right AV groove, although the posterior septum remains a common location.

Figure 10-6 ECG of infant from Figure 10-3 during sinus rhythm. The infant has Wolff–Parkinson–White syndrome with a short PR interval and a delta wave.

Atrioventricular Nodal Reentrant Tachycardia The pathways used as substrate for the reentrant circuit in AVNRT are electrical approaches to the AV node located in the atrium. Two such pathways have been described. The

“slow” pathway has slower conduction and longer refrac-toriness and is anatomically located in the mid- to pos-teroseptal region of the AV groove. The “fast” pathway has faster conduction and shorter refractory periods and is located in the anteroseptal aspect of the AV groove.

Typical AVNRT involves conduction antegrade down the slow pathway and retrograde up the fast pathway. Atypical AVNRT involves the opposite sequence. Interestingly, in some patients with AVNRT, dual AV node pathways are not manifest during electrophysiology testing. In addition, many individuals who do not have AVNRT may have dual AV node physiology on electrophysiology testing.

The incidence of AVNRT increases with age. In fact, AVNRT represents the most common type of reentrant SVT in adults. In the pediatric population, it is most com-monly seen during adolescence. A possible explanation for this is that the AV node undergoes electrophysiologic alterations with age. In one study, dual AV node pathways were found in 15% of children younger than 13 years of age and in 44% of children older than 13 years of age.⁷

Atrial Flutter

Atrial flutter is observed in two distinct groups in the pediatric population: newborns and patients with con-genital heart disease, especially following atrial surgery for congenital heart defects.

The reentrant circuit of atrial flutter of infancy is gen-erally confined to the right atrium. Similar to typical adult atrial flutter, it may utilize the isthmus between the tri-cuspid valve and the inferior vena cava as the area of slow conduction. Characteristic flutter waves may be present in leads II, III, and aVF (Fig. 10-7). Atrial rates may reach over 400 beats per minute (bpm). There is frequently 2:1 or greater AV block, which results in a ven-tricular response rate of ≥ 200 beats per minute.

Atrial flutter of infancy is rare. It may present in utero or in the newborn period. Congenital heart defects may coexist with the arrhythmia, so echocardiography is rec-ommended as part of the workup. Typical associated defects include atrial septal defect, aneurysm of atrial septum, and Ebstein’s anomaly. Conversion to normal sinus rhythm is often spontaneous. If it persists, the arrhythmia is generally quite responsive to therapy. The prognosis for atrial flutter in infancy with a structurally normal heart is promising, with recurrences infrequent after the initial conversion.

Atrial flutter is also seen in patients who have under-gone surgical correction of congenital heart disease.

Atrial flutter seen in this setting is usually referred to as intraatrial reentrant tachycardia (IART) or as incisional atrial tachycardias to highlight its differences from typical atrial flutter. The surgical procedures that predispose patients to IART generally involve surgery in the atria, the most common being repair of an atrial septal defect, atrial baffling procedures (i.e., the Senning or the Mustard

Figure 10-7 ECG showing atrial flutter with variable 1:1, 2:1, and 3:1 conduction in a newborn.

Note the typical flutter waves in leads II, III, and aVF.

operation) for D-transposition of the great arteries, and Fontan procedures for single ventricle physiology (Fig. 10-8). Older age at surgery is an important risk factor for the development of atrial arrhythmias following a repair of an atrial septal defect.⁸ The reentrant circuit of IART may occur anywhere within the atria and frequently incorporates an anatomic barrier or a surgical scar. During electrophysiology testing, more than one reentrant circuit may be defined. On surface EGGs, the typical saw-tooth flutter waves are absent. In fact, noninvasive monitoring may reveal multiple flutter wave morphologies, indicative of multiple reentrant circuits.

The incidence of IART following congenital heart sur-gery increases with age. Possible predisposing factors include surgical scars within the atrium, elevated atrial pressure, abnormal atrial anatomy associated with the pri-mary lesion, and sinus node dysfunction.⁹Atrial rates in IART range considerably, from 150 to 450 bpm with variable conduction to the ventricle. Symptomatology is related to the ventricular response rate and myo-cardial function. A fast ventricular response may result in palpitations, syncope, or sudden death. A slower ventricular response rate may result in fatigue or exercise intolerance, especially in the Fontan patient in whom maintenance of AV synchrony may be crucial for adequate cardiac output. If the ventricular res-ponse rate is slow enough, patients may not even per-ceive the arrhythmia. Long-standing IART in a patient after a Fontan procedure may result in sluggish blood flow in the Fontan baffle and the potential for clot formation.

