Effect of brief bursts of carotid sinus stimuli on heart rate and atrioventricular conduction.

The effect of a single brief stimulus burst applied simultaneously to both carotid sinus nerves on atrioventricular conduction (PR) was examined in paced and unpaced preparations of anesthetized open-chest dogs. The relative timing of the stimulus burst was varied to encompass the complete cardiac cycle. Carotid sinus/vagal effect curves were constructed to identify the time course of the response. In paced preparations the maximum increase in PR was 20.3 +/- 2.7 msec and this occurred 458.0 +/- 22.8 msec after the stimulus. There was a latency of 246.0 +/- 12.4 msec after the electrical stimulus before the PR began to increase. In unpaced heart preparations the effect of single carotid sinus nerve stimuli on heart period (PP) and PR was also determined. PP was maximally lengthened by 251.4 +/- 49.4 msec at 644.2 +/- 50.9 msec after the stimulus. There was a latency of 251.4 +/- 10.5 msec before the first noticeable change in PP occurred. The PR response was biphasic. The maximum lengthening of the PR interval was 16.2 +/- 4.2 msec. This response occurred at 361 +/- 22.0 msec after the electrical stimulus. The PR decreased to a minimum value of 13.6 +/- 2.2 msec below control values at 767.1 +/- 44.0 msec after the stimulus. The overall effect of carotid sinus activity on atrioventricular conduction depends not only on the direct nodal effect of acetylcholine but also on the indirect heart rate changes. We conclude that brief bursts of carotid sinus nerve stimulation produce cardiac electrophysiological effects qualitatively similar to the effects of direct vagal stimulation, differing only in having a lower amplitude, a longer latency and a somewhat wider time dispersion.