Summated circulatory responses of thermal and baroreflexes in humans

Baroreceptor and thermal receptor mediated reflexes are important neural control systems that can markedly alter blood pressure in humans. These two systems have different afferent limbs and are integrated at different central sites. Although the circulatory effects of activating these individual reflexes have been examined, the effects of simultaneous activation of two or more separate reflexes have received relatively little attention. We used noninvasive methods in 13 normal men (20-27 yr) to examine the responses mediated by cardiopulmonary and arterial baroreceptors; the reflexes were elicited by alterations in central blood volume [lower body negative pressure (LBNP) and leg elevation (LE)] in the presence or absence of local thermal receptor stimulation induced by hand immersion (HI) in 10 degree C water. During all levels (-10, -25, and -40 Torr) of LBNP and during LE, forearm vascular resistance (FVR) was altered. At the two higher levels of LBNP, stroke volume, cardiac index, and blood pressure (BP) decreased, and heart rate (HR) and total peripheral resistance (TPR) increased. TPR, FVR, and BP increased during HI. Our analyses indicated that responses to combined stimuli, i.e., local cold during LBNP or LE, resulted in simple additive responses in all variables except HR. A central interaction between thermal and baroreflex control of HR is possible.