Chapter 485. Arrhythmias

Anatomy and Physiology

The heart has specialized cells collected into nodes and tracts. The sinoatrial node, near the junction of the superior vena cava and right atrium, has a rich vagal and sympathetic nerve supply and controls heart rate. Conduction of impulses from the sinoatrial node to the atrioventricular node occurs without a specialized conducting system. However, there are preferential pathways that have been termed internodal tracts. In addition, preferential conduction from the sinus node to the roof of the left atrium occurs over the Bachmann bundle, which is also not part of a specialized conduction system. The Bachmann bundle is important, however, in timing of left atrial contraction in relation to mitral valve opening. The slow cell-to-cell conduction through atrial myocardium explains the relatively long duration of the P wave.

The atrioventricular (AV) node is in the interatrial septum just anterior and superior to the mouth of the coronary sinus; it is also innervated by vagal and sympathetic fibers and consists of a mesh of very thin fibers that conduct impulses very slowly. As a result, the AV node delays conduction, giving time for ventricular filling. Also, in the presence of atrial fibrillation, the AV node limits the number of impulses reaching the ventricles. From the AV node, the bundle of His passes through the central fibrous body into the ventricular septum just behind and below its membranous portion.1 The bundle of His has large, rapidly conducting fibers and has vagal nerves only proximally; more distally, there is only sympathetic innervation of conduction tissues. Near the summit of the muscular ventricular septum, the bundle of His gives off the compact right bundle branch with wide fibers and then the left branch bundle with a diffuse fan of thinner fibers. The bundle is insulated from surrounding myocardium and normally does not activate the ventricular myocardium until it branches and ramifies into the Purkinje fibers. These peripheral conducting fibers ramify just beneath the endocardium so that the ventricular walls are depolarized from subendocardium to subepicardium. Rapid conduction down the His-Purkinje system allows the entire ventricular myocardium to contract nearly simultaneously, explaining the narrow QRS complex in normal hearts.

Some people have accessory pathways connecting atrial and ventricular myocardium. In patients with Wolff-Parkinson-White syndrome, the bundle of Kent is a muscular bridge spanning the atrioventricular groove.2 With some Kent bundles, conduction is possible in both anterograde and retrograde directions; in others, it is exclusively retrograde. Anterograde conduction causes early depolarization of the ventricles (preexcitation); retrograde conduction causes rapid reentry between the atria and the ventricles, causing sustained tachyarrhythmias. A second type of accessory pathway, the Mahaim fiber, is thought to be made up of specialized conducting fibers and to connect directly to the specialized conducting system.3 The most common type, called the atriofascicular connection, connects atrial myocardium with elements of the right bundle branch. In general, only anterograde conduction is thought to be possible in Mahaim pathways; tachycardia occurs as a result of anterograde conduction from the atrium to the distal conducting system and then retrograde conduction to the bundle of His, atrioventricular node, and back to the atrium.

Some individuals have dual atrioventricular (AV) node pathways.4 These pathways are functionally and anatomically distinct, with the atrial approaches to the slow pathway being posterior and inferior to the compact AV node and to the fast pathway being anterior and superior to the node. The effective refractory period is usually shorter in the slow pathway than in the fast. Dual AV node pathways provide the basis for atrioventricular node reentry tachycardia; after a premature atrial contraction blocks in the fast pathway and conducts down the slow, the impulse may reenter the fast pathway retrograde, setting up sustained reentrant tachycardia.

Cellular Electrophysiology

If a microelectrode is placed in a cardiac muscle cell, the potential between it and an electrode outside the cell is the transmembrane potential. In diastole this “resting” potential is –80 to –90 mV. When the cell is stimulated, an action potential results (eFig. 485.1). Rapid depolarization (phase 0) results from rapid sodium influx; the rapid return to near zero potential (phase 1) results from chloride influx. The less negative the transmembrane potential at the onset of depolarization, the slower the rate of depolarization (Vmax) in phase 0; in turn, reduced Vmax slows the velocity of propagation of the impulse through the fibers. Antiarrhythmic agents with class I activity (eg, procainamide, flecainide) block sodium influx, reduce Vmax, and therefore slow conduction. Phase 1 is followed by a plateau (phase 2), during which slow inward calcium and sodium currents are nearly balanced by potassium leaking out of the cell. Potassium conductance then increases, and potassium efflux causes the potential to become more negative rapidly (phase 3) until the resting membrane potential is reached. During phase 4, sodium is actively pumped out of the cell in exchange for potassium.

Once the action potential has begun, the cell is completely refractory to stimulation until it attains a transmembrane potential of about 55 mV in phase 3, and from this value, until it reaches a resting potential of 85 mV in phase 4, the cell gradually regains excitability. Between absolute refractoriness and complete responsiveness is the relative refractory period; in keeping with the relationship between Vmax and transmembrane potential, a stimulus early in the refractory period produces a slow and poorly conducted action potential. Prolongation of action potential duration and absolute and relative refractory periods occurs with slowing of the rate of depolarization, a long R-R interval thus giving a longer refractory period of the next beat. Refractory periods are also prolonged by disease (myocarditis, ischemia) and by some antiarrhythmic drugs (ibutilide, sotalol).

Certain cells have a different pattern (eFig. 485.1C). The resting potential starts at about 80 mV but then automatically becomes more positive. When the potential reaches a threshold of about 50 mV, the action potential begins spontaneously; thus, these cells are called automatic cells and are said to have automaticity. Normally, most cardiac muscle cells do not have automaticity; those that do may be found anywhere in the heart but are mainly in the sinoatrial node, the lower part of the atrioventricular node, the muscle in the mitral and tricuspid valves, and the conduction system. The repetition rate at which groups of automatic cells depolarize depends mainly on the slope of spontaneous depolarization; the more rapid it is, the sooner the threshold is reached. Diastolic depolarization rate is accelerated by catecholamines, vagal blockade, a raised temperature, lowered extracellular potassium or calcium, or a fall in tissue pH or oxygen tension; these changes thus tend to increase heart rate.

Diastolic depolarization rate and thus automaticity are reduced by vagal stimulation; β-adrenergic blockade; increased extracellular potassium; and drugs such as quinidine, procainamide, lidocaine, and phenytoin. The effect of these drugs in reducing automaticity makes them useful in treating many arrhythmias. It is important to note that different automatic cells have different sensitivities for changes induced by these agents. Thus, phenytoin increases atrial diastolic depolarization, but decreases ventricular diastolic depolarization, and lidocaine has a marked effect on ventricular automaticity, but not on atrial automaticity. Normally, the discharge rate is highest for automatic cells in the sinoatrial node, which is thus the usual pacemaker for the heart. Collections of automatic cells (latent pacemakers) lower in the conducting system have slower diastolic depolarization, the slowest being those in the ventricles. Normally, these lower (or ectopic) pacemakers are discharged by impulses from the sinoatrial node before they can discharge spontaneously, and thus they are usually suppressed by impulses from above. The typical discharge rate of each pacemaker varies with age (eTable 485.1).

Most normal cardiac muscle and automatic cells show rapid depolarization and also conduct rapidly; thus, they are known as fast-responding cells. However, some cells show a slow response. Their transmembrane resting potential is only about 60 mV, depolarization is slow, as is conduction, and in phase 4 there is an after-depolarization that normally does not reach the threshold for initiating another action potential. Slow-responding cells are found normally in the sinoatrial and atrioventricular nodes and the atrioventricular valves. Fast-responding cells can be converted into slow-responding cells by ischemia, digitalis toxicity, excess potassium or catecholamines, or chronic dilatation. In all these cells, the rapid sodium influx of phase 1 is absent; depolarization is achieved mainly by the slow inward calcium current and can be blocked by calcium channel blockers (verapamil, diltiazem).

There are 3 major types of disorders: abnormal automaticity (impulse formation), abnormal conduction, and fibrillation.

Disorders of Automaticity

If the sinoatrial node discharges abnormally slowly or the impulse from it is not conducted, an ectopic lower pacemaker, either atrial or more commonly junctional, takes over. If the block is in the atrioventricular node or bundle of His, a ventricular pacemaker may take over. In this way, lower pacemakers escape from suppression by higher pacemakers. Escape may occur for a single beat, for a few beats, or may result in a permanent lower pacemaker rhythm if the higher pacemaker does not regain control. Escape beats or rhythms indicate abnormality of the higher pacemaker or abnormal conduction from it, and the escape is a normal response. An escape beat is recognized by its late appearance (R-R interval longer than normal) and evidence of an ectopic focus (abnormal P-wave axis and morphology for an atrial ectopic focus; no P wave, very short PR; or retrograde P wave for junctional focus). An escape rhythm is characterized by an ectopic rhythm that is slower than a normal sinus rhythm (eFig. 485.2).

