Effects of Mild Hyperbaric Normoxia on Physiological Recovery After Exercise in a Hot Environment

Background Mild hyperbaric normoxia (MHN), defined as exposure to increased ambient pressure while breathing normal air, is a non-oxygen-enriched hyperbaric recovery approach. Simulated firefighting activities impose substantial cardiovascular strain and thermal strain, providing a relevant model for recovery research. This study examined cardiac and thermoregulatory responses during recovery following simulated firefighting under MHN conditions. Methods Twelve healthy young male participants performed a 20-minute cycling exercise at 60% of age-predicted maximal heart rate in a controlled hot environment (31°C, 60% relative humidity) while wearing full firefighting gear. Post-exercise recovery was conducted under three atmospheric pressure conditions: normobaric normoxia (1.0 atm absolute [ATA] and hyperbaric normoxia (1.5 and 1.7 ATA) for 20 minutes in a randomized order. Heart rate, cardiac output, stroke volume, core and skin temperatures, and skin blood flow were continuously measured, whereas blood pressure and subjective thermal responses were obtained every 5 minutes. Results MHN at 1.5 ATA significantly reduced heart rate during recovery compared with 1.0 and 1.7 ATA ( p  = 0.027), indicating enhanced autonomic recovery, whereas blood pressure showed no significant differences across conditions. Core body temperature remained stable across pressures; however, mean skin temperature was higher at 1.5 and 1.7 ATA than at 1.0 ATA ( p  = 0.048). Skin blood flow declined similarly over time regardless of condition. Thermal comfort differed between pressure conditions (p < 0.001), with the 1.7 ATA condition differing from both 1.0 ATA and 1.5 ATA, whereas 1.0 ATA and 1.5 ATA showed similar ratings corresponding to comfortable conditions. Thermal sensation changed significantly over time but was generally perceived as slightly cool across all pressures. Conclusion MHN at 1.5 ATA appears to facilitate faster cardiac recovery after exercise-induced heat stress, potentially through enhanced parasympathetic activation, while altered skin temperature responses may reflect changes in peripheral heat exchange. These results provide preliminary evidence supporting the use of mild hyperbaric normoxia as a recovery strategy for firefighters and other individuals working in extreme heat, although additional studies in real operational settings are needed to confirm its practical applicability.