Abstract
The purpose of this study was to identify the dose-dependent effects of heat strain and orthostasis [via lower body negative pressure (LBNP)], with and without mild hypohydration, on systemic function and cerebral perfusion. Eleven men (means +/- SD: 27 +/- 7 y; body mass 77 +/- 6 kg), resting supine in a water-perfused suit, underwent progressive passive heating [0.5 degrees C increments in core temperature (T-c; esophageal to +/- 2.0 degrees C)] while euhydrated (EUH) or hypohydrated (HYPO; 1.5-2% body mass deficit). At each thermal state, mean cerebral artery blood velocity (MCAv(mean); transcranial Doppler), partial pressure of end-tidal carbon dioxide (PETCO2), heart rate (HR) and mean arterial blood pressure (MAP; photoplethysmography) were measured continuously during LBNP (0, -15, -30, and -45 mmHg). Four subjects became intolerant before +2.0 degrees C T-c, unrelated to hydration status. Without LBNP, decreases in PETCO2 accounted fully for reductions in MCAv(mean) across all T-c. With LBNP at heat tolerance (+1.5 or +2.0 degrees C), PETCO2 accounted for 69 +/- 25% of the change in MCAv(mean). The HYPO condition did not affect MCAv(mean) or any cardiovascular variables during combined LBNP and passive heat stress (all P > 0.13). These findings indicate that hypocapnia accounted fully for the reduction in MCAv(mean) when passively heat stressed in the absence of LBNP and for two- thirds of the reduction when at heat tolerance combined with LBNP. Furthermore, when elevations in T-c are matched, mild hypohydration does not influence cerebrovascular or cardiovascular responses to LBNP, even when stressed by a combination of hyperthermia and LBNP.