The other disorder of automaticity is premature discharge of a lower pacemaker. One or 2 of these beats are termed premature beats or extrasystoles, but 3 or more in a row constitute ectopic tachycardia. If the ectopic focus is in the atrium or is junctional, the descending impulse reaches the ventricles through the His-Purkinje system, and a normal QRS complex usually occurs (but see Aberrant Conduction, below). If the ectopic focus is ventricular, the impulse spreads slowly through muscle cells and produces a widened, bizarre QRS-T complex with the polarities of the QRS complex and the T wave in opposition to each other. If the premature beat begins in the right ventricle, then this ventricle is depolarized before the left ventricle and the QRS pattern resembles a left bundle branch block. Conversely, a left ventricular ectopic beat resembles a right bundle branch block. The basic difference between bundle branch block and ectopic patterns is that in the former only the terminal part of depolarization is slow, whereas in the latter the whole QRS complex is widened and distorted. It may be difficult to make this differentiation in practice. However, in children, most aberrantly conducted beats with bundle branch block follow a premature atrial beat. Therefore, a wide QRS complex without a discernable premature P wave is likely to be ventricular in origin.

With a ventricular premature beat, the retrograde impulse is often blocked at the atrioventricular node and the basic rhythm of the sinoatrial node is usually unaltered, thus the first sinus beat after a ventricular extrasystole follows a full compensatory pause. That is, the interval between the sinus beat preceding the extrasystole and the sinus beat following it is at least twice the normal R-R interval (Fig. 485-1). However, with atrial or junctional premature beats, the sinoatrial node is usually discharged; thus, the next sinus beat usually occurs after an incomplete compensatory pause (less than twice the normal R-R interval).

Ectopic tachycardias have QRS complexes like those of the corresponding ectopic beats: usually normal if the focus is supraventricular, almost always widened if it is ventricular, but widened with a supraventricular focus and aberrant conduction. These are differentiated below.

The effect of premature atrial beats on P-wave morphology depends on the site of the atrial focus. If it is near the sinoatrial node, an almost normal P wave occurs (eFig. 485.3); if it is near the atrioventricular (AV) node, then the atrial axis is directed superiorly, leading to inverted P waves in leads II, III, and aVF (Fig. 485-1B). Therefore, low atrial and high His-bundle premature beats give similar P waves, although, because of the time taken for retrograde conduction through the AV node to the atria, an infranodal pacemaker is more likely to give P waves buried in the QRS complex or following it. If the ectopic focus is ventricular, there can be an abnormal P wave after the QRS complex due to retrograde AV node conduction, but often the retrograde impulse is blocked in the AV node and the atrium is activated normally from the sinoatrial node at a different rate.

Premature beats may be caused by increased automaticity of an ectopic focus; they have many causes, including endogenous catecholamines, drugs, and disease. Frequently, the mechanism of ectopic tachycardias relates to the slow response that can occur in damaged fibers. Unlike fast-response fibers, which can be inhibited by rapid impulses from higher pacemakers, the slow-response fibers react to rapid stimuli with increasing amplitude of after-depolarizations that may reach threshold and initiate the repetitive action potentials characteristic of an ectopic tachycardia.

Tachycardias may also result from reentry, a circular form of depolarization that occurs when there are 2 parallel pathways with unidirectional block and often a protected zone of slow conduction. For example, consider a strip of damaged ventricular muscle that does not conduct anterograde, but does conduct slowly retrograde. The normal sinus impulse descends and depolarizes the normal, but not the abnormal, muscle. The wave of depolarization eventually passes slowly retrograde up the abnormal strip that has not been depolarized; by the time the impulse emerges from the abnormal muscle, the rest of the muscle is responsive and is depolarized prematurely—a ventricular premature beat. Usually, the wave of depolarization does not continue to circulate because the abnormal muscle, being depolarized, is then refractory to impulses from any direction. The sequence may be repeated to produce ventricular bigeminy. Because normally there is slow conduction through the atrioventricular node, reentry through it is believed to explain many types of extrasystoles. If the recirculating wave of depolarization is itself slowed in some other portion of muscle, then the wave may return to the original abnormal muscle strip when it has regained excitability, and a rapid repetitive depolarization (reentry tachycardia) will occur. Reentry is the cause of tachycardia in Wolff-Parkinson-White syndrome, in which a premature atrial impulse blocks in the accessory pathway, but descends through the slowly conducting atrioventricular node, returns to the atrium through the rapidly conducting bundle of Kent, and continues around the circuit in an endless loop.

Disorders of Conduction

Conduction may be impaired either by an abnormal increase in the refractory period (pathologic or primary) or by interference from another impulse that alters the refractory period (physiologic or secondary). Physiologic factors that affect conduction include concealed conduction and physiological interference. When conduction through tissue cannot be detected on the electrocardiogram but is inferred by its effect on conduction of a subsequent impulse, then it is termed concealed conduction. This may well be shown, for example, if there is an interpolated (very early) ventricular premature beat (eFig. 485.4). The retrograde impulse from the ventricular focus enters the atrioventricular (AV) node and prolongs the AV nodal refractory period, but does not capture the atrium, which is still refractory from the last beat. The next impulse from the sinoatrial node is conducted more slowly than normal through the AV node because it is still partially refractory. The resulting long PR interval is the evidence for concealed retrograde conduction.

Interference is shown most simply if there is a very early isolated atrial premature beat. In eFigure 485.3, the ninth P wave is seen on the T wave of the preceding QRS complex, but because the AV node or ventricle is still refractory, the impulse is not conducted. This is a physiologic, not pathologic, failure of conduction and so does not imply any abnormality in the AV node. A more prolonged episode of physiologic interference may occur if the sinus pacemaker slows or a junctional pacemaker speeds up so that the atria and ventricles beat at roughly similar rates but are controlled by different pacemakers (see eFigs. 485.1 and 485.5). This is one form of atrioventricular dissociation (the other main form being complete atrioventricular block). Despite a normal AV conduction system, the descending impulse from the sinoatrial node does not pass the AV node, which has been made refractory by the retrograde impulse from the ventricle or bundle of His. In turn, this retrograde impulse does not capture the atrium, which has been made refractory by the sinus beat. Failure of the sinoatrial node to capture the ventricle is thus not an abnormality of conduction but a physiologic effect on refractory periods. An electrocardiogram with this type of interference shows unrelated P waves and QRS complexes with the ventricular rate faster than the atrial rate. Occasionally, ventricular and atrial rates are the same, but the PR interval is too short to diagnose a sinus rhythm; this is termed isorhythmic atrioventricular dissociation. Sometimes the interference occurs in the atrium or the ventricle rather than in the atrioventricular node, and then it produces atrial or ventricular fusion beats, the latter being more important. Typically, a ventricular premature beat has a wide QRS complex. However, if the ventricular premature beat begins while the normal sinus impulse is descending the His-Purkinje system, then some of the ventricle will be depolarized normally and the rest slowly; the resulting QRS complex (the fusion beat) will usually be intermediate in form between the normal complex and that associated with the ventricular ectopic beat (eFig. 485.6; see also Fig. 485-8). A fusion beat essentially proves that wider beats are ventricular in origin.

Aberrant Conduction

Whenever a supraventricular impulse reaches the atrioventricular (AV) node or bundle of His in the relative refractory period, the impulse may be transmitted aberrantly because of uneven loss of refractoriness in the AV node or bundle branches. The impulse may arrive at the AV node or bundle at this time because of a premature supraventricular beat, supraventricular tachycardia, or a prolonged QT interval of the preceding beat. In most people, the right bundle branch normally has a longer refractory period than the left bundle. Therefore, premature supraventricular beats or tachycardias may be conducted normally to the left ventricle but slowly to the right ventricle, giving the pattern of right bundle branch block and a wide QRS complex (eFig. 485.3). However, left bundle aberration occurs occasionally. Bundle branch aberration after a single premature supraventricular beat is quite common, particularly if the beat follows a long pause (Ashman phenomenon). However, sustained bundle branch aberration with supraventricular tachycardia is less common in children than adults, and most sustained wide QRS tachycardias in children are caused by ventricular tachycardia.

