CHAPTER 84: SUPRAVENTRICULAR TACHYCARDIA: ATRIAL TACHYCARDIA, ATRIOVENTRICULAR NODAL REENTRY, AND WOLFF-PARKINSON-WHITE SYNDROME

Supraventricular tachycardias (SVTs) include all tachyarrhythmias that either originate from or incorporate supraventricular tissue in a reentrant circuit. The ventricular rate may be the same or less than the atrial rate, depending on the atrioventricular (AV) nodal conduction. The term paroxysmal supraventricular tachycardia (PSVT) refers to a clinical syndrome characterized by a rapid, regular tachycardia with an abrupt onset and termination. Approximately two-thirds of cases of PSVT result from AV nodal reentrant tachycardia (AVNRT). Orthodromic AV reentrant tachycardia (AVRT), which involves an accessory pathway (AP), is the second most common cause of PSVT, accounting for approximately one-third of cases. The term Wolff-Parkinson-White (WPW) syndrome designates a condition comprising both preexcitation and tachyarrhythmias. Atrial tachycardias, which arise exclusively from atrial tissue, account for approximately 5% of all cases of PSVT.¹ The purpose of this chapter is to review the mechanism, clinical features, and approach to diagnosis and treatment of patients with AVNRT and AP-mediated tachycardias (including WPW syndrome). Particular attention is focused on reviewing management guidelines developed by the American Heart Association (AHA), American College of Cardiology (ACC), and Heart Rhythm Society (HRS; formerly known as the North American Society of Pacing and Electrophysiology).¹ We will also provide a brief overview of atrial tachycardia.

AVNRT is an important arrhythmia for several reasons. First, AVNRT is extremely common. AVNRT occurs in approximately 10% of the general population and accounts for up to two-thirds of all cases of PSVT. Although AVNRT can occur at any age, it is extremely uncommon before 5 years of age. The usual age of onset is beyond the fourth decade of life and is later than the usual age of onset of AP-mediated tachycardias. Women are affected twice as often as men. A second reason for the importance of AVNRT is the fact that it can result in significant debility and decreased quality of life.

Pathophysiologic Basis of Atrioventricular Nodal Reentrant Tachycardia

Anatomic Considerations of the Atrioventricular Node

The AV node is located epicardially just underlying the right atrial epicardium anterior to the nodal artery and between the coronary sinus (CS) and medial tricuspid valve leaflet. It comprises three different components: the transitional cell zone, the compact node, and the penetrating bundle of His (Fig. 84–1). The compact AV node refers to the most easily histologically distinguishable tissue located at the apex of the triangle of Koch (TOK). A zone of transitional cells is interposed between the compact node and the atrial myocardium. Transitional cells enter the TOK to join the compact node superiorly, inferiorly, posteriorly, and from the left. At its distal extent, the AV node is distinguished from the penetrating bundle, not so much by cellular characteristics as by the presence of a fibrous collar surrounding the specialized cells. Systematic anatomic investigation of the AV node in patients with AVNRT is lacking. No obvious histologic abnormalities have been identified among patients with AVNRT versus patients without AVNRT. Several recent autopsy studies have reported that the sites of successful slow pathway ablation were clearly away from the histologic compact AV node, approximately 1 or 2 cm inferior and posterior to it.

Concept of Dual Pathways

The concept of dual AV nodal physiology was introduced in the 1950s in an effort to explain AVNRT. It was proposed that dual AV node physiology results from functional dissociation within the compact node into fast and slow pathways, and that AVNRT resulted from reentry within the AV node involving the fast and slow pathways. Subsequently, several investigators developed a catheter-based ablative technique that used direct current shocks or radiofrequency (RF) energy to eliminate the fast AV pathway and permanently eliminate AVNRT. Despite the excellent results of this procedure, it was complicated by a small, but definite, risk of AV block. A technique to interrupt slow pathway, conduction without affecting normal AV conduction through the fast pathway by ablating small segments of myocardium in the posterior septum near the CS ostium, was subsequently described.²This experience paved the way for the current approach using catheter ablation of this arrhythmia.

Types of Atrioventricular Nodal Reentrant Tachycardia

Three types of AVNRT have been described (Table 84–1).³ Typical or slow/fast AVNRT is the most prevalent type, accounting for 85% to 90% of cases. Representing the other 10% to 15% of cases, atypical AVNRT can be further differentiated into fast/slow and slow/slow (or intermediate) AVNRT. Induction of typical and atypical AVNRT in the same patient is possible, but unusual. The typical or slow/fast AVNRT is thought to use the slow pathway for antegrade conduction and the fast pathway for retrograde conduction (Fig. 84–2). When an atrial premature complex blocks the fast pathway and proceeds slowly along the slow pathway, the fast pathway has enough time to recover from its refractoriness. This allows the impulse to activate the fast pathway retrogradely and return to the atrium, giving rise to an AV nodal reentrant echo beat. The impulse then travels down along the slow pathway again, continuation giving rise to AVNRT. It has been proposed that the fast/slow AVNRT uses the fast pathway for anterograde conduction and the slow pathway for retrograde conduction. The slow/slow AVNRT, conversely, requires presence of two or more slow pathways with different conduction properties and refractory periods; one slow pathway is used for antegrade conduction and the other slow pathway for retrograde conduction.

