PHASE DEPENDENCIES OF THE HUMAN BARORECEPTOR REFLEX

We studied the influence of respiratory and cardiac phase on responses of the cardiac pacemaker to brief (0.35-s) increases of carotid baror eceptor afferent traffic provoked by neck suction in seven healthy you ng adult subjects. Cardiac responses to neck suction were measured ind irectly from electrocardiographic changes of heart period. Our results show that it is possible to separate the influences of respiratory an d cardiac phases at the onset of a neck suction impulse by a product o f two factors: one depending only on the respiratory phase and one dep ending only on the cardiac phase. This result is consistent with the h ypothesis that efferent vagal activity is a function of afferent baror eceptor activity, whereas respiratory neurons modulate that medullary throughput independent of the cardiac phase. Furthermore, we have show n that stimulus broadening and stimulus cropping influence the outcome of neck suction experiments in a way that makes it virtually impossib le to obtain information on the phase dependency of the cardiac pacema ker's sensitivity to vagal stimulation without accurate knowledge of t he functional shape of stimulus broadening.