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Introduction: Hypoxia caused by high altitude exposure can impair cerebral and mental functions. Blood flow and oxygenation of the buccal tissue can be reliable markers to detect hypoxia. In this study, near infrared spectroscopy was used in combination with a novel optical probe to evaluate the applicability of the novel probe in measuring hypoxia markers in buccal tissue under a hypoxic condition., Methods: Six healthy participants were tested at altitudes from 2000 to 16,000 ft inside a hypobaric chamber. The buccal reference measurements of blood flow and oxygen saturation were synchronized with the spectral measurements of the novel near infrared probe and the relationship between the reference measurements and spectral data were evaluated by multivariate partial least square method. In addition, finger oxygen saturation was measured during the experiment and the recordings were compared with buccal oxygen saturation., Results: The spectral analysis illustrated that the spectral data from the near infrared probe correlated strongly with the absorption features of both buccal flow and oxygenation measured by the reflectance sensors (average R(2) = 0.89). The results showed probably overestimated values for buccal oxygen saturation recorded by the reference pulse oximeter in comparison with finger oxygen saturation, with the mean difference increasing from 1.8% at 2000 ft to 11.4% at 16,000 ft., Conclusion: The novel near infrared probe showed promising results for simultaneous measurement of blood flow and oxygen saturation in the buccal tissue. The suggested method can be used as a new technique for early indication of hypoxia in future clinical applications.

Introduction: An important problem for pilots is visual disturbances occurring under +Gz acceleration. Assessment of the degree of intensification of these disturbances is generally accepted as the acceleration tolerance level (ATL) criterion determined in human centrifuges. The aim of this research was to evaluate the visual-motor responses of pilots during rapidly increasing acceleration contained in cyclic intervals of +6 Gz to the maximum ATL., Methods: The study involved 40 male pilots ages 32-41 yr. The task was a quick and faultless response to the light stimuli presented on a light bar during exposure to acceleration until reaching the ATL. Simple response time (SRT) measurements were performed using a visual-motor analysis system throughout the exposures which allowed assessment of a pilot's ATL., Results: There were 29 pilots who tolerated the initial phase of interval acceleration and achieved +6 Gz, completing the test at ATL. Relative to the control measurements, the obtained results indicate a significant effect of the applied acceleration on response time. SRT during +6 Gz exposure was not significantly longer compared with the reaction time between each of the intervals. SRT and erroneous reactions indicated no statistically significant differences between the "lower" and "higher" ATL groups., Conclusion: SRT measurements over the +6-Gz exposure intervals did not vary between "lower" and "higher" ATL groups and, therefore, are not useful in predicting pilot performance. The gradual exposure to the maximum value of +6 Gz with exposure to the first three intervals on the +6-Gz plateau effectively differentiated pilots.

Introduction: Continuous positive airway pressure (CPAP) is used in air ambulances to treat patients with impaired oxygenation. Differences in mechanical principles between CPAP devices may affect their performance at different ambient air pressures, as will occur in an air ambulance during flight., Methods: Two different CPAP systems, a threshold resistor device and a flow resistor device, at settings of 5 and 10 cm H₂O were examined. Static pressure, static airflow, and pressure during simulated breathing were measured at ground level and at three different altitudes [2400 m (7874 ft), 3000 m (9843 ft), and 10,700 m (35,105 ft)]., Results: When altitude increased, the performance of the two CPAP systems differed during both static and simulated breathing pressure measurements. With the threshold resistor CPAP, measured pressure levels were close to the preset CPAP level. Static pressure decreased 0.71 ± 0.35 cm H₂O at CPAP 10 cm H₂O compared to ground level and 35,105 ft (10,700 m). With the flow resistor CPAP, as the altitude increased, CPAP produced pressure levels increased. At 35,105 ft (10,700 m), the increase was 5.13 ± 0.33 cm H₂O at CPAP 10 cm H₂O., Discussion: The velocity of airflow through the flow resistor CPAP device is strongly influenced by reduced ambient air pressure, leading to a higher delivered CPAP effect than the preset CPAP level. Threshold resistor CPAP devices seem to have robust performance regardless of altitude. Thus, the threshold resistor CPAP device is probably more appropriate for CPAP treatment in an air ambulance cabin, where ambient pressure will vary during patient transport.

Background: A major part of testing of rebreather apparatuses for underwater diving focuses on the oxygen dosage system., Methods: A metabolic simulator for testing breathing apparatuses was built and evaluated. Oxygen consumption was achieved through catalytic combustion of propene. With an admixture of carbon dioxide in the propene fuel, the system allowed the respiratory exchange ratio to be set freely within human variability and also made it possible to increase test pressures above the condensation pressure of propene. The system was tested by breathing ambient air in a pressure chamber with oxygen uptake (Vo₂) ranging from 1-4 L · min(-1), tidal volume (VT) from 1-3 L, breathing frequency (f) of 20 and 25 breaths/min, and chamber pressures from 100 to 670 kPa., Results: The measured end-tidal oxygen concentration (Fo₂) was compared to calculated end-tidal Fo₂. The largest average difference in end-tidal Fo₂during atmospheric pressure conditions was 0.63%-points with a 0.28%-point average difference during the whole test. During hyperbaric conditions with pressures ranging from 100 to 670 kPa, the largest average difference in Fo₂was 1.68%-points seen during compression from 100 kPa to 400 kPa and the average difference in Fo₂during the whole test was 0.29%-points., Conclusion: In combination with a breathing simulator simulating tidal breathing, the system can be used for dynamic continuous testing of breathing equipment with changes in VT, f, Vo2, and pressure.