Diagnosis

Clinical Features

Patients with AVNRT typically present with the clinical syndrome of paroxysmal SVT. Episodes may last from seconds to several hours. Patients often learn to use certain maneuvers such as carotid sinus massage or the Valsalva maneuver to terminate the arrhythmia, but many patients require pharmacologic treatment to achieve this. There is no significant association of AVNRT with other types of structural heart disease. The physical examination is usually remarkable only for a rapid, regular heart rate. At times, because of the simultaneous contraction of atria and ventricles, cannon A-waves can be seen. Clinical variables that are predictive of AVNRT rather than APs as the cause of paroxysmal SVT include older age at onset of symptoms (> 30 years), the presence of palpitations in the neck during tachycardia, and female gender.

Electrocardiographic Characteristics

AVNRT is characterized by a tachycardia with a narrow QRS complex with sudden onset and termination generally at regular rates between 120 and 200 bpm. Uncommonly, the rate can be as low as 110 bpm; occasionally, especially in children, it may exceed 200 bpm. The rate of tachycardia may vary from episode to episode. In typical or slow/fast AVNRT, anterograde AV node conduction usually exceeds 200 ms. Because the retrograde conduction is through the fast pathway, the ventriculoatrial (VA) interval is short, resulting in superimposition of the P wave onto the QRS complex on the surface electrocardiogram (ECG). Usually, the P wave is obscured by the QRS or may be seen slightly before or after the QRS complex. The presence of a pseudo r wave in lead V₁ or pseudo S wave in leads II, III, and aVF suggests typical AVNRT (Fig. 84–3). Because of fast retrograde conduction, the RP interval is shorter than the PR interval.

In atypical fast/slow or slow/slow AVNRT, the atrial to His bundle (AH) interval is relatively short and the His bundle to atrial (HA) interval long. The P wave on surface ECG hence can be well delineated and is inverted in II, III, and aVF. The RP interval is equal to or longer than the PR interval (Fig. 84–4). This is one of the causes of long R–P tachycardia (see Table 84–1).

Functional bundle branch block (BBB) may develop, producing a wide QRS tachycardia. However, functional BBB should not affect the rate of tachycardia. Less commonly, dual pathways can be manifest on the ECG during sinus rhythm by sudden prolongation of the PR interval, PR alternans, and two QRS complexes in response to a single P wave (Fig. 84–5).

Other ECG changes may be seen during or after the termination of AVNRT. Significant ST-segment depressions can be observed during tachycardia in nearly 25% to 50% of patients with AVNRT, and this is not predictive of ischemia.

Newly acquired T-wave inversions after termination of AVNRT, commonly in anterior or inferior leads, can be seen in nearly 40% of patients. They may be seen immediately after termination of tachycardia or may develop within 6 hours, and they can persist for a variable duration. This occurrence is also not related to rate or duration of tachycardia. This is also not the result of coronary artery disease, but is caused by repolarization abnormalities, probably because of ionic current alterations resulting from the rapid rate.

A beat-to-beat oscillation in the QRS amplitude (ie, QRS alternans) can be observed, although uncommonly, during episodes of AVNRT. Studies have reported that QRS alternans is observed more frequently in association with AP-mediated tachycardias than during AVNRT.

Electrophysiologic Testing

Dual AV nodal physiology can be demonstrated by two pacing techniques. Atrial pacing with introduction of premature atrial contractions (PACs) with increasing prematurity shows a gradual and progressive conduction delay in the AH interval. At a critical atrial coupling interval, a 10-ms decrement in the A₁A₂ results in a marked (> 50-ms) prolongation of the A₂H₂ interval. It is a well-accepted convention that a 50-ms or greater increase in the AH interval in response to a 10-ms shortening of the A₁A₂ is considered evidence of dual AV node physiology. A plot of the A₁A₂ versus the A₂H₂ or the A₂H₂ versus the H₁H₂ shows a discontinuous curve (Fig. 84–6). The fast pathway has a shorter conduction time and longer refractory period, and the slow pathway has a longer conduction time and shorter refractory period. The abrupt increase in AV nodal conduction time is caused by block in the conduction of fast pathway with a selective conduction over the slow pathway (Fig. 84–7). Some patients with AVNRT may not have discontinuous refractory period curves, and some patients without AVNRT exhibit discontinuous refractory period curves. It has been estimated that dual AV node physiology is present in approximately 10% of the general population. Such existence in the latter patients is a benign finding. Multiple AV nodal pathways can be demonstrated in occasional patients. More than one pathway may be involved in clinical tachycardia. The development of a PR interval that is greater than the atrial pacing cycle length during stable 1:1 AV conduction is a quite specific sign of slow AV nodal conduction.

Whereas typical AVNRT is usually not inducible with ventricular pacing, this is the rule with atypical AVNRT. Lack of reproducible arrhythmia induction is most often caused by block in retrograde fast pathway. Other causes include slow-pathway block and an inability to achieve a critical delay in the AH interval.

During typical AVNRT, the atrial and ventricular activation is nearly simultaneous, hence causing short HA intervals. An HA interval below 70 ms is virtually diagnostic of AVNRT (see Fig. 84–7). Differentiation of AVNRT, particularly of the atypical type, from other forms of SVT can be challenging, requiring the use of several maneuvers during the electrophysiologic study (EPS).

