Responses of the cardiovascular system to carotid sinus nerve stimulation.

The changes in blood pressure, heart rate, cardiac output and blood flow in the femoral and common carotid arteries on carotid sinus nerve stimulation (CSNS) were studied in chloralose anaesthetized dogs, both with spontaneous heart rhythm and during atrial pacing. Stimulation of the sinus nerves with impulse trains and with impulses of constant frequency had almost equal effects on the blood pressure. The former had a greater effect on the heart rate; these findings verified earlier observations. The reductions in cardiac output followed those in heart rate. During atrial pacing the stroke volume was reduced on CSNS. The total peripheral resistance, regional peripheral resistances and input impedance of the vascular bed of the femoral artery were calculated. The initial effects of CSNS varied in relation to the prestimulation total peripheral resistance and to stimulation frequency. Differences between stimulation with a constant frequency and the intermittent types, with the same number of impulses per cardiac cycle, were negligible as regards effects on stroke volume, blood flow and regional vascular resistances. CSNS caused changes in input impedance of the vascular bed of the femoral artery which were very similar to those observed earlier on intraarterial injection of vasodilator drugs. The different effects of intermittent and constant frequency CSNS on the heart rate in dogs with intact vagal nerves and no clamping of the common carotid arteries might be caused by asymmetries in the autonomic effects on the S.A. node. The neurophysiological mechanisms are discussed. It is deduced that greater reductions in blood pressure with intermittent stimulation are only obtained in the pre-existence of a high sympathetic tone.