Wolff-Parkinson-White Syndrome

Wolff-Parkinson-White (WPW) syndrome is a part of a group of conduction abnormalities that are classified under the heading preexcitation. The classic feature of these is the presence of an abnormal conduction pathway originating in the atria, bypassing the atrioventricular (AV) node and entering the upper portion of the ventricular tissue. Conduction through this abnormal pathway results in early depolarization of the ventricles. The presence of this abnormal pathway (termed accessory pathway) was initially described in 1893; a report in 1914 indicated the abnormal conduction pathway is able to create a circuit, allowing for the development of atrial tachyarrhythmias.1,2 In 1930, Wolff, Parkinson, and White reported on 11 patients who had a slurred upstroke of the QRS complex and a short PR interval.2 Later, a 1944 report discussed the presence of a short PR interval (less than 0.12 seconds) and a slurred upstroke of the QRS complex (referred to as a delta wave) that was associated with the development of tachyarrhythmias.2 The clinical significance of WPW syndrome is its relationship to the incidence of atrial and ventricular tachyarrhythmias and sudden cardiac death.3 The purpose of this column is to review the clinical significance, electrocardiographic (ECG) criteria, and management of WPW syndrome.The incidence of an accessory pathway is estimated to be in 0.1% to 0.3% of the population.3 However, the actual prevalence is unknown, because approximately 50% of the individuals who have WPW syndrome do not exhibit symptoms.4 Often, WPW syndrome is diagnosed in adolescents or young adults because of reports of symptomatic palpitations.Researchers have shown that this abnormal pathway can exist anywhere between atrial and ventricular tissue.2 The accessory pathway in WPW syndrome (formally named the Kent bundle) connects the atrial tissue to the ventricular tissue, bypassing the AV node. The pathway may be located on the right or left side of the conduction system. The most common location for the accessory pathway is on the left lateral wall of the heart, occurring in 50% of people with WPW syndrome. Other locations include the back wall of the heart between the atrial and ventricular septum (30%) and in the right lateral or anterior wall between the right atrium and right ventricular free wall (20%).2There are other variants of bypass tracts. For example, James fibers connect the atrial tissue directly to the His bundle, contributing to a preexcitation variant called Lown-GanongLevine syndrome.5 This syndrome is characterized by having a short PR interval (< 0.12 s) with a normal QRS duration.The ECG characteristics of WPW syndrome are unique because of the accessory pathway. In a normal heart with no accessory pathway, electrical conduction starts in the sinus node, travels through the AV node, then to the His bundle, to the right and left bundles, and into the ventricles (Figure 1A). One purpose of the AV node is to slow electrical impulses in order to protect the ventricles from the high atrial rates often seen with atrial fibrillation or flutter. When an accessory pathway is present and the patient is in sinus rhythm, the electrical impulse usually travels down both the accessory pathway and the AV node to the ventricles (Figure 1B).2 The unusual shape of the QRS complex is a result of the ventricles being depolarized via both the AV node and the accessory pathway.6 As the impulse travels over the accessory pathway, the first unique ECG feature found with WPW syndrome is a PR interval of less than 0.12 seconds. The electrical impulse stimulates part of the ventricular myocardium early because of the accessory pathway, which accounts for the second unique feature seen on the ECG: the delta wave. A delta wave is a “slurring” of the initial wave of the QRS complex (either a Q wave or R wave). This slurring of the QRS complex creates the third unique ECG finding: a widening of the QRS complex to greater than 0.12 seconds. Figure 2 provides an example of the typical QRS waveform characteristic of WPW syndrome.The AV node and the accessory pathway not only conduct impulses in an antegrade fashion but also can conduct in a retrograde way as well. Some people have an accessory pathway but never manifest these unique ECG features because the accessory pathway is incapable of antegrade conduction. However, a pathway capable of retrograde conduction can play a role in the development of tachyarrhythmias. This is referred to as a concealed bypass tract.2,4 Sometimes people exhibit switching or alternating conduction between the normal pathway and the accessory pathway (Figure 3 and Figure 4). Some people have more than one accessory pathway, which is more common in people with structural heart disease.3,7 Such people make up approximately 12% of patients with WPW syndrome and 9% of the pediatric population3 and are at greater risk for supraventricular tachyarrhythmias and ventricular fibrillation.People with WPW syndrome