Treatment of Reentrant Supraventricular Tachycardia

Treatment of reentrant SVT can be divided into acute and chronic therapy. Acute therapy seeks to interrupt the reentrant circuit and to restore sinus rhythm immedi-ately. The selection of the type of acute therapy depends upon the clinical situation. A patient who presents in shock as a result of SVT should be treated differently from someone who has SVT but is in stable condition. In the emergent setting, the ABCs of resuscitation must be followed. (Once the Airway is secured and Breathing is assured, attention can be paid to Circulation.) Chronic therapy attempts to prevent recurrence of SVT and depends significantly upon the type of SVT.

AVRT and AVNRT

For AVRT and AVNRT, adenosine is a rapid pharmacologic means of interrupting the reentrant circuit at the AV node.

Adenosine requires that intravenous access is swiftly achieved. Its effectiveness needs a sufficient bolus to reach the heart quickly because it has a very short half-life.

If IV access cannot readily be obtained, if the patient is in extremis, or both, then electrical direct current car-dioversion is the preferred next option. Carcar-dioversion should be performed with an energy output of 0.5–1 J/kg.

The output can be doubled to a maximum of 5–6 J/kg until the treatment is effective.

In the acute but stable patient with AVRT or AVNRT, the first-line therapy is adenosine. Digoxin is effective and especially useful in the patient with decreased myocardial function, but it may require several hours for conversion.

More rapid digitalization can be performed with careful

Figure 10-8 ECG showing intra-atrial reentrant tachycardia with variable conduction in a 4-year-old with hypoplastic left heart syndrome status post Fontan operation.

intravenous administration of digoxin. Other pharmaco-logic therapies used in the acute setting include intra-venous β-blockers such as esmolol, intravenous procainamide, and intravenous amiodarone. These med-ications should be used with caution as they all have neg-ative inotropic effects. Other therapeutic modalities include transesophageal pacing and vagal maneuvers.

Vagal maneuvers for adolescents and older children include the Valsalva maneuver and the headstand. In infants, a bag of ice to the center of the face elicits the div-ing reflex and often is successful in terminatdiv-ing the SVT.

Although IV calcium-channel blockers are an important therapy for SVT in adults, they are contraindicated in chil-dren, especially in those under 1–3 years of age. There have been reports of hemodynamic decompensation and sudden death in infants who were given verapamil.¹⁰

Chronic therapy for AVRT and AVNRT is similar. Digoxin and β-blockers are first-line oral therapy agents. Digoxin and calcium-channel blockers are contraindicated in patients with WPW. The reason is that these medicines slow AV nodal conduction and can enhance antegrade conduction down the accessory pathway, allowing for a more rapid ventricular response during atrial flutter or fib-rillation. For AVRT or AVNRT refractory to first-line ther-apy, other agents such as flecainide, procainamide, sotalol, amiodarone, and verapamil can be employed. Generally, during initiation of anti-arrhythmics, children are admitted to the hospital for at least five half-lives of the medicine, which is the amount of time required to achieve steady-state pharmacologic levels. During this time, the families are educated about the signs and symptoms of SVT, and they learn how appropriately to dose the medication.

Young children must be monitored for hypoglycemia and hypotension when β-blockers are started.

An increasingly popular therapeutic modality for recurrent SVT is catheter ablation. Radiofrequency abla-tion involves applicaabla-tion of radiofrequency energy by a steerable electrode catheter to the arrhythmia substrate or location (e.g., an accessory pathway or an AV nodal slow pathway). Radiofrequency energy causes tissue heating and necrosis. Radiofrequency catheter ablation has been used successfully to cure various types of arrhythmias in pediatric patients. Success rates for AVRT have been reported between 86% and 97%, depending on the location of the accessory pathway, and at greater than 95% for AVNRT. Complication rates are low, at 3–4%.

