Specific effect of hypobaria on cerebrovascular hypercapnic responses in hypoxia

Integrating physiological monitoring systems in military aviation: a brief narrative review of its importance, opportunities, and risks

Military pilots risk their lives during training and operations. Advancements in aerospace engineering, flight profiles, and mission demands may require the pilot to test the safe limits of their physiology. Monitoring pilot physiology (e.g. heart rate, oximetry, and respiration) inflight is in consideration by several nations to inform pilots of reduced performance capacity and guide future developments in aircraft and life-support system design. Numerous challenges, however, prevent the immediate operationalisation of physiological monitoring sensors, particularly their unreliability in the aerospace environment and incompatibility with pilot clothing and protective equipment. Human performance and behaviour are also highly variable and measuring these in controlled laboratory settings do not mirror the real-world conditions pilots must endure. Misleading or erroneous predictive models are unacceptable as these could compromise mission success and lose operator trust. This narrative review provides an overview of considerations for integrating physiological monitoring systems within the military aviation environment.Practitioner summary: Advancements in military technology can conflictingly enhance and compromise pilot safety and performance. We summarise some of the opportunities, limitations, and risks of integrating physiological monitoring systems within military aviation. Our intent is to catalyse further research and technological development.Abbreviations: AGS: anti-gravity suit; AGSM: anti-gravity straining manoeuvre; A-LOC: almost loss of consciousness; CBF: cerebral blood flow; ECG: electrocardiogram; EEG: electroencephalogram; fNIRS: functional near-infrared spectroscopy; G-forces: gravitational forces; G-LOC: gravity-induced loss of consciousness; HR: heart rate; HRV: heart rate variability; LSS: life-support system; NATO: North Atlantic Treaty Organisation; PE: Physiological Episode; PCO2: partial pressure of carbon dioxide; PO2: partial pressure of oxygen; OBOGS: on board oxygen generating systems; SpO2: peripheral blood haemoglobin-oxygen saturation; STANAG: North Atlantic Treaty Organisation Standardisation Agreement; UPE: Unexplained Physiological Episode; WBV: whole body vibration.

Independent effects of acute normobaric hypoxia and hypobaric hypoxia on human physiology

The purpose of this study was to examine the effects of acute normobaric (NH, decreased FiO₂) and hypobaric (HH, 4200 m ascent) hypoxia exposures compared to sea level (normobaric normoxia, NN). Tissue oxygenation, cardiovascular, and body fluid variables measured during rest and a 3-min step-test following 90-min exposures (NH, HH, NN). Muscle oxygenated hemoglobin (O₂Hb) decreased, and muscle deoxygenated hemoglobin (HHb) increased environmentally independent from rest to exercise (p < 0.001). During exercise, brain O₂Hb was lower at HH compared to NN (p = 0.007), trending similarly with NH (p = 0.066), but no difference between NN and NH (p = 0.158). During exercise, HR at NH (141 ± 4 beats·min^-1) and HH (141 ± 3 beats·min^-1) were higher than NN (127 ± 44 beats·min^-1, p = 0.002), but not each other (p = 0.208). During exercise, stroke volume at HH (109.6 ± 4.1 mL·beat^-1) was higher than NH (97.8 ± 3.3 mL·beat^-1) and NN (99.8 ± 3.9 mL·beat^-1, p ≤ 0.010) with no difference between NH and NN (p = 0.481). During exercise, cardiac output at NH (13.8 ± 0.6 L) and HH (15.5 ± 0.7 L) were higher than NN (12.6 ± 0.5 L, p ≤ 0.006) with HH also higher than NH (p = 0.001). During acute hypoxic stimuli, skeletal muscle maintains oxygenation whereas the brain does not. These differences may be mediated by environmentally specific cardiovascular compensation. Thus, caution is advised when equating NH and HH.

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.

No effect of patent foramen ovale on acute mountain sickness and pulmonary pressure in normobaric hypoxia

What is the central question to this study? Is there a relationship between a patent foramen ovale and the development of acute mountain sickness and an exaggerated increase in pulmonary pressure in response to 7-10 hours of normobaric hypoxia? What is the main finding and its importance? Patent foramen ovale presence did not increase susceptibility to acute mountain sickness or result in an exaggerated increase in pulmonary artery systolic pressure with normobaric hypoxia. This data suggest hypobaric hypoxia is integral to the increased susceptibility to acute mountain sickness previously reported in those with patent foramen ovale, and patent foramen ovale presence alone does not contribute to the hypoxic pulmonary pressor response. ABSTRACT: Acute mountain sickness (AMS) develops following rapid ascent to altitude, but its exact causes remain unknown. A patent foramen ovale (PFO) is a right-to-left intracardiac shunt present in ∼30% of the population that has been shown to increase AMS susceptibility with high altitude hypoxia. Additionally, high altitude pulmonary edema (HAPE), is a severe type of altitude illness characterized by an exaggerated pulmonary pressure response, and there is a greater prevalence of PFO in those with a history of HAPE. However, whether hypoxia, per se, is causing the increased incidence of AMS in those with a PFO and whether a PFO is associated with an exaggerated increase in pulmonary pressure in those without a history of HAPE is unknown. Participants (n = 36) matched for biological sex (18 female) and the presence or absence of a PFO (18 PFO+) were exposed to 7-10 hours of normobaric hypoxia equivalent to 4755 m. Presence and severity of AMS was determined using the Lake Louise AMS scoring system. Pulmonary artery systolic pressure, cardiac output, and total pulmonary resistance were measured using ultrasound. We found no significant association of PFO with incidence or severity of AMS and no association of PFO with arterial oxygen saturation. Additionally, there was no effect of a PFO on pulmonary pressure, cardiac output, or total pulmonary resistance. These data suggest that hypobaric hypoxia is necessary for those with a PFO to have increased incidence of AMS and that presence of PFO is not associated with an exaggerated pulmonary pressor response. This article is protected by copyright. All rights reserved.

