Effect of postganglionic vagal stimulation on the organization of atrioventricular nodal conduction in isolated rabbit heart tissue.

Postganglionic stimulation of vagal terminals (PGVS) in the isolated rabbit heart atrioventricular (AV) node was used to study the effects of cholinergic influence on AV nodal conduction. Standard microelectrode techniques were used to record action potentials, predominantly from cells located in the N region of the AV node. In addition, programmed stimulation was used in conjunction with PGVS to initiate or terminate AVN reentry. The introduction of a single short burst of PGVS (total duration 50 to 100 msec, impulse duration 1 msec, and interimpulse interval 6 msec) with subthreshold amplitude for AV node fibers caused reproducible disorganization of the prevailing excitation front. This was manifest as local nonuniform depression of conduction, hump formations in the action potentials, and alteration in the sequence of depolarization. The introduction of repetitive bursts of PGVS revealed a triphasic time course of changes in AV nodal conduction time, representing initial maximal prolongation, relative shortening, and secondary inhibition. It was found that these phases corresponded to vagally induced initial disorganization and a subsequent rebound process. Vagally induced disorganization of the sequence of action potential depolarization was also a triggering mechanism for concealed as well as manifest AV nodal reentry. In the latter case the reentry circuit usually involved the AN region and perinodal atrial tissue. PGVS-induced depression of the N region was also able to block the retrograde wavefront, thereby terminating reentry. The possible relationship of PGVS-induced disorganization of conduction and the inhomogeneous structure of AV node are discussed. The present results provide additional information for better understanding of the AV nodal conduction abnormalities observed clinically and particularly those related to AV node-vagus interaction.