Demonstration of a Dual A-V Nodal Conduction System in the Isolated Rabbit Heart

The intranodal propagation of atrial and ventricular echoes in in-vitro preparations of the rabbit heart was traced from microelectrode records at many puncture sites. Composite records indicate that the upper part of the node can be functionally and spatially dissociated into two pathways (α and β), which communicate with a final common pathway (FCP) about halfway between the atrium and the His bundle. Atrial echoes, induced by premature atrial stimulation, traverse the α pathway and return to the atrium from the middle part of the node over the β route; echoes can occur without propagation of the premature response to the ventricle. Ventricular echoes, induced by premature stimulation of the His bundle, pass from the FCP to the atrium over the α route, and back to the FCP over the β route. Repetitive reciprocation, in the nature of an intranodal circus movement, was commonly observed. The results completely substantiate previous conclusions based on indirect observations in the dog heart.