Effect of voluntary hypocapnic hyperventilation on cutaneous circulation in resting heated humans

Abstract

Hypocapnia attenuates the sweat response normally seen in hyperthermic resting subjects, but its effect on the blood flow response in their nonglabrous skin under the same hyperthermic conditions remains unclear. In the present study, we investigated whether hypocapnia induced by voluntary hyperventilation affects the blood flow response to heat stress in the nonglabrous skin of resting humans. Nine healthy male subjects were passively heated using legs-only hot water immersion and a water-perfused suit, which caused esophageal temperature (Tes) to increase by as much as 1.0°C. During normothermia and at +0.6°C Tes and +1.0°C Tes, the subjects performed two voluntary 7-min hyperventilation (minute ventilation = 40 l/min) trials (hypocapnic and eucapnic) in random order. End-tidal CO2 pressure was reduced by 23–25 torr during hypocapnic hyperventilation, but it was maintained at the spontaneous breathing level during eucapnic hyperventilation. Cutaneous blood flow was evaluated as the cutaneous red blood cell flux in the forearm (CBFforearm) or forehead (CBFforehead) and was normalized to the normothermic spontaneous breathing value. Hypocapnic hyperventilation at +0.6°C Tes was associated with significantly reduced CBFforearm, compared with eucapnic hyperventilation, after 5–7 min of hyperventilation (395 to 429 vs. 487 to 525% baseline, P < 0.05). No significant difference in CBFforehead was seen during hypocapnic hyperventilation compared with eucapnic hyperventilation at +0.6°C Tes or +1.0°C Tes. These results suggest that in resting humans, hypocapnia achieved through voluntary hyperventilation attenuates the increase in cutaneous blood flow elicited by moderate heat stress in the nonglabrous skin of the forearm, but not the forehead.