Wolf Parkinson White (WPW) syndrome
Wolf Parkinson White (WPW) syndrome is due to an accessory atrioventricular pathway which bypasses the normal atrioventricular (AV) nodal delay. ECG manifests a short PR interval and a delta wave. They are prone for supraventricular tachycardias (SVT) due to reentry mechanism. There are at least 10 different locations of the pathway around the mitral and tricuspid valve annulus. Each pathway gives a different manifestation in the routine ECG. The amplitude of the delta wave in each lead helps us to identify the location of the accessory pathway.
SVT in WPW syndrome can be orthodromic or antidromic. Orthodromic tachycardia has a narrow QRS complex and is the commonest variety accounting for 90%. The remaining 10% is antidromic and has a wide QRS. In orthodromic tachycardia the impulse travels down the normal AV pathway into the ventricle and back into the atrium through the accessory pathway. Vice versa occurs in antidromic tachycardia. Antidromic tachycardia is often mistaken for ventricular tachycardia due to the wide QRS. It may be noted that preexcitation (delta wave) does not manifest during orthodromic tachycardia. Hence a diagnosis of WPW syndrome cannot be made during the tachycardia. An ECG taken after termination of the tachycardia will show the delta wave so that it is mandatory to take an ECG after termination of SVT. SVT in WPW syndrome can be terminated by carotid sinus massage, intravenous adenosine or by intravenous verapamil.
Atrial fibrillation can rarely occur in WPW syndrome and can be life threatening due to the high ventricular rates. Ventricular rate is very high because the impulses are transmitted down both normal AV pathway and the accessory pathway. Moreover, the refractory period of accessory pathway decreases with increasing rates, permitting rapid conduction. The ventricle may not be able to track the fast rate and go into ventricular fibrillation. Hence atrial fibrillation in WPW syndrome requires immediate termination by electrical cardioversion.
WPW syndrome can be treated medically with anti-arrhythmic drugs like amiodarone. But the current day treatment of choice in symptomatic persons is radiofrequency catheter ablation of the accessory pathway. This is more important for pathways with lower refractory period as they have a tendency to conduct atrial fibrillation at very fast rates.