INTRODUCTION: Cabin decompression presents a threat in high-altitude-capable aircraft. A chamber study was performed to compare effects of rapid (RD) vs. gradual decompression and gauge impairment at altitude with and without hypoxia, as well as to assess recovery.

METHODS: There were 12 participants who completed RD (1 s) and Gradual (3 min 12 s) ascents from 2743–7620 m (9000–25000 ft) altitude pressures while breathing air or 100% O₂. Physiological indices included oxygen saturation (S_Po₂), heart rate (HR), respiration, end tidal O₂ and CO₂ partial pressures, and electroencephalography (EEG). Cognition was evaluated using SYNWIN, which combines memory, arithmetic, visual, and auditory tasks. The study incorporated ascent rate (RD, gradual), breathing gas (air, 100% O₂) and epoch (ground-level, pre-breathe, ascent-altitude, recovery) as factors.

RESULTS: Physiological effects in hypoxic “air” ascents included decreased S_Po₂ and end tidal O₂ and CO₂ partial pressures (hypocapnia), with elevated HR and minute ventilation ($\dot{\text{V}}$_E); S_Po₂ and HR effects were greater after RD (−7.3% lower and +10.0 bpm higher, respectively). HR and $\dot{\text{V}}$_E decreased during recovery. SYNWIN performance declined during ascent in air, with key metrics, including composite score, falling further (−75% vs. −50%) after RD. Broad cognitive impairment was not recorded on 100% O₂, nor in recovery. EEG signals showed increased slow-wave activity during hypoxia.

DISCUSSION: In hypoxic exposures, RD impaired performance more than gradual ascent. Hypobaria did not comprehensively impair performance without hypoxia. Lingering impairment was not observed during recovery, but HR and $\dot{\text{V}}$_E metrics suggested compensatory slowing following altitude stress. Participants’ cognitive strategy shifted as hypoxia progressed, with efficiency giving way to “satisficing,” redistributing effort to easier tasks.

Beer J, Mojica AJ, Blacker KJ, Dart TS, Morse BG, Sherman PM. Relative severity of human performance decrements recorded in rapid vs. gradual decompression. Aerosp Med Hum Perform. 2024; 95(7):353–366.

INTRODUCTION: While there are numerous factors that may affect pilot attentional performance, we hypothesize that an increased expiratory work of breathing experienced by fighter pilots may impose a “distraction stimulus” by creating an increased expiratory effort sensation. Therefore, the purpose of this study was to determine the extent to which increasing expiratory pressure time product or expiratory effort sensation impacts attentional performance.

METHODS: Data was collected on 10 healthy participants (age: 29 ± 6 yr). Participants completed six repetitions of a modified Masked Conjunctive Continuous Performance Task protocol while breathing against four different expiratory threshold loads. Repeated measures analysis of variances and generalized additive mixed effects models were used to investigate the effects of expiratory threshold load conditions on expiratory pressure time product, expiratory effort sensation, and the influence of altered end tidal gases on Masked Conjunctive Continuous Performance Task scores.

RESULTS: The overall median hit reaction times were significantly longer as the expiratory threshold loads increased. Specific shape-conjunctive and non-conjunctive median hit reaction times were longer with increased expiratory effort sensation. Additionally, increased expiratory effort sensation did not significantly change commission error rates, but did significantly increase omission error rates.

DISCUSSION: The findings of our work suggest that both progressively greater expiratory threshold loads during spontaneous breathing and expiratory effort sensation may impair subjects’ attentional performance due to longer reaction times and increased stimuli recognition error rates.

Kelley EF, Cross TJ, Johnson BD. Expiratory threshold loading and attentional performance. Aerosp Med Hum Perform. 2024; 95(7):367–374.

INTRODUCTION: Modular organization in brain regions often performs specific biological functions and is largely based on anatomically and/or functionally related brain areas. The current study aimed to explore changes in whole-brain modular organization affected by flight training.

METHODS: The study included 25 male flight cadets and 24 male controls. The first assessment was performed in 2019, when the subjects were university freshmen. The second assessment was completed in 2022. High spatial resolution structural imaging (T1) and resting-state functional MRI data were collected. Then, 90 cerebral regions were organized into 6 brain modules. The intensity of intra- and intermodular communication was calculated.

RESULTS: Mixed-effect regression model analysis identified significantly increased interconnections between the parietal and occipital modules in the cadet group, but significantly decreased interconnections in the control group. This change was largely attributed to flight training.

DISCUSSION: Pilots need to control the aircraft (e.g., attitude, heading, etc.) using the stick and pedal in response to the current state of the aircraft displayed by the instrument panel; as such, flying requires a large amount of hand–eye coordination. Day-to-day flight training appeared to intensify the connection between the parietal and occipital modules among cadets.

Chen X, Jiang H, Meng Y, Xu Z, Luo C. Increased functional connectivity between the parietal and occipital modules among flight cadets. Aerosp Med Hum Perform. 2024; 95(7):375–380.

INTRODUCTION: Most airline pilots reported having suffered from sleep disorders and fatigue due to circadian disruption, a potential risk to flight safety. This study attempted to uncover the actual scenario of circadian disruption and working load status among airline pilots.

