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

Abstract

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 water-perfused suit during 5 min of 70 degrees head-up tilt (HUT). The spontaneous variability in arterial pressure was quantified by power spectrum analysis. The sensitivity of the arterial baroreceptor-cardiac reflex was calculated from the spontaneous changes in beat-to-beat arterial pressure and heart rate (fc). During supine rest the variability of arterial pressure decreased during cooling, while it remained unchanged during heating. The variability increased with HUT; it was greater (P < 0.05) with heating than with cooling. In the supine condition, the arterial baroreflex sensitivity of fc 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 during 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 significant negative correlations between the sensitivities and the amplitude of the arterial pressure variability during normothermia and heating (P < 0.0001). The results suggest that the spontaneous baroreflex response of fc is a modulatory factor for the changes of arterial pressure variability brought about by thermal stress during orthostatic stress.

Spontaneous arterial baroreflex control of the heart rate during head-down tilt in heat-stressed humans

“…Bahjaoui- Bouhaddi et al (2000) have reported that spontaneous cardiac baroreex sensitivity was not changed by 10°HDT during normothermia. However, LBNP or head-up tilt reduces spontaneous cardiac barore¯ex sensitivity (Hughson et al 1994;Yamazaki and Sone 2001). It has been shown that the alterations in the cardiopulmonary baroreceptor loading due to LBNP or LBPP result in changes in the gain of the carotid baroreceptor-cardiac re¯ex (Pawelczyk and Raven 1989;Shi et al 1993).…”

Effects of whole-body and local thermal stress on hydrostatic volume changes in the human calf

“…However, the amount of information on the effect of thermal stress on the magnitude and pattern of change in leg volume during postural change remains small. Since orthostatic hypotension during heat stress has been observed frequently even during a short-term orthostatic stress for 5 min or less (Horvath and Botelho 1949;Lind et al 1968;Yamazaki and Sone 2001), there is a need to observe the change of leg volume within a few minutes during postural change.…”

Heat stress and orthostatic tolerance

“…Acute skin surface cooling immediately before upright tilting in heat-stressed individuals improved orthostatic tolerance relative to heat stress without cooling 102) . This effect is likely related to profound cutaneous vasoconstriction and the associated redistribution of blood flow from the cutaneous circulation to the central circulation, resulting in increased CVP 103,104) , pulmonary capillary wedge pressure 104) , and baroreflex sensitivity 52,61) during cold stimulus.…”