Effect of hypercapnia on skin blood flow during normothermia and whole‐body heating

Abstract

Normoxic hypercapnia increases sympathetic outflow to muscle, presumably causing vasoconstriction. Whether similar responses occur in the cutaneous vasculature is unknown. This study tested the hypothesis that normoxic hypercapnia decreases cutaneous vascular conductance (CVC) during normothermia (NT) and whole-body heating (WBH), with greater reductions in CVC during WBH. Four men and 3 women (mean±SD; age: 34±7 yr) rested supine while wearing a tube-lined suit perfused with 34 °C water (NT), followed by perfusion of 50 °C water to increase internal temperature ~1 °C (WBH). During each thermal condition, a normoxic 5% CO2 gas mixture was inspired for ~4 min while forearm skin blood flow (SkBF; laser Doppler flowmetry) and mean arterial blood pressure (MAP) were measured continuously for the subsequent calculation of CVC (SkBF/MAP). End-tidal CO2 increased similarly during hypercapnic exposure in both NT and WBH (+7.9±2.4 and +8.3±1.9 mmHg, respectively; P=0.66). No differences in CVC were observed between pre-hypercapnic baseline and hypercapnia in either NT (P=0.33) or WBH (P=0.87). These results indicate that CVC is not affected by hypercapnia under normothermic and heat stress conditions. Mild hypercapnia, therefore, is unlikely to impair human thermoregulation by means of reducing SkBF. Funded by NIH grants HL-61388 and HL-84072.