Background: At +1 Gz, electrical muscle stimulation (EMS) has been shown to increase systemic blood pressure similarly to a standard G-suit or lower body muscle straining. It was hypothesized that EMS might improve G protection at increased G levels., Methods: An EMS suit was developed with electrodes over the calves, thighs, gluteal, and abdominal muscles. Using nine subjects, the EMS suit was compared to a standard five-bladder G-suit during various G profiles up to +9 Gz in a human-rated centrifuge with EMS activated by electrical muscle stimulators at G levels at or above +4 Gz. The optimal EMS stimulation for a solid muscle contraction was determined for each muscle group in each subject prior to the G exposures., Results: The mean maximal G level attained in the standard suit was 1.1 G higher during a relaxed gradual onset profile, 1.5 G higher during a relaxed rapid onset profile, and 2.0 G higher during a straining rapid onset profile when compared to the EMS suit. During a simulated aerial combat maneuver (SACM) ride, duration was 46 s longer with the standard suit compared to the EMS. During the SACM, the average heart rate was 23 bpm lower with the standard suit compared to EMS. All of the above differences were statistically significant. Finally, there were four G-LOCs with the EMS and none with the standard suit., Conclusion: The tested EMS suit did not give sufficient G protection at high Gs for pilots, nor substitute for a standard G-suit, as indicated by lower G protection and the episodes of G-LOC.

Introduction: During an acute hypoxia exposure, impairment of memory is one of the most frequently reported symptoms, either during hypoxia awareness training of aircrews or after an in-flight hypoxic incident. However, the effects of acute hypoxia on memory have been little studied in laboratory-controlled conditions. Moreover, none of these studies were performed in hypobaric conditions. The main aim of our study was to investigate the effects of acute hypobaric hypoxia on working memory (WM). This study also aimed to find links between physiological measurements and cognitive performance during acute hypoxia exposure., Methods: During hypoxia awareness training, 28 subjects (experimental group) were exposed to a simulated altitude level of 10,000 m (31,000 ft) in a hypobaric chamber, while 29 subjects (control group) stayed at sea level. WM was assessed in both groups with the Paced Auditory Serial Addition Test (PASAT). Peripheral oxygen saturation (SpO2) and heart rate were recorded., Results: WM was strongly impaired in the hypoxic group. One major finding is that hypoxia highly increased the mean error frequency rate. WM performance decreased linearly with hypoxemia, but SpO2 was weakly predictive of PASAT performance and vice versa., Discussion: WM is impaired by acute hypobaric hypoxia. Given the importance of WM in aircraft piloting and its sensitivity to hypoxia, the PASAT, in association with SpO2 and EEG recordings, could improve both hypoxia training and our understanding of the effects of hypoxia on memory.

Introduction: Skeletal muscle discomfort commonly occurs during long-distance air travel. Although the effects of high altitude on muscle have been well-studied, the effects of short-term exposure to the altitude at which aircraft cabins are pressurized, 2438 m, have not. The primary aim of this study was to examine global gene expression in the gastrocnemius muscle after simulated flight. Inflammatory indicators were also assessed in the muscle., Methods: Thirty-five mice were evenly exposed to normobaria or hypobaria (2438 m) for -8-9 h. Microarray and ribonucleic acid (RNA) analyses were performed. Additionally, macrophage and neutrophil presence was examined., Results: Fourteen genes were downregulated in females after hypobaria. These genes included those related to epithelial homeostasis, such as the keratins, and genes activated by cellular insult. In contrast, four noncoding, regulatory RNAs were upregulated in males. No difference in proinflammatory cytokine gene and messenger RNA (mRNA) expression was detected between normobaria and hypobaria. The mean number of CD68-positive leukocytes per mm2 and mean area percentage of the CD68 antigen in muscle of normobaric (NB) and hypobaric (HB) mice were 53-54 and -0.2%, respectively., Discussion: Simulated flight does not activate a proinflammatory response in healthy muscle. However, epithelial and cellular defense genes may be downregulated in females, whereas regulatory RNAs may be upregulated in males.

