Threat of Hyperthermia During Live-Fire Combat Training in the UH-60 Helicopter: An Important Tool for Operator State Monitoring of US Warfighter Aircrew.

INTRODUCTION: US warfighters operating aircraft are often exposed to cockpit temperatures exceeding 80 °F, introducing the potential risk of aircrew reaching dangerously high body temperatures. Hyperthermia induces significant decrements in physical and cognitive functions subsequently threatening the survivability of warfighters. However, the changes in body temperature and ensuing negative physiological effects are understudied, especially during combat training. Therefore, the purpose of the current study was to evaluate the temporal changes in body temperature and autonomic nervous system (ANS) activity in US warfighter aircrew operating the UH-60 helicopter during combat training.

MATERIALS AND METHODS: Data from 2, military study cohorts were analyzed. Thirty-four participants from one study performed 7 in-flight operations in a UH-60 helicopter during a 540-minute live-fire combat training in US warfighters. Twenty-four participants from another study performed simulated combat training drills in a thermoneutral laboratory for 134 minutes. Body temperature (°F), heart rate (HR), respiration rate (RR), and HR variability (HRV), specifically rMSSD: the root mean square of successive differences in the NN intervals (noise-free R-R intervals) and SDNN: standard deviation of NN intervals, were continuously measured using the Warfighter Monitor (Tiger Tech Solutions, Inc., Miami Beach, FL, United States). Baseline measures for body temperature, HR, HRV, and RR, in both studies, were collected over a 5-minute period prior to soldiers' performance any activities. Between- and within-group differences in body temperature, HR, RR, and HRV were evaluated via studentized independent and dependent t-tests, respectively. All study protocols were approved by the US Army Aeromedical Research Laboratory (IRB#2022-024 and 2020-012).

RESULTS: At baseline, no significant between-group differences were observed for body temperature (98.58 vs. 98.70 °F, P = .20), HR (64.75 vs. 66.66 beats per minute; bpm, P = .35), HRV (rMSSD: 87.44 vs. 89.35 ms, P = .80; SDNN: 100.85 vs. 112.16 ms, P = .31), or RR (15.93 vs. 15.55 breaths/minute, P = .50) between the UH-60 and Simulation Lab groups. The following maximum values were reached for body temperature (102.99 vs. 98.45 °F, P < .00001), HR (146.64 vs. 75.38 bpm, P < .00001), and RR (35.47 vs. 26.49 breaths/minute, P < .00001). The UH-60 group elicited significant within-group increases in body temperature (4.41 vs. -0.25 °F, P < .00001) and HR (81.89 vs. 8.72 bpm, P = < .0001). UH-60 and Simulation Lab groups showed significant within-group increases in RR (19.54 vs. 10.94 breaths/minute, P < .00001) and decreases in HRV (rMSSD: 43.80 vs. 54.01 ms, P < .00001; SDNN: 40.43 vs. 71.02 ms, P < .00001).

DISCUSSION AND CONCLUSIONS: Live-fire combat training in the UH-60 helicopter elicited larger increases in body temperature, HR, and RR relative to the simulated combat training. UH-60 aircrew reached hyperthermic body temperatures and hyperthermic-induced hyperventilation. Our findings highlight failure of simulation laboratories to accurately mimic live-fire combat training with severe underestimations of body temperature, ventilation, and ANS activity. Consequently, the poor accuracy of simulation combat training may underprepared US warfighters for combat, threatening their survivability.