Management

Because AVNRT is generally a benign arrhythmia that does not influence survival, the main reason for treating it is to alleviate symptoms. The threshold for initiation of therapy varies based on patient preference and on the frequency and duration of the episodes of tachycardia as well as the associated symptoms. The threshold for treatment also reflects whether the patient is a competitive athlete, a woman considering pregnancy, or someone with a high-risk occupation such as scuba diving. The ACC/AHA/HRS have recently published updated guidelines on the management of supraventricular arrhythmias. The recommendations made by this task force are reviewed at the end of this section.¹

Acute Management

The dependence of AVNRT on AV nodal conduction has important therapeutic implications. Because of the known influence of autonomic tone on AV nodal conduction, maneuvers that increase vagal tone, such as the Valsalva and Mueller maneuvers, gagging, carotid sinus massage, and occasionally exposing the face to ice water, can be used to terminate the tachycardia. The effect of vagal maneuvers is most pronounced on the slow pathway. Hence, in the slow/fast form of AVNRT, the tachycardia typically terminates in the anterograde direction; in the fast/slow type of AVNRT, the block is in the retrograde limb of the tachycardia circuit.

Adenosine, a purinergic blocking agent that causes acute and transient AV nodal blockade, is the drug of choice for acute termination of AVNRT. Adenosine is nearly 100% effective in terminating AVNRT. It has a rapid onset of action (seconds) and a short half-life (< 10 seconds); moreover, it does not impair contractility. When used for termination of a narrow-complex tachycardia, an initial dose of 6 mg may be followed by 12 mg if the first dose is ineffective. Because of its short duration of action, it is essential that adenosine be administered as a rapid bolus. This is best achieved by having a syringe with adenosine and a 5-mL flush attached to the patient using a stopcock. In this fashion, the dose of adenosine can be followed immediately by a saline flush to ensure rapid delivery of the drug. Minor side effects, including transient dyspnea or chest pain, are common with adenosine. Sinus arrest or bradycardia may occur, but resolve quickly if appropriate upward dosing is used. Adenosine shortens the atrial refractory period, and atrial ectopy may induce atrial fibrillation. This may be dangerous if the patient has an AP capable of rapid antegrade conduction. Because adenosine is cleared so rapidly, reinitiation of tachycardia after initial termination may occur. Either repeat administration of the same dose of adenosine or substitution of a calcium channel blocker (CCB) will be effective. Adenosine mediates its effects via a specific cell-surface receptor, the A₁ receptor. Theophylline and other methylxanthines block the A₁ receptor. Caffeine levels achieved after beverage ingestion may be overcome by the doses of adenosine used to treat patients with PSVT. Dipyridamole blocks adenosine elimination, thereby potentiating and prolonging its effects. Cardiac transplant recipients are also unusually sensitive to adenosine. If adenosine is chosen in these latter situations, much lower starting doses (ie, 1 mg) should be selected. Adenosine should also be used with great caution in patients with a history of asthma because it may trigger bronchospasm.

Several other drugs that affect the AV node can also be used for acute termination of AVNRT. Verapamil, a CCB, can terminate AVNRT and prevent induction. It slows conduction both in the slow and fast pathways, and termination is usually caused by anterograde block. A 5-mg bolus of verapamil may be followed by one or two additional 5-mg boluses 10 minutes apart if the initial dose does not terminate the tachycardia. Drugs that enhance vagal tone, such as digoxin, or that block the sympathetic effect, like β-blockers, can also be used to terminate AVNRT. Digoxin, which has a slower onset of action than the other AV nodal blockers, is not favored for the acute termination of AVNRT, except if there are relative contraindications to the other agents. Class Ia and Ic sodium channel blockers can also be used in treating an acute event of AVNRT, a strategy that is rarely used when other regimens have failed. Unlike the other agents, the sodium channel blocking agents depress retrograde fast-pathway conduction. If a patient’s tachycardia cannot be terminated with intravenous drugs, direct-current shock cardioversion can always be used. Energies in the range of 10 to 50 J are usually adequate.

The recommendations for the acute management of patients with recurrent AVNRT, which were developed by the ACC/AHA/HRS, are shown in Table 84–2.¹ Catheter ablation is considered class 1 therapy for treatment of patients with symptomatic AVNRT. The indications for pharmacologic management are also provided.

Long-Term Management

Catheter Ablation

Catheter ablation of AVNRT is performed by ablating the slowly conducting portion of the AV node using the posterior approach. After the diagnosis of AVNRT has been established and the end point for ablation defined (either the presence of dual AV node physiology or reproducibly inducible AVNRT), the ablation catheter is positioned across the tricuspid valve at the level of the CS ostium (Fig. 84–8). The ablation catheter is then slowly withdrawn with clockwise catheter torque until a small multicomponent atrial electrogram is observed together with a large ventricular electrogram.³ The most common site of successful ablation is at the level of the superior aspect of the CS ostium (see Fig. 84–8). In patients with unusual forms of AVNRT, the site of successful ablation is slightly inferior to the CS ostium or, at times, within the floor of the CS. After an appropriate site has been identified, RF energy is applied. The development of a junctional rhythm during RF application is a marker for a successful ablation site. If a junctional rhythm develops, VA conduction should be monitored carefully; if VA or AV conduction block is observed, RF energy should immediately be discontinued. Some centers, including ours, use an even more conservative strategy for RF energy delivery in an effort to further reduce the risk of AV block.⁴

Successful ablation of AVNRT with the posterior approach is usually characterized by an increase in the AV block cycle length and in the AV nodal effective refractory period; a normal PR interval on ECG and lack of inducibility of AVNRT, both at baseline and during isoproterenol infusion, are also seen. Retrograde conduction is generally not affected. The presence of more than one AV nodal echo beat or a PR prolongation that exceeds the paced atrial cycle length during stable 1:1 AV conduction suggests that the AVNRT is likely to recur and will require further RF applications.

