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Transmembrane potentials of A-V nodal cells in isolated rabbit hearts were recorded while the route of entry of atrial activity into the A-V node was changed by stimulating the atrium at a high, constant rate on eitherthe crista terminalis or the interatrial septum. Atrial activity arising from the interatrial septum provided a less effective input to the A-V node, as shown by the occurrence of 2 : 1 A-V conduction block, compared to 1 : 1 during driving on the crista terminalis at the same rate. The amplitude and rate of rise of the action potentials of most A-V nodal cells was smaller during driving on the interatrial septum, and many cells developed alarger or smaller "hump" in the repolarization phase, which was absent during stimulation on the crista terminalis. The pattern of A-V nodal excitation during stimulation on the crista terminalis was similar to that during spontaneous sinus rhythm, but was profoundly altered during stimulation on the interatrial septum. The results can be explained by assuming that an atrial wave front coming from the interatrial septum excites the atrial margin of the A-V node asynchronously, resulting in inhoinogeneous conduction within the node, causing longitudinal dissociation. The results show that successful propagation through the A-V node is at least partly determined by the direction of the atrial wave front. This factor may play an important role during atrial fibrillation.

Influence of the Direction of the Atrial Wave Front on A-V Nodal Transmission in Isolated Hearts of Rabbits