Influence of nonthermal baroreceptor modulation of heat loss responses during uncompensable heat stress.

We evaluated the hypothesis that with increasing levels of hyperthermia, thermal influences would predominate over nonthermal baroreceptor control of cutaneous vascular conductance (CVC) and local sweat rate (LSR). On separate days, eight male participants were positioned in either an upright seated posture (URS) or a 15 degrees head-down tilt (HDT) posture in a thermoneutral condition and during passive heating, until mean body temperature (T(body)) increased by 1.5 degrees C. Hemodynamic [heart rate (HR), cardiac output, mean arterial pressure (MAP)] and thermal responses [T(re), CVC, LSR] were measured continuously. MAP showed a gradual decrease in the early- to mid-stages of heating for both HDT and URS. At a T(body) > 0.6 degrees C, MAP achieved a stable, albeit reduced level from baseline resting for the duration of the heating, whereas MAP decreased significantly throughout the heating period in the URS position (p < 0.001). CVC increased rapidly in the early stages of heating and achieved a stable elevated level in both HDT and URS at the mid-stage of heating (T(body) increase