Introduction: Military aircrew are trained to recognize the signs and symptoms of hypoxia in a safe environment using a variety of methods to simulate altitude. In order to investigate the effectiveness of hypoxia training, this study compared the recall of hypoxia symptoms in military aircrew between two consecutive hypobaric chamber hypoxia training sessions conducted, on average, 4.5 yr apart., Methods: Previously trained subjects completed a questionnaire immediately before and after they underwent refresher hypoxia training and recorded the occurrence, order, and severity of symptoms experienced. Responses from refresher training were compared with their recall of symptoms experienced during previous training., Results: There was no difference in the recall of most hypoxia symptoms between training sessions. Slurred speech was recalled more frequently from previous training compared to refresher training (14 vs. 4 subjects), whereas hot/cold flushes were recalled less frequently from previous training compared to refresher training (5 vs. 17 subjects). There was a statistically significant difference in overall hypoxia score (10.3 vs. 8.3), suggesting that from memory subjects may underestimate the level of hypoxia experienced in previous training., Discussion: A high level of similarity between the recall of previously experienced hypoxia symptoms and recent experience supports the effectiveness of hypoxia training. These results replicate the finding of a 'hypoxia signature' reported by a previous study. Small differences in the recall of some symptoms and in overall hypoxia score highlight the importance of drawing attention to the more subtle symptoms of early hypoxia, and of using training techniques which optimize aircrew recall.

Introduction: Almost 2 billion people travel aboard commercial airlines every year, with about 20% developing symptoms of the common cold within 1 wk after air travel. We hypothesize that hypobaric hypoxic conditions associated with air travel may contribute to immune impairment., Methods: We studied the effects of hypobaric hypoxic conditions during a simulated flight at 8000 ft (2438 m) cruising altitude on immune and stress markers in 52 healthy volunteers (mean age 31) before and on days 1, 4, and 7 after the flight. We did a cohort study using a generalized estimating equation to examine the differences in the repeated measures., Results: Our findings show that the hypobaric hypoxic conditions of a 10-h overnight simulation flight are not associated with severe immune impairment or abnormal IgA or cortisol levels, but with transient impairment in some parameters: we observed a transient decrease in lymphocyte proliferative responses combined with an upregulation in CD69 and CD14 cells and a decrease in HLA-DR in the immediate days following the simulated flight that normalized by day 7 in most instances., Discussion: These transient immune changes may contribute to an increased susceptibility to respiratory infections commonly seen after long-haul flights.

Introduction: Most helicopter operations are carried out at altitudes below 10,000 ft. At these altitudes, the risk of the crew experiencing hypoxia is low. For that reason, supplementary oxygen is not standard equipment on board most helicopters. Due to developments in military missions, high-altitude operations have become more frequent-as have the chances of the crew experiencing hypoxia. Helicopter crews are subjected to a higher load of whole-body vibration compared to fixed-wing aircraft crews. Whole-body vibration increases muscle work, with increased oxygen consumption as a result. We hypothesized that whole-body vibration, as experienced by helicopter crews, causes additional lowering of arterial oxygen levels under hypoxic conditions., Methods: Data were collected from 10 subjects. They were all exposed to six different pressure altitudes in a hypobaric chamber, ranging from 1000 ft to 16,000 ft (approximately 305 m to approximately 4877 m). Arterial blood samples were drawn on two occasions at each altitude: after 14 min of rest and followed by 15 min of whole-body vibration (17 Hz, at 1.1 m x s(-2) in the z-axis) at each altitude., Results: There was no significant effect of whole-body vibration on arterial oxygen pressure at altitudes up to 16,000 ft (approximately 4877 m), nor was there any effect on ventilation, seen as changes in arterial pressure of CO2., Discussion: We contribute the lack of effect to the low vibration intensity used in this study. Since this vibration intensity was higher than experienced by helicopter crews during flight, we conclude that whole-body vibration does not contribute to hypoxia during high-altitude operations in helicopters.

Some manufacturers of reduced oxygen (O2) breathing devices claim a comparable hypobaric hypoxia (HH) training experience by providing F1O2 < 0.209 at or near sea level pressure to match the ambient oxygen partial pressure (iso-PO2) of the target altitude. I conclude after a review of literature from investigators and manufacturers that these devices may not properly account for the 47 mmHg of water vapor partial pressure that reduces the inspired partial pressure of oxygen (P1O2), which is substantial at higher altitude relative to sea level. Consequently, some devices claiming an equivalent HH experience under normobaric conditions would significantly overestimate the HH condition, especially when simulating altitudes above 10,000 ft (3048 m). At best, the claim should be that the devices provide an approximate HH experience since they only duplicate the ambient PO2 at sea level as at altitude. An approach to reduce the overestimation and standardize the operation is to at least provide machines that create the same P1O2 conditions at sea level as at the target altitude, a simple software upgrade.