An important form of aberration occurs when the ventricle during tachycardia is activated entirely or in part from an accessory AV connection. Activation from a Kent bundle does not use the His-Purkinje system, thus conduction through the ventricle is slow, as with a ventricular ectopic focus. This phenomenon is seen in Wolff-Parkinson-White syndrome with atrial tachycardia or fibrillation or in antidromic reentry with conduction anterograde in the pathway and retrograde in the AV node. It is also seen during reentry involving a Mahaim atriofascicular connection, in which conduction is down the pathway to the right bundle branch with retrograde activation to the atrium via the bundle of His and AV node.

Atrioventricular Block

If conduction is delayed through the atrioventricular (AV) node or the bundle of His, then various degrees of AV block occur. If there is merely a long PR interval but all beats are conducted, first-degree AV block is diagnosed (Fig. 485-2). If no atrial beats are conducted so that the ventricles are driven by a junctional or ventricular focus, then there are normal P waves at one rate and QRS complexes at a slower rate, usually with no fixed relationship between P waves and QRS complexes. This is complete or third-degree AV block (Fig. 485-3).

Whenever atria and ventricles are controlled by independent pacemakers, there is AV dissociation. This may result either from complete AV block or from junctional or ventricular tachycardia or accelerated rhythm without retrograde conduction. The former has faster atrial than ventricular rates; the descending impulse cannot reach the lower pacemaker because a conduction block exists. The latter has faster ventricular than atrial rates, but coexistent anterograde conduction block is not excluded. The absence of anterograde conduction block is demonstrated when a critically timed atrial impulse conducts to the ventricles and suddenly shortens the R-R interval. In ventricular tachycardia, this shortened R-R interval is associated with shortening of, or normalization of, the QRS complex. In junctional tachycardia, the QRS remains relatively unchanged in capture beats. If there is doubt about AV conduction, one can determine if block is present by increasing atrial rate with exercise or hyperventilation and noting whether or not ventricular capture occurs (eFig. 485.5).

If some sinus beats are conducted to the ventricle but others do not reach it, there is second-degree atrioventricular (AV) block. Mobitz type I second-degree AV block is also known as Wenckebach conduction and is generally due to failure of conduction in the AV node. The first atrial impulse of a group of beats is conducted normally, but the next atrial impulse reaches the AV node while it is still partly refractory and thus is conducted more slowly, giving a longer PR interval. The next atrial impulse arrives even earlier in the AV nodal refractory period, with an even longer PR interval as a result. Eventually the atrial impulse reaches the AV node in its absolute refractory period and is blocked so that no QRS complex follows (Fig. 485-4). The effect of this on the QRS complexes is to make them occur with progressively shorter R-R intervals until an unusually long R-R interval indicates that 1 ventricular complex is missing. Two other features are typical of this arrhythmia. The largest increment in PR intervals is in the second beat after the dropped beat; the first has the shortest PR interval because the long pause has allowed the AV node to recover. Furthermore, the duration of the longest R-R interval is always less than twice the short R-R intervals because these cycles contain the longest PR intervals. This feature of progressive shortening of the R-R intervals until a long R-R interval less than the length of 2 of the shorter R-R intervals occurs is the hallmark that allows Wenckebach phenomenon to be diagnosed even when P waves are not easily seen. Finally, the phenomenon of group beating provides a clue to Wenckebach conduction. Atrioventricular block occurs with a repetitive pattern, often 3:2 or 4:3, so that there are repetitive groups of 2 or 3 QRS complexes followed by pauses.

Mobitz type II second-degree atrioventricular (AV) block occurs when a QRS complex drops out without prior lengthening of the PR intervals and usually occurs distal to the AV node. It is less common but more serious than type I second-degree AV block and is more likely to lead to complete AV block.

Second-degree AV block does not imply a fixed relationship between atrial and ventricular complexes. There may be 2:1 second-degree AV block, in which only alternate atrial impulses are conducted to the ventricle; 3:1 second-degree AV block indicates that every third atrial impulse is conducted to the ventricles. There may be higher grades of second-degree block, such as 4:1 and 5:1, or the block may vary, with no consistent relationship between the numbers of conducted and blocked atrial impulses. This lack of group beating helps to differentiate type II from type I block. In addition, there is no shortening of PR interval after a blocked beat in type II second-degree AV block.

Atrial and Ventricular Fibrillation

Sometimes a stimulus to a region that has repolarized is conducted at different rates in some directions and blocked in others. As a result, an irregular wave front of depolarization takes a tortuous path through the muscle and leaves behind it an irregular path of refractory cells. Portions of the wave front return to regions that were formerly refractory but are now excitable. The wave of depolarization becomes more irregular, and eventually there is chaotic fragmented contraction that is termed fibrillation.

To produce fibrillation 2 conditions are needed. First, there must be asymmetric slowing of conduction or asymmetric shortening of refractoriness due to hypoxemia, ischemia, electrolyte abnormalities, or many drugs. Second, there must be a premature stimulus, such as an external electric shock, a premature beat, or tachycardia that occurs very early, before all the muscle has repolarized. The fact that an extrasystole can occur so early is itself evidence of decreased refractoriness of some muscle.

If the chaotic rhythm occurs in the atria, there is atrial fibrillation, which is easily initiated by sustained rapid atrial pacing. However, in children, atrial fibrillation is usually not sustained and reverts to sinus rhythm unless the atria are enlarged. If the chaotic rhythm occurs in the ventricles, there is ventricular fibrillation: The vulnerable period in which a shock (or rarely, an ectopic beat) can cause fibrillation is at the apex of the T wave.

Arrhythmias are best diagnosed with knowledge of the history and physical findings and with all previous electrocardiograms from that patient. The electrocardiogram of the arrhythmia under study should be a routine full electrocardiogram plus some very long strips, lasting about 1 minute or more. In sinus rhythm, leads II and V1 are most likely to show good P waves, but with ectopic atrial foci, other leads may be better. Sometimes special maneuvers are needed to bring out P waves, such as using modified chest, esophageal, or intracardiac leads.

The electrocardiogram should be examined carefully for P waves. These should be inspected for variations in rate, morphology, and mean axis, for PR intervals and their possible variation, and to find out if the P waves are likely to be the source of the impulse causing the QRS complex. Then, the QRS complex should be examined for its rate, rhythm, and morphology.

Maneuvers that increase atrioventricular block (eg, vagal stimuli, certain drugs such as adenosine) may slow ventricular rate and allow better definition of atrial and ventricular complexes. They should be performed only while the electrocardiogram is running, for safety as well as for future analysis.

Most arrhythmias can be diagnosed by careful diagnosis of the electrocardiogram during the occurrence of the arrhythmia as well as in the baseline state. Often, however, additional methods are helpful. Recordings of paroxysmal arrhythmias are often difficult to obtain, particularly at their onset or termination, which are the most important features for analysis. If episodes occur daily, a 24-hour ambulatory electrocardiogram (Holter monitor) may capture the episode. If they are less frequent, transtelephonic event recorders, with or without looping memory, that are activated by the patient when symptoms occur may capture an episode. Another option is surgically implanted loop recorder that has a battery life of up to several years.5 Sometimes tachycardias are precipitated by exercise, and an exercise test may elicit the arrhythmia. Invasive electrophysiologic study is the definitive method of diagnosing cardiac arrhythmias. As many as 4 electrode catheters are inserted via veins into the heart for pacing and recording the intracardiac electrograms. Both sinoatrial and atrioventricular node function are assessed, and arrhythmias may be induced, diagnosed, and mapped to determine their origin and route of conduction by programmed electrical stimulation.

Arrhythmias with Regular Rhythm at Normal Rate

A regular rhythm at normal rate is most likely to be sinus rhythm, but the electrocardiogram should be inspected carefully to exclude atrial tachycardia with 2:1 atrioventricular block, atrial flutter with 4:1 atrioventricular block, accelerated junctional or idioventricular rhythms, or atrial fibrillation with an independent idioventricular rhythm caused by a coexistent atrioventricular block.

Pauses

Occasional pauses are most commonly caused by nonconducted very early atrial extrasystoles; these may not always be readily apparent, as the P waves may be superimposed on the preceding T wave. They can also result from marked sinus arrhythmia, second-degree atrioventricular block, or sinoatrial exit block as well as concealed conduction of junctional extrasystoles that have made the atrioventricular node refractory.