are prone to symptomatic supraventricular tachyarrhythmias (SVT). Because the AV node and the accessory pathway may be able to conduct impulses in both an antegrade and retrograde fashion, it sets up the possibility of a re-entrant loop tachyarrhythmia, where the electrical impulse gets caught in an endless loop around the AV node to the accessory pathway or vice versa (Figure 5A and Figure 6A). It is possible that a critically timed sinus impulse may be conducted normally through the conduction system and by the time the impulse reaches the ventricular area near the accessory pathway, the impulse may be conducted retrogradely up the bypass tract, depolarizing the atrial tissue. This is called an “atrial echo,” “reciprocating tachycardia,” or “orthodromic reciprocating tachycardia” (Figure 5A). If this impulse finds the AV node repolarized, it may be conducted back down to the ventricles. If this cycle continues, the result is a re-entrant atrioventricular tachycardia, which is a narrow complex tachycardia (Figure 5B).An antidromic tachycardia is one that depolarizes the ventricles via the accessory pathway and has retrograde conduction to the atria via the AV node (Figure 6A). The rhythm causes a wide QRS complex tachycardia2 (Figure 6B) and can mimic ventricular tachycardia. In this case, if there is a resting ECG that shows the unique ECG features described above, the likely diagnosis would be SVT instead of ventricular tachycardia.6 If there is no known history of an accessory pathway or a previous ECG, the clinician should assume ventricular tachycardia until proven otherwise.People with an accessory pathway who develop one of the above tachyarrhythmias may experience symptoms such as palpitations, dizziness, syncope, and shortness of breath, but these arrhythmias usually are not life-threatening. Atrial fibrillation occurs in about 15% of people with an accessory pathway, which can be life-threatening.7 If a person with an accessory pathway develops atrial fibrillation and conducts down the accessory pathway and back up to the atria via the AV node (antidromic conduction), the ventricles may respond in a 1:1 fashion because there is no slowing of the electrical impulses in the AV node. In atrial fibrillation, the atrial electrical impulses “fire” at more than 300 times per minute, so the ventricles may respond with ventricular fibrillation or ventricular tachycardia, which could lead to sudden cardiac death (Figure 7).For patients who present with atrial fibrillation and have antegrade conduction over the accessory pathway (a wide QRS tachycardia), but who are hemodynamically stable, intravenous ibutilide or procainamide can be used to slow conduction over the accessory pathway to stop atrial fibrillation. Atrioventricular node blocking agents should be avoided (eg, diltiazem, verapamil, digoxin, adenosine, and β-blockers) in this situation because blocking the AV node helps direct electrical impulses down the accessory pathway. Intravenous amiodarone is not recommended because there have been reports that it can also direct impulses down the accessory pathway and cause ventricular fibrillation.8 If the patient is hemodynamically unstable, cardioversion should be performed.7Some people with documented WPW syndrome on the ECG are asymptomatic and never develop tachyarrhythmias. Even so, these patients should be thoroughly questioned about any history of palpitations or other symptoms. An electrophysiology (EP) evaluation should be recommended regardless of a history of symptoms to determine the risk of developing a tachyarrhythmia. If the patient is deemed to have a high risk of developing a tachyarrhythmia (through noninvasive and invasive testing), then there are 2 general options: catheter ablation and medications. Catheter ablation uses radiofrequency impulses to permanently “block” the accessory pathway by burning and destroying the tissue. This treatment has a high success rate (93%-95%) with a low complication rate.7 Figure 8 shows a patient’s ECG before and after ablation. For patients who are unwilling or unable to undergo the invasive catheter ablation, medications can be used to prevent the tachycardias. Oral agents such as flecainide or propafenone can be used to prevent WPW syndrome–associated tachycardias in patients without structural heart disease or coronary artery disease.8Wolff-Parkinson-White syndrome is an uncommon condition. Some, but not all, patients with preexcitation on the ECG will develop tachyarrhythmias. Most of the tachyarrhythmias associated with WPW syndrome are not life-threatening. However, atrial fibrillation in the setting of WPW syndrome can lead to ventricular fibrillation and sudden cardiac death if the accessory pathway is capable of antegrade conduction. Patients who have preexcitation on the ECG should be referred to an EP provider to assess the risk of developing tachyarrhythmias. In symptomatic patients, or those at high risk of developing tachyarrhythmias, catheter ablation can be a curative therapy.