The most common serious complications are AV block, catheter perforation, pericardial effusion, and thrombi or emboli.¹¹Common indications for radiofrequency abla-tion of AVRT and AVNRT include life-threatening arrhyth-mias, medically resistant tachycardias, adverse drug reactions, tachycardia-induced cardiomyopathy, impend-ing cardiac surgery, and patient choice. Recently, cryoen-ergy has also been used to ablate arrhythmia substrates, especially when they are located close to the AV node.

Atrial Flutter

Atrial flutter in the newborn is generally converted to sinus rhythm with medications, transesophageal pacing, or car-dioversion. Medications include intravenous preparations of digoxin, procainamide, amiodarone, and sotalol.

Procainamide should be used in conjunction with digoxin, which will increase the degree of AV block as procainamide may slow the flutter rate, resulting in more rapid AV duction. Once atrial flutter in the newborn period is con-verted to sinus rhythm, recurrence is uncommon.

Treatment of IART in the postoperative heart can be divided into acute and chronic therapy. Prior to any attempts at converting IART to sinus rhythm, the presence of an atrial thrombus must be ruled out. This usually requires transesophageal echocardiography, especially in older children and adolescents, as transtho-racic windows may not be adequate to visualize an intra-atrial thrombus. Acute therapy includes medications, transesophageal pacing, and cardioversion. Medications that have been proven successful in this setting include procainamide, propafenone, amiodarone, and sotalol. An agent to slow AV nodal conduction, such as digoxin, should be used in conjunction with Class IA antiarrhyth-mics such as procainamide.

Chronic therapy for IART is often quite difficult, necessitating multiple medications and modalities.

Digoxin is usually the first-line therapy. Because it may have little direct effect on the arrhythmia substrate, it often must be supplemented with a second agent.

Second-line medications include procainamide, disopyra-mide, flecainide, propafenone, amiodarone, and sotalol.

Other modalities include overdrive pacing techniques and radiofrequency ablation. Antibradycardia pacing has proven useful in those patients who have a significant bradycardia component to their disease (i.e., bradycardia-tachycardia syndrome). Antibradycardia-tachycardia pacing devices have recently been reintroduced into the market and have shown some modest benefit.¹² Radiofrequency ablation has been used to interrupt the reentrant circuits of IART. Due to various complicating factors—multi-ple reentrant circuits, comfactors—multi-plex atrial anatomy, or thick atrial muscle—success rates have been in the range of 70–80%, with recurrence rates as high as 50%.⁹ New technologies, such as improved mapping systems and cooled catheter tips, may help to improve success rates and to decrease postablation recurrence.

Automatic Supraventricular Tachycardia Presentation

Automatic SVT results from a focus or foci of cells that have increased automaticity. Atrial automatic tachycar-dias tend to present in children less than 6 years of age;

they are not uncommon in older children and adoles-cents. Automatic tachycardias arising from the region of

the AV node, or junctional ectopic tachycardia, present in two distinct settings. The first is during infancy and childhood in a familial form and the second is in patients immediately following intracardiac surgery.

Automatic tachycardias initiate with a gradual increase in heart rate (the warm-up phase) until they reach their maximum heart rate. Termination involves a gradual decrease in heart rate (the cool-down phase). The heart rate is inappropriately high for the patient’s activity level.

The rate of the tachycardia is catecholamine-sensitive.

During sleep or under the influence of sedation, the rate will be slower or the tachycardia may even be sup-pressed by the normal sinus node activity. During times of stress, the tachycardia rate is faster. These tachycar-dias tend to be chronic and incessant. Because of the warm-up phase and their incessant nature, these arrhyth-mias are often not perceived by the patient. Persistence of the tachycardia may eventually lead to myocardial dys-function. Presentation for many patients includes signs and symptoms of congestive heart failure.

Types of Automatic Supraventricular Tachycardia Ectopic Atrial Tachycardia

EAT, or AAT, represents between 10% and 20% of the SVT seen in the pediatric population. Ectopic atrial tachycar-dia arises from a single focus of increased automaticity

EAT, or AAT, represents between 10% and 20% of the SVT seen in the pediatric population. Ectopic atrial tachycar-dia arises from a single focus of increased automaticity