Groups of Beats

Bigeminy means that beats occur in pairs. Most commonly this is because of extrasystole coupled to the previous normal beat, but it can also indicate a 3:2 atrioventricular block (every third atrial beat not conducted) or an escape beat paired with the next normal beat. Similarly, trigeminy means groups of 3 beats. These could be 2 normal beats and 1 premature beat, 1 normal and 2 premature beats, 4:3 atrioventricular block, and so on. Wenckebach periods often produce beats that are grouped in pairs, triplets, or larger groups.

Irregular Rhythms

Occasional irregularities result from premature beats or escape beats, the latter occurring with sinus pauses, blocked atrial premature beats, or second-degree atrioventricular block. These are usually easy to differentiate. At times it is difficult to distinguish atrial fibrillation from a second-degree atrioventricular block with Wenckebach periods occurring in a tachycardia, where P waves are not easy to find. The diagnosis is made by the characteristic features of Wenckebach periods: progressive shortening of the R-R interval before a long pause, with the longest R-R interval being less than 2 of the shortest intervals.

Tachycardias

A rapid heart rate with normal P waves and PR intervals, and with variations in rate from moment to moment, is a sinus tachycardia. A rapid fixed rate with complete regularity, normal QRS complexes, and either P waves on the T waves or else no clear P waves is probably a supraventricular tachycardia; vagal stimulation causes either no change or an abrupt reversion to sinus rhythm. A rapid rate, usually fixed and regular, with a wide QRS complex is either a ventricular tachycardia or a supraventricular tachycardia with ventricular aberration or preceding bundle branch block. A ventricular tachycardia can be diagnosed by examining the QRS patterns (see above), by finding fusion beats, or by noting that the QRS complexes are similar to ventricular ectopic beats found in other electrocardiograms. Vagal stimulation usually does not affect this arrhythmia. If a slower atrial rhythm is noted from P waves usually deforming the T waves variously from beat to beat, then ventricular tachycardia is likely. However, retrograde P waves at the same rate as the ventricles can occur with junctional or ventricular tachycardias. A rapid supraventricular tachycardia can also be caused by atrial flutter with a regular block; this is differentiated from a plain supraventricular tachycardia by the typical sawtooth flutter waves, although these waves may be seen in only 1 or 2 leads. It could also result from atrial fibrillation, detected by slight variations in the R-R intervals. In both these arrhythmias the rate is slowed and the variability is increased by increasing atrioventricular nodal block with vagal stimulation or digitalis.

Principles of Treatment

In addition to correctly identifying the arrhythmia, proper management of arrhythmias requires attention to 4 questions.

1. 1. Does the arrhythmia need treatment? In many children no treatment is needed. Many antiarrhythmic agents may cause new, or worsen existing, arrhythmias or even cause sudden death. Arrhythmias that require treatment are those that threaten to cause ventricular fibrillation or asystole (eg, ventricular tachycardia, multiform ventricular ectopic beats) and those in which the ventricular rate is too slow or too fast for effective cardiac output. In general, the effects of abnormal ventricular rates depend not only on the rates but also on how much they differ from the patient’s usual rate. Thus, a chronically slow ventricular rate of 50 beats per minute is accompanied by ventricular dilatation and hypertrophy and well-maintained cardiac output. On the other hand, a sudden drop from 100 to 50 beats per minute may be poorly tolerated.

   2. Can underlying causes be corrected? In children, particularly neonates, most clinically significant arrhythmias are caused by apnea, hypoxemia, acidemia, electrolyte disturbances, or drugs such as digitalis or catecholamines. Acute digoxin toxicity can cause life-threatening arrhythmias that are best treated with digoxin immune Fab.6 If the arrhythmia is treated without attention to the underlying cause, the patient may deteriorate.

   3. Do secondary effects need treatment? Arrhythmias that reduce cardiac output may cause hypotension and acidemia, with subsequent increase in sympathetic tone, which helps to maintain the arrhythmia.

   4. What specific therapeutic strategy is needed for the arrhythmia? Consider 2 patients with 2:1 atrioventricular block, 1 with an atrial rate of 260 beats per minute, the other with an atrial rate of 100 beats per minute. The first patient has an adequate ventricular rate of 130 beats per minute, and therapy should be directed at slowing the atrial rate; the atrioventricular block should not be treated because it is useful in preventing too many impulses from reaching the ventricles and causing a harmful rapid ventricular rate. In the second patient, therapy should be directed to increasing the ventricular rate of 50 beats per minute by improving atrioventricular conduction or by pacing the ventricles.

Pharmacologic Management

Methods of controlling arrhythmias may be chosen with the mechanism of the specific arrhythmia in mind. However, caution is needed when extrapolating pharmacologic effects observed in normal cardiac muscle to diseased muscle. For example, in ischemic heart muscle, lidocaine prolongs conduction and refractoriness, whereas it increases conduction velocity in normal fibers; in fact, its effectiveness against some ventricular arrhythmias may depend on this action (blocking reentry) rather than on its depression of excitability. For this reason, selection of the appropriate antiarrhythmic agent should be based on clinical experience as well as theory or the effects in single cardiac cells.

Bradycardias

A slow ventricular rate may be caused by severe sinus bradycardia or arrest, or second- or third-degree atrioventricular (AV) block. The slowing either causes a cardiac output that is too low or allows breakthrough of potentially dangerous ventricular escape beats or tachycardia. Treatment is aimed at increasing the ventricular rate. Atropine (0.01 mg/kg subcutaneously or intravenously) may increase sinoatrial nodal discharge rate or accelerate AV conduction, but it does not speed up low ventricular pacemakers because the vagus does not innervate the conduction tissue below the bundle of His. Isoproterenol (0.05 to 0.5 μg/kg/min) may increase pacemaker discharge rates and conduction at any level but should be used with care if there are ventricular arrhythmias as well. Slowed conduction through the AV node, especially if induced by digoxin, may be improved by phenytoin. If drug therapy is ineffective or contraindicated, then temporary or permanent artificial cardiac pacing may be needed.

Ectopic Beats and Ectopic Tachycardias

Arrhythmias with premature beats or tachycardias have abnormalities related to increased automaticity of cardiac cells, to increases after depolarization of slow-response fibers, or to uneven conduction times that result in reentry. Proposed mechanisms of antiarrhythmic action include raising the threshold at which cells depolarize, decreasing sodium conductance so that phase 0 depolarization is slowed and conduction velocity along the fiber is reduced, thereby converting unidirectional block to bidirectional block and stopping reentry; decreasing the slow calcium current and preventing after-depolarization; prolonging the effective refractory period relative to the action potential duration so that at any heart rate there is less time for a reentrant or ectopic impulse to depolarize the cell; and decreasing phase 4 depolarization so that it takes longer to reach the threshold for depolarization. Many drugs act in more than 1 way but often have 1 major action, so that it is possible to avoid using 2 drugs with the same basic mode of action.

Vagal stimulation (carotid sinus massage, Valsalva maneuver, baroreceptor reflex to hypertension, anticholinesterase drugs) reduces automaticity and shortens the action potential duration and the effective refractory period of atrial muscle; it also slows atrial and atrioventricular (AV) nodal conduction and lengthens the refractory period of the AV node. These actions may abolish paroxysmal supraventricular tachycardia, and they decrease ventricular response in atrial flutter or fibrillation. Because of sparse ventricular innervation, vagal stimulation has little effect or no on ventricular arrhythmias.

Should vagal stimulation fail, the initial drug of choice for treating supraventricular arrhythmias is digoxin. Digoxin may increase ventricular automaticity and thus increases the chances of getting ventricular premature beats and tachycardias. This explains in part why digoxin is not a good choice for treating ventricular arrhythmias. At therapeutic doses, both excitability and conduction velocity are depressed in the atrium. At all doses, AV nodal conduction is slowed so that first the PR interval lengthens; with larger doses, second- or third-degree AV nodal block occurs. Digoxin shortens the action potential duration and the refractory period in atria and ventricles, thus shortening the QT interval. For acute termination of paroxysmal supraventricular tachycardias that rely on the AV node as part of the reentrant circuit, adenosine is the drug of choice.7 Some clinicians use propranolol or verapamil to slow the ventricular response in atrial flutter or fibrillation; verapamil is ineffective in this regard when Wolff-Parkinson-White syndrome is present, as is digoxin, thus both are contraindicated in patients with Wolff-Parkinson-White syndrome8.

Antiarrhythmic drugs are classified by their major electrophysiological properties; the major groups are described in Table 485-1, as are their main indications and contraindications.

