Changes in metabolic gas kinetics on exposure to heat stress

Objectives: Heat stress is one of the major stressors in military aviation with the potential to adversely affect the efficiency of the aircrew and hence flight safety. Metabolic rate (MR) increases on exposure to heat and metabolic gases are considered as a proxy for MR. This study examined the influence of heat stress on metabolic gas kinetics in healthy Indian males and assessed the duration of attaining normal baseline values of metabolic gases post-exposure. Materials and Methods: In 16 healthy male volunteers, cardiorespiratory variables, including metabolic gases (oxygen uptake [VO2], carbon dioxide output [VCO2], minute ventilation [VE], breathing frequency [BF], and heart rate [HR]), were recorded before and approximately 2 h after a standard meal. The subjects were then exposed to a simulated temperature of 40°C with a relative humidity of 70% for 1 h in the environmental chamber. Same physiological parameters were recorded at the end of 30 min and 60 min during heat exposure and up to 90 min following exposure to heat stress at an interval of 30 min. Results: A significant increase (P < 0.001) in mean VO2 (ml/Kg/min) was observed post-meal (1.49 ± 0.95) as well as at 30 min (1.17 ± 0.96) and at 60 min (2.14 ± 1.19) of heat exposure. A similar increase (P < 0.05) in mean VCO2 was observed post-meal and following heat exposure. VE (L/min) increased by 12.12% post-meal (P = 0.01), 16.16% (P < 0.001) at 30 min, and 19.65% (P < 0.001) at 60 min of heat exposure. There was a significant increase in mean BF (per min) during heat exposure (2.31 ± 1.24 at 30 min and 3.53 ± 1.05 at 60 min) and till 60 min of the recovery period compared to baseline (P < 0.001). HR (bpm) increased by 14 bpm at 30 min and 17 bpm at 60 min of exposure and till 30 min after elimination of heat stress (P < 0.001). Conclusion: A statistically significant increase was observed in VO2, VCO2, VE, BF, and HR on exposure to heat stress. Optimal recovery was observed after 30 min of eliminating the heat stress for VO2 and VCO2. Similar recovery was observed after 60 min of eliminating the heat stress for HR and following 90 min for VE and BF. Hence, if the crew is required to continue to operate in the heat stress environment, a minimum period of 90 min of a break in between the sorties must be ensured in a relatively cooler environment.