Hypoxic Hypoxia and Brain Function in Military Aviation: Basic Physiology and Applied Perspectives

Acute hypobaric hypoxia (HH) is a major physiological threat during high-altitude flight and operations. In military aviation, although hypoxia-related fatalities are rare, incidences are common and are likely underreported. Hypoxia is a reduction in oxygen availability, which can impair brain function and performance of operational and safety-critical tasks. HH occurs at high altitude, due to the reduction in atmospheric oxygen pressure. This physiological state is also partially simulated in normobaric environments for training and research, by reducing the fraction of inspired oxygen to achieve comparable tissue oxygen saturation [normobaric hypoxia (NH)]. Hypoxia can occur in susceptible individuals below 10,000 ft (3,048 m) in unpressurised aircrafts and at higher altitudes in pressurised environments when life support systems malfunction or due to improper equipment use. Between 10,000 ft and 15,000 ft (4,572 m), brain function is mildly impaired and hypoxic symptoms are common, although both are often difficult to accurately quantify, which may partly be due to the effects of hypocapnia. Above 15,000 ft, brain function exponentially deteriorates with increasing altitude until loss of consciousness. The period of effective and safe performance of operational tasks following exposure to hypoxia is termed the time-of-useful-consciousness (TUC). Recovery of brain function following hypoxia may also lag beyond arterial reoxygenation and could be exacerbated by repeated hypoxic exposures or hyperoxic recovery. This review provides an overview of the basic physiology and implications of hypoxia for military aviation and discusses the utility of hypoxia recognition training.

Specific effect of hypobaria on cerebrovascular hypercapnic responses in hypoxia

It remains unknown whether hypobaria plays a role on cerebrovascular reactivity to CO₂ (CVR). The present study evaluated the putative effect of hypobaria on CVR and its influence on cerebral oxygen delivery (cDO₂) in five randomized conditions (i.e., normobaric normoxia, NN, altitude level of 440 m; hypobaric hypoxia, HH at altitude levels of 3,000 m and 5,500 m; normobaric hypoxia, NH, altitude simulation of 5,500 m; and hypobaric normoxia, HN). CVR was assessed in nine healthy participants (either students in aviation or pilots) during a hypercapnic test (i.e., 5% CO₂). We obtained CVR by plotting middle cerebral artery velocity versus end‐tidal CO₂ pressure (P_ETCO₂) using a sigmoid model. Hypobaria induced an increased slope in HH (0.66 ± 0.33) compared to NH (0.35 ± 0.19) with a trend in HN (0.46 ± 0.12) compared to NN (0.23 ± 0.12, p = .069). P_ETCO₂ was decreased (22.3 ± 2.4 vs. 34.5 ± 2.8 mmHg and 19.9 ± 1.3 vs. 30.8 ± 2.2 mmHg, for HN vs. NN and HH vs. NH, respectively, p < .05) in hypobaric conditions when compared to normobaric conditions with comparable inspired oxygen pressure (141 ± 1 vs. 133 ± 3 mmHg and 74 ± 1 vs. 70 ± 2 mmHg, for NN vs. HN and NH vs. HH, respectively) During hypercapnia, cDO₂ was decreased in 5,500 m HH (p = .046), but maintained in NH when compared to NN. To conclude, CVR seems more sensitive (i.e., slope increase) in hypobaric than in normobaric conditions. Moreover, hypobaria potentially affected vasodilation reserve (i.e., MCAv autoregulation) and brain oxygen delivery during hypercapnia. These results are relevant for populations (i.e., aviation pilots; high‐altitude residents as miners; mountaineers) occasionally exposed to hypobaric normoxia.