Nonpharmacologic Management

Acute Management

Drug therapy is not always successful in abolishing arrhythmias. Electrical cardioversion by a depolarizing shock is mandatory in ventricular fibrillation and is the method of first or second choice in treating acute tachycardias (paroxysmal atrial or ventricular tachycardias, atrial flutter, or fibrillation), especially if they cause serious cardiovascular difficulties. Rapid overdrive pacing, using temporary electrodes inserted by a transvenous or esophageal route, may be used to drive the heart fast enough to prevent the emergence of ventricular ectopic beats, to abolish reentrant tachycardias,9 or to prevent the recurrence of tachycardias.

Permanent Antibradycardia Pacing

Permanent pacemaker implantation is often needed to manage bradyarrhythmias. Implantation may be transvenous, by the subclavian or cephalic vein, with the pulse generator placed infraclavicularly. Implantation may, alternatively, be epicardial, with a subxiphoid generator. The epicardial route is reserved for the smallest children and those in whom transvenous pacing is contraindicated because of the risk of thromboembolism (eg, those with an intracardiac shunt and those with no venous access to the ventricle, such as those with the Fontan procedure).

Pacemakers are classified by a 3-letter code that describes the chamber paced, the chamber sensed, and the response to a sensed event: V, ventricle; A, atrium; D, dual; I, inhibit; T, triggered. For example, VVI means that the ventricle is both paced and sensed, and generator output is inhibited when intrinsic ventricular activity is sensed. With the advent of the fully automatic DDD pacemaker, only 3 configurations are now commonly implanted: VVI, AAI, and DDD.

Recently, the management of heart failure in adults has been revolutionized by the developed of cardiac resynchronization therapy in which 2 sets of leads are placed on the ventricle for biventricular pacing. Increasingly, this technology is finding a place in the management of children with a variety of clinical problems.10

As a pulse generator nears the end of its battery life (typically 5 to 10 years), it exhibits a characteristic behavior, depending on the manufacturer and model. This is commonly a prolongation of pulse width, slowing of the rate, or the onset of asynchronous pacing. Another common cause of unusual behavior is lead fracture, when the pulse generator may oversense and fail to fire, fail to capture, or revert to asynchronous pacing. All children with implanted pacemakers should have regular follow-up, including transtelephonic monitoring, so that such problems can be detected promptly.

Antitachycardia Pacing

For selected patients who can be easily converted from a reentrant supraventricular tachycardia by rapid pacing, permanent antitachycardia pacing may be used. In terms of classification, the letter T is added for those pulse generators that have antitachycardia pacing capability. For example, an AAI-T device provides atrial demand pacing but also is capable of converting sensed episodes of paroxysmal tachycardia back into sinus rhythm by burst pacing or programmed stimulation; the modalities used are programmed by the clinician.

Implantable Cardioverter-Defibrillator

Children with life-threatening ventricular arrhythmias unresponsive to medical management may need implantation of a device that senses the onset of ventricular fibrillation and delivers 1 or more direct current defibrillation shocks directly to the heart. The lead configuration may be similar to a standard transvenous pacemaker, with a coil in the right ventricular chamber, or the energy may be delivered via epicardial patches or subcutaneous coils.11 The later systems are used in smaller children and in those with single ventricles or a lack of reasonable venous access. These devices have the capability of antibradycardia pacing (in either VVI or DDD configurations), antitachycardia pacing, and cardioversion or defibrillation.

Catheter Ablation

The development of techniques to map and ablate abnormal foci or pathways in the heart using transcatheter radiofrequency energy revolutionized the management of patients with abnormal tachycardia. Currently, ablation may be carried out using either transcatheter radiofrequency energy or via cryoablation (tissue freezing).12 The catheter ablation procedure is performed in the cardiac catheterization laboratory as part of an electrophysiology study. Once the diagnosis is made and the abnormal pathway or focus is located precisely, a special electrode catheter is placed at that site, and ablation energy is delivered. With transcatheter radiofrequency ablation, current passes between the electrode of the catheter and a large chest patch, creating a tiny region of thermal injury on the endocardium. When cryoablation is used, the tip of the catheter is cooled to about –80°C for at least 4 minutes, destroying a small area of endocardial tissue. If properly placed, such lesions may interrupt conduction in an accessory pathway, modify the atrioventricular node, or eliminate an automatic focus. All forms of abnormal tachycardia are amenable to catheter ablation at relatively low risk, although risks are higher in infants. Success rates are over 90% for the common forms of supraventricular tachycardia but are lower for other forms of abnormal tachycardia.13 Radiofrequency ablation has a higher success rate, but cryoablation has a better safety profile, particularly when addressing anatomic substrates that are in close proximity to the atrioventricular node or other critical structures.

Surgery

Although catheter ablation has replaced cardiac surgery for definitive cure of the mechanism of abnormal tachycardia in most patients, surgery may be recommended for patients who have failed catheter ablation and in infants with incessant ventricular tachycardia secondary to cardiac hamartomas. Another setting in which arrhythmia surgery may be recommended is in the situation of late postoperative atrial tachycardia/atrial flutter. For example, patients who have undergone an atriopulmonary connection Fontan procedure for palliation of a single ventricle are at high risk for the development of intractable atrial tachycardia. The surgical maze procedure along with conversion to an external conduit Fontan may offer the best hope of long-term control in such patients.14

Sinus Tachycardia

Sinus tachycardia is most often caused by anxiety or exercise but may result from fever, anemia, hypovolemia, shock, congestive heart failure, hyperthyroidism, and many medications. It is often confused with abnormal tachycardias, particularly in sick patients, and differentiation may be difficult. The rate seldom exceeds 200 beats per minute, even in newborns, and varies with time or activity. The electrocardiogram usually shows normal P waves, but at fast rates the P wave may be superimposed on the previous T wave. Vagal stimulation gradually slows the heart rate, in contrast to the abrupt slowing that it may cause with a paroxysmal supraventricular tachycardia. The best diagnostic modality is to treat the likely underlying cause of sinus tachycardia and observe slow resolution.

Sinus Bradycardia

Sinus bradycardia is physiologic in well-trained athletes as well as with vagal stimulation; it is also seen in pathologic states, including increased intracranial pressure, hyperkalemia, hypothermia, hypothalamic seizures, hypothyroidism, hypoxemia, obstructive jaundice, muscarinic poisoning, vagal effects of digitalis intoxication, and surgical damage to the sinoatrial node. It is occasionally seen for no known reason. Note that a slow sinus rhythm implies that the lower pacemakers have an even lower discharge rate than the sinoatrial node.

Sinus Arrhythmia

Phasic sinus arrhythmia (Fig. 485-5) is a normal variant in which the sinus rate varies; usually, but not always, the rate increases with inspiration and slows with expiration. This arrhythmia usually occurs when there is good vagal tone with a slow or normal heart rate; it is rare in newborns. During expiration the sinus rate may be sufficiently slowed to allow escape beats from an atrial or junctional pacemaker. Sinus arrhythmia with atrial escape beats has also been called wandering atrial pacemaker. The variants of sinus arrhythmia are important only in that they should not be mistaken for serious arrhythmias.

Sinus Arrest

Sinus arrest is a prolonged cessation of sinoatrial nodal pacemaker activity for more than 2 cycles (eFig. 485.7). With no sinus beats, there may be escape beats from a lower pacemaker, but if there are long pauses with no atrial or ventricular activity, then there is failure of the lower pacemakers as well as the sinoatrial node.

In sinoatrial block (sinoatrial exit block), the sinoatrial pacemaker is discharging, but occasionally an impulse does not depolarize the atria. This is recognized by pauses that approximate multiples of a normal P-P interval or with Wenckebach-type group beating.

Some children have sick sinus syndrome or bradycardia-tachycardia syndrome. They have episodes of marked sinus slowing or prolonged sinus arrest in which the lower pacemakers do not take over appropriately. As a result, there may be profound slowing of the heart rate or even asystole for several seconds. At other times there may be episodes of tachycardia. This syndrome is seen most often after atrial surgery (eg, repair of atrial septal defects, atrial baffles in transposition of the great arteries, or the atriopulmonary Fontan procedure), but it may rarely arise spontaneously. Attempts to suppress the tachycardias with drugs may lead to unacceptable episodes of slowing or asystole, and permanent implantable pacemakers may be required.

Because atrial and junctional ectopic pacemakers can produce similar QRS complexes and either similarly abnormal P waves or else no visible P waves, they are usually discussed together.

