Time course of recovery from acute hypoxia exposure as measured by vigilance and event-related potentials

Relative Severity of Human Performance Decrements Recorded in Rapid vs. Gradual Decompression

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 (SPo₂), 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 SPo₂ and end tidal O₂ and CO₂ partial pressures (hypocapnia), with elevated HR and minute ventilation (V˙E); SPo₂ and HR effects were greater after RD (-7.3% lower and +10.0 bpm higher, respectively). HR and 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 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.

Personal Hypoxia Symptoms Vary Widely Within Individuals

INTRODUCTION: Exposure to high ambient altitudes above 10,000 ft (3048 m) over sea level during aviation can present the risk of hypobaric hypoxia. Hypoxia can impair sensory and cognitive functions, degrading performance and leading to mishaps. Military aircrew undergo regular hypoxia familiarization training to recognize their symptoms and understand the consequences of hypoxia. However, over the years, aviators have come to believe that individuals have a "personal hypoxia signature." The idea is that intraindividual variability in symptom experience during repeated exposure is low. In other words, individuals will experience the same symptoms during hypoxia from day to day, year to year.METHODS: We critically reviewed the existing literature on this hypothesis. Most studies that claim to support the notion of a signature only examine group-level data, which do not inform individual-level consistency. Other studies use inappropriate statistical methods, while still others do not control for accuracy of recall over the period of years. To combat these shortcomings, we present a dataset of 91 individuals who completed nearly identical mask-off, normobaric hypoxia exposures days apart.RESULTS: We found that for every symptom on the Hypoxia Symptom Questionnaire, at least half of the subjects reported the symptom inconsistently across repeated exposure. This means that, at best, 50% of subjects did not report the same symptom across exposures.DISCUSSION: These data provide compelling evidence against the existence of hypoxia signatures. We urge that hypoxia familiarization training incorporate these findings and encourage individuals to expect a wide range of hypoxia symptoms upon repeated exposure.Cox BD, McHail DG, Blacker KJ. Personal hypoxia symptoms vary widely within individuals. Aerosp Med Hum Perform. 2024; 95(1):54-58.

Acute hypoxia alters visuospatial attention orienting: an electrical neuroimaging study

Our study investigated the effects of hypoxia on visuospatial attention processing during preparation for a single/double-choice motor response. ERPs were recorded in two sessions in which participants breathed either ambient-air or oxygen-impoverished air. During each session, participants performed four cue-target attention orienting and/or alerting tasks. Replicating the classic findings of valid visuospatial attentional orienting modulation, ERPs to pre-target cues elicited both an Anterior directing attention negativity (ADAN)/CNV and a posterior Late directing attention positivity (LDAP)/TP, which in ambient air were larger for attention orienting than for alerting. Hypoxia increased the amplitude of both these potentials in the spatial orienting conditions for the upper visual hemifield, while, for the lower hemifield, it increased ADAN/CNV, but decreased LDAP/TP for the same attention conditions. To these ERP changes corresponded compensatory enhanced activation of right anterior cingulate cortex, left superior parietal lobule and frontal gyrus, as well as detrimental effects of hypoxia on behavioral overt performance. Together, these findings reveal for the first time, to our knowledge, that (1) these reversed alterations of the activation patterns during the time between cue and target occur at a larger extent in hypoxia than in air, and (2) acute normobaric hypoxia alters visuospatial attention orienting shifting in space.

Sensitivity of cognitive function tests to acute hypoxia in healthy subjects: a systematic literature review