Calkins and colleagues.⁵ reported the largest multicenter experience with the posterior approach. In this trial of more than 1000 patients with AV tachycardias, 373 patients had AVNRT. The overall success rate of ablation was 97%. AV block is the most common complication, occurring in 0.5% to 1% of patients. The incidence of recurrence after successful ablation is approximately 3%. Because of the higher efficacy and lower incidence of AV block and arrhythmia recurrence, and the greater likelihood of maintaining a normal PR interval during sinus rhythm, the posterior approach is now considered the preferred approach to ablation of AVNRT.⁶

Cryoablation has become available as an alternative energy source for creation of myocardial lesions that can be used for treatment of AVNRT.⁶ The main advantage of cryoenergy as compared with RF energy is that the risk of heart block appears to be lower.^7,8,9,10 This potential, but unproven, benefit must be balanced against longer procedure times and lower acute and long-term efficacy. Because of the implications of heart block in children, cryoablation is used by an increasing number of pediatric electrophysiologists.⁸ In contrast, RF energy has remained the dominant ablation energy source for treatment of AVNRT in adults.

Medical Therapy

Most pharmacologic agents that depress AV nodal conduction may be demonstrated to reduce the frequency of recurrences of AVNRT. These pharmacologic agents include β-blockers, CCBs, class 1a antiarrhythmic agents such as procainamide and disopyramide, class 1c antiarrhythmic agents such as flecainide and propafenone, and class 3 antiarrhythmic agents such as sotalol (Betapace) and amiodarone. The best studied among these have been the class Ic antiarrhythmic agents. Although the 1c antiarrhythmic agents are effective for treatment of AVNRT, they are used rarely because of the curative nature of catheter ablation.

Management Strategy

Patients with AVNRT typically present with the clinical syndrome of PSVT. The diagnosis of PSVT can be made with a high degree of certainty based on the clinical history alone, even if the tachycardia has never been documented by an ECG. Physicians may try to document the tachycardia with an ECG using either a 30-day event monitor or by instructing the patient to go to an emergency department or physician’s office if another episode of tachycardia occurs. Because AVNRT is not a life-threatening arrhythmia, the primary indication for its treatment relates to its impact on the patient’s quality of life. Patients who develop a highly symptomatic episode of PSVT, particularly if it requires an emergency room visit for termination, may elect to initiate therapy after a single episode. By contrast, a patient who presents with minimally symptomatic episodes of PSVT that terminate spontaneously or with a Valsalva maneuver may elect to be followed clinically without specific therapy.

After it has been decided to initiate treatment for AVNRT, the question arises of whether to initiate pharmacologic therapy or to use catheter ablation (Table 84–3). Because of its greater than 95% efficacy and low incidence of complications, catheter ablation is now considered first-line therapy. The ACC/AHA/HRS SVT Guidelines include catheter ablation of AVNRT using the posterior approach as a class 1 indication for catheter ablation.¹ It is therefore reasonable to discuss catheter ablation with all patients suspected of having AVNRT. Depending on the frequency and severity of tachycardia episodes as well as the patient’s lifestyle and preferences, an individual may elect to be followed clinically without specific therapy, to begin a trial of pharmacologic therapy with a β-blocker or CCB, or to undergo EPS and catheter ablation.

The recommendations for the long-term management of patients with recurrent AVNRT, which were developed by the ACC/AHA/HRS, are shown in Table 84–4.¹ Catheter ablation is considered class 1 therapy for treatment of patients with symptomatic AVNRT. The indications for pharmacologic management are also provided.

APs are important because they provide a substrate for antidromic and orthodromic AV reciprocating tachycardia, are associated with sudden cardiac death, and may be detected in asymptomatic patients on a routine screening ECG. The sections that follow cover the pathophysiology, diagnosis, and management of this fascinating clinical entity.

Pathophysiology

APs are anomalous, typically extranodal connections that connect the epicardial surfaces of the atrium and ventricle along the AV groove. Accessory bypass tracts, which conduct antegrade from the atrium to the ventricle and, therefore, are detectable on an ECG, are reportedly present in 0.15% to 0.25% of the general population. A higher prevalence of 0.55% has been reported in first-degree relatives of patients with WPW syndrome.

Classification

APs can be classified based on their site of origin and insertion, location along the mitral or tricuspid annulus, type of conduction, and properties of conduction (decremental or nondecremental). APs usually exhibit rapid, nondecremental conduction, similar to that which is present in normal His-Purkinje tissue and atrial or ventricular myocardium. Approximately 8% of APs display decremental antegrade or retrograde conduction. Whereas APs that are capable only of retrograde conduction are referred to as concealed, those capable of antegrade conduction are referred to as manifest, demonstrating preexcitation on a standard ECG (Fig. 84–9). APs usually conduct both antegrade and retrograde. Antegrade-only APs are particularly uncommon. When present, they are usually right sided and frequently demonstrate decremental conduction (Mahaim fiber). Concealed APs are less common, accounting for approximately 15% of all APs. Patients with Ebstein anomaly who also have WPW syndrome frequently have more than one AP.

Variant APs include those that connect the atrium to the distal or compact AV node (James fibers), the atrium to the His bundle (the Brechenmacher fiber), and the AV node or His bundle to the distal Purkinje fibers or ventricular myocardium (the Mahaim fibers) (see Fig. 84–1B).

Concept of Preexcitation

The hallmark of an AP function during sinus rhythm is depolarization of all or part of the ventricles earlier than expected if conduction has occurred only over the normal AV conduction system, resulting in preexcitation (Fig. 84–10).