Introduction: The well-established technique of mask-off hypoxia training in a hypobaric training environment elicits symptoms that are correlated with in-flight symptoms reported by aircrew. Aircrew receive training on recognition of symptoms and response early in their flying career and accomplish refresher training on a 5-yr cycle. The symptoms reported after acute hypoxia represent cognitive and psychomotor impairment. The purpose of this study was to evaluate the correlation of symptoms experienced during hypoxia training and recall of symptoms S from the training sessions 5 yr previously., Methods: A survey listing 18 symptoms of hypoxia and severity of condition was presented to 1123 aircrew attending refresher training at 10 U.S. Air Force Aerospace Physiology Training Units prior to and immediately following hypoxia training in the hypobaric chamber., Results: The five symptoms most commonly reported following hypoxia training are: lightheaded/dizzy, dizziness, mental confusion, visual impairment, and tingling. The hypoxia symptom "lightheaded/dizzy" recorded the highest frequency of all 18 symptoms. Lightheaded/dizzy frequencies for both previous and current hypoxia training were 67.2% and 72.3%, respectively. This symptom remained consistent throughout all data analysis, retaining the highest frequency in all levels of severity (mild, moderate, and extreme) for both the previous hypoxia training and current hypoxia training., Discussion: The similarity of symptoms recalled between hypoxia training events provides strong evidence that hypoxia training is an effective method of establishing recognized decrements that may influence performance in flight.

Introduction: Cerebral vascular response via local and reflex adjustments is part of the integrated response to hypoxia and is coupled with changes in systemic vascular resistances that allow a redistribution of blood flow toward the brain. The cerebral vascular response in airmen exposed to simulated high altitude is not clear, thus we sought to investigate this aspect., Methods: Four healthy military airmen were exposed to simulated high altitude in a hypobaric chamber according to a standard training protocol. Blood saturation (SpO2) and blood flow velocity with transcranial Doppler from the left middle cerebral artery (Vm) were continuously recorded. Pulsatility Index (PI), resistance index (RI), and systolic/diastolic ratio (S/D ratio) were computed. Alternate hypoxia-hyperoxia trials for 2 and 1 min, respectively, were used to assess the cerebrovascular response., Results: Acute hypoxia induced an increase in Vm that promptly recovered when the oxygen supply was restored (mean increase of 5.5% at 18,000 ft and 17.2% at 25,000 ft). Alternate hypoxia-hyperoxia at 25,000 and 18,000 ft elicited changes in both SpO2 and Vm. In hypoxia, PI significantly decreased (mean decrease o" 25.6% at 18,000 ft and 39.5% at 25,000 ft), as did RI (mean decrease of 18.7% at 18,000 ft and 34.4% at 25,000 ft), while S/D ratio increased., Discussion: The standard altitude training protocol induced a transient cerebrovascular response. The response was as expected, with hypoxia-induced vasodilation and opposite changes when breathing pure oxygen.

This is a report of two cases of arterial gas embolism (AGE) occurring during the course of routine pressure tolerance testing (PTT) of Canadian Forces divers in a dry hyperbaric chamber. PTT is used by many military organizations as a means to determine whether divers can sustain a hyperbaric challenge similar to that to which they will be exposed during their diving duties. Problems arising from such testing are usually limited to issues of equalization and minor otic barotraumas. Incidents of AGE resulting from hyperbaric chamber exposures in general are very unusual. The incidents reported here are the first such cases arising in military divers during PTT to be reported in the extant literature. In one case a potential precipitating pulmonary lesion was identified during post-event chest imaging. In the other case, while presenting with all the usual hallmarks of an AGE, no predisposing pulmonary lesion was identified. Ascent rates were within the limits considered acceptable by the Canadian Forces for PTT. The cases are useful in examining the pathophysiologic mechanisms underlying pulmonary barotrauma and AGE, and raise questions as to the appropriate screening procedures for military divers in this regard.

Introduction: The risk of severe decompression sickness (DCS) increases rapidly above 6248 m (20,500 ft) and is greater when breathing higher proportions of inert gas. Contemporary aircrew may be exposed to higher cabin altitudes while breathing molecular sieve oxygen concentrator (MSOC) product gas containing variable concentrations of oxygen, nitrogen, and argon. This study assessed the risk of DCS at 6553 m (21,500 ft) breathing two simulated MSOC product gas mixtures., Methods: In a hypobaric chamber, 10 subjects each undertook 2 4-h exposures at 6553 m breathing either 75% O2:21% N2:4% Ar or 56% 02:42% N2:2% Ar. Subjects undertook regular activities simulating in-flight movements of fast jet aircrew. Venous gas emboli (VGE) "bubble" load was graded every 15 min using 2D and Doppler echocardiography by experienced operators blinded to breathing gas composition., Results: DCS occurred in five exposures (25%), the earliest after less than 90 min at altitude. All were minor, single-site, uncomplicated limb bends that resolved with recompression. VGE occurred in 85% of exposures with some early-onset, heavy loads. Survival (Probit) analysis indicated that breathing 56% oxygen significantly decreased VGE latency relative to breathing 75% oxygen (relative potency 3.05)., Conclusions: From 20 experimental exposures, the risk of DCS at 6553 m is estimated at 5% by 90 min and 20% at 3 h. Exploiting the negative predictive value of VGE latency as a surrogate measure of protection from DCS, at high cabin altitudes better MSOC performance (higher product gas oxygen concentrations) will protect more aircrew for longer.

