Bypass Tracts & the Wolff-Parkinson-White Syndrome
Congenital bands of tissue that can conduct impulses but lie outside the normal conduction system are called accessory pathways, or bypass tracts. These pathways are responsible for a variety of mechanistically distinct tachycardias by providing preferential conduction between different areas within the heart.
Accessory pathways are quite prevalent in the general population, with a 2:1 male-to-female predominance. The presence of a bypass tract, however, does not mean that a tachyarrhythmia is a certainty because less than half of persons with documented bypass tracts ever sustain an arrhythmia. The actual number depends on the population studied and varies from 13% in a healthy outpatient population to 80% in the hospital setting.
Approximately 5–10% of patients with documented bypass tracts have concomitant structural heart disease. Ebstein anomaly is the most common, accounting for 25–50% of the anomalies in this group. Of patients with Ebstein anomaly, 8–10% have coexistent bypass tracts, mostly on the right side. The association of right-sided accessory pathways with structural heart disease is strong: 45% of patients with right-sided (and only 5% of those with left-sided) pathways display some type of heart disease.
A familial tendency toward bypass tracts has been seen in some instances, with a fourfold to tenfold increase in incidence among first-degree family members.
Anatomically, the atria and ventricles are in apposition, separated by an invagination known as the AV groove. Paroxysmal tachycardias mediated by accessory pathways that cross the groove and electrically link the atria and ventricles, when combined with a short P-R interval (< 0.12 seconds), a wide QRS, and secondary repolarization abnormalities, define the Wolff-Parkinson-White syndrome. When this ECG pattern is seen without the tachycardia, it is called Wolff-Parkinson-White pattern or ventricular preexcitation.
Although the most common site of insertion is between the lateral aspect of the left atrium and left ventricular myocardium, pathways can cross the AV groove anywhere in its course (except the region between the aortic and mitral valves) to connect the left or right atrium to its respective ventricle (Figure 11–5). In noting the distribution of accessory pathways, 46–60% are located in the left free wall, 25% within the posteroseptal space, 13–21% in the right free wall, and 2% in the anteroseptal space. Females have right-sided accessory pathways and Asians have right anterior accessory pathways more frequently than other groups, suggesting that pathogenesis of these pathways has a genetic component. Each location produces a distinct ECG pattern (Figure 11–6), but in the 13% of patients with two or more bypass tracts, the ECG tracing can be confounding and show multiple QRS morphologies.
Cross-sectional diagram of the atrioventricular groove. Atrioventricular bypass tracts may cross the groove anywhere in its course except in the region bounded by the left and right fibrous trigones. (Reprinted, with permission, from Cox JL, et al. J Thorac Cardiovasc Surg. 1985;90:490.)
Single atrioventricular bypass-tract localization based on maximally preexcited electrocardiographic morphology of the QRS. LA/LL, left anterior or left lateral accessory pathways; LPL/LP, left posterolateral or left posterior accessory pathways; PS, posteroseptal accessory pathways; RAL/RL, right anterolateral or right lateral accessory pathways; RAS/RA, right anteroseptal or right anterior accessory pathways; RP/RPL, right posterior or right posterolateral accessory pathways; +, positive delta wave; -, negative delta wave; ±, isoelectric delta wave. (Reprinted, with permission, from Fananapazir L, et al. Circulation. 1990;81:578.)
Cardiac Electrical Conduction
Unlike the AV node, whose function is to delay atrial impulses en route to the ventricles, most bypass tracts conduct rapidly and without delay, which accounts for the short P-R interval often seen in sinus rhythm in these patients.
Impulses that reach the ventricles over a bypass tract spread through cell-to-cell conduction within the myocardium, activating the ventricles in series rather than in parallel. This relatively slow process is manifested as a wide QRS complex.
