Effects of acute hypobaric hypoxia on thermoregulatory and circulatory responses during cold air exposure

The thermoregulatory responses during simultaneous exposure to hypoxia and cold are not well understood owing to the opposite reactions of vasomotor tone in these two environments. Therefore, the purpose of this study was to investigate the influences of hypobaric hypoxia on various thermoregulatory responses, including skin blood flow (SkBF) during cold exposure.

Ten subjects participated in two experimental conditions: normobaric normoxia with cold (NC, barometric pressure (P_B) = 760 mmHg) and hypobaric hypoxia with cold (HC, P_B = 493 mmHg). The air temperature was maintained at 28 °C for 65 min and gradually decreased to 19 °C for both conditions. The total duration of the experiment was 135 min.

The saturation of percutaneous oxygen (SpO₂) was maintained at 98-99% in NC condition, but decreased to around 84% in HC condition. The rectal and mean skin temperatures showed no significant differences between the conditions; however, the forehead temperature was higher in HC condition than in NC condition. The pulse rate increased in HC condition, and there was a strong negative relationship between SpO₂ and pulse rate (r = - 0.860, p = 0.013). SkBF and blood pressure showed no significant differences between the two conditions.

These results suggest that hypobaric hypoxia during cold exposure did not alter the overall thermoregulatory responses. However, hypobaric hypoxia did affect pulse rate regardless of cold exposure.