Introduction: Military pilots frequently report ear pain with onset several hours after altitude exposure while breathing pure oxygen, but the prevalence of this problem is unknown. A similar problem is described in divers after breathing hyperbaric oxygen and it is related to the oxygen contained in the middle ear., Methods: In order to assess the prevalence of delayed ear pain after altitude exposure and investigate the effectiveness of preventive use of a nasal balloon (NB), we studied 88 healthy military jet pilots who were asymptomatic after altitude chamber exposure which included 100% oxygen breathing. A group of 44 subjects received the NB shortly after the chamber and they were advised to use it every hour before going to sleep. A control group of 44 subjects was requested to perform the Valsalva maneuver alone over the same period. All subjects underwent clinical examination by an otolaryngologist and tympanometry just before the chamber exposure and again the day after., Results: The day after the altitude exposure, 53.4% of subjects reported ear pain. In the treated group, 61.4% of subjects were free of symptoms, compared to 31.8% in the control group (P < 0.01). Tympanogram was abnormal in eight symptomatic subjects and in six asymptomatic., Conclusion: Our data suggest that in our subjects there is high prevalence of delayed ear barotrauma after altitude chamber exposure while breathing pure oxygen and the tympanogram may improve the accuracy of the diagnosis in asymptomatic subjects. The nasal balloon appears to be effective for prevention.

Introduction: The U.S. Navy has replaced segments of refresher low-pressure chamber instruction with normobaric hypoxia training using a reduced oxygen breathing device (ROBD). A previous training evaluation revealed that this alternative instructional paradigm is a preferred means of training experienced jet aviators to recognize and recover from hypoxia. However, findings from this earlier work also indicated that air hunger was the most commonly reported symptom during ROBD training. This finding raised concern that air hunger could have resulted from a training artifact caused by the lower breathing-gas flow rate produced by the ROBD when compared to more familiar jet aircraft breathing systems. In an effort to address this issue, a software change was made that increased ROBD mask flow from 30 to 50 L x min(-1) (LPM). The purpose of this retrospective study was to determine if there are differences in the hypoxia symptoms reported by aviators trained on the ROBD upgrade (ROBD-50) compared to those trained on the original device (ROBD-30)., Methods: Hypoxia training was provided to 156 aviators using the ROBD-50, and survey results were compared to those obtained from 121 aviators trained on the ROBD-30., Results: There was a significant decrease in the number of aviators who reported experiencing air hunger while training on the ROBD-50 (44.2%) as compared to the ROBD-30 (59.4%) [Pearson chi2 (1) = 5.45, P < 0.051., Discussion: These findings suggest that the ROBD breathing-gas flow rate is an important contributor to the symptom of air hunger and, therefore, may impact training fidelity.

Introduction: Residence at high altitude modifies thremoregulatory responses to cold stress upon return to lower altitude. These changes are difficult to explain since several stresses related to high altitude may interact, including hypoxia, cold, solar radiation, and physical exertion. We hypothesized that adaptation to hypoxia without cold exposure would produce at least part of the observed changes., Methods: Five men underwent acclimation to intermittent hypoxia (AIH) in a hypobaric chamber (8 h daily for 4 d, and 6 h on the last day, 4500 to 6000 m) at 24 degrees C. Cold stress responses were tested during a whole-body standard cold air test (1 degrees C, 2 h at rest at sea level) both before and after AIH., Results: Increased reticulocyte counts and percentages confirmed acclimation to hypoxia after AIH. Changes in thermoregulation during the cold test included lower mean skin temperature after 60-80 min (18.8 +/- 0.7 degrees C vs. 19.4 +/- 0.7 degrees C); higher mean metabolic heat production (127 +/- 8 W x m(-2) vs. 118 +/- 6 W x m(-2)); and lower heat debt (7.7 +/- 1.3 kJ x kg(-1) vs. 10.3 +/- 1.2 kJ x kg(-1)), without significant change in rectal temperature. Time to onset for continuous shivering decreased after AIH (12 +/- 5 min vs. 21 +/- 6.3 min), and shivering activity occurred at higher mean skin but not rectal temperatures., Conclusion: AIH in comfortable ambient temperature leads to a normothermic-insulative-metabolic general cold adaptation. We conclude that AIH modifies the thermoregulatory responses to cold at sea level without cold exposure leading to a cross-adaptation.

