Heat stress reduces cerebral blood velocity and markedly impairs orthostatic tolerance in humans

Abstract

Orthostatic tolerance is reduced in the heat-stressed human. This study tested the following hypotheses: 1) whole body heat stress reduces cerebral blood velocity (CBV) and increases cerebral vascular resistance (CVR); and 2) reductions in CBV and increases in CVR in response to an orthostatic challenge will be greater while subjects are heat stressed. Fifteen subjects were instrumented for measurements of CBV (transcranial ultrasonography), mean arterial blood pressure (MAP), heart rate, and internal temperature. Whole body heating increased both internal temperature (36.4+/-0.1 to 37.3+/-0.1 degrees C) and heart rate (59+/-3 to 90+/-3 beats/min); P<0.001. Whole body heating also reduced CBV (62+/-3 to 53+/-2 cm/s) primarily via an elevation in CVR (1.35+/-0.06 to 1.63+/-0.07 mmHg.cm-1.s; P<0.001. A subset of subjects (n=8) were exposed to lower-body negative pressure (LBNP 10, 20, 30, 40 mmHg) in both normothermic and heat-stressed conditions. During normothermia, LBNP of 30 mmHg (highest level of LBNP achieved by the majority of subjects in both thermal conditions) did not significantly alter CBV, CVR, or MAP. During whole body heating, this LBNP decreased MAP (81+/-2 to 75+/-3 mmHg), decreased CBV (50+/-4 to 39+/-1 cm/s), and increased CVR (1.67+/-0.17 to 1.92+/-0.12 mmHg.cm-1.s); P<0.05. These data indicate that heat stress decreases CBV, and the reduction in CBV for a given orthostatic challenge is greater during heat stress. These outcomes reduce the reserve to buffer further decreases in cerebral perfusion before presyncope. Increases in CVR during whole body heating, coupled with even greater increases in CVR during orthostasis and heat stress, likely contribute to orthostatic intolerance.

Fig. 1

Effects of whole body heating on cerebral blood velocity from the middle cerebral artery (A) and cerebral vascular resistance (B). The symbols with error bars denote mean responses, while lines denote individual responses. *P < 0.001.

Fig. 2

Effects of normothermia and heat stress on the change in cerebral blood velocity (CBFV) during lower body negative pressure (LBNP). Baseline represents 0 mmHg LBNP. *Significant difference from baseline. #Significant difference from the previous LBNP. n = 5 given that only five subjects tolerated 30 mmHg LBNP while heat stressed. The three subjects who did not tolerate 30 mmHg LBNP were excluded from this figure to appropriately compare the paired and interactive effects of whole body heating and LBNP.

Fig. 3

Effects of normothermia and heat stress on the change of cerebral vascular resistance (CVR) during LBNP. Baseline represents 0 mmHg LBNP. *Significant difference from baseline. #Significant difference from the previous LBNP. n = 5 given that only five subjects tolerated 30 mmHg LBNP. The three subjects who did not tolerate 30 mmHg LBNP were excluded from this figure to appropriately compare the paired and interactive effects of whole body heating and LBNP.