Sinus impulses are not restricted to using the AV node or the bypass tract only to reach the lower chambers. Instead, both may contribute to ventricular activation. This produces a QRS that is initially wide, reflecting conduction over the bypass tract, with the latter portion of the QRS appearing normal and narrow, indicating that the remainder of the ventricle has been depolarized via the normal conduction system (the AV node and His-Purkinje system). The initial slurred upstroke of the QRS, a delta wave, indicates ventricular preexcitation, which can be defined as ventricular depolarization that begins earlier than would be expected by conduction over the AV node alone. The degree of preexcitation and P-R shortening depends on the proportion of ventricular activation occurring over the AV node and the bypass tract. This, in turn, is related to two factors. The first is the conduction velocity of the bypass tract relative to the AV node. The faster the bypass tract can conduct impulses to the ventricles in relation to the AV node, the earlier the ventricle will preexcite, and vice versa. The second factor is the location of the tract, and more specifically, its proximity to the sinus node and AV node. A sinus impulse will encounter a right-sided free-wall bypass tract earlier than it will the AV node, and this favors a short P-R interval with a high degree of ventricular preexcitation (Figure 11–7A). On the other hand, a sinus beat will encounter the AV node early in its course while traveling to a pathway in the lateral left atrium, allowing ventricular activation to occur primarily by the normal conduction system. A narrow, minimally (if at all) preexcited QRS complex with a normal or near-normal P-R interval may be seen (Figure 11–7B). Changes in autonomic tone, by modifying the conduction velocity and refractoriness over both the pathway and the AV node, can produce varying degrees of preexcitation at different times in the same patient (Figure 11–7C).
Ventricular preexcitation over a bypass tract in sinus rhythm. Note the short P-R interval. A: Right anterior bypass tract. The delta wave is positive in most leads (arrow), and negative in aVR and V1–V3. B: Left lateral bypass tract. The isoelectric delta wave in V1 gives the appearance of a normal PR interval. Inspection of the simultaneously recorded rhythm strip leads (lower three panels) reveals delta wave onset to be at the end of the P wave in leads II and V5. C: Posterior septal bypass tract. A short time after this tracing was obtained, the patient exhibited minimal to no preexcitation. This was due to fluctuations in autonomic tone causing enhanced conduction through the AV node.
If the delta wave axis of a maximally preexcited beat is discordant from the accompanying preexcited QRS axis, or if more than one preexcited QRS morphology is noted, there may be multiple bypass tracts.
Atrioventricular Reciprocating Tachycardia
An inherent property of accessory pathways is their ability to conduct in a retrograde direction more easily than antegrade. The AV node, on the other hand, conducts more efficiently antegrade. For this reason, reentrant rhythms in this setting most commonly use the AV node to go from atrium to ventricle and the bypass tract to return to the atrium. Orthodromic AVRT (antegrade conduction over the AV node) accounts for 70–80% of arrhythmias in patients with AV bypass tracts, with heart rates of 140–250 bpm (Figure 11–8). Antidromic AVRT, in which the atrial impulse is carried to the ventricle over the bypass tract and reenters the atrium via retrograde conduction over the AV node, is rare, occurring in approximately 5–10% of cases. Because conduction to the ventricles during orthodromic AVRT occurs over the normal conduction system, the QRS is narrow, unless bundle branch aberrancy is present. During antidromic AVRT, the QRS is wide and maximally preexcited as a result of the complete lack of AV nodal contribution to ventricular depolarization. When two or more bypass tracts are present, each tract may act as the antegrade or retrograde limb (or both), especially with involvement of the AV node. There is a higher incidence of ventricular fibrillation in patients with multiple accessory pathways. Additionally, multiple pathways are more common in patients with antidromic SVT and in patients with Ebstein anomaly.
Orthodromic atrioventricular (AV) reciprocating tachycardia (O-AVRT) in a patient with a left-sided bypass tract. The circuit conducts from atria to ventricles over the AV node and from ventricles to atria retrograde over the bypass tract. This mechanism accounts for the narrow QRS and the retrograde P waves inscribed in the early portion of the T waves. Although the electrocardiogram with atrioventricular nodal reentrant tachycardia (AVNRT) may appear similar, a ventriculoatrial conduction time of more than 100 ms, as measured from QRS onset to P wave onset, greatly favors O-AVRT. The time in this tracing is 110 ms.
Tachycardia is usually initiated by a premature atrial or ventricular beat. In orthodromic tachycardia, a premature atrial beat conducts down the AV node to depolarize the ventricle, and the bypass tract carries the impulse back to the atrium (Figure 11–9). A ventricular premature beat finding the AV node refractory might initiate an identical tachycardia by first conducting up the bypass tract to the atrium. Antidromic tachycardia initiates in an identical fashion but with a reversed direction of conduction.
