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Effect of whole‐body heat stress on peripheral vasoconstriction during engagement of the venoarteriolar response

The aim of this study was to test the hypothesis that whole‐body heat stress attenuates venoarteriolar response (VAR) mediated vasoconstriction of the cutaneous and whole‐leg circulations. During normothermia (NT), cutaneous blood flow (CBF) and femoral artery blood flow (FBF) were obtained during supine and leg dependent conditions. These measurements were repeated following whole‐body passive heat stress (WBH) that increased internal temperature by 1.4±0.2 °C. Cutaneous (CVC) and femoral vascular conductances (FVC) were calculated by dividing CBF and FBF by mean arterial pressure. During leg dependency, the absolute decrease in CVC (NT: 0.22 ± 0.1; WBH: 0.19 ± 0.3 arbitrary units·mmHg−1; P=0.97) and FVC (NT: 1.90 ± 1.7; WBH: 0.70 ± 2.1 ml·min−1·mmHg−1; P=0.71) were similar between thermal conditions. However, since WBH substantially increased CVC and FVC (P<0.001 for both variables), the percent reduction in CVC (NT: 58 ± 9%; WBH: 9.4 ± 9.1%, P<0.001) and FVC (NT: 33 ± 21%; WBH: 6.0 ± 17%; P=0.02) during leg dependency were attenuated during WBH relative to NT. These data suggest that heat stress reduces the vasoconstrictive response in the cutaneous and whole leg circulations during engagement of the VAR via leg dependency. These attenuated responses may contribute to reduced blood pressure control in hyperthermic subjects leading to impaired orthostatic tolerance.