Cognitive Impairment During Combined Normobaric vs. Hypobaric and Normoxic vs. Hypoxic Acute Exposure

INTRODUCTION: Exposure to hypoxia has a deleterious effect on cognitive function; however, the putative effect of hypobaria remains unclear. The present study aimed to evaluate cognitive performance in pilot trainees who were exposed to acute normobaric (NH) and hypobaric hypoxia (HH). Of relevance for military pilots, we also aimed to assess cognitive performance in hypobaric normoxia (HN).METHODS: A total of 16 healthy pilot trainees were exposed to 4 randomized conditions (i.e., normobaric normoxia, NN, altitude level of 440 m; HH at 5500 m; NH, altitude simulation of 5500 m; and HN). Subjects performed a cognitive assessment (KLT-R test). Cerebral oxygen delivery (cDO₂) was estimated based middle cerebral artery blood flow velocity (MCAv) and pulse oxygen saturation (S_po₂) monitored during cognitive assessment.RESULTS: Percentage of errors increased in NH (14.3 9.1%) and HH (12.9 6.4%) when compared to NN (6.5 4.1%) and HN (6.0 4.0%). Number of calculations accomplished was lower only in HH than in NN and HN. When compared to NN, cDO₂ decreased in NH and HH.DISCUSSION: Cognitive performance was decreased similarly in acute NH and HH. The cDO₂ reduction in NH and HH implies insufficient MCAv increase to ensure cognitive performance maintenance. The present study suggests negligible hypobaric influence on cognitive performance in hypoxia and normoxia.Aebi MR, Bourdillon N, Noser P, Millet GP, Bron D. Cognitive impairment during combined normobaric vs. hypobaric and normoxic vs. hypoxic acute exposure. Aerosp Med Hum Perform. 2020; 91(11):845851.

Minimal Influence of Hypobaria on Heart Rate Variability in Hypoxia and Normoxia

Introduction

The present study evaluated the putative effect of hypobaria on resting HRV in normoxia and hypoxia.

Methods

Fifteen young pilot trainees were exposed to five different conditions in a randomized order: normobaric normoxia (NN, P_B = 726 ± 5 mmHg, F_IO₂ = 20.9%), hypobaric normoxia (HN, P_B = 380 ± 6 mmHg, F_IO₂≅40%), normobaric hypoxia (NH, P_B = 725 ± 4 mmHg, F_IO₂≅11%); and hypobaric hypoxia (HH at 3000 and 5500 m, HH3000 and HH5500, P_B = 525 ± 6 and 380 ± 8 mmHg, respectively, F_IO₂ = 20.9%). HRV and pulse arterial oxygen saturation (SpO₂) were measured at rest seated during a 6 min period in each condition. HRV parameters were analyzed (Kubios HVR Standard, V 3.0) for time (RMSSD) and frequency (LF, HF, LF/HF ratio, and total power). Gas exchanges were collected at rest for 10 min following HRV recording.

Results

SpO₂ decreased in HH3000 (95 ± 3) and HH5500 (81 ± 5), when compared to NN (99 ± 0). SpO₂ was higher in NH (86 ± 4) than HH5500 but similar between HN (98 ± 2) and NN. Participants showed lower RMSSD and total power values in NH and HH5500 when compared to NN. In hypoxia, LF/HF ratio was greater in HH5500 than NH, whereas in normoxia, LF/HF ratio was lower in HN than NN. Minute ventilation was higher in HH5500 than in all other conditions.

Discussion

The present study reports a slight hypobaric effect either in normoxia or in hypoxia on some HRV parameters. In hypoxia, with a more prominent sympathetic activation, the hypobaric effect is likely due to the greater ventilation stimulus and larger desaturation. In normoxia, the HRV differences may come from the hyperoxic breathing and slight breathing pattern change due to hypobaria in HN.

Cardio-respiratory, oxidative stress and acute mountain sickness responses to normobaric and hypobaric hypoxia in prematurely born adults

PURPOSE

We compared the effects of hypobaric and normobaric hypoxia on select cardio-respiratory responses, oxidative stress and acute mountain sickness (AMS) severity in prematurely born individuals, known to exhibit blunted hypoxic ventilatory response.

METHODS

Sixteen prematurely born but otherwise healthy males underwent two 8-h hypoxic exposures under: (1) hypobaric hypoxic [HH; terrestrial altitude 3840 m; P_iO₂:90.2 (0.5) mmHg; BP: 478 (2) mmHg] and (2) normobaric hypoxic [NH; P_iO₂:90.6 (0.9) mmHg; F_iO₂:0.142 (0.001)] condition. Resting values of capillary oxyhemoglobin saturation (SpO₂), heart rate (HR) and blood pressure were measured before and every 2 h during the exposures. Ventilatory responses and middle cerebral artery blood flow velocity (MCAv) were assessed at rest and during submaximal cycling before and at 4 and 8 h. Plasmatic levels of selected oxidative stress and antioxidant markers and AMS symptoms were also determined at these time points.

RESULTS

HH resulted in significantly lower resting (P = 0.010) and exercise (P = 0.004) SpO₂ as compared to NH with no significant differences in the ventilatory parameters, HR or blood pressure. No significant differences between conditions were found in resting or exercising MCAv and measured oxidative stress markers. Significantly lower values of ferric-reducing antioxidant power (P = 0.037) were observed during HH as opposed to NH. AMS severity was higher at 8 h compared to baseline (P = 0.002) with no significant differences between conditions.

CONCLUSION

These data suggest that, in prematurely born adults, 8-h exposure to hypobaric, as opposed to normobaric hypoxia, provokes greater reductions in systemic oxygenation and antioxidant capacity. Further studies investigating prolonged hypobaric exposures in this population are warranted.

REGISTRATION

NCT02780908 (ClinicalTrials.gov).