The reentry circuit of orthodromic atrioventricular (AV) reciprocating tachycardia (O-AVRT). The atrioventricular (AV) node serves as the antegrade limb of the reentry circuit, and an accessory pathway serves as the retrograde limb. In this case, the accessory pathway is located in the free wall of the right ventricle. The wave of depolarization travels from the AV node to the accessory pathway through the ventricle and from the accessory pathway to the AV node through the atrium. Because the ventricles are depolarized by the normal conduction system, the QRS is narrow unless there is a bundle branch block. Also shown is an ECG example of O-AVRT, at a rate of about 210 bpm. A P wave is present in the left half of the RR cycle (arrow) because retrograde conduction through the accessory pathway is more rapid than antegrade conduction through the AV node.
Atrial fibrillation accounts for only 19–38% of arrhythmias in the population with accessory pathways, but it is potentially more lethal than the reciprocating tachycardias discussed earlier. It is more common in patients with antegrade conducting accessory pathways and in pathways with a short antegrade refractory period. By virtue of their short refractory periods, bypass tracts (unlike the AV node) have the potential to conduct very rapidly to the ventricles at ventricular rates of 250–350 bpm (Figure 11–10) with the possibility of degeneration to ventricular fibrillation. A reputed marker for sudden death in patients with atrial fibrillation is a shortest preexcited R-R interval of ≤ 250 ms (corresponding to a heart rate of ≥ 240 bpm) between two fully preexcited beats. The finding of a short R-R interval actually has a low positive predictive value, however, because sudden death in this syndrome is still rare.
Atrial fibrillation with antegrade conduction over a left posterolateral bypass tract. Although most beats are fully preexcited, several of the beats in the rhythm strip are narrower, indicating combined conduction over both the bypass tract and the atrioventricular node. Antidromic atrioventricular reciprocating tachycardia (A-AVRT) would have a similar appearance on 12-lead electrocardiogram, but the rhythm irregularity and the varying degrees of preexcitation nullify this possibility. (Reprinted, with permission, from Zipes DP. Specific arrhythmias: diagnosis and treatment. In: Braunwald E, ed. Heart Disease. A Textbook of Cardiovascular Medicine. Philadelphia: WB Saunders; 1988.)
During atrial fibrillation, the ECG reveals an irregularly irregular rhythm with QRS complexes of varying morphologies, representing conduction to the ventricles via the AV node (normally conducted narrow complexes), the bypass tract (wide, preexcited complexes), and both (fusion beats, harboring elements of both the normally conducted and preexcited beats). In this setting, the bypass tract may be called a bystander since it is not integral to the tachycardia. Patients with AV bypass tracts have a higher incidence of atrial fibrillation than does the general population, possibly because of the degeneration of reentrant tachycardia or of microreentry within the atrial portion of the bypass tract. It has been shown that ablation of the bypass tract can frequently also eliminate atrial fibrillation.
Between 15% and 50% of patients with no evidence of preexcitation during sinus rhythm are found to have bypass tracts that conduct only in the retrograde direction. By definition, concealed bypass tracts (their presence cannot be detected by standard ECG) do not display delta waves on the ECG during sinus rhythm, but they can still support an orthodromic AVRT and account for about 30% of orthodromic tachycardias.
Differentiating orthodromic AVRT from AVNRT on the ECG can be difficult. The incidence of both tachycardias being operative at different times in the same person is reported to be between 1.7% and 7%. Therefore, although the presence of a delta wave on the nontachycardiac tracing makes it statistically unlikely that AVNRT was the documented tachycardia, it does not exclude the possibility completely.
Because of the simultaneous atrial and ventricular activation that occurs during AVNRT, the P waves formed as a result of retrograde conduction to the atrium are usually obscured by or merged with the QRS complex. Likewise, because of the short retrograde conduction time via the bypass tract, orthodromic AVRT usually is a short R-P tachycardia, albeit the R-P interval is somewhat longer than with AVNRT (see Figure 11–8). In AVRT, the P waves are usually located within the ST segment and are not obscured or merged with the QRS.
The management of orthodromic AVRT or antidromic AVRT is similar to that of AVNRT. Since the AV node is an integral part of the reentry circuit in AVRT, blocking the AV node terminates the tachycardia. Therefore, vagal maneuvers such as the Valsalva maneuver or carotid sinus massage can be tried first.
Intravenous adenosine almost always terminates these tachycardias. Other AV nodal blocking agents such as β-blockers, calcium channel blockers, and digoxin can also be helpful. Just as in AVNRT, vagal maneuvers can be retried after each dose of these longer acting AV nodal blocking agents.
