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The pattern of atrioventricular (AV) nodal activation in isolated rabbit hearts was studied by recording transmembrane action potentials in about 200 AV nodal fibers per heart. A brush electrode consisting of ten microelectrodes was used. Both antegrade and retrograde spread of excitation were mapped: retrograde conduction was not an exact mirror image of antegrade conduction. The major input during antegrade conduction was from the crista terminalis, but the major output during retrograde conduction was through the interatrial septum. Some parts of the AV node were excited without directly participating in either antegrade or retrograde transmission of the impulse. Simultaneous pacing of the atrium and the His bundle generated antegrade and retrograde wave fronts which collided at different levels of the AV node. In this way, cells which transmitted the impulse in both directions were distinguished from those which did not (dead-end pathways). Two types of dead-end pathways were found: type A probably consisted of superficial atrial fibers terminating in the base of the tricuspid valve and type B branched off from cells in the middle node. The shape of the action potential of AV nodal cells strongly depended on the direction of propagation. In some parts of the AV node, the amplitude of the action potential was larger during antegrade activation than it was during retrograde activation; in other parts, the amplitude was smaller during antegrade conduction. The largest amplitudes occurred when wave fronts collided.

Spread of Excitation in the Atrioventricular Node of Isolated Rabbit Hearts Studied by Multiple Microelectrode Recording