Thermal stress modulates arterial pressure variability and arterial baroreflex response of heart rate during head-up tilt in humans

To examine the effects of thermal stress on the blood pressure variability and the arterial baroreceptor-cardiac reflex during orthostatic stress, 11 male volunteers underwent whole body thermal stress using a cool or hot wat er-perfused suit during 5 min of 70 degrees head-up tilt (HUT). The spontan eous variability in arterial pressure was quantified by power spectrum anal ysis. The sensitivity of the arterial baroreceptor-cardiac reflex was calcu lated from the spontaneous changes in beat-to-beat arterial pressure and he art rate (f(c)). During supine rest the variability of arterial pressure de creased during cooling, while it remained unchanged during heating. The var iability increased with HUT; it was greater (P < 0.05) with heating than wi th cooling. In the supine condition, the arterial baroreflex sensitivity of f(c) increased during cooling, while it did not change during heating. The sensitivities decreased (P < 0.05) with HUT during both thermal stresses; the decreased rate of sensitivity from the pre-tilt value was greater durin g heating [mean 63 (SEM 4)%] and smaller during cooling [mean 11 (SEM 24)%] than during normothermia [mean 47 (SEM 4)%] (both, P < 0.05). There were s ignificant negative correlations between the sensitivities and the amplitud e of the arterial pressure variability during normothermia and heating (P < 0.0001). The results suggest that the spontaneous baroreflex response off, is a modulatory factor for the changes of arterial pressure variability br ought about by thermal stress during orthostatic stress.