ABSTRACT
SE29 an impulse arriving at the atrioventricular node will normally split and travel down both pathways at the same time, but the impulse travelling via the fast pathway arrives at the bundle of His first and depolarizes the ventricles. By the time the impulse travelling down the slow pathway arrives at the bundle of His, the bundle is refractory and so this impulse goes no further. ● However, if a supraventricular ectopic beat happens to occur during the refractory period of the fast pathway, this ectopic will travel down the slow pathway and, by the time it reaches the end of the slow pathway, the fast pathway may have repolarized. If so, this impulse will then travel back up along the fast pathway, and then back down the slow pathway, ad infinitum. In the common form of AVNRT, this slow-fast re-entry circuit gives rise to the arrhythmia. Fast-slow and slow-slow re-entry circuits are also possible. ● In AVNRT, P waves are often hard or even impossible to discern. In around a quarter of cases, they are hidden within the QRS complexes. In another two-thirds of
QRS complex in lead V1. In a small number of cases, the P wave can be found just before the QRS complex. ● AVNRT is around 10 times commoner than atrioventricular re-entry tachycardia (AVRT – the result of an atrioventricular accessory pathway as seen in Wolff-Parkinson-White syndrome). The ECG in sinus rhythm in AVNRT is usually normal, but in AVRT an ECG in sinus rhythm may reveal a short PR interval or delta wave, suggesting Wolff-Parkinson-White syndrome. The distinction between AVRT and AVNRT can be difficult, however, and may require electrophysiological studies.
