Changes in metabolic gas kinetics on exposure to heat stress

INTRODUCTION

Temperature variations have a significant impact on living organisms in tropical countries. In humans, ambient temperature primarily affects the core body temperature (T_c) and thus metabolic rate (MR). These changes in T_c and MR have a detrimental effect on the physical and mental performance of the individual. Similarly, military aviators who invariably get exposed to extremes of temperatures are prone to experience impairment of their ability to perform a task in the cockpit.^[1] This may have a critical effect on flight safety and mission accomplishment. Significant effects of heat stress are seen among aviators, especially during high-speed low-level flying, where the cockpit temperatures could exceed 40°C.^[2-4] Any physical activity of an individual will increase the level of heat production and accounts for the total energy expenditure.^[1] Replenishment of these energy stores occurs during the recovery period or the post-exposure period. Various cardiological and respiratory changes, namely, increase in heart rate (HR), increased sweat rate, and increase in oxygen uptake (VO₂), occur due to heat stress.^[5] Body generally takes anywhere from 15 min to 48 h to fully recover to a resting state. The duration of recovery or replenishment of the energy stores not only depends on the duration of exposure to stress but is also directly proportional to the intensity of stress and physical activity after eliminating the stressor.^[6]

In military aviation, aircrew may be required to undertake multiple or long-haul sorties causing prolonged exposure to heat and thus making them vulnerable to the effects of heat stress. Heat stress causes an increase in MR expressed in terms of metabolic gas kinetics (VO₂ and carbon dioxide elimination). It is necessary to restore the expended energy with adequate rest before any further physical activity or exposure to stressors to prevent exhaustion. Hence, with a hypothesis that a significant increase in metabolic gas kinetics occurs on exposure to heat stress, this study was conducted to assess the changes and the duration required for the optimal recovery of cardiorespiratory variables such as VO₂, carbon dioxide elimination (VCO₂), minute ventilation (V_E), breathing frequency (BF), and HR.

MATERIALS AND METHODS

RESULTS

Mean values of cardiorespiratory and physiological parameters (VO₂, VCO₂, V_E, BF, and HR) at different conditions are mentioned in [Table 1].

VO₂ showed significant changes on exposure to heat stress/ different conditions with F (6, 90) = 22.52 and P < 0.001. Post hoc analysis revealed that VO₂ was significantly higher post-meal and on exposure to a hot environment compared to baseline and post-exposure values [Table 2].

VCO₂ showed significant changes on exposure to heat stress/ different conditions with F (6, 90) = 7.228 (P < 0.001). Levene’s test for homogeneity was found to be significant (P = 0.029). Hence, Wilcoxon post hoc analysis was carried out. Similar to VO₂, post hoc analysis revealed that VCO₂ was also significantly higher during post-meal and on exposure to a hot environment compared to baseline and post-exposure values. VCO₂ reached below the baseline in 30 min of post-exposure and increased to near baseline in the next 1 h of recovery [Table 3].

BF showed significant changes on exposure to heat stress/ different conditions with F (6, 90) = 30.24 (P < 0.001). Post hoc analysis revealed that the increase in BF on exposure to heat was significant throughout the duration of exposure and till 1 h of the post-exposure recovery period when compared to the baseline recordings [Table 4].

VE showed significant changes on exposure to heat stress/ different conditions with F (6, 90) = 7.342 and P < 0.001. Post hoc analysis revealed that VE was significantly higher during post-meal, exposure to a hot environment, and till 1 h of post-exposure recovery period when compared to the baseline value [Table 5].

HR showed significant changes on exposure to heat stress/ different conditions with F (6, 90) = 58.26 and P < 0.001. Post hoc analysis revealed that HR was significantly higher during exposure to a hot environment and till 30 min of the post-exposure recovery period when compared to the baseline recordings [Table 6].

DISCUSSION

Heat stress has already been a recognized physiological threat among military aviators. It is also a known fact that heat stress influences the performance of the aircrew during flying. In modern warfare, where “low-level high-speed flying” is required, there is an imminent risk of heat stress to aircrew. Of the numerous physiological responses that occur in the body on exposure to heat, cardiorespiratory variables such as VO₂, VCO₂, V_E, BF, and HR are considered to have an impact on the physical and mental abilities of the aircrew. This study was conducted with an aim to study such changes in the form of the metabolic gas kinetics on exposure to heat stress. VO₂ and VCO₂ are considered primary parameters and V_E, BF, and HR are considered secondary parameters in this study. In the past, studies had revealed that the cardiorespiratory variables vary with the ambient temperature and the energy expenditure depends on the intensity and duration of exposure to heat stress.^[6] This study was undertaken to assess the effects of heat stress on various gas kinetic parameters to understand the duration of optimal performance of the individual in hot conditions and the time required for the replenishment of energy expended post-exposure.

