Because of an abnormal delay in conduction through the AV node, first-degree AV block results in prolongation of the PR interval above the upper limits of normal for age and heart rate (Figure 45-2). It is important to note that bradycardia does not occur because of first-degree AV block alone. This type of block can appear in otherwise healthy children as a benign phenomenon, usually related to increased vagal tone. Other causes may include cardiac surgery, rheumatic fever, Lyme disease, digoxin toxicity, and electrolyte imbalance. Isolated first-degree AV block does not require treatment unless there is progression to more advanced AV block.

Figure 45-2 Atrioventricular conduction variations.

Second-Degree Atrioventricular Block

Intermittent failure of AV conduction results in two forms of second-degree AV block: Mobitz type I and Mobitz type II. Each type of second-degree AV block occurs at a different level within the conduction system, thus yielding different surface ECG morphologies. Pathologic causes include myocarditis, cardiomyopathy, myocardial infarction, digitalis toxicity, congenital heart defects, and cardiac surgery.

Mobitz Type I (Wenckebach phenomenon) occurs at the level of the AV node, yielding progressive lengthening in the PR interval until one QRS complex or ventricular depolarization fails to occur (see Figure 45-2). This may occur in otherwise healthy children with parasympathetic dominance as well as in trained athletes and is frequently observed during monitoring of heart rates during sleep. After confirmation via surface ECG, management should focus on treating any underlying pathologic causes.

Mobitz type II occurs at the level of the bundle of His and is defined as the intermittent loss of AV conduction without preceding lengthening of the PR interval. This form of second-degree AV block is less common and suggests a more serious form of AV conduction disorder in which progression to complete heart block with hemodynamic compromise is more likely. Although similar causes exist between Mobitz type I and type II, consultation with a pediatric cardiologist is indicated (see Figure 45-2).

Tachyarrhythmias

Supraventricular Tachycardias

Supraventricular tachycardias (SVT) involve conduction system tissue both above and within the AV node and is the most common tachycardia seen in the pediatric population with published incidence of 1 in 250 to 1000 live births. There are three forms of SVT: reentry tachycardias with an accessory pathway, reentry tachycardias without an accessory pathway (AV nodal and intraatrial), and automatic tachycardias (ectopic).

Reentry Tachycardia with an Accessory Pathway

Wolff-Parkinson-White (WPW) syndrome accounts for 10% to 20% of cases of SVT (Figure 45-3). It is diagnosed by the presence of preexcitation (early depolarization via the accessory pathway) on a surface ECG. This is manifested by a short PR, a widened QRS, and a delta wave. WPW may present at any age and can be associated with Ebstein’s anomaly, heterotaxy syndrome, and L-transposition of the great arteries. Sudden death may occur because of rapid ventricular responses with 1 : 1 conduction in the setting of more rapid atrial tachyarrhythmias such as atrial fibrillation. The risk of sudden death in asymptomatic patients incidentally discovered to have WPW is yet unknown, thus making risk stratification a challenge.

Figure 45-3 Accessory pathways and the Wolff-Parkinson-White syndrome.

During SVT, impulses may be carried both in an antegrade (orthodromic conduction) or retrograde fashion (antidromic conduction) through the AV node, with an accessory pathway completing the circuit. Orthodromic conduction results in a normal QRS duration with retrograde, inverted P waves after each QRS complex. Antidromic conduction results in a wide QRS duration and is also followed by inverted P waves, although R-P duration is variable.

The SVT associated with WPW occurs with abrupt onset and cessation (paroxysmal) and with a rapid and regular heart rate (usually >200-240 beats/min). Clinical symptoms in young infants and toddlers are often difficult to detect because many children tolerate episodes of SVT very well. If SVT persists or rates are exceptionally elevated, symptoms consistent with CHF may develop (irritability, tachypnea, poor feeding, pallor, fever, and hepatomegaly). Pharmacologic and electrical management strategies are similar to those without an accessory pathway and are discussed below.

Reentry Tachycardia without an Accessory Pathway

AV nodal reentrant tachycardia (AVNRT) occurs when dual conduction pathways exist within the AV node creating substrate for reentry pathophysiology similar to WPW and other accessory pathway SVT (Figure 45-4). Seen more commonly in older children and adolescents, rates can range from 140 to 200 beats/min.

Figure 45-4 Atrioventricular nodal reentry tachycardia.

Acute management of hemodynamically unstable patients with reentry tachycardia should seek to restore sinus rhythm either pharmacologically or electrically with synchronized cardioversion. In non-urgent situations, vagal stimulation by placement of an iced water bag over the face may abort paroxysmal episodes of SVT. Other vagal maneuvers include the Valsalva maneuver, breath-holding, blowing balloons, and even standing upside down. Pharmacologic treatment involves rapid infusion of adenosine, which interrupts reentry circuits by blocking AV conduction. Maintenance medications that can be used include digoxin (contraindicated in patients with WPW), β-blockers, amiodarone, and (less commonly) verapamil. These second-line agents have negative inotropic effects and should be used with caution and only under the supervision of a cardiologist.

Definitive treatment for SVT involves radiofrequency ablation of the accessory pathways, bypass tracts, or ectopic foci in the heart. Success rates range from 86% to 96% depending on the location of the accessory pathway and type of SVT.

