Cardiovascular Effects of Direct Vagal Stimulation in Man.

Summary. The vagal control of the heart has been studied in man during total neck dissections. After that the normal tonic discharge of the fibres to the heart had been blocked by locally injected xylocaine, the nerve could be directly stimulated at maximal strength and increasing frequencies. Plotting the effector response, as measured by the pulse rate, ECG and blood pressure, against the stimulation frequencies gives a hyperbolic curve, typical of autonomic neuroeffectors, with almost maximal effects on the heart already at 10 impulses per second. By comparing the heart response to known stimulation rates with the heart rate before nerve block, the normally present vagal tone could be roughly evaluated to about 2–4 impulses per second. No evidence was obtained in support of vagal fibres with a negative inotropic effect on the heart ventricles, as has previously been suggested. It was further observed that afferent fibres in the vagal nerves with reflex connections to the vasomotor centre are very sensitive to mechanical stimuli so that dissections of the nerve can induce drastic reflex effects on the cardiovascular system.