Wolff-Parkinson-White syndrome

Wolff-Parkinson-White (WPW) syndrome is the second most common cause of paroxysmal tachycardias in the developed world. The three most common accessory pathways associated with paroxysmal tachycardias are the bundle of Kent, the Mahaim pathway and the James pathway.

In these conditions, extra electrical conduits exist between the atrial and ventricular myocardium, allowing pre-excitation of the ventricular muscle and, in some cases, a short PR interval and delta wave on the ECG. Re-entry tachycardias may result from anterograde electrical activity along the atrio-ventricular (AV) node and retrograde conduction through the accessory pathways, resulting in a circus rhythm.

ECG in a patient with Wolff-Parkinsons-White syndrome. There is a left-sided accessory pathway causing a short PR interval and delta wave (slurred upstroke to the QRS complex) most clearly seen in lead II and leads V2 to V6

Accessory pathways

In WPW syndrome, the accessory pathway is separate from the AV node, whereas Mahaim fibres arise within the AV node or the bundle of His and terminate in the ventricular myocardium. Only part of the AV node is bypassed by Mahaim fibres and hence, the PR interval tends to be normal.

In the uncommon Lown-Ganong-Levine syndrome, accessory James fibres bypass the AV node and reach the bundle of His, rather than directly stimulating the ventricular myocardium. Pre-excitation of the ventricles occurs, resulting in a shortened PR interval on the ECG but no delta wave.

The accessory pathway is a strand of normal myocardium that results from incomplete separation of the atria and ventricles during embryological development. Although the pathway can occur anywhere between the atria and ventricles, it commonly lies on the left side of the heart (type A) and is associated with a positive complex in V1 on the ECG.

A negative complex in V1 is seen in type B WPW, where the accessory pathway is found on the right. ECG algorithms and complex electrophysiological techniques are used to define the exact location of the Kent bundle. Fibrosis of the accessory pathway may render it ineffective over time. In sinus rhythm, atrial electrical activity is propagated along the accessory pathway at a greater velocity than through the AV node, creating a short PR interval. The Kent bundle does not insert into the conducting system, but into myocardium, leading to premature activation of part of the ventricle and hence the delta wave. Conduction via the AV node soon catches up and depolarises the rest of the ventricle in the normal way.

Arrhythmias most commonly seen in WPW sysndrome are AF and atrio-ventricular re-entry tachycardia (AVRT). Without the protection of the AV node, impulses from fibrillating atria can be transmitted directly to the ventricles via the Kent bundle, resulting in a rapid ventricular response. Delta waves will be seen on the ECG. Acute heart failure and shock may ensue. The arrhythmia may deteriorate to ventricular fibrillation.

Drugs such as digoxin and verapamil block the AV node and encourage conduction via the accessory, and are contraindicated in WPW syndrome. Treatments include sotalol, flecainide, disopyramide, amiodarone and electrical cardioversion. Internal cardioverter-defribillator devices may be required.

The AV node remains refractory for a shorter time than the accessory pathway. Atrial ectopics can propagate through the AV node at a time before the accessory pathway has recovered. By the time the impulse reaches the ventricles, the accessory pathway has recovered and can transmit the impulse back towards the atria and then again down the AV node. A circus rhythm is set up, leading to an AVRT. Ventricular ectopics can also trigger AVRTs. Delta waves are not seen in this condition.

Asymptomatic patients in high-risk situations, high-risk patients and those refractory to medical treatment may be candidates for destruction of the accessory pathway.

- Dr Raj Thakkar is a GP in Wooburn Green, Buckinghamshire, and a hospital practitioner in echocardiography at Stoke Mandeville Hospital, South Bucks NHS Trust