VO₂ is considered and calculated as a proxy for MR.^[5] MR increases post-meal and the end product of it is heat. Studies in the past reported that the increase in MR was due to the thermic effect of food.^[7-11] Similar findings were also observed in this study. The increase in VO₂ post-meal was 1.49 ± 0.95 ml/Kg/min compared to baseline. Thus, the percentage of post-meal increase in VO₂ was 33.13% (p < 0.001). Similarly, a statistically significant increase of VO₂ of 27.02% was observed after 30 min of exposure to heat while 48.35% increase was recorded at the end of 1 h of heat exposure. Such a rise in VO₂ on exposure to heat stress was also documented by National Research Council (USA) in a laboratory study of VO₂.^[12] This increase may be attributed to increased energy requirement to replenish the oxygen debt caused due to heat stress.^[6,13,14] During the post-exposure period, the recovery was measured in the form of a percentage of approximation to the baseline value. Approximation levels of mean VO₂ to the baseline were found to be 95.98% at the end of 30 min and 96.77% at the end of 60 min depicting the level of recovery. Interestingly, there was a reinclination of VO₂ away from baseline from 96.77% at the end of 60 min to 94.59% at the end of 90 min of post-exposure.

An increase in VO₂ increases the gaseous exchange at the cellular level increasing carbon dioxide output (VCO₂). The mean difference in the amount of VCO₂ post-meal was 23.43% higher than the baseline. Furthermore, a significant increase of 8.54% at the end of 30 min of heat exposure and 12.41% at the end of 60 min of heat exposure was observed. During the recovery period, a significant decline was observed wherein the levels reached below the baseline (−2.75%) within 30 min of post-exposure (indicating 100% recovery). This finding of a rise in VCO₂ may be attributed to excess post-exposure oxygen consumption, leading to increased VCO₂.^[14]

The increase in BF post-meal was not statistically significant. BF continued to increase significantly throughout heat exposure. The increase was of the magnitude of 13.36% at the end of 30 min and 20.37% at the end of 60 min of exposure. During recovery, BF remained statistically higher than the baseline at the end of 30 min and 60 min of post-exposure to heat stress. BF at 90 min of post-exposure was insignificant depicting optimal recovery (P > 0.05). These findings were in contrast to a study conducted by Raulph et al. (2012), where the respiratory rate remained constant throughout the exposure period.^[15]

A 12.12% significant increase in V_E was observed post-meal. Similarly, V_E was also found to be significantly higher during 30 min and 60 min of heat exposure. Like BF, V_E values also did not reach the baseline values even after 60 min of the recovery period. However, the value at 90 min post-exposure was insignificant (P = 0.63) depicting an optimal recovery of VE. Gaudio Raulph who conducted a study on heat-induced hyperventilation and had also observed an increase in V_E though not statistically significant. Consolazio et al.^[16] and Henderson et al.^[5] had also reported similar findings in the past. The increase in V_E has been related to increase in tidal volume during heat exposure.^[15]

No significant increase in HR was observed post-meal. However, HR increased by 10 bpm (14.46%) and 17 bpm (23.71%) on exposure to heat stress for 30 min and 60 min, respectively. This increase was statistically significant. Post-exposure, approximation of HR to baseline (a sign of recovery) was 92.05% and 97.27% after 30 min and 60 min of elimination of heat exposure, respectively. In comparison to baseline, the HR measured at the end of 30 min of the recovery period was statistically significant whereas, at 60 min post-exposure, there was no significant difference. Hence, it can be stated that optimal recovery of HR occurred at about 60 min of the recovery period. The findings of changes in HR observed in this study were similar to various studies in the past.^[5,16-19]

CONCLUSION

Our study brings out a few important findings; that have relevance to aviation operations. The cardiorespiratory parameters were found to be significantly affected by exposure to heat stress. It was also observed that most of the parameters returned to baseline values in about 60 min post-exposure indicating recovery of the physiological functions. Hence, it is recommended that a minimum of 90 min of recovery time must be followed by the crew before undertaking any sensitive task following exposure to similar heat stress conditions. Furthermore, 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.