METHODS: In study 1, 21 pilots were invited to participate in a 14-d sleep monitoring and a dual 2-back test to monitor their sleep patterns and cognitive function level. To provide an in-depth view, data from scheduled flights, including 567 airline pilots, was analyzed in Study 2. The present study used cluster analysis to reflect the distribution of the flight scheduling characteristics, including working time and actual working hours. A simulation model was then developed to predict the pilots’ 1-mo sleep-wake pattern.

RESULTS: The results indicated that sleep problems were prevalent in this population, especially the night before an earlier morning shift. Regarding the cognitive test, they scored the lowest on earlier morning shifts compared with daytime and evening shifts. It was found that over 70% of the flight schedules can lead to circadian disruption, and 47.44% of the pilots worked under high-load status.

DISCUSSION: Airline pilots inevitably work irregular hours and the current policies for coping with circadian disruption seem inefficient. This study thus calls for urgency in improving scheduling and fatigue management systems from the circadian rhythm perspective.

Yang SX, Cheng S, Sun Y, Tang X, Huang Z. Circadian disruption in civilian airline pilots. Aerosp Med Hum Perform. 2024; 95(7):381–389.

INTRODUCTION: Spatial disorientation (SD) remains the leading contributor to Class A mishaps in the U.S. Navy, consistent with historical trends. Despite this, SD training for military aircrew is largely confined to the classroom and experiential training replicating SD illusions is limited and infrequent. Static flight simulators are most commonly used for training but offer no vestibular stimulation to the flight crew, omitting the source of vestibular-mediated SD.

BACKGROUND: We first cover vestibular-mediated SD illusions which may be replicated through galvanic vestibular stimulation (GVS) in a static environment. GVS is a safe, reliable, low-cost avenue for providing vestibular sensory stimulation. We review the underlying mechanisms of GVS such as the excitement and inhibition of the afferent neurons innervating the vestibular system, particularly in the binaural bipolar electrode montage.

APPLICATIONS: Two approaches for how GVS may be used to enhance SD training are examined. The first is a means for providing unreliable vestibular sensory perceptions to pilots, and the second details how GVS can be leveraged for replicating vestibular-mediated SD illusions.

DISCUSSION: We recommend GVS be pursued as an enhancement to existing SD training. The ability to disorient aircrew in the safe training environment of a static flight simulator would allow for aircrew familiarization to SD, serving as an opportunity to practice life-saving checklist items to recover from SD. A repeatable training profile that could be worn by military aircrew in a static flight simulator may afford a low-cost training solution to the number one cause of fatalities in military aviation.

Allred AR, Lippert AF, Wood SJ. Galvanic vestibular stimulation advancements for spatial disorientation training. Aerosp Med Hum Perform. 2024; 95(7):390–398.

INTRODUCTION: The original Spacecraft Maximal Allowable Concentrations (SMACs) for toluene (set for 1 h, 24 h, 7 d, 30 d, and 180 d) were first established by NASA in 1996 based on a human study in which no irritation or neurotoxicity was reported following 6-h exposure to 40 ppm toluene vapors. While the toluene SMACs were updated in 2008 to account for auditory, visual, and hormonal effects (for 7 d, 30 d, and 180 d) and to include a long-term SMAC (1000 d) in anticipation of longer spaceflight exploration missions, the short-term SMAC limits (1 h and 24 h) remained unchanged. Acute toluene exposure is reported to result in ocular and nasal irritation, although it is not a primary irritant, as well as central nervous system effects including headaches and dizziness. Long-term exposure to toluene can elicit hepatotoxicity, nephrotoxicity, neurotoxicity, and endocrine toxicity.

RESULTS AND DISCUSSION: Since publication of the original and revised toluene SMACs, the National Academy of Sciences developed interim Acute Exposure Guideline Limits reviewed by the National Research Council Committee. Based on these data, we have increased the limits for toluene in crewed spacecraft to 40 ppm for 1 h, 24 h, 7 d, and 30 d. SMACs for durations of 180 and 1000 d will remain unchanged.

Tapia CM, Langford SD, Ryder VE. Revisions to limits for toluene in spacecraft air. Aerosp Med Hum Perform. 2024; 95(7):399–402.

INTRODUCTION: This paper describes the method for assigning medical diagnostic and treatment capabilities and resources to the database which assists with an updated probabilistic risk analysis (PRA) tool for exploration class medical system planning. The National Aeronautics and Space Administration has used PRA since 2011 to inform mission medical system design, but existing tools are designed only for low Earth orbit. An updated PRA tool was needed to assist with exploration class missions.

METHODS: A team of medical experts with a wide range of expertise and experience, including Space Medicine, was assembled to build capability and resource tables for the new PRA tool. This team met over 8 mo and used practice guidelines, literature, and experience to build capability and resource tables (CRTs) for each condition in the new PRA tool database.

RESULTS: This process led to CRTs for each condition and a total of 617 distinct capabilities and 839 discrete resources.

CONCLUSION: The CRT method is an effective way to translate medical practice guidelines into capabilities and resources usable by PRA tools for exploration class medical system planning. This same method may be used in commercial space ventures and in other applications in which medical predictive analytics are informative.

Levin DR, Nelson AM, Zahner C, Stratton ER, Anderson A, Steller J. A method to determine capabilities and resources for spacecraft medical systems. Aerosp Med Hum Perform. 2024; 95(7):403–408.