Effect Of Hypoxia On The Thermoregulatory Responses To Cold

Abstract

Acute hypoxia has been suggested to alter thermoregulation and thermal perception which may increase cold injury susceptibility. However, the effect of typical mountaineering sojourns on thermoregulation and thermal comfort to subsequent hypoxic, cold exposure is unknown. PURPOSE: To determine thermoregulation and thermal comfort during hypoxic and mild cold exposure before and after an 18 day Alpine mountaineering expedition (maximum height: 4127 ± 144 m) (mean ± SD). METHODS: In shorts only, ten males completed a 2.5 hour cold air test (CAT 15°C, RH 44%) in a supine position. Three CAT were performed in an environmental chamber: 1. sea-level (SL: FIO2 = 20.9%); 2. unacclimatized in normobaric hypoxia (UN: FIO2 = 12.5%); 3. post expedition acclimatised in normobaric hypoxia (AC: FIO2 = 12.5%). To avoid an order effect SL and UN were performed before the expedition in half of the participants and 2 months after the expedition in the remainder. Body weight and composition were determined prior to each CAT. Prior and during CAT core temperature, mean weighted skin temperature, metabolic heat production, shivering activity (observed and electromyography), thermal comfort (McGinnis) and arterial oxygen saturation were assessed. Data were analysed by ANOVA. RESULTS: Body weight and composition were unchanged (Body fat: SL 9 ± 6, UN 9 ± 6, AC 9 ± 6 %; P > 0.05). During the CAT core and skin temperature were not different between SL, UN and AC. However compared with SL, metabolic heat production was lower during UN CAT (SL 93 ± 36, UN 68 ± 25 W·m-2; P = 0.000) and remained lower AC (65 ± 25 W·m-2; P = 0.000) despite an increase in arterial oxygen saturation during AC CAT (SL 97 ± 2, UN 87 ± 9 and AC 91 ± 5%; P = 0.001). Shivering activity was lower AC compared with UN and SL (SL 1.1 ± 0.1, UN 1.0 ± 0.2 and AC 0.6 ± 0.1; P = 0.001). Thermal comfort was increased AC compared with SL and UN (SL 5.0 ± 1.4, UN 5.6 ± 1.1 'uncomfortably cool', AC 6.1 ± 0.8 'cool but fairly comfortable; P = 0.008). CONCLUSION: Following an 18 day mountaineering expedition a lack of change in core and skin temperature despite a decrease in shivering is consistent with a greater reliance on non-shivering thermogenesis. The alterations observed post expedition in thermal comfort may suggest a greater thermal tolerance but an increase in susceptibility to cold injury.