In contrast to orthodromic or antidromic AVRT, patients with atrial fibrillation or atrial flutter and a bystander bypass tract should not be given AV nodal blocking agents. This point is crucial since these types of medicines can precipitate ventricular fibrillation. Blocking the AV node enhances conduction down the bypass tract, making all the complexes wide. Furthermore, the bypass tract can often conduct faster than the AV node, increasing ventricular rates to sometimes over 200 bpm. The aberrant conduction and rapid rate can lead to disorganized ventricular conduction, resulting in ventricular fibrillation. The drug of choice for patients with atrial fibrillation or atrial flutter and a bystander bypass tract is intravenous procainamide or another drug that preferentially blocks the bypass tract. This shunts more conduction through the AV node, which narrows the QRS complexes and often slows down the overall ventricular rate.
Asymptomatic patients showing delta waves on the ECG generally do not require treatment unless they are involved in a high-risk profession such as commercial pilots, police officers, and firefighters. Patients with occasional or rare bouts of minimal or mildly symptomatic palpitations from orthodromic or antidromic AVRT can often be safely treated with such agents as β-blockers or calcium channel blockers to prevent recurrent episodes. However, due to the potential of ventricular fibrillation, these AV node blocking agents should be used with great caution or not at all in patients who have also demonstrated atrial fibrillation or atrial flutter.
Radiofrequency Catheter Ablation Therapy
Patients who experience significant symptoms such as dizziness, presyncope, or syncope should undergo an electrophysiologic study with concomitant radiofrequency ablation. In addition, patients with frequent symptoms who do not respond to or who wish to avoid drug therapy can also undergo ablative therapy. Recent guidelines indicate that ablation can be considered first-line therapy for patients with symptomatic Wolff-Parkinson-White syndrome.
The right internal jugular or femoral vein ablation approach is used for accessory pathways located on the right side of the heart. Left-sided pathways can be approached from the left ventricle with the retrograde technique or transseptally from within the left atrium using the Brockenbrough technique. A steerable catheter is moved around the mitral or tricuspid annulus until the site of shortest impulse transit between the atrium and ventricle is found. This mapping process localizes the bypass tract. Frequently, an impulse can be recorded directly from the bypass tract, further confirming its localization. Once identified, radiofrequency energy delivered to the tract through the mapping catheter permanently destroys the tract and prevents further transmission of electric impulses over it.
Given its curative potential, a high success rate (95% in experienced hands, even with multiple bypass tracts), and a low complication rate, radiofrequency catheter ablation is now a very common treatment for accessory pathways (Table 11–2). As with other supraventricular arrhythmias, new mapping systems have been developed that decrease fluoroscopy and procedure time as well as allowing the operator to return to specific locations if needed.
Table 11–2. Complications of Radiofrequency Ablation of Accessory Pathways ||Download (.pdf)
Table 11–2. Complications of Radiofrequency Ablation of Accessory Pathways
Coronary artery spasm
Mild mitral regurgitation
Coronary artery thrombosis
Mild aortic regurgitation
Transient neurologic deficit
Femoral artery complications
Surgical Ablation Therapy
In rare instances, patients will have multiple pathways or pathways that are inaccessible to an ablation catheter. These patients may undergo surgical division of their tracts.
A variety of bypass tracts other than AV tracts (Kent fibers) also exist. Atriohisian fibers connecting the atrium to the His bundle have been demonstrated. This led to the description of the Lown-Ganong-Levine (LGL) syndrome, which refers to those patients with a short P-R interval, normal QRS, and recurrent SVT. However, current data suggest that LGL syndrome does not truly exist since atriohisian pathways have not been shown to support any type of reentry tachycardia.
Atriofascicular fibers, which are also known as Mahaim fibers, typically run from the lateral right atrium to the right bundle branch. The tract is capable of antegrade conduction only, and therefore, only antidromic AVRT is possible. Because the antegrade reentrant circuit engages the right bundle branch, the tachycardia QRS complex typically has a left bundle branch block pattern. Treatment considerations are the same as for Wolff-Parkinson-White syndrome.
Hsu JC, et al. Differences in accessory pathway location by sex and race. Heart Rhythm.
Kalarus Z, et al. Influence of reciprocating tachycardia on the development of atrial fibrillation in patients with preexcitation syndrome. Pacing Clin Electrophysiol.
Kothari S, et al. Atriofascicular pathways: where to ablate? Pacing Clin Electrophysiol.