Introduction: Hypoxia familiarization instruction has been an integral part of military aviation training for more than 60 yr. Traditionally, aircrew learn hypoxia recognition while being exposed to hypobaric conditions in a low-pressure chamber (LPC). A training device has been developed that induces hypoxia using mixed gas delivered through an aviator's oxygen mask. The reduced oxygen breathing device (ROBD) simulates the diminished oxygen present at altitude by mixing breathing air and nitrogen under normobaric conditions. The purpose of this paper is to describe an alternate hypoxia training paradigm that combines the ROBD with tactical flight simulators and to present results from student surveys., Methods: Dynamic hypoxia instruction was provided inside F/A-18 tactical flight simulators using a second generation ROBD (ROBD-2). There were 121 naval aviators who were individually exposed to a simulated altitude of 7620 m (25,000 ft) while performing complex flight duties. Subsequent to ROBD training, all students completed a 19-question, anonymous survey that asked them to rate the quality of the instruction., Results: Of the 121 students trained with the ROBD-2 in combination with a flight simulator, 114 (94.2%) were able to recognize their hypoxia symptoms and recover the aircraft, 117 (96.7%) rated ROBD training as more realistic, and 110 (90.9%) as more effective than traditional LPC training., Discussion: Student feedback from this instructional evaluation indicates that using an ROBD in combination with actual flight duties is a safe, effective, and preferred means of training experienced tactical jet aviators to recognize and recover from hypoxia.

Introduction: This study proposes an ethological overview of the social behavior of confined and isolated teams within three experimental chambers and at a polar base during long-duration campaigns. These were undertaken as analogous conditions of the mobility restraint and social constraint to which Mars teams would have to adapt., Method: The ethological method consisted of weekly observations and descriptions of the subjects' motor actions and spatial positions at dinnertime, then evaluating their levels of frequency in terms of distances, orientations, and dispersions, as well as facial expressions and body movements., Results: Changes in social behavior indicate different adaptive strategies over time and according to the situation. In large or open areas, interindividual distances are constant. In reduced habitats, the frequency of personal distances decreases and the frequency of public distances increases with high levels of social distance and body mobility from the initial period to the final period. Increasing spatial dispersion and decreasing social orientations indicate low levels of sociality within the teams at the mid-period. Frequent collateral activities during long-duration confinement campaigns indicate high levels of stress in the team members., Discussion: Such ethological indicators could be used in future interplanetary mission scenarios.

Introduction: The zero prebreathe altitude threshold for developing 5% decompression sickness (DCS) symptoms in men has been reported to be 6248 m (20,500 ft). However, such an altitude threshold when 1 h of oxygen prebreathe is used has not been well documented and was the primary purpose of this study., Methods: The 51 male human subjects were exposed to 9144 m (30,000 ft), 8382 m (27,500 ft), 7620 m (25,000 ft), and/or 6858 m (22,500 ft) for 8 h. They were monitored for symptoms of DCS and venous gas emboli (VGE)., Results: DCS symptom incidence after 4 h of exposure decreased with exposure altitude from 87% at 9144 m to 26% at 6858 m. VGE were lower during the 4-h 6858-m exposures (32%) than at the higher altitudes (76-85%). The symptom incidences during the first 4 h of exposure were lower at 6858 m and 7620 m following a 1-h prebreathe as compared with analogous zero-prebreathe exposures. There were no differences between incidences of VGE or DCS at any of the four altitudes after 8 vs. 4 h of exposure., Conclusion: The altitude threshold for 5% DCS symptoms is below 6858 m after 1 h of prebreathe. However, during 6858-m and 7620-m exposures, a 1-h prebreathe is highly beneficial in reducing DCS incidence and delaying the onset of DCS, keeping the incidence to less than 6% during the first 90 min of exposure. Use of 4-h vs. 8-h exposures does not appear to underestimate DCS risk at or above 7620 m.

Introduction: Exposure to altitude reduces oxygen supply to the central nervous system and may cause a variety of neuropsychological impairments. We investigated the relationship between certain cognitive functions and cardiovascular and respiratory variables during acute hypobaric hypoxia., Methods: There were three groups of seven men who were each exposed to a 2-h altitude profile (AP) involving 30 min at each of the following simulated altitudes (m): AP1, 450-1500-3000; AP2, 450-1500-4500; Control 450-650-650. The neuropsychological tests included word fluency and three word-association tasks tapping processes of cognitive flexibility and emotion regulation. A lateralized tachistoscopic lexical decision task with high and low emotional target words was also administered to assess possible shifts in hemispheric superiorities for positive and negative affect., Results: No significant differences in word fluency, word association, or lateralized lexical decision performances were found, despite a significant oxygen desaturation and a drop in diastolic BP at 4500 m, indicating the beginning of central hypoxia in terms of a functional impairment of the vasomotor center., Conclusion: During acute exposure to hypobaric hypoxia, selected cognitive and affective functions mediated by the frontal lobe were preserved. Functional hemispheric asymmetries for emotional processes remained unchanged.

Introduction: Elevated hearing thresholds have been documented in some astronauts after long-term spaceflights although noise levels were lower than those normally associated with noise-induced hearing loss in ground-based operations. The present study was conducted to determine whether prolonged exposure (70 h) to levels (72 dBA) recorded on the International Space Station (ISS) service module would impact diverse measures of auditory function, as well as cognition and memory, motivation, and cardiovascular function., Method: Five mixed gender subgroups of five normal-hearing subjects, aged 20-50 yr, were sequestered for 70 h in an environment that modeled conditions on the ISS. They were assigned to one of three background conditions: quiet (n = 5), continuous noise from the ISS service module (n = 10), or continuous noise during the day only (n = 10). Subjects were tested repeatedly within and across days as individuals or pair mates., Results: There were no negative effects of the noise on any of the outcome measures. Introduction of a delay or noise in a communication channel used in the assessment of speech communicability significantly affected the time taken for joint problem solving by partners., Discussion: The results of this study were not consistent with the observation of hearing loss measured after spaceflights. Nor were changes evident in cognition, motivation, or cardiovascular function. Factors which might account for the discrepancy are discussed.