Intraatrial reentrant tachycardia (atrial flutter or fibrillation) is caused by reentrant or “circus” movements within the atria. Atrial flutter usually exhibits regular and regularly irregular intervals with atrial rates greater than 250 beats/min. Variable ventricular responses can be observed because of varying degrees of AV block. Risk factors for atrial flutter include structural heart disease with dilated atria, atrial scarring from prior surgery for congenital heart disease (e.g., Fontan, Mustard, or Senning operations), myocarditis, hyperthyroidism, and digitalis toxicity. ECG reveals rapid and regular atrial sawtoothed flutter waves, which are diagnostic for atrial flutter (Figure 45-5).

Figure 45-5 Atrial flutter.

Evaluation for intraatrial thrombus is required before performing synchronized cardioversion. Radiofrequency ablation in the congenital heart disease population is much more challenging considering multiple reentrant circuits from postoperative scarring, complex atrial anatomy, and thicker atria leading to lower success rates and higher recurrence rates. Long-term anticoagulation with warfarin should be considered in patients with recalcitrant atrial flutter because of the higher incidence of thromboembolic disease.

Atrial fibrillation is much less common in the pediatric population and results from multiple chaotic atrial foci, resulting in an irregularly irregular ventricular response. Risk factors for atrial fibrillation are similar to those for atrial flutter. Acute management of atrial fibrillation in hemodynamically stable patients should focus on ventricular rate control and determination of underlying etiology. Normal sinus rhythm may also be restored with DC synchronized cardioversion after an appropriate evaluation for intracardiac thrombus. As in patients with chronic atrial flutter, long-term anticoagulation with warfarin should be considered for patients with chronic atrial fibrillation.

Automatic Tachycardias

Automaticity refers to the ability of conduction tissue to spontaneously depolarize. When present outside of the sinus and AV nodes, automaticity can lead to cells firing repetitively, thus resulting in tachycardia and suppression of dominant pacemaker impulses. Such tachycardias are often susceptible to catecholaminogenic stimulation and have usual “warm-up” and “cool-down” phases compared with paroxysmal forms of reentrant tachycardia. As a group, automatic tachycardias tend to be chronic and incessant with resulting myocardial depression and dysfunction. Therapies target two strategies: slowing the ventricular response rate and decreasing automaticity of the abnormal focus or foci. Three types of automatic tachycardias are observed in the pediatric population:

Ectopic atrial tachycardia accounts for approximately 10% to 20% of all SVT observed in the pediatric population. Heart rates are variable, ranging from 140 to 200 beats/min, with a surface ECG revealing identifiable P waves with an abnormal axis. Vagal maneuvers and adenosine serve as diagnostic tools revealing a gradual slowing in rate with subsequent acceleration after cessation of each maneuver. Spontaneous resolution may occur in some patients, but most patients require chronic therapy with multiple agents.

Multifocal or chaotic atrial tachycardia is a much rarer form of SVT and is characterized by multiple foci of increased automaticity in the atria, resulting in three or more distinct P-wave morphologies on surface ECG. Often confused with atrial fibrillation, this arrhythmia occurs most often in the newborn period without associated cardiac disease. Spontaneous resolution frequently occurs during the first year of life.

Junctional ectopic tachycardia (JET) occurs when a focus of automaticity is present in the region of the AV node. In JET, AV dissociation occurs with a ventricular rate greater than the atrial rate. Congenital, or familial JET often presents in the neonatal period with symptoms of CHF and in association with congenital heart disease. The majority of patients require medical therapy with multiple agents. Radiofrequency ablation has also been used to treat JET but is reserved for more resistant cases because of the high risk of inducing complete heart block from ablation points near the AV node. When occurring in the postoperative period, JET is usually transient and self-limited, lasting from 24 to 72 hours. Although brief in duration, postoperative JET may cause significant hemodynamic compromise and can be fatal if not controlled.

Ventricular Tachycardia

Ventricular tachycardia (VT) is defined as three or more PVCs in series at a heart rate greater than 120 beats/min. These may be self-limited or may present with sudden death (Figure 45-6). Etiologies of VT can be divided into acute and chronic causes. Acute causes include electrolyte imbalance, infections (myocarditis, pericarditis, rheumatic fever), use of toxins or drugs (cocaine, antiarrhythmics, general anesthetics, sympathomimetics, psychotropics), trauma, and myocardial ischemia (infarction, anomalous coronary arteries, Kawasaki disease). Chronic causes of VT include postoperative congenital heart disease (e.g., tetralogy of Fallot), cardiomyopathies (especially hypertrophic cardiomyopathy), tumors or infiltrative processes, primary channelopathies (long QT syndrome, Brugada syndrome), and idiopathic forms. The clinical diagnosis of VT must be distinguished from other arrhythmias that may present with a wide QRS complex tachycardia such as SVT with aberrant conduction. Any wide QRS complex tachycardia, however, should first be considered VT until proven otherwise. Hemodynamically unstable patients with mental status changes or evidence of low cardiac output must be treated promptly with DC synchronized cardioversion. For hemodynamically stable infants and older pediatric patients, intravenous amiodarone and lidocaine are the initial drugs of choice. Ventricular arrhythmias caused by acute etiologies such as electrolyte imbalance, hypoxia, or drug toxicity resolve after the offending abnormality has been corrected. For patients with chronic VT, medical therapies should be aimed at preventing recurrence.