The degree of shortening of the PR interval and the extent of ventricular preexcitation depend on several factors, including location of the AP, the relationship between antegrade conduction times and refractory periods of the AV bypass tract, and the normal AV conduction system. A bypass tract that crosses the AV groove in the left lateral region may also result in inapparent preexcitation and minimal PR interval shortening during sinus rhythm because of greater interatrial distance for impulse propagation from the sinus node to this site of atrial input into the AP. Conversely, an AP on the right side is more likely to demonstrate marked preexcitation. Preexcitation may be less apparent during sinus tachycardia, when sympathetic tone is high and vagal tone low, resulting in faster AV node conduction time than that in the AP. On the other extreme, during conditions of slowed conduction through the AV node by intrinsic nodal factors, withdrawal of sympathetic tone, or increased vagal tone, the amount of preexcitation apparent on the 12-lead ECG is maximized because of relatively greater conduction through the AP. Rapid intravenous administration of adenosine causing blocking or slowing of AV node conduction and exposing the anterograde AP conduction has been used as a diagnostic maneuver. The degree of preexcitation can also be enhanced with atrial pacing directly over the AP, eliminating the intra-atrial conduction delay from the sinus node to the atrial insertion site of AP (Fig. 84–11).

Intermittent preexcitation is characterized by abrupt loss of delta wave, normalization of the QRS duration, and an increase in the PR interval during a continuous ECG recording, often despite only minor variations in resting sinus rhythm heart rate. This should be distinguished from day-to-day variability in preexcitation or inapparent preexcitation caused by factors described above. The presence of intermittent preexcitation has been considered to suggest that the refractory period in the AP is long, making them very unlikely to mediate a rapid, preexcited ventricular response during atrial fibrillation.

Tachycardias Associated with Accessory Pathways

Tachycardias associated with APs can be subdivided into those in which the AP is necessary for initiation and maintenance of tachycardia and those in whom the AP acts as a bystander.

AVRT is a macro-reentrant tachycardia involving the atrium, the AP, the AV node, and the ventricle. AVRT is further subclassified into orthodromic and antidromic AVRT (Fig. 84–12). During orthodromic AVRT, the reentrant impulse uses the AV node and specialized conduction system for conduction from the atrium to the, ventricle and uses the AP for conduction from the ventricle to the atrium. Orthodromic AVRT can be initiated by atrial or ventricular premature depolarizations (atrial premature beats [APDs] or ventricular premature beats [VPDs]). APDs initiating the tachycardia block antegradely in the AP and conduct relatively slowly over the AV nodal tissue to the ventricles. The impulse then retrogradely conducts over the AP reentering the atria at the atrial insertion site of the pathway, thus completing the reentrant loop (Fig. 84–13). A VPD, conversely, blocks in the His-Purkinje system and retrogradely reaches the atria through retrogradely conducting AP. The impulse then antegradely conducts through the AV nodal tissue, completing the circuit. The QRS complex during orthodromic AVRT hence is not preexcited.

During antidromic AVRT, however, the reentrant impulse travels in the reverse direction, with conduction from the atrium to the ventricle occurring via the AP. Antidromic AV reciprocating tachycardia is rare, occurring in only 5% to 10% of patients with WPW syndrome. APDs that occur at a coupling interval that is longer than the refractory period of the AP and shorter than the AV nodal refractory period can initiate the antidromic AVRT; the converse is true with a VPD. Susceptibility of antidromic AVRT appears to depend on the existence of adequate separation between the AP and the AV nodal tissue. Hence, most of the antidromic AVRTs seem to occur only with left-sided bypass tracts.

Other forms of SVTs, including atrial tachycardia, junctional tachycardia, AVNRT, and even ventricular tachycardia, may occur in patients with bypass tracts. Dual AV nodal physiology has been noted in nearly 12% of patients with WPW syndrome. Coexisting ventricular tachycardia is less likely because patients with WPW syndrome tend to present at a younger age and have less structural heart disease.

Atrial fibrillation is a less common, but potentially more serious, arrhythmia in patients with the WPW syndrome. If an AP has a short antegrade refractory period, atrial fibrillation may result in a rapid ventricular response with subsequent degeneration to ventricular fibrillation.^11,12,13 The risk of sudden death has been shown to be higher if the shortest R-R interval is less than 250 ms during spontaneous or induced atrial fibrillation.¹¹ It has been estimated that one-third of patients with WPW syndrome also have atrial fibrillation. APs appear to play a pathophysiologic role in the development of atrial fibrillation in these patients, because most are young and do not have structural heart disease. Furthermore, surgical or catheter ablation of APs usually results in elimination of atrial fibrillation as well.

Diagnosis

Clinical Features

Preexcitation occurs in the general population at a frequency of around 1.5 per 1000. Of these, 50% to 60% of patients become symptomatic. Approximately one-third of all patients with PSVT are diagnosed as having an AP-mediated tachycardia. Patients with AP-mediated tachycardias most commonly present with the syndrome of PSVT. Population-based studies have demonstrated a bimodal distribution of symptoms for patients with preexcitation, with a peak in early childhood followed by a second peak in young adulthood. Nearly 25% of patients will become asymptomatic over time. More than half the patients with an episode will experience a recurrence.

