Human sinus node responses to repetitive, ramped carotid baroreceptor stimuli

When arterial pressure rises spontaneously, baroreceptor areas are presented with repetitive stimuli of progressively increasing intensities. We simulated rising arterial pressure in 18 healthy young adults by applying trains of repetitive, ramped, R-wave-coupled neck suction, and we related P-P interval responses to intensities of stimuli with least-squares linear regression. We found that the P-P interval response to the stimulus train we used is immediate and highly linear (most correlation coefficients were above 0.90). In studies in which the base-line heart rate was less than 75 beats/min, cardiac inhibition was maximal in the cardiac cycle in which the stimulus was delivered. Decay of inhibition commences rapidly after the end of baroreceptor stimulation and proceeds with time constants that are independent of maximum levels of inhibition. P-P interval shortening (cardioacceleration) comprises an integral feature of baroreflex responses, such that inhibitory stimuli set in motion oscillations of heart period above and below base-line levels. Our data suggest that the pattern of sinus node responses during transitions of arterial pressure results from a complex interplay among several factors, including baroreflex responsiveness, base-line heart rate, the rate decay of baroreflex inhibition, and the biphasic nature of sinus node responses to inhibitory cholinergic interventions.