Acute exposure to hypoxia can lead to cognitive impairment. Therefore, hypoxia may become a safety concern for occupational or recreational settings at altitude. Cognitive tests are used as a tool to assess the degree to which hypoxia affects cognitive performance. However, so many different cognitive tests are used that comparing studies is challenging. This structured literature evaluation provides an overview of the different cognitive tests used to assess the effects of acute hypoxia on cognitive performance in healthy volunteers. Less frequently used similar cognitive tests were clustered and classified into domains. Subsequently, the different cognitive test clusters were compared for sensitivity to different levels of oxygen saturation. A total of 38 articles complied with the selection criteria, covering 86 different cognitive tests. The tests and clusters showed that the most consistent effects of acute hypoxia were found with the Stroop test (where 42% of studies demonstrated significant abnormalities). The most sensitive clusters were auditory/verbal memory: delayed recognition (83%); evoked potentials (60%); visual/spatial delayed recognition (50%); and sustained attention (47%). Attention tasks were not particularly sensitive to acute hypoxia (impairments in 0%-47% of studies). A significant hypoxia level-response relationship was found for the Stroop test (p = 0.001), as well as three clusters in the executive domain: inhibition (p = 0.034), reasoning/association (p = 0.019), and working memory (p = 0.024). This relationship shows a higher test sensitivity at more severe levels of hypoxia, predominantly below 80% saturation. No significant influence of barometric pressure could be identified in the limited number of studies where this was varied. This review suggests that complex and executive functions are particularly sensitive to hypoxia. Moreover, this literature evaluation provides the first step towards standardization of cognitive testing, which is crucial for a better understanding of the effects of acute hypoxia on cognition.

Auditory function in humans at high altitude. A scoping review

PURPOSE

High-altitude (HA) affects sensory organ response, but its effects on the inner ear are not fully understood. The present scoping review aimed to collect the available evidence about HA effects on the inner ear with focus on auditory function.

METHODS

The scoping review was conducted following the guidelines of the Preferred Reporting Items for Systematic Review and Meta-Analysis extension for scoping reviews. PubMed, Scopus, and Web of Science electronic databases were systematically searched to identify studies conducted in the last 20 years, which quantified in healthy subjects the effects of HA on auditory function.

RESULTS

The systematic search identified 17 studies on a total population of 888 subjects (88.7% male, age: 27.8 ± 4.1 years; median sample size of 15 subjects). Nine studies were conducted in a simulated environment and eight during real expeditions at HA. To quantify auditory function, six studies performed pure tone audiometry, four studies measured otoacoustic emissions (OAE) and eight studies measured auditory evoked responses (AER). Study protocols presented heterogeneity in the spatio-temporal patterns of HA exposure, with highly varying maximal altitudes and exposure durations.

CONCLUSION

Most studies reported a reduction of auditory function with HA in terms of either elevation of auditory thresholds, lengthening of AER latencies, reduction of distortion-product and transient-evoked OAEs. Future studies in larger populations, using standardized protocols and multi-technique auditory function evaluation, are needed to further characterize the spatio-temporal pattern of HA effects along the auditory pathways and clarify the pathophysiological implications and reversibility of the observed changes.

Delayed Drowsiness After Normobaric Hypoxia Training in an F/A-18 Hornet Simulator

BACKGROUND: In military aviation, due to high-altitude flight operations, hypoxia training is mandatory and nowadays is mainly done as normobaric hypoxia training in flight simulators. During the last decade, scientific data has been published about delayed recovery after normobaric hypoxia, known as a "hypoxia hangover." Sopite syndrome is a symptom complex that develops as a result of exposure to real or apparent motion, and it is characterized by yawning, excessive drowsiness, lassitude, lethargy, mild depression, and a reduced ability to focus on an assigned task.CASE REPORT: In this study, we present the case of a 49-yr-old pilot who participated in normobaric hypoxia refreshment training in an F/A-18C Hornet simulator and experienced delayed drowsiness, even 3 h after the training.DISCUSSION: This case report demonstrates the danger of deep hypoxia. Hypoxia training instructions should include restrictions related to driving a car immediately after hypoxia training. In addition, hypoxia may lower the brain threshold for sopite syndrome.Varis N, Leinonen A, Perälä J, Leino TK, Husa L, Sovelius R. Delayed drowsiness after normobaric hypoxia training in an F/A-18 Hornet simulator. Aerosp Med Hum Perform. 2023; 94(9):715-718.

