TY - JOUR

T1 - Normobaric hypoxia does not alter the critical environmental limits for thermal balance during exercise-heat stress

AU - Coombs, Geoff B

AU - Cramer, Matthew N

AU - Ravanelli, Nicholas

AU - Imbeault, Pascal

AU - Jay, Ollie

N1 - © 2020 The Authors. Experimental Physiology © 2020 The Physiological Society.

PY - 2021/1

Y1 - 2021/1

N2 - NEW FINDINGS: What is the central question of this study? Hypoxia reportedly does not impair thermoregulation during exercise in compensable heat stress conditions: does it have an impact on maximal heat dissipation and therefore the critical environmental limit for the physiological compensability of core temperature? What is the main finding and its importance? Although skin blood flow was higher in hypoxia, no differences in sweat rates or the critical environmental limit for the physiological compensability of core temperature - an indicator of maximal heat loss - were found compared to exercise in normoxia, indicating no influence of normobaric hypoxia on thermoregulatory capacity in warm conditions.ABSTRACT: Altered control of skin blood flow (SkBF) in hypoxia does not impair thermoregulation during exercise in compensable conditions, but its impact on maximal heat dissipation is unknown. This study therefore sought to determine whether maximum heat loss is altered by hypoxia during exercise in warm conditions. On separate days, eight males exercised for 90 min at a fixed heat production of ∼500 W in normoxia (NORM) or normobaric hypoxia (HYP, FIO2 = 0.13) in a 34°C environment. Ambient vapour pressure was maintained at 2.13 kPa for 45 min, after which it was raised 0.11 kPa every 7.5 min. The critical ambient vapour pressure at which oesophageal temperature inflected upward (Pcrit ) indicated that maximum heat dissipation had been reached. Neither local sweat rates on the upper arm, back and forehead (average NORM: 1.46 (0.15) vs. HYP: 1.41 (0.16) mg cm-2 min-1 ; P = 0.59) nor whole-body sweat losses (NORM: 1029 (137) g vs. HYP: 1025 (150) g; P = 0.95) were different between trials. Laser-Doppler flux values (LDF; arbitrary units), an index of SkBF, were not different between NORM and HYP on the forearm (P = 0.23) or back (P = 0.73); however, when normalized as a percentage of maximum, LDF values tended to be higher in HYP compared to NORM at the forearm (condition effect, P = 0.05) but not back (P = 0.19). Despite potentially greater SkBF in hypoxia, there was no difference in Pcrit between conditions (NORM: 3.67 (0.35) kPa; HYP: 3.46 (0.39) kPa; P = 0.22). These findings suggest that hypoxia does not independently alter thermoregulatory capacity during exercise in warm conditions.

AB - NEW FINDINGS: What is the central question of this study? Hypoxia reportedly does not impair thermoregulation during exercise in compensable heat stress conditions: does it have an impact on maximal heat dissipation and therefore the critical environmental limit for the physiological compensability of core temperature? What is the main finding and its importance? Although skin blood flow was higher in hypoxia, no differences in sweat rates or the critical environmental limit for the physiological compensability of core temperature - an indicator of maximal heat loss - were found compared to exercise in normoxia, indicating no influence of normobaric hypoxia on thermoregulatory capacity in warm conditions.ABSTRACT: Altered control of skin blood flow (SkBF) in hypoxia does not impair thermoregulation during exercise in compensable conditions, but its impact on maximal heat dissipation is unknown. This study therefore sought to determine whether maximum heat loss is altered by hypoxia during exercise in warm conditions. On separate days, eight males exercised for 90 min at a fixed heat production of ∼500 W in normoxia (NORM) or normobaric hypoxia (HYP, FIO2 = 0.13) in a 34°C environment. Ambient vapour pressure was maintained at 2.13 kPa for 45 min, after which it was raised 0.11 kPa every 7.5 min. The critical ambient vapour pressure at which oesophageal temperature inflected upward (Pcrit ) indicated that maximum heat dissipation had been reached. Neither local sweat rates on the upper arm, back and forehead (average NORM: 1.46 (0.15) vs. HYP: 1.41 (0.16) mg cm-2 min-1 ; P = 0.59) nor whole-body sweat losses (NORM: 1029 (137) g vs. HYP: 1025 (150) g; P = 0.95) were different between trials. Laser-Doppler flux values (LDF; arbitrary units), an index of SkBF, were not different between NORM and HYP on the forearm (P = 0.23) or back (P = 0.73); however, when normalized as a percentage of maximum, LDF values tended to be higher in HYP compared to NORM at the forearm (condition effect, P = 0.05) but not back (P = 0.19). Despite potentially greater SkBF in hypoxia, there was no difference in Pcrit between conditions (NORM: 3.67 (0.35) kPa; HYP: 3.46 (0.39) kPa; P = 0.22). These findings suggest that hypoxia does not independently alter thermoregulatory capacity during exercise in warm conditions.

KW - Adult

KW - Body Temperature/physiology

KW - Body Temperature Regulation/physiology

KW - Exercise/physiology

KW - Female

KW - Heat Stress Disorders/physiopathology

KW - Heat-Shock Response/physiology

KW - Hot Temperature/adverse effects

KW - Humans

KW - Hypoxia/physiopathology

KW - Male

KW - Regional Blood Flow/physiology

KW - Sweating/physiology

U2 - 10.1113/EP088466

DO - 10.1113/EP088466

M3 - Article

C2 - 32190934

VL - 106

SP - 359

EP - 369

JO - Experimental Physiology

JF - Experimental Physiology

SN - 0958-0670

IS - 1

ER -