Symptoms range from palpitations to syncope. As in AVNRT, episodes of tachycardia may be associated with dyspnea, chest pain, decreased exercise tolerance, anxiety, dizziness, or syncope. Although syncope is often considered a bad prognostic sign, the evidence is not clear. Physical examination demonstrates a fast, regular pulse with a constant-intensity first heart sound. The jugular venous pressure waveform is usually constant, but it can sometimes be elevated. The incidence of sudden cardiac death in patients with WPW syndrome has been estimated to range from 0.15% to 0.39%.^11,12,13 It is distinctly unusual for cardiac arrest to be the first symptomatic manifestation of WPW syndrome. A recent report by Bunch et al compared the outcomes of 872 WPW patients who underwent ablation, 1461 WPW patients who did not have ablation, and 11,175 control patients.¹⁴ Among all WPW patients, the mortality and incidence of sudden death was no different than in the control population. However, the risk of atrial fibrillation was greater in WPW patients. It is also notable that patients with WPW who underwent ablation had a lower mortality than those who did not. Surprisingly, however, there was no difference in the incidence of sudden death in these two groups. Given the high prevalence of atrial fibrillation among patients with WPW syndrome and the concern for sudden cardiac death resulting from rapid preexcited atrial fibrillation, the low annual incidence of sudden death among patients with WPW syndrome is reassuring.

Patients with functioning AV bypass tracts tend to have certain congenital abnormalities, particularly Ebstein anomaly of the tricuspid valve. Nearly 10% of patients with Ebstein anomaly have preexcitation. APs also commonly occur in patients with corrected transposition of great vessels. In this case, the Ebstein anomaly of the left (tricuspid) valve is associated with APs to the functioning systemic ventricle (anatomic right ventricle). In addition, multiple bypass tracts are frequently seen in patients with Ebstein anomaly. Conversely, of patients with WPW syndrome presenting with SVT early in childhood, only 5% have Ebstein anomaly, even though it is the most common from of congenital heart disease associated with WPW syndrome.

Electrocardiographic Characteristics

The ECG hallmark of an antegradely conducting AP is the delta wave along with a shorter than usual PR interval. Conversely, the presence of retrograde conduction only in an AP will not be apparent on a surface ECG during sinus rhythm. Whereas ECG during orthodromic AVRT has a normal QRS complex with retrogradely conducting P wave after the completion of the QRS complex in the ST segment or early in the T wave (see Fig. 84–4), the QRS during antidromic AVRT is fully preexcited.

Numerous algorithms have been described to localize the site of the AP using the axis of the delta wave and QRS morphology. The location of the AP along the AV ring is classified variously into five or ten regions, which can be broadly divided into those on the left and the right of the AV groove. Distribution along these lines is not homogenous. Some 46% to 60% of the pathways are found on the left free wall space. Nearly 25% are within the posteroseptal and midseptal spaces, 15% to 20% in the right free wall space, and 2% in the anteroseptal space. A simple algorithm that includes both the delta wave axis and the QRS axis is shown in Fig. 84–14.

ST-segment depression may also occur during orthodromic AVRT. It may occur even in young individuals, who are unlikely to have coronary artery disease. The location of the ST-segment depression may vary with the location of the AP. ST-segment depression in V₃ to V₆ is almost invariably seen with a left lateral pathway; a negative T wave in the inferior leads is associated with a posteroseptal or posterior pathway; and a negative or notched T wave in V₂ or V₃ with a positive retrograde P wave in at least two inferior leads suggests an anteroseptal pathway. However, ST-segment depression occurring during orthodromic AVRT episodes in older patients or associated with symptoms of ischemia mandate the consideration of coexisting coronary artery disease.

Electrophysiologic Testing

EPS in patients with AVRT is done to not only confirm the presence of an AP and differentiate this condition from other forms of SVT, but also to find the pathway participating in the tachycardia and aid in ablative therapy.

By definition, if an AP is present and conducting antegradely, some part of the ventricle begins activation earlier than expected, so that the HV interval is less than normal at rest (see Fig. 84–10). Because the QRS complex is a fusion complex of conduction down both the AV node and the AP, slowing of conduction down the normal pathway results in an increasing degree of preexcitation.

Eccentric atrial activation with ventricular pacing makes it easy to identify the presence of an AP (Fig. 84–15). Retrograde conduction over most APs is nondecremental. Hence, in the absence of intraventricular conduction delay or the presence of multiple bypass tracts, the VA conduction time is the same over a range of pace cycle lengths (see Fig. 84–15). The exception to this is the slowly conducting decremental posteroseptal pathway found in the permanent form of junctional reciprocating tachycardia, in which the VA conduction time increases with increasing ventricular pacing rate.

It is important and often challenging to differentiate retrograde conduction over septal pathway from conduction over the normal AV system. One maneuver that can make this differentiation is differential pacing (ie, pacing both from the right ventricular apex and the right ventricular base) and measuring the VA conduction time. Retrograde conduction over the normal AV conduction system is fastest when pacing from the apex because conduction can occur rapidly over the His-Purkinje system. VA intervals are longer when the pacing site is moved from the apex to the base. The converse is true in the presence of an AP, with VA intervals shortest when pacing from the base, closer to the site of pathway insertion, than from the apex. The technique of para-Hisian pacing is useful in differentiating the anteroseptal pathway from AVNRT.

Development of BBB aberration during tachycardia can be useful in determining both presence of, and participation of, an AP in tachycardia (Fig. 84–16). An increase in tachycardia cycle length caused by an increase in VA conduction time with functional BBB is consistent with the presence of an AP ipsilateral to the BBB.