Recovery from acute hypoxia: A systematic review of cognitive and physiological responses during the 'hypoxia hangover'

Recovery of cognitive and physiological responses following a hypoxic exposure may not be considered in various operational and research settings. Understanding recovery profiles and influential factors can guide post-hypoxia restrictions to reduce the risk of further cognitive and physiological deterioration, and the potential for incidents and accidents. We systematically evaluated the available evidence on recovery of cognitive and basic physiological responses following an acute hypoxic exposure to improve understanding of the performance and safety implications, and to inform post-hypoxia restrictions. This systematic review summarises 30 studies that document the recovery of either a cognitive or physiological index from an acute hypoxic exposure. Titles and abstracts from PubMed (MEDLINE) and Scopus were searched from inception to July 2022, of which 22 full text articles were considered eligible. An additional 8 articles from other sources were identified and also considered eligible. The overall quality of evidence was moderate (average Rosendal score, 58%) and there was a large range of hypoxic exposures. Heart rate, peripheral blood haemoglobin-oxygen saturation and heart rate variability typically normalised within seconds-to-minutes following return to normoxia or hyperoxia. Whereas, cognitive performance, blood pressure, cerebral tissue oxygenation, ventilation and electroencephalogram indices could persist for minutes-to-hours following a hypoxic exposure, and one study suggested regional cerebral tissue oxygenation requires up to 24 hours to recover. Full recovery of most cognitive and physiological indices, however, appear much sooner and typically within ~2-4 hours. Based on these findings, there is evidence to support a 'hypoxia hangover' and a need to implement restrictions following acute hypoxic exposures. The severity and duration of these restrictions is unclear but should consider the population, subsequent requirement for safety-critical tasks and hypoxic exposure.

Carbon dioxide protects simulated driving performance during severe hypoxia

PURPOSE

We sought to determine the effect of acute severe hypoxia, with and without concurrent manipulation of carbon dioxide (CO2), on complex real-world psychomotor task performance.

METHODS

Twenty-one participants completed a 10-min simulated driving task while breathing room air (normoxia) or hypoxic air (PETO2 = 45 mmHg) under poikilocapnic, isocapnic, and hypercapnic conditions (PETCO2 = not manipulated, clamped at baseline, and clamped at baseline + 10 mmHg, respectively). Driving performance was assessed using a fixed-base motor vehicle simulator. Oxygenation in the frontal cortex was measured using functional near-infrared spectroscopy.

RESULTS

Speed limit exceedances were greater during the poikilocapnic than normoxic, hypercapnic, and isocapnic conditions (mean exceedances: 8, 4, 5, and 7, respectively; all p ≤ 0.05 vs poikilocapnic hypoxia). Vehicle speed was greater in the poikilocapnic than normoxic and hypercapnic conditions (mean difference: 0.35 km h-1 and 0.67 km h-1, respectively). All hypoxic conditions similarly decreased cerebral oxyhaemoglobin and increased deoxyhaemoglobin, compared to normoxic baseline, while total hemoglobin remained unchanged.

CONCLUSIONS

These findings demonstrate that supplemental CO2 can confer a neuroprotective effect by offsetting impairments in complex psychomotor task performance evoked by severe poikilocapnic hypoxia; however, differences in performance are unlikely to be linked to measurable differences in cerebral oxygenation.

Lingering Altitude Effects During Piloting and Navigation in a Synthetic Cockpit

INTRODUCTION: A study was performed to evaluate a cockpit flight simulation suite for measuring moderate altitude effects in a limited subject group. Objectives were to determine whether the apparatus can detect subtle deterioration, record physiological processes throughout hypobaric exposure, and assess recovery.METHODS: Eight subjects trained to perform precision instrument control (PICT) flight and unusual attitude recovery (UAR) and completed chamber flights dedicated to the PICT and UAR, respectively. Each flight comprised five epochs, including ground level pressure (GLP), ascent through altitude plateaus at 10,000, 14,000, and 17,500 ft (3050, 4270, and 5338 m), then postexposure recovery. PICT performance was assessed using control error (FSE) and time-out-of-bounds (TOOB) when pilots exited the flight corridor. UARs were assessed using response times needed to initiate correction and to achieve wings-level attitude. Physiological indices included S_po₂, heart rate (HR), end tidal O₂ and CO₂ pressures, and respiration metrics.RESULTS: Seven subjects completed both flights. PICT performance deteriorated at altitude: FSE increased 33% at 17,513 ft and 21% in Recovery vs. GLP. Mean TOOB increased from 11 s at GLP to 60 s in Recovery. UAR effects were less clear, with some evidence of accelerated responses during and after ascent.CONCLUSIONS: The test paradigm was shown to be effective; piloting impairment was detected during and after exposure. Physiological channels recorded a combination of hypoxia, elevated ventilation, and hypocapnia during ascent, followed by respiratory slowing in recovery. Findings indicate precision piloting and respiration are subject to changes during moderate altitude exposure and may remain altered after S_po₂ recovers, and changes may be linked to hypocapnia.Beer J, Morse B, Dart T, Adler S, Sherman P. Lingering altitude effects during piloting and navigation in a synthetic cockpit. Aerosp Med Hum Perform. 2023; 94(3):135-141.

