The combined effects of artificial gravity, temperature, and hypoxia on haemodynamic responses and limb blood flow

Under simultaneous environmental and gravitational stressors, integrated vascular responses maintain homeostatic balance via coordinated baro- and thermo-regulatory action. The effect of temperature and hypoxia at an elevated gravitational vector on the interaction of these systems was examined. Ten male participants experienced either cool (18.4 °C) or warm (29.1 °C) ambient temperatures in normoxia (partial pressure of oxygen, P_IO₂ = 133 mmHg) or hypoxia (P_IO₂ = 92 mmHg). Cardiovascular (heart rate, HR; arterial pressure, MAP; cardiac output, CO; stroke volume, SV; skin blood flow, SkBF) and thermoregulatory (skin temperature; core temperature) responses were monitored during standing (NG), and supine centrifugation at ground reaction forces (GRF) measured with a force platform at 1GRF and 2GRF. At 2GRF, warm and hypoxic conditions reduced the test duration by 16%. No differences were observed between NG and 1GRF in any variable; however, 2GRF significantly raised HR by 29.3% and MAP by 12.6%, and lowered SV by 22.2%. Warm condition significantly increased HR, and significantly decreased MAP and SV compared to the cool condition, by 17.8%, 6.1%, and 5.8%, respectively. Hypoxia had no effect on any variable. Arm SkBF significantly decreased by 33.3% with increasing artificial gravity, whereas leg SkBF increased by 38.7%. Higher ambient temperatures had no effect on leg SkBF, but significantly increased arm SkBF by 38.7%. Human tolerance to passive centrifugation is significantly lower at 2GRF, and further affected by the ambient conditions. Haemodynamic and leg SkBF responses in higher temperature and Gz conditions were frequently unable to prevent pre-syncopal symptoms. Finally, arm SkBF was modulated by both baroreflex and thermoregulation, and the baroreflex alone in leg SkBF.