Effects of Acute Hypoxia on Early Visual and Auditory Evoked Potentials

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.

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.

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.

Sex differences during a cold-stress test in normobaric and hypobaric hypoxia: A randomized controlled crossover study

Cold and hypoxia are two stressors that are frequently combined and investigated in the scientific literature. Despite the growing literature regarding normobaric hypoxia (NH) and hypobaric hypoxia (HH), responses between females and males are less often evaluated. Therefore, this study aims to investigate the physiological sex differences following a cold-stress test under normoxia, normobaric- and hypobaric hypoxia. A total of n = 10 females (24.8 ± 5.1 years) and n = 10 males (30.3 ± 6.3 years) from a university population volunteered for this study. The cold-stress test (CST) of the right hand (15°C for 2 min) was performed using a randomised crossover design in normobaric normoxia, NH and HH. The change (∆) from baseline to post-CST up to 15 min was analysed for cutaneous vascular conductance (CVC) and the hands’ skin temperature, whilst the mean values across time (post-CST up to 15 min) were assessed for peripheral oxygen saturation (SpO₂), thermal sensation- and comfort. Pressure pain threshold (PPT) was assessed after the post-CST 15 min period. The hands’ skin temperature drop was higher (p = 0.01) in the female group (∆3.3 ± 1.5°C) compared to the male group (∆1.9 ± 0.9°C) only in NH. Females (−0.9 ± 0.5) rated this temperature drop in NH to feel significantly colder (p = 0.02) compared to the males (−0.2 ± 0.7). No differences were observed between sexes in NN, NH, and HH for ∆CVC, SpO₂, thermal comfort and PPT. In conclusion, females and males show similar reactions after a CST under normoxia and hypoxia. Sex differences were observed in the local skin temperature response and thermal sensation only in NH.

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.