Voluntary Urinary Retention Effects on Cognitive Performance

INTRODUCTION: Aircrew in-flight bladder relief remains an understudied stressor; specifically the effects of withholding urination on flight-relevant cognitive performance. This quasi-experimental study investigated whether voluntary urinary retention over a 3-h period negatively impacted cognitive performance.METHODS: We assessed vigilance using the psychomotor vigilance task (PVT) and measured the P3b event-related potential (ERP) in response to PVT stimuli. We also measured working memory (WM) performance using a change detection task and assessed the contralateral delay activity during the WM task using electroencephalography (EEG). Subjects (N = 29) completed a baseline test on both tasks, following bladder voiding and immediately after consuming 0.75 L of water. Subjects performed tasks at 1, 2, and 3 h post-void and urgency to void one's bladder was assessed regularly. A total of 17 subjects were able to complete the entire study protocol. Repeated-measures ANOVAs assessed changes in PVT and WM outcomes.RESULTS: Reaction time (RT) on the PVT was significantly impaired (5% slower) with longer urinary retention time and showed a 2.5-fold increase in the number of lapses (RT > 500 ms) with increased retention time. Together these results indicate that sustained attention was impaired with increased voluntary urine retention. We did not see significant changes in WM performance with our manipulations. Additionally, neural measures acquired with EEG for both tasks did not show any significant effect.DISCUSSION: As measured with the PVT, sustained attention was impaired during 3 h of voluntary urinary retention, highlighting the need for further development of adequate bladder relief systems in military aviation.Griswold CA, Vento KA, Blacker KJ. Voluntary urinary retention effects on cognitive performance. Aerosp Med Hum Perform. 2023; 94(2):79-85.

Sex comparisons in physiological and cognitive performance during hypoxic challenge

Within the tactical aviation community, human performance research lags in considering potential psychophysiological differences between male and female aviators due to little inclusion of females during the design and development of aircraft systems. A poor understanding of how male and female aviators differ with respect to human performance results in unknown potential sex differences on aeromedically relevant environmental stressors, perchance leading to suboptimal performance, safety, and health guidelines. For example, previous hypoxia studies have excluded female participants or lacked a sizeable sample to examine sex comparisons. As such, progress toward sensor development and improving hypoxia familiarization training are stunted due to limited knowledge of how individual differences, including sex, may or may not underlie hypoxia symptoms and performance impairment. Investigating sex differences bridges the gap between aerospace medicine and operational health, and addressing hypoxia is one of many facets yet to be studied. In the current study, we retrospectively examined N = 6 hypoxia studies with male-female participant samples (total, N = 189; male, n = 118; female, n = 71). We explored sex as a predictor of physiological response, sensory deficits, the severity of cognitive performance declines, and symptom manifestation via linear and binary logistic regression models. We found that the female sex predicted lower peripheral oxygen saturation and the likelihood of headache reporting in response to hypoxic challenge, yet explained little variance when combined with age and body mass index. The sensory and cognitive performance models did not converge, suggesting high intra-individual variability. Together, sex, age, and body mass index were not the most robust predictors in responses to hypoxic challenge; we cannot infer this for sensory deficits and cognitive performance within an experimentally induced hypoxic environment. The findings have implications for improving hypoxia familiarization training, monitoring sensor development, and emergency response and recovery protocols in case of a hypoxia occurrence suitable for all aircrew. We recommend continuing to elucidate the impact of sex and intrapersonal differences in hypoxia and other aeromedically relevant stressors in tactical aviation.

