Cardiac electrical responses to vagal stimulation of fibers to discrete cardiac regions.

We stimulated regional intracardiac parasympathetic nerve fibers to the atrioventricular (AV) nodal area (AVP stimuli) or to the sinoatrial (SA) nodal area (SAP stimuli) in autonomically decentralized, anesthetized dogs. AVP stimuli increased the AV interval (AV conduction time); the magnitude of the response depended directly on the stimulation frequency. AVP stimuli did not alter the atrial interval (heart period) in spontaneously beating hearts. The magnitude of the negative dromotropic response increased when the atrial interval was shortened. SAP stimuli increased the atrial interval, and the magnitude of the response depended directly on the stimulation frequency. SAP stimuli evoked little or no change in the AV interval in the unpaced heart. When the atrium was paced, SAP stimuli did not change the AV interval in about half of the preparations, and there was a small but significant change in the remaining preparations. The negative dromotropic or chronotropic response to AVP or SAP stimuli was potentiated by physostigmine and blocked by atropine. These results suggest that 1) AVP stimuli induce a selective negative dromotropic response, and 2) this negative dromotropic effect is secondarily affected by heart period, by the pacemaking site, and by atrial activation patterns, but it does not affect the sinus cycle length in the dog heart.