AV Re-entrant Tachycardias

These are due to an aberrant pathway (rarely multiple pathways) linking atrium and ventricle, bypassing the normal conducting system.

The best-known clinical syndrome is Wolff-Parkinson-White Syndrome (WPW) where the aberrant pathway is known as the Bundle of Kent (see image above).

The key ECG features of WPW when the patient is in sinus rhythm are a short PR interval and the slurred upstroke of the R wave (delta wave).

In sinus rhythm conduction is usually via both pathways, with rapid transmission through the accessory pathway causing ventricular pre-excitation, but most ventricular depolarisation occurring normally via the Bundle of His.

This is represented on the resting ECG (see Figure 8) with:

• Short PR interval (120ms)
• Slurred upstroke of the R wave known as a delta wave (from ventricular pre-excitation via accessory pathway, bypassing the slower AV node)
• Wide QRS complex (>120ms)
• Discordance (in the opposite direction to the QRS) of the ST-segment & T-wave is also common, due to the abnormal direction of depolarisation & repolarisation

Sometimes the accessory pathway can only conduct retrogradely, in which case normal AV conduction occurs through the AV node and the ECG is normal in sinus rhythm (a concealed accessory pathway).