Hyperventilation and Hypoxia Hangover During Normobaric Hypoxia Training in Hawk Simulator

Introduction: In military aviation during high-altitude operations, an oxygen or cabin pressure emergency can impair brain function and performance. There are variations in individuals’ physiological responses to low partial pressure of oxygen and hypoxia symptoms can vary from one exposure to another. The aim of this study was to evaluate how normobaric hypoxia (NH) affects pilots’ minute ventilation and 10 min afterwards on Instrument Landing System (ILS) flight performance in Hawk simulator during a tactical flight sortie.

Methods: Fifteen volunteer fighter pilots from the Finnish Air Force participated in this double blinded, placebo controlled and randomized study. The subjects performed three flights in a tactical Hawk simulator in a randomized order with full flight gear, regulators and masks on. In the middle of the flight without the subjects’ knowledge, 21% (control), 8% or 6% oxygen in nitrogen was turned on. Minute ventilation (VE) was measured before, during NH and after NH. Forehead peripheral oxygen saturation (SpO₂), wireless ECG and subjective symptoms were documented. The flights were conducted so that both subjects and flight instructors were blinded to the gas mixture. The pilots performed tactical maneuvers at simulated altitude of 20,000 ft or 26,000 ft until they recognized the symptoms of hypoxia. Thereafter they performed hypoxia emergency procedures with 100% oxygen and returned to base (RTB). During the ILS approach, flight performance was evaluated.

Results: The mean VE increased during NH from 12.9 L/min (21% O2 on the control flight) to 17.8 L/min with 8% oxygen (p < 0.01), and to 21.0 L/min with 6% oxygen (p < 0.01). Ten minutes after combined hyperventilation and hypoxia, the ILS flight performance decreased from 4.4 (control flight) to 4.0 with 8% oxygen (p = 0.16) and to 3.2 with 6% oxygen (p < 0.01). A significant correlation (r = -0.472) was found between the subjects’ VE during 6% oxygen exposure and the ILS flight performance.

Discussion: Hyperventilation during NH has a long-lasting and dose-dependent effect on the pilot’s ILS flight performance, even though the hypoxia emergency procedures are executed 10 min earlier. Hyperventilation leads to body loss of carbon dioxide and hypocapnia which may even worsen the hypoxia hangover.

Effects of Acute Hypoxia on Early Visual and Auditory Evoked Potentials

Reduced levels of environmental oxygen lead to hypoxic hypoxia and are a primary threat in tactical aviation. The visual system is particularly vulnerable to hypoxia, and its impairment can severely impact performance. The auditory system is relatively spared by hypoxia, although which stages of auditory processing are most impacted by hypoxia remains unclear. Previous work has used electroencephalography (EEG) to assess neural markers of cognitive processing for visual and auditory stimuli and found that these markers were sensitive to a normobaric hypoxic exposure. In the current study, we assessed whether early sensory evoked potentials, that precede cognitive activity, are also impaired by normobaric hypoxia. In a within-subjects design, we compared visual (P100) and auditory evoked potentials (sensory gating for the P50, N100, and P200) in 34 healthy adults during normoxic (21% O2) and two separate hypoxic (9.7% O2) exposures. Self-reported symptoms of hypoxia were also assessed using the Hypoxia Symptom Questionnaire (HSQ). We found that P100 mean amplitude was not reduced under hypoxic compared to normoxic conditions, suggesting no statistically significant impairment of early visual processing. The sensory gating ratio for auditory stimuli was intact for paired responses of the P50 and N100. However, the P200 sensory gating ratio was attenuated under hypoxic compared to normoxic conditions, suggesting disruption of the auditory system specific to the level of allocating attention that follows basic auditory processing. Exploratory analyses of HSQ scores identified a robust effect of hypoxia. However, consistency of symptoms reported between the two hypoxia exposures exhibited high intra-individual variability, which may have implications for the theory that individuals have a consistent hypoxia signature or reliable constellation of responses to hypoxia. These findings suggest that early sensory processing is not impaired during hypoxia, but for the auditory system there is impairment at the level of attentional processing. Given the previous findings of impaired visual performance under hypoxia, these results suggest that this impairment does not stem from early visual processing deficits in visual cortex. Together these findings help focus the search on when and where hypoxia-induced deficits occur and may guide the development of countermeasures for hypoxia in tactical aviation.