Introduction: High altitude exposure has an inherent risk of altitude decompression sickness (DCS). A predictive DCS model was needed to reduce operational risk. To be operationally acceptable, such a theoretical model would need to be validated in the laboratory using human subjects., Methods: The Air Force Research Laboratory (AFRL) has conducted numerous studies on human subjects exposed to simulated altitudes in hypobaric chambers. The database from those studies was used to develop a statistical altitude DCS model. In addition, a bubble growth model was developed using a finite difference method to solve for bubble radius as a function of time. The bubble growth model, integrated with the statistical model, constitutes the AFRL DCS Risk Assessment Model. Validation of the model was accomplished by comparing computer predictions of DCS risk with results from subsequent prospective human subject exposures. There were five exposure profiles, not previously found in the database, covering a wide parameter of ranges of altitude (18,000-35,000 ft), exposure time (180-360 min), prebreathe time (0-90 min), and activity level (rest-strenuous) that were used. The subjects were monitored for DCS symptoms and venous gas emboli., Results: There were 30 subjects who were exposed to each of the 5 altitude profiles. The DCS incidence onset curves predicted by the model were not significantly different from the experimental values for all scenarios tested and were generally within +/- 5% of the actual values., Conclusion: A predictive altitude DCS model was successfully developed and validated.

Background: High-altitude hypoxia may induce oxidative stress in humans. However, the effect of acute, severe, and non-acclimatized short-term hypobaric hypoxia exposure in humans has not been described. Additionally, little is known regarding the confounding role of reoxygenation in the extent of oxidative stress and damage markers in hypoxia. Our goals were to analyze the effect of of hypobaric hypoxia and reoxygenation on plasma oxidative stress and oxidative damage., Methods: There were six male volunteers exposed to a simulated altitude of 5500 m (52.52 kPa) in the INEFC-UB hypobaric chamber over 4 h and returned to sea level (SL) in 30 min. Data were collected at baseline SL at 1 h and 4 h of hypoxia at 5500 m and immediately after return to sea level (RSL)., Results: Elevated scores of acute mountain sickness (13) and significant changes in arterial oxygen saturation (97.5 +/- 0.5; 53.3 +/- 1.9; 97.1 +/- 0.3%, p < 0.05 at SL, 4 h, and RSL, respectively) were observed. Significant reductions (p < 0.05) on total glutathione (TGSH) content were measured from SL and 1 h vs. 4 h and RSL. The percentage of oxidized glutathione (%GSSG) as an indicator of redox oxidative changes increased significantly (SL vs. 1 h; 1 h vs. 4 h, and RSL). Lipid peroxidation (TBARS), protein oxidation (SH protein groups), and total antioxidant status (TAS) followed the redox changes suggested by the glutathione system throughout the protocol., Conclusions: Hypobaric hypoxia increased the burden of plasma oxidative stress and damage markers all through the hypoxia period. However, no additional changes were observed with reoxygenation at the end of the reoxygenation period.

Introduction: Adverse reactions during hypobaric chamber operations result from changes in barometric pressure per se and to the related reductions in the partial pressure of inspired oxygen. Previous studies have indicated that an adverse reaction may occur in approximately 6% of exposures in chambers used for flight training. The purpose of this study was to determine the rate of adverse reactions in a chamber used exclusively for terrestrial altitude research studies., Methods: Data for incidence of ear block, sinus block, toothache, and other adverse reactions were analyzed using a master history file that contained information such as annual and total number of studies and human exposures, and onset and outcome of adverse reactions. Incidence for a given time period was calculated as (# of reactions x 100)/(# of exposures)., Results: In 33 yr, there have been 23,656 human exposures and 296 adverse reactions. The overall incidence was 1.25 reactions per 100 exposures (0.72 for research volunteers and 0.53 for staff). The majority (75% of all reactions) were ear blocks (0.93 reactions per 100 exposures). Staff members were more likely than research volunteers to have more than one reaction., Discussion: The incidence of 1.25 reactions per 100 exposures for our chamber is considerably lower than that reported in the literature. This is due primarily to our use of lower altitudes and slower rates of ascent and descent compared with other facilities whose emphasis is on aircraft flight training.

