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To elucidate the mechanism by which stimulation of the carotid sinus nerves prolongs the R-R interval, the effects of activating an implanted carotid sinus nerve stimulator were studied in eight patients at varying levels of background autonomic activity and with varying types of efferent autonomic blockade. Beta-receptor blockade was induced with intravenous propranolol, 0.20 mg/kg, and parasympathetic blockade with intravenous atropine, 0.04 mg/kg. In the supine position, average prolongation of the R-R interval due to stimulation of the carotid sinus nerves was 269 ± 56 msec prior to autonomic blocking drugs, 442 ± 214 msec after propranolol (NS), and 44 ± 13 msec after propranolol and atropine (P < 0.025). Comparable changes were produced in the standing position. With moderate treadmill exercise, stimulation of the carotid sinus nerves prolonged the R-R interval by only 40 ± 30 msec prior to blocking drugs, 62 ± 19 msec after propranolol, and 40 ± 30 msec after propranolol and atropine. It is concluded that the prolongation of the R-R interval produced by stimulation of the carotid sinus nerves is secondary to augmented parasympathetic activity, and the attenuation of this response during erect exercise appears to be due to a centrally mediated reduction in the responsiveness of the parasympathetic nervous system to baroreceptor stimuli.

Mechanism of Prolongation of the R-R Interval with Electrical Stimulation of the Carotid Sinus Nerves in Man