Usually the heart rhythm is controlled by a small pacemaker situated in the right upper chamber of the heart known as the sinus node (SA node). The signals from the sinus node are transmitted down the upper chambers to the lower chamber through the conduction system of the heart. In order to have the upper chambers contracting earlier than the lower chambers, a delay to the conducted signal is given at a relay station in the middle of the heart known as the AV node or the atrioventricular node situated at the junction between the upper and lower chambers, somewhat in the middle of the heart. Wolf Parkinson White (WPW) syndrome is due to an accessory atrioventricular pathway which bypasses the normal atrioventricular (AV) nodal delay. ECG (electrocardiogram) manifests a short PR interval and a delta wave. They are prone for supraventricular tachycardias (SVT – fast rhythm of the heart originating from from the upper chambers) due to reentry mechanism. This means that signals passing down the abnormal pathway travels back along the normal pathway or vice versa to produce a circus movement producing an oscillation of electrical signals within the heart. This causes a very rapid heart action known as tachycardia. 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 means the downward conduction of the signals is along the normal pathway while the return signal is along the abnormal pathway. In antidromic, the downward signal path is the abnormal pathway and the return is by the normal pathway. The word dromic means in relation to conduction. Orthodromic tachycardia has a narrow QRS complex (electrical activity of the ventricles or the lower chamber of the heart) and is the commonest variety accounting for 90%. The remaining 10% is antidromic and has a wide QRS because the downward conduction is through the abnormal pathway. 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 (fast rhythm originating in the lower chambers of the heart) due to the wide QRS complex. 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 (pressure along the upper part of the blood vessel carrying oxygenated blood to the head), intravenous adenosine or by intravenous verapamil (medicines used to block conduction along the atrioventricular node or junction).
Atrial fibrillation (irregular fast rhythm originating from the upper chamber of the heart) 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 (electrical signals) are transmitted down both normal AV pathway and the accessory pathway. Moreover, the refractory period (a short period during which a second signal is not let through after the first one – a built in safety mechanism to prevent unduly frequent signal transmission) of accessory pathway decreases with increasing rates, permitting rapid conduction (in the normal conduction system, refractory period increases with increasing rate). The ventricle may not be able to track the fast rate and go into ventricular fibrillation (an irregular fast, life threatening rhythm of the lower chambers of the heart). Hence atrial fibrillation in WPW syndrome requires immediate termination by electrical cardioversion (electrical high voltage counter shock to stop the abnormal rhythm and allowing the pacemaker of the heart to take over with normal rhythm).
WPW syndrome can be treated medically with anti-arrhythmic drugs (drugs used to correct irregular heart rhythm) 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. In radiofrequency catheter ablation, small electrodes are introduced into various parts of the heart to record electrical signals from various parts of the heart to locate the site of the abnormal pathway. Once the abnormal pathway has been located, radiofrequency energy is delivered to produce a tiny highly localized superficial burn which destroys the abnormal pathway. This procedure is painless and can be done under local anaesthesia to avoid pain at the site of electrode introduction into the body, usually in the groin.