Management

Catheter Ablation of Accessory Pathways

Catheter ablation of APs is performed in conjunction with a diagnostic EPS. After the AP has been localized to a region of the heart, precise mapping and ablation are performed using a steerable electrode catheter. No prospective, randomized clinical trials have evaluated the safety and efficacy of catheter ablation of APs. However, the results of catheter ablation of APs have been reported in a large number of other trials.^{5,8,9,15,16,17,18,19,20,21} The largest prospective, multicenter clinical trial to evaluate the safety and efficacy of RF ablation was reported by Calkins and colleagues.⁵ This study involved analysis of 1050 patients, of whom 500 had APs. Overall success of catheter ablation in curing APs was 93%. The success rate for catheter ablation of left free wall APs was slightly higher than for catheter ablation of right-sided APs (95% vs 90%, P = .03). After an initially successful procedure, recurrence of AP conduction was found in approximately 5% of patients. The recurrence-free interval postablation was also best with left-sided pathways (Fig. 84–17). APs that recur can usually be successfully ablated again. Complications associated with catheter ablation of APs may result from obtaining vascular access (hematomas, deep venous thrombosis, perforation of the aorta, arteriovenous fistula, pneumothorax), catheter manipulation (valvular damage, microemboli, perforation of the CS or myocardial wall, coronary dissection or thrombosis), or delivery of RF energy (AV block, myocardial perforation, coronary artery spasm or occlusion, transient ischemic attacks, or cerebrovascular accidents).

Calkins and coworkers,⁵ reported the incidence of major complications in their trial to be 3% and of minor complications around 8%. The procedure-related mortality associated with catheter ablation of APs has ranged from 0% to 0.2%. The two most common types of major complications reported during catheter ablation of APs are inadvertent complete AV block and cardiac tamponade. The incidence of inadvertent complete AV block ranges from 0.17% to 1.0%. Most instances of complete AV block occur in the setting of the ablation of septal and posteroseptal APs. The frequency of cardiac tamponade as a result of the ablation of APs varies between 0.13% and 1.1%.

In the past several years, cryoablation has become available as an alternative energy source for creation of myocardial lesions, which can be used for catheter ablation of APs. The main advantage of cryoenergy compared with RF energy is that the risk of heart block appears to be lower.¹⁶ This potential benefit must be balanced against longer procedure times and lower acute and long-term efficacy.¹⁶ Because of the lower acute and long-term success rates of catheter ablation of APs using cryoenergy, this energy source is generally used only for ablation of APs located in the anteroseptal and para-Hisian locations.

Medical Therapy

Antiarrhythmic drugs represent one therapeutic option for management of patients with AP-mediated arrhythmias (see Table 84–4). Antiarrhythmic drugs that primarily modify conduction through the AV node include verapamil, β-blockers, and adenosine. In contrast, the antiarrhythmic drugs, which primarily modify conduction across the AP, consist of class 1 drugs such as procainamide, propafenone, and flecainide as well as class 3 antiarrhythmic drugs such as sotalol and amiodarone. The approach to acute termination of these arrhythmias generally differs from that used for long-term suppression and prevention of further episodes of SVT. In general, the approach used does not vary based on the specific tachycardia mechanism, which generally is unknown when the patient first presents to an emergency department. Pharmacologic agents are in general more effective in terminating an acute episode of tachycardia than in preventing future recurrences. Verapamil or diltiazem should not be administered intravenously to patients with atrial fibrillation and preexcitation because they may accelerate conduction through the AP and precipitate a cardiac arrest.

There have been no controlled trials of drug prophylaxis involving patients with AV reentry. However, a number of small, nonrandomized trials have been performed which have demonstrated reasonable effectiveness of propafenone, flecainide, and amiodarone.

Management Considerations

Management of Asymptomatic Preexcitation

Most patients with asymptomatic preexcitation have a good prognosis. Because of the small, but real, risks associated with invasive procedures, EPS is not mandated for risk stratification or ablative therapy. The ACC/AHA/HRS Guidelines for Management of Patients with Supraventricular Arrhythmias gives electrophysiologic testing and catheter ablation when indicated a 2A classification for treatment of patients with asymptomatic preexcitation.¹ A 2A designation means that it is reasonable to offer EPS with or without ablation in selected patients after a thorough discussion of the risks and benefits of the procedure. Similar recommendations have been made in the PACES/HRS Expert Consensus Statement on the Management of Asymptomatic Young Patients with a WPW ECG Pattern.¹⁴

Several noninvasive and invasive tests have been proposed as useful in stratifying patients for the risk of sudden death.^1,17 The detection of intermittent preexcitation—which is characterized by an abrupt loss of the delta wave, normalization of the QRS complex, and an increase in the PR interval during a continuous ECG recording—is evidence that an AP has a relatively long refractory period and is unlikely to precipitate ventricular fibrillation. The loss of preexcitation after administration of antiarrhythmic drugs such as procainamide or ajmaline has also been used to indicate a low-risk subgroup. These noninvasive tests are generally considered inferior to EPS in the assessment of risk of sudden cardiac death. Because of this, they play little role in patient management at present.

When screening studies are performed in patients with asymptomatic preexcitation, approximately 20% of those who are asymptomatic will demonstrate a rapid ventricular rate during atrial fibrillation induced at EPS. Studies have identified markers that identify patients at increased risk. These include (1) a short preexcited R-R interval below 250 ms during spontaneous or induced atrial fibrillation, (2) a history of symptomatic tachycardia, (3) multiple APs, and (4) Ebstein anomaly.^1,17,18 A short preexcited R-R interval during atrial fibrillation above 250 ms has been reported to have a negative predictive value greater than 95%.

