Continuous Physiological Monitoring of the Combined Exposure to Hypoxia and High Cognitive Load in Military Personnel

SIMPLE SUMMARY: The health and safety of military aviators is paramount during flight operations. In flight, aviators experience extreme environmental conditions such as high altitude, which reduces oxygen availability to the brain and compromises the function of all bodily systems. The autonomic nervous system (ANS) regulates many of the bodily systems, and therefore its function is a strong indicator of the physiological consequences to prolonged exposure to less oxygen. Importantly, aviators spend most of their flight time at less severe altitudes. However, even mild decrements in oxygen may elicit suboptimal function of the ANS, compromising aviator safety. What remains less clear is how the exposure to mild reductions in oxygen while simultaneously performing simulated flight tasks affects the ANS. The current study investigated this question by exposing aviators to varying levels of oxygen while carrying out simulated flight tasks. The aviators’ ANS responses were measured throughout the nearly two hours of trials. Our study observed heightened sympathetic nervous system activity (e.g., “fight or flight”) and found suggestions of increased anxiety. Lastly, we found that the timing and extent of the ANS responses differed between conditions. These observations highlight the importance of monitoring several markers of ANS function to avoid deteriorating aviator function when flying at mild altitudes. ABSTRACT: Military aviators endure high cognitive loads and hypoxic environments during flight operations, impacting the autonomic nervous system (ANS). The synergistic effects of these exposures on the ANS, however, are less clear. This study investigated the simultaneous effects of mild hypoxia and high cognitive load on the ANS in military personnel. This study employed a two-factor experimental design. Twenty-four healthy participants aged between 19 and 45 years were exposed to mild hypoxia (14.0% O(2)), normoxia (21.0% O(2)), and hyperoxia (33.0% O(2)). During each epoch (n = 5), participants continuously performed one 15 min and one 10 min series of simulated, in-flight tasks separated by 1 min of rest. Exposure sequences (hypoxia–normoxia and normoxia–hyperoxia) were separated by a 60 min break. Heart rate (HR), heart rate variability (HRV), and O(2) saturation (SpO(2)) were continuously measured via an armband monitor (Warfighter Monitor(TM), Tiger Tech Solutions, Inc., Miami, FL, USA). Paired and independent t-tests were used to evaluate differences in HR, HRV, and SpO(2) within and between exposure sequences. Survival analyses were performed to assess the timing and magnitude of the ANS responses. Sympathetic nervous system (SNS) activity during hypoxia was highest in epoch 1 (HR: +6.9 bpm, p = 0.002; rMSSD: −9.7 ms, p = 0.003; SDNN: −11.3 ms, p = 0.003; SpO(2): −8.4%, p < 0.0000) and appeared to slightly decline with non-significant increases in HRV. During normoxia, SNS activity was heightened, albeit non-significantly, in epoch 1, with higher HR (68.5 bpm vs. 73.0 bpm, p = 0.06), lower HRV (rMSSD: 45.1 ms vs. 38.7 ms, p = 0.09 and SDNN: 52.5 ms vs. 45.1 ms, p = 0.08), and lower SpO(2) (−0.7% p = 0.05). In epochs 2–4, HR, HRV, and SpO(2) trended towards baseline values. Significant between-group differences in HR, HRV, and O(2) saturation were observed. Hypoxia elicited significantly greater HRs (+5.0, p = 0.03), lower rMSSD (−7.1, p = 0.03), lower SDNN (−8.2, p = 0.03), and lower SpO(2) (−1.4%, p = 0.002) compared to normoxia. Hyperoxia appeared to augment the parasympathetic reactivation reflected by significantly lower HR, in addition to higher HRV and O(2) relative to normoxia. Hypoxia induced a greater ANS response in military personnel during the simultaneous exposure to high cognitive load. The significant and differential ANS responses to varying O(2) levels and high cognitive load observed highlight the importance of continuously monitoring multiple physiological parameters during flight operations.