Reentry mechanisms

In order to have a reentry mechanism in the AV node, a patient must have dual pathways leading to the compact zone of the AV node. Approximately 70% of patients have just a single pathway, known as the alpha or slow pathway. About 30% of patients have a dual pathway with a beta or faster conducting pathway, that has a relatively longer refractory period than the alpha pathway.

Note that in sinus rhythm (A), the impulse conducts preferentially antegrade down the fast pathway and any conduction down the slow pathway is extinguished. As seen in B, a PAC may not conduct through the fast pathway due to the longer refractory period, and may preferentially conduct down the slow pathway to reach the ventricle. However, since the fast pathway has now had time to recover, the impulse may now conduct retrograde to activate the atria from below, via the fast beta pathway. When this reentry loop becomes self-perpetuating, it results in a rapid regular tachycardia, with retrograde P waves usually either hidden in or just following the QRS.

Animation