Ectopic atrial rhythm, also called coronary sinus rhythm, is similar to a normal sinus rhythm except for a superior P vector that gives P waves in leads II, III, and aVF. This is a normal variant, and heart rates are normal and may show respiratory variation. It is common in patients with sinus venosus atrial septal defects and in the polysplenia syndromes.

Atrial Premature Beats

Atrial premature beats (extrasystoles) are characterized by P waves that occur prematurely, usually differ in shape from normal P waves, and are followed by a near-normal PR interval and either a normal or sometimes an aberrant QRS complex (eFigs. 485.1 and 485.3). In older children there is usually an incomplete compensatory pause, but in infants the pause is occasionally complete. Very premature atrial extrasystoles may not be conducted because the impulse reaches the atrioventricular node or ventricles while these are still refractory (eFig. 485.3). Very early, blocked atrial extrasystoles are the most common causes of occasional pauses in rhythm. Isolated infrequent atrial extrasystoles are unimportant. They are often seen in newborns and usually disappear by 2 to 6 weeks of age; even if they remain, they are usually of no concern. Multiple atrial premature beats may indicate an underlying mechanism for supraventricular tachycardia, such as an accessory pathway, or can cause transient atrial fibrillation that will normally not be sustained if the atrium is normal. Atrial bigeminy, in which every other P wave is a blocked premature atrial beat, may cause a persistently low ventricular rate.

Paroxysmal Supraventricular Tachycardia

Paroxysmal supraventricular tachycardia includes several mechanisms of tachycardia. It is characterized by rapid atrial rates, generally 180 to 300 beats per minute, and usually each beat is conducted to the ventricles (Fig. 485-6). Initial episodes are most frequent in early infancy, but they can occur at any age and even in utero. Brief episodes may cause no symptoms, although the parents may note mild pallor and a dynamic precordium. Older children may be aware of rapid heartbeats or fluttering in the chest. The episodes usually start and end abruptly. Short episodes of tachycardia usually do not harm the patient, but episodes of several hours or more may cause congestive heart failure, particularly in infants. Infants may even present with shock and heart failure, and the tachycardia may not be recognized as the cause of illness.

Specific causes of the tachycardia usually are not found, although many infants have their paroxysm during a respiratory illness. In adolescents and adults, emotional stress, fatigue, excess caffeine ingestion, tobacco, and drugs such as amphetamines may precipitate attacks. Underlying congenital heart disease is uncommon, although there is a clear association of accessory pathways (including Wolff-Parkinson-White syndrome) and both Ebstein anomaly and hypertrophic cardiomyopathy. About 5% to 10% of patients with paroxysmal supraventricular tachycardias show ventricular preexcitation between the attacks and therefore are said to have Wolff-Parkinson-White syndrome (Fig. 485-6).

Patients with Wolff-Parkinson-White syndrome have one or more bridges of muscle (bundle of Kent) between the atria and ventricles, thus impulses from the atrium can reach the ventricles through 2 pathways. While the impulse is delayed in the atrioventricular node, there is rapid transmission to the ventricles via the bundle of Kent. There is early but slow depolarization of the ventricle adjacent to the bundle of Kent, which produces a short PR interval and then a slow initial part of the QRS complex—the wave. During this slow spread through part of the ventricular muscle, the normal activation by the conduction system occurs, and the rest of the ventricles depolarize normally. The 2 waves of ventricular depolarization meet to produce 1 form of fusion beat. During the tachycardia the impulse descends through the atrioventricular node and reenters by the abnormal connection; thus, the QRS complex is normal. Approximately 50% of patients with preexcitation have tachyarrhythmias.

Other patients, without preexcitation, may have a concealed accessory pathway, in which the bundle of Kent is capable only of retrograde conduction. The tachycardia circuit is identical to that in Wolff-Parkinson-White syndrome. One important form of concealed accessory pathway is the permanent form of junctional reciprocating tachycardia. There is a concealed, slowly conducting accessory pathway near the mouth of the coronary sinus, and slow retrograde conduction in this pathway leads to incessant tachycardia, which may in turn lead to cardiomyopathy. Patients without preexcitation may alternatively have atrioventricular node reentry tachycardia, in which there are 2 atrioventricular node pathways, 1 fast and 1 slow. Reentry occurs within the atrioventricular node, with conduction down the slow and up the fast pathway.

The harmful cardiac effects of supraventricular tachycardias are caused by inadequate ventricular filling because of the short diastole and also loss of atrial contraction at the right time in the cycle. In addition, there can be myocardial ischemia from impaired coronary flow; ischemia probably causes the ST depression and T-wave inversion that are common in the attack. Finally, incessant tachycardias such as atrial ectopic tachycardia, junctional ectopic tachycardia (below), and permanent junctional reciprocating tachycardia, if allowed to continue for weeks or months, may lead to cardiomyopathy. Sometimes atrial tachycardia is seen with 2:1 atrioventricular block. Although this may represent atrial flutter or tachycardia with a very rapid atrial rate, in children taking digoxin digitalis intoxication must be considered (eFig. 485.8). Digoxin causes the ectopic tachycardia and, by its effect on the atrioventricular node, allows only every second impulse to reach the ventricles. As a result, these patients have ventricular rates of about 100 to 150 beats per minute; they may easily be thought to have sinus tachycardia if the alternating blocked P waves are not seen or if the lack of variation of the ventricular rate is not noted. This arrhythmia is important because it may be a prelude to more serious digitalis-induced arrhythmias, including ventricular fibrillation.

Treatment of paroxysmal supraventricular tachycardia is aimed at stopping the attack and preventing future attacks. Treatment of the acute episode depends on the clinical severity. If the child is not very ill, begin by stimulating the vagus nerve. The electrocardiogram should always be recorded before and during vagal stimulation so that there will be a record of the arrhythmia and of vagal effects on it. The vagus nerve can be stimulated mechanically by massaging 1 carotid sinus at a time, by causing gagging with posterior pharyngeal stimulation, by inserting a gloved finger into the anus, by placing a damp cold cloth wrapped around crushed ice on the face and nose for 30 to 40 seconds (diving reflex), or by placing ice water in the stomach with a nasogastric tube. Although stimulating the diving reflex has a high success rate, it should be done with caution in young infants, in whom ventricular fibrillation has, rarely, followed its use. Eyeball pressure should be never be used. If these simple measures are ineffective, then the vagus can be stimulated by raising the blood pressure suddenly and producing a baroreceptor reflex. To do this, inject phenylephrine (Neo-Synephrine), 0.02 mg/kg intravenously, slowly. The vagus may also be stimulated by giving edrophonium chloride (Tensilon), an anticholinesterase. If this is given rapidly intravenously and undiluted at a dose of 2 mg for infants, 5 mg for those 1 to 5 years old, and 10 mg for older children, there will be a sudden but temporary vagal stimulation that may stop the attack. Atropine should be available to treat any bronchospasm that might occur, and edrophonium must not be used if there is a history of asthma. In practice, the use of such drugs to elicit a vagal response has become rare because of the availability of adenosine.

Adenosine is valuable in acute management of supraventricular tachycardias that involve the atrioventricular node as part of the reentrant circuit, such as atrioventricular node reentrant tachycardia and atrioventricular reciprocating tachycardia of Wolff-Parkinson-White syndrome, concealed accessory pathways, and permanent junctional reciprocating tachycardia. When given rapidly by intravenous injection, adenosine induces a very brief episode of complete atrioventricular block, thereby interrupting the reentrant circuit. The onset of effect is usually less than 15 seconds, the half-life in blood is under 10 seconds, and the resulting atrioventricular blockade is, therefore, brief and self-limited. Other forms of abnormal tachycardia such as atrial tachycardia and ventricular tachycardia may also be sensitive to adenosine, independent of the effect on the atrioventricular node, but the effect is not as reliable as in the tachycardias listed above. Successful conversion of supraventricular tachycardia with adenosine may provide evidence against other forms of tachycardia, such as atrial tachycardia, that do not involve the atrioventricular node as part of the reentrant circuit.

Digoxin is a well-tested drug for treating supraventricular tachycardias in children. It is often successful in stopping the attack even if no other therapy is given, but because it may take some time to act, there is no reason not to try vagal stimulation while digoxin is being absorbed. Furthermore, because digoxin sensitizes the heart to vagal stimulation, repeating the vagal stimulation after digitalization is often successful in stopping a persistent attack.

