Determinants of physiological response to exertional heat stress in humans

INTRODUCTION AND AIMS Previous field studies of exercise undertaken in hot conditions reported dissociation between heat-related symptoms and body core temperature (Tc) elevation. This prompted us to examine physiological mechanisms underpinning responses to exertional heat exposure in a controlled laboratory experiment. METHODS Thirty-eight apparently healthy participants had body composition (DEXA) and fitness (V̇O2peak) measured in a preliminary visit. Tc, heart rate (HR), exercise intensity (V̇O2) and echocardiographic measures of end-diastolic volume (EDV), stroke volume (SV) and cardiac output (Q̇) were measured at rest and at 30-minute intervals throughout a two-hour walk (5km/h and 2% gradient) in a climate-controlled chamber (40⸰C and 50% relative-humidity). RESULTS Twenty-seven participants completed the experiment (Completers; Com), 7 were stopped due to Tc exceeding 39⸰C (Hyperthermics; Hyp). Four participants did not complete the protocol due to adverse symptoms and were excluded from this analysis. Hyp had significantly greater Tc at 60-minutes (38.6±0.5⸰C vs. 38.0±0.3⸰C; P=<0.001) and 90-minutes (39.0±0.3⸰C vs. 38.3±0.3⸰C; P=<0.001) than Com. Baseline V̇O2peak (ml.kg-1.min-1) did not differ between Hyp and Com (P=0.248). Whilst body weight and lean body mass did not significantly differ between Hyp and Com, visceral adipose tissue (VAT) volume (463.2±194.3cm3 vs. 259.9±284.4cm3 P=0.014) and mass (437.0±183.1g vs. 245.2±268.2g P=0.013) were significantly greater in Hyp vs Com. In Hyp, HR change from baseline was greater at 60-minutes (Δ57±15bpm vs. Δ36±18bpm; P=0.01) and 90-minutes (Δ63±20bpm vs. Δ42±19bpm; P=0.017), whilst change in EDV (Δ-22.7±11.4ml vs. Δ-6.92±11.2ml; P=0.016) and SV (Δ-19.4±6.1ml vs. Δ-0.7±7.5ml; P=<0.001) were significantly reduced in Hyp compared to Com. CONCLUSION During walking in the heat, subjects who became hyperthermic had significantly higher baseline visceral fat mass and early signs of exaggerated haemodynamic burden during exertional heat exposure. This study has implications for the identification of appropriate variables for establishing safe work limits during heat exposure in military and work-related contexts.