Synchronization of the Cardiac Pacemaker with Repetitive Stimulation of the Carotid Sinus Nerve in the Dog

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Abstract
In anesthetized open-chest dogs, the right carotid sinus nerve was stimulated with brief bursts of electrical impulses. When a single burst was given with each heart beat, the reflexly induced change in heart rate depended on the time within the cardiac cycle that the stimulus was applied. The maximum response was obtained when the P-St interval (i.e., the time from the beginning of the P wave to the beginning of the stimulus burst) was approximately zero. At this P-St interval the cardiac cycle (P-P interval) was prolonged to a mean value of 583 ± 145 (SD) msec from a prestimulation control value of 408 ± 80 msec. The minimum response, a mean P-P interval 45 ± 20 msec less than the maximum, was obtained at a mean P-St interval of 322 ± 48 msec. As a consequence of this phase dependency, over a definite but small range of stimulation frequencies, the discharge of the cardiac pacemaker became synchronized with the neural activity in the baroreceptor reflex arc. Within this critical range of stimulation frequencies, the cardiac pacemaker responded paradoxically to carotid sinus nerve stimulation: a change in the frequency of stimulation evoked a parallel, rather than the well-known inverse, change in heart rate.