If the infant is severely ill with shock and heart failure, simple maneuvers such as an ice pack on the face or carotid sinus massage may be tried for a very brief period. If there is no response, the arrhythmia may be stopped by an electric shock synchronized so that it does not fall in the vulnerable period (cardioversion). It is then essential to insert an intravenous line for administration of drugs, including sodium bicarbonate to combat acidemia, and to attempt to raise blood pressure with phenylephrine.

Infants with supraventricular tachycardias may have recurrences for the next 6 months, but tachycardias often cease by 1 year of age. Children whose tachycardias begin later are more likely to have recurrent attacks for many years, and many of those who lose their tachycardia in the first year experience recurrences later, often around 4 to 6 years of age.15 Digoxin is usually the most useful agent for preventing recurrences in the first year, but if it is ineffective, then propranolol may be added. If the patient has ventricular preexcitation, digoxin is generally contraindicated. Occasionally, other drugs such as sotalol, flecainide, or even amiodarone may be needed. Some older children have recurrent attacks of supraventricular tachycardia with several months between attacks. If the attacks do not cause heart failure, and if the patient and family can be reassured, it may be better for them to tolerate the attacks rather than take daily medications to prevent a rare event. These patients can often be taught how to stop an attack with a Valsalva maneuver or some other form of vagal stimulation. They can also be given a prescription for propranolol to be used if an attack persists.

On the other hand, patients occasionally have frequent, disabling attacks that do not respond to any form of medical therapy; they may need electrophysiological investigation and radiofrequency ablation for interruption of anomalous pathways or other forms of nonpharmacologic management.16

Supraventricular tachycardia in the fetus requires treatment because of the high incidence of fetal death and nonimmune hydrops fetalis. If the infant is near term, consideration should be given to immediate delivery, after which conventional treatment can be given. If the infant is very premature, a trial of antiarrhythmic agents that cross the placenta is warranted. Digitalizing the mother rapidly (0.25 mg intravenously twice hourly for 3 doses) is the first choice, but if it fails to restore sinus rhythm, other agents may be needed. In particular, flecainide has been used with success in fetuses who have already developed hydrops fetalis. Recent reports suggest that potent antiarrhythmic agents that do not cross the placenta in adequate concentrations, such as amiodarone, may be given to the fetus by direct fetal cordocentesis.

Junctional Tachycardia

An accelerated junctional pacemaker produces a ventricular rate that may be normal for a sinus pacemaker but is high for a junctional pacemaker. This rhythm is seen most often after surgery near the atrioventricular node (such as closure of ventricular septal defect), but there is a congenital form that is incessant and may lead to tachycardia-induced cardiomyopathy. When an accelerated junctional focus is very fast, it is termed junctional ectopic tachycardia. In the postoperative period, intravenous amiodarone is most often employed, along with atrial pacing to restore atrioventricular synchrony, and cooling of the patient to around 34°C may also be effective. In the congenital variety, the use of beta blockers, flecainide, sotalol, and amiodarone have all been reported,17 and catheter ablation is also effective, but with some risk of causing complete atrioventricular block.

Atrial Flutter

Atrial flutter is characterized by rapid atrial rates of about 300 per minute in older children and adolescents and as high as 450 to 500 beats per minute in newborns. In the typical form, there is a sawtooth configuration of atrial waves (F waves) best seen in leads II, III, and V1, with variable atrioventricular block (Fig. 485-7). However, after extensive atrial surgery, atrial flutter may be atypical, and the typical sawtooth F waves may not be present. The ventricular rate may be regular or irregular; if regular, the ratio of atrial rate to ventricular rate can range from 2:1 (most frequent) to 8:1. With a reasonable ventricular rate, the arrhythmia may be tolerated well for a long time, but with rapid ventricular responses, severe symptoms can occur. Treatment should initially be with digoxin to increase the atrioventricular block and slow ventricular rate; if this does not suffice, then propranolol can be added. Sometimes this treatment will cause sinus rhythm to return, but if it does not, electrical cardioversion may be successful. Atrial flutter is rare in infants and children and is usually associated with structural heart disease, particularly with dilated atria. It sometimes occurs transiently in otherwise normal newborn infants. Because atrial flutter has been implicated in sudden death after the Mustard or Senning repair of transposition of the great arteries, an aggressive approach to abolishing the arrhythmia has been recommended. Interruption of the flutter circuit by catheter ablation or surgery has been successful.

Atrial Fibrillation

Atrial fibrillation is rare in children. It is characterized by disordered atrial activity, and the electrocardiogram shows no P waves but instead has fine or coarse fibrillatory or f waves (eFig. 485.9). The ventricular response is very irregular and usually rapid; generally, it is hard to find 2 exactly equal R-R intervals. Initial treatment is with digoxin or other agents such as diltiazem infusion to decrease ventricular rate. If the atrium is enlarged, then atrial fibrillation may be sustained. To restore sinus rhythm, antiarrhythmic agents such as procainamide, sotalol, or ibutilide may be employed, but cardioversion is generally more effective. After sinus rhythm is restored, sotalol or amiodarone may be used to maintain it. If the atrial fibrillation keeps recurring, it is usually better to aim only for control of the ventricular rate with digoxin; if ventricular rates still rise unduly with exercise, then propranolol or verapamil may be added. When atrial fibrillation occurs in the presence of an accessory pathway (Wolff-Parkinson-White syndrome), conduction to the ventricle over the pathway may be dangerously rapid with wide QRS complexes and is thought to be the mechanism of sudden death in this syndrome. Because digoxin may shorten the anterograde effective refractory period of the accessory pathway, allowing even faster ventricular rates, it is contraindicated for the acute and chronic treatment of patients of any age with the Wolff-Parkinson-White syndrome except when an electrophysiologic study has documented that digoxin may be used safely.

Ventricular Extrasystoles

Ventricular extrasystoles may occur infrequently or in a fixed ratio with normal beats: bigeminy, trigeminy, quadrigeminy, and so on (eFigs. 485.1 and 485.6). If there is a single ectopic focus, then the coupling interval (the interval between the normal QRS complex and the ectopic one that follows it) is constant or almost constant, and the wide QRS complex is uniform in configuration. These uniform ventricular extrasystoles are common in normal adolescents. When the heart rate increases with exercise or anxiety, the extrasystoles usually disappear because the faster sinus rate discharges the ectopic focus before it can produce the premature beats. With mild exercise the sympathetic stimulation may actually increase the frequency of these ectopic beats, and it is only when heart rates exceed 150 to 160 beats per minute that these benign ectopic beats may disappear. Alcohol, caffeine, tobacco, fatigue, amphetamines, or other sympathomimetic drugs may increase the frequency of ventricular extrasystoles. In clinically healthy people, uniform ventricular ectopic beats are usually benign. However, if there is underlying heart disease, more attention should be paid to these extrasystoles, especially if they are of recent onset. If the extrasystoles appear in a patient receiving catecholamines or digoxin, they should be regarded as possible signs of toxicity, and the dosage should be reduced or the drug should be stopped. Should there be frequent ventricular extrasystoles, particularly if they are known to be of recent onset, cardiac consultation should be obtained in order to exclude underlying heart disease such as mitral valve prolapse, myocarditis, or cardiomyopathy.

More importance should be attached to multiform ventricular extrasystoles (ie, those with different coupling intervals and varying QRS complexes and axes); these are particularly common in digitalis intoxication in adults, but are unusual in children. They cause concern for 2 reasons. First, they are often a sign of significant underlying cardiac disease, especially with poor myocardial function. Second, multiform ventricular ectopy is more likely to be associated with unstable ventricular tachycardia or ventricular fibrillation.

Couplets in which 2 extrasystoles follow a normal beat may be serious and require cardiac consultation. Ventricular premature beats sometimes occur early after cardiac surgery, particularly ventriculotomy. Unless they are very frequent, multiform, or repetitive, they need no treatment. They usually disappear with time. If they appear later, or if exercise testing brings out frequent or multiform ventricular ectopic beats, then cardiac consultation should be sought.

Ventricular Tachycardia

Ventricular tachycardia is defined as a series of 3 or more ventricular ectopic beats, often manifesting as a rapid tachycardia with wide QRS complexes (Fig. 485-8A). Although in children the QRS complex can be relatively narrow (Fig. 485-8B), it always differs from the QRS in sinus rhythm (Fig. 485-8C). Often there is atrioventricular dissociation, with the P waves occurring less often than QRS complexes; this may be seen by slight variations of the T waves caused by varying superimposition of P waves on them. However, in about one half of pediatric patients, there may be retrograde capture of the atria, particularly at slower ventricular rates, so that there is a 1:1 relationship between atrial and ventricular contractions. The major differential diagnosis is from supraventricular tachycardia with aberrant conduction. Proof of the ventricular origin of the arrhythmia can be obtained if the QRS complexes resemble typical ventricular extrasystoles in previous electrocardiograms or if ventricular fusion beats are seen (eFig. 485.6 and Fig. 485-8).