More recent evidence makes a stronger case for use of EPS in risk stratifying all asymptomatic patients with preexcitation.²³ Pappone and coworkers²⁴ studied 212 consecutive asymptomatic WPW patients after a baseline EPS over 5 years. After a mean follow-up of 37.7 months, 33 patients became symptomatic. Of these, 29 had inducible SVT on EPS, and only 4 were not inducible. More importantly, there were three sudden deaths in the entire population, and all of them occurred in patients in whom AVRT and atrial fibrillation were inducible during EPS. In a more recent study, Pappone and colleagues (24) examined the role of prophylactic catheter ablation in children with asymptomatic preexcitation. Of the 165 eligible children, 60 were determined to be at high risk of an arrhythmia based on their results of EPS. Of these 60 patients, 20 underwent prophylactic catheter ablation, 27 had no treatment, and 13 withdrew from the study. During a mean follow-up of 34 months, 1 child in the ablation group (5%) and 12 in the control group (44%) had arrhythmic events. Among these 12 patients in the control group, 2 experienced ventricular fibrillation, and one died suddenly.

Santinelli and coworkers^25,26 published two papers describing the natural history of asymptomatic preexcitation. Among 293 adults with asymptomatic preexcitation followed for a median of 67 months, 31 patients (10.6%) developed a first arrhythmic event. Among these patients, the event was classified as potentially life threatening in 17 patients. One of these patients experienced a cardiac arrest. Multivariate analysis identified inducibility, an anterograde effective refractory period shorter than 250 ms as predictive of potentially life-threatening arrhythmias.²⁴ And among 184 children with asymptomatic preexcitation followed for a median of 57 months, 51 patients (28%) developed a first arrhythmic event. Among these patients, the event was classified as potentially life threatening in 19 patients. Three of these patients experienced a cardiac arrest. Multivariate analysis identified an anterograde effective refractory period shorter than 240 ms and the presence of multiple APs as predictive of potentially life-threatening arrhythmias.^25,26 Pappone et al went on to publish an outcome registry involving 2169 WPW patients followed for 8 years²⁷; among this group, 1001 (550 asymptomatic) did not undergo ablation and 1168 (206 asymptomatic) did have ablation. VF occurred during follow-up in 1.5% of patients who did not have catheter ablation. In contrast, no patients with WPW who underwent ablation experienced VF during follow-up. Based on the results of these studies, as well as the well-established safety and efficacy of catheter ablation of APs, an increasing proportion of electrophysiologists, particularly pediatric electrophysiologists, now advocate screening EPS and prophylactic catheter ablation when a high-risk AP is uncovered. The recommendations made for management of asymptomatic preexcitation in the AHA/ACC/HRS 2015 SVT Guidelines are summarized in Table 84–5.

Management of Symptomatic Wolff-Parkinson-White Syndrome

The ACC/AHA/HRS Guidelines for Management of Patients with SVT states that catheter ablation is considered first-line therapy (class 1) and the treatment of choice for patients with WPW syndrome (ie, patients with manifest preexcitation along with symptoms).^1,19,20 It is curative in more than 95% of patients and has a low complication rate. It also obviates the unwanted side effects of antiarrhythmic agents. Catheter ablation is also considered first-line therapy (class 1) for patients with PSVT involving a concealed AP. However, because concealed APs are not associated with an increased risk of sudden cardiac death in these patients, catheter ablation can be presented as one of a number of potential therapeutic approaches, including pharmacologic therapy and clinical follow-up alone (see discussion of management of AVNRT). When pharmacologic therapy is selected for patients with concealed APs, it is reasonable to consider a trial of β-blocker therapy, CCB therapy, or a class 1C antiarrhythmic agent. It is important to note that β-blockers and CCBs generally are not recommended for the management of patients who have evidence of preexcitation.

The recommendations for the acute and long-term management of patients with AP-mediated arrhythmias developed by the ACC/AHA/HRS are shown in Tables 84–6 and 84–7.¹ Catheter ablation is considered class 1 therapy for treatment of patients with WPW syndrome and for those with AVRT in the absence of preexcitation.

Atrial tachycardia is an arrhythmia that arises from the left or right atrium or the musculature of the great cardiac veins.¹ There are many types of atrial tachycardias. Some are paroxysmal while others are continuous. The mechanisms of atrial tachycardias vary widely and include enhanced automaticity, triggered activity, and reentry. Although atrial fibrillation and atrial flutter can be considered under the broad umbrella of atrial tachycardias, in my opinion it is best to consider atrial flutter and atrial fibrillation as distinct arrhythmias, in part because of the marked increase in stroke risk that accompanies atrial fibrillation and atrial flutter.

Atrial tachycardias are generally benign arrhythmias where the main indication for treatment is symptom relief. One important exception is incessant atrial tachycardia with a rapid rate. It is well established that incessant atrial tachycardias that result in a persistently rapid ventricular response can lead to a rate related cardiomyopathy. It is important to recognize that this potentially serious condition is reversible once the atrial tachycardia is eliminated with an associated return of the ventricular rate to normal.

There are many approaches to treatment of atrial tachycardia. Pharmacologic therapy with β-blockers or CCBs is effective in a proportion of patients. If these first-line medications fail, membrane active antiarrhythmic medications such as flecainide, propafenone, sotolol, or rarely amiodarone can be considered. Catheter ablation is an important treatment strategy that is appropriate to use first line based on patient’s values and preferences. With the use of three-dimensional computerized mapping systems, the cure rate associated with ablation of atrial tachycardia exceeds 90%. One important exception to this rule is multifocal atrial tachycardia, which is far more difficult to approach with ablation because of its multifocal origin.