Ventricular tachycardia may cause severe symptoms and, particularly in patients with structural heart disease, may lead to ventricular fibrillation. It should usually be treated as an emergency. Because most wide QRS tachycardias in children represent ventricular tachycardia, they should generally be treated as ventricular tachycardia unless one is certain of a supraventricular origin. Inappropriate treatment of ventricular tachycardia (with digoxin or verapamil, for example) may be fatal. Cardioversion is usually effective. If the patient is hemodynamically stable, amiodarone, lidocaine, or procainamide may be given intravenously. Future attacks may be prevented with propranolol, mexiletine, sotalol, flecainide, or amiodarone. Such decisions should always be made with the involvement of a cardiology consultant. Occasionally, ventricular tachycardia occurs at a rate similar to a normal sinus rate and with a relatively narrow QRS complex (Fig. 485-8); this is called ventricular accelerated rhythm. If it is seen in the presence of an otherwise normal heart and causes no circulatory difficulties, it is benign. It will usually not require treatment, and when seen in the newborn, episodes usually disappear within several weeks or months.

Ventricular Fibrillation

Ventricular fibrillation produces a recording with no QRS complexes on it; there is just a wavy line (Fig. 485-9). P waves can be present. There is no cardiac output, and cardiopulmonary resuscitation should be commenced while preparations are made for electrical defibrillation.

Ventricular fibrillation is often the terminal event in many illnesses. It may follow hypoxemia, hyperkalemia, digitalis or quinidine intoxication, myocarditis, myocardial infarction, catecholamine infusions, anesthetics, and many drugs. It may be seen in patients with dilated cardiomyopathy. It is also a consequence of other arrhythmias, particularly ventricular tachycardia or multiple multiform ventricular ectopic beats.

Ventricular fibrillation may occur in 2 other settings. People with complete (third-degree) atrioventricular block have episodes of transient ventricular fibrillation that cause syncope (Adams-Stokes attacks), which indicates the need for artificial pacemakers. Occasionally, these attacks occur when the complete heart block is intermittent and is brought on by vagal or other stimuli. Ventricular fibrillation is also the cause of the syncope and death that occurs in the long QT syndromes. In this condition, the arrhythmia starts with an unusual form of ventricular tachycardia called torsades de pointes, in which there are polymorphous complexes that seem to rotate around the isoelectric line. Several of these patients have died suddenly of fright or on hearing an unexpected loud noise. In nearly all of the congenital long QT syndromes, the prolongation of the QT interval is related to specific heritable abnormalities in cardiac potassium or sodium channels. Multiple specific genes coding for channels have been identified that may be abnormal because of mutations or deletion, but a significant minority of patients with long QT syndrome have none of the known mutations. Treatment of these syndromes generally requires chronic β-adrenergic blockade, and some patients will require an implantable cardioverter-defibrillator, particularly those with a history of aborted sudden death, those who continue to have syncope despite appropriate beta-blocker therapy, and those with the sodium-channel mutation (long QT 3).18

First-Degree Atrioventricular Block

First-degree atrioventricular block (Fig. 485-2) may occasionally occur without any evidence of heart disease or with congenital heart disease such as endocardial cushion defect or corrected transposition of the great arteries. It may also occur with acute infections such as in acute rheumatic fever, diphtheria, and other infections, and its appearance does not necessarily mean that there is myocarditis. It also occurs with strong vagal stimulation and with digoxin administration. The mechanism may be delay of conduction through the atrium, atrioventricular node, or His-Purkinje system. It needs no treatment, but it should be considered carefully to determine if there is an underlying problem and followed to find out if the conduction defect will get worse and produce higher degrees of heart block.

Second-Degree Atrioventricular Block

Mobitz type I second-degree atrioventricular (AV) block (Wenckebach conduction) (Fig. 485-4) may occur in healthy people, particularly athletes with slow sinus rates. In this setting it occurs at night and is caused by increased vagal tone. If it is asymptomatic in someone with no structural or functional heart disease, it does not require aggressive evaluation or treatment. If it causes symptoms, it usually responds to atropine. This type of AV block can also be caused by drugs such as digoxin. Mobitz type II second-degree AV block usually results from disease of the His-Purkinje system, and in the pediatric population this is most commonly due to cardiac surgery. It always requires careful evaluation. As there is a significant risk of progression to third-degree AV block or syncope, with the chance of sudden death, permanent pacemaker implantation is generally recommended.

Third-Degree Atrioventricular Block

Third-degree (complete) atrioventricular block is often congenital or occurs early in childhood. In two thirds of these children there is no underlying heart disease, but the others may have a variety of congenital lesions, the most common being corrected transposition of the great arteries. The block often occurs in utero, and at times cesarean section has been done because of fetal distress inferred from the slow heart rate. There is a strong association with systemic lupus erythematosus or mixed connective tissue disease in the mother, who may even develop overt disease later, after the child’s birth. Complete heart block may also follow surgery when patches are placed in the region of the atrioventricular node and the bundle of His. Occasionally, drugs may cause complete heart block, and it has been seen in digitalis intoxication. Finally, it is a manifestation of the NKX 2.5 mutation, in which there is an association with congenital heart disease.19

Although some infants with congenital heart block develop congestive heart failure, most remain asymptomatic for several decades. These patients adjust to the slow rate by increasing stroke volume and by cardiac hypertrophy; they often have systolic flow murmurs over the base and middiastolic flow murmurs over the mitral and tricuspid valves because of the increased stroke volume. The first heart sound varies in intensity because of varying PR interval, and there are cannon waves in the jugular veins. On the electrocardiogram there are regular P waves at a normal rate, but these will be completely unrelated to the less-frequent QRS complexes (Figs. 485-3 and 485-10). The QRS complexes are usually narrow and normal, indicating that the escape focus generating the ventricular rhythm is high in the conducting system. Sometimes the QRS complexes are wide because the escape focus is in the ventricles.

Permanent implanted pacemakers are needed when the ventricular rate is too slow to sustain an adequate cardiac output or if these patients have syncope that may be from Adams-Stokes attacks (episodes of syncope from sudden asystole, ventricular tachycardia, or ventricular fibrillation; see Fig. 485-10). Usually, these episodes last between 6 and 60 seconds, and then the ventricles resume their slow beat spontaneously; but sometimes the episodes are fatal. The attacks are more common after surgical block than with congenital block and more common with a wide than a narrow QRS complex. Should they occur, a ventricular pacemaker should be inserted promptly. Isoproterenol at 0.05 to 0.5 μg/kg/min may be infused to speed the ventricles before inserting the permanent pacemaker. All drugs that are potentially cardiac depressants should be avoided, as they can depress the idioventricular pacemaker. Another group that should be considered for permanent pacing is that of asymptomatic patients whose mean daytime ventricular rates are below 50 beats per minute or who have long pauses with R-R intervals greater than 3 seconds, because of their tendency to have syncope or sudden death. By adulthood most if not all patients with complete congenital atrioventricular block will have pacemakers implanted because of the development of symptoms or to prevent them.

In fetuses with isolated complete atrioventricular block, a favorable outcome has usually been found at heart rates over 55 beats per minute. If the fetal heart rate is lower, perinatologists have attempted to increase it, either with tocolytic agents that are also β-sympathomimetic agents (ritodrine or terbutaline) or by fetal placement of a pacemaker. These treatments are still undergoing clinical trials.

Surgical Bundle Branch Block

After closure of a ventricular septal defect, either isolated or in a tetralogy of Fallot, there may be damage to the right bundle branch and the anterior fibers of the left bundle. There will then be a right bundle branch block and a left anterior hemiblock manifested by a left superior QRS axis deviation of about –30� to –90�. These patients have only the posterior fibers of the left bundle for atrioventricular (AV) conduction and so are at risk of developing complete AV block should these remaining fibers be injured (eg, by slow fibrosis). No treatment is indicated, but care must be paid to any symptoms that might suggest episodes of complete AV block. Electrophysiologic study to determine if the residual fibers can conduct normally may be indicated, particularly if the PR interval is also prolonged.