We have designed a lower body negative pressure (LBNP) chamber and control system that makes it possible to apply LBNP either very rapidly (< 300 ms), or more gradually, according to predefined protocols. The capability of the new, high-performance agile aircraft to reach a high-G onset rate makes it highly desirable to be able to study immediate, rapid, and transient cardiovascular responses to simulated gravitational stress. Our new LBNP chamber has been used to study the main cardiovascular variables during onset and release of mild LBNP (-20 mmHg). We have revealed large transient physiological responses during the onset and release of mild LBNP. This new finding was largely made possible by precise control of the onset and release of LBNP during the experiments. The purpose of this paper is therefore to describe some of the technical solutions which made rapid and controlled changes in LBNP possible, focusing on the importance of precise control of the LBNP chamber.

Introduction: Current space suits have limited movement due to gas pressurization during exposure to the vacuum of space. Alternatively, if pressure is applied by an elastic garment vs. pneumatic garment to produce mechanical counter pressure (MCP), several advantages are possible. In this study, we investigate local microcirculatory and other effects produced with and without a prototype MCP glove and sleeve during exposure to varying levels of vacuum., Methods: The entire arms of eight male volunteers were studied at normal ambient pressure and during 5 min exposures to -50, -100, and -150 mm Hg with and without the MCP glove and sleeve. Pressure distribution, skin microvascular flow, and temperature were measured., Results: The MCP glove and sleeve generated over 200 mm Hg on the middle finger, dorsum of the hand, and the wrist. However, pressure was significantly lower on the forearm and the upper arm. Without the glove and sleeve, only two of eight subjects tolerated -100 mm Hg. Also, no subject tolerated -150 mm Hg. However, subjects tolerated all vacuum pressures wearing the glove and sleeve. Skin microvascular flow and temperature remained within control values with the glove and sleeve at a chamber pressure of -150 mm Hg., Discussion: The MCP glove and sleeve counteracts adverse effects of vacuum exposures due to lower pressure differentials. Pressure levels over the hand and wrist are similar to those of the current U.S. space suit glove and sleeve, but additional development is required to increase MCP over the forearm and upper arm.

Background: All naval aviators, navigators, and aircrewmen are required to participate in hypoxia familiarization training. This training is performed in a hypobaric chamber and is considered high risk due to the potential for barotrauma and/or decompression sickness (DCS). Prior analysis of the DCS in U.S. Navy hypobaric chambers revealed a significantly higher incidence among inside observers (IOs) compared with students. In response to these reports, all IOs are required to denitrogenate by breathing 100% oxygen for 30 min prior to altitude exposure (prebreathing). Although the Army, Navy, and Air Force prebreathe for 30 min prior to most hypobaric training exposures, there have been no reports validating the efficacy of this measure. This study examined the incidence of altitude DCS during training exposures to simulated altitudes of 25,000 ft (25k) and 35,000 ft (35k) in IOs and students, some of whom prebreathed and some of whom did not., Methods: Exposures and DCS cases for a period of 9 yr were tabulated from training reports maintained at the Naval Operational Medicine Institute in Pensacola, FL. Chi-square or Fisher's Exact test was used to compare the data sets and p < or = 0.05 was considered significant., Results: The overall DCS incidence for students and IOs for all chamber profiles was 0.25%. The incidence for 25k was 0.29% for students who did not prebreathe and 0.15% for IOs who did (p = 0.10). Within the student group there was a 0.44% DCS incidence for 25k with no prebreathe and a 0.17% DCS incidence for 35k with prebreathe (p = 0.004)., Conclusions: A 30-min prebreathe prior to altitude exposure appears to contribute to a reduction in the risk of DCS during hypobaric chamber training.

Introduction: Early, retrospective reports of the incidence of altitude decompression sickness (DCS) during altitude chamber training exposures indicated that women were more susceptible than men. We hypothesized that a controlled, prospective study would show no significant difference., Methods: We conducted 25 altitude chamber decompression exposure profiles. A total of 291 human subjects, 197 men and 94 women, underwent 961 exposures to simulated altitude for up to 8 h, using zero to 4 h of preoxygenation. Throughout the exposures, subjects breathed 100% oxygen, rested or performed mild or strenuous exercise, and were monitored for precordial venous gas emboli (VGE) and DCS symptoms., Results: No significant differences in DCS incidence were observed between men (49.5%) and women (45.3%). However, VGE occurred at significantly higher rates among men than women under the same exposure conditions, 69.3% and 55.0% respectively. Women using hormonal contraception showed significantly greater susceptibility to DCS than those not using hormonal contraception during the latter two weeks of the menstrual cycle. Significantly higher DCS incidence was observed in the heaviest men, in women with the highest body fat, and in subjects with the highest body mass indices and lowest levels of fitness., Conclusion: No differences in altitude DCS incidence were observed between the sexes under our test conditions, although men developed VGE more often than women. Age and height showed no significant influence on DCS incidence, but persons of either sex with higher body mass index and lower physical fitness developed DCS more frequently.

Background: Transient neurological disorders are often observed at high altitude but are poorly documented under field conditions. The mechanism usually invoked is a hypocapnic vasoconstriction due to severe hypoxic hyperventilation. During a simulated ascent of Mount Everest in a hypobaric chamber by eight volunteer alpinists (Operation Everest III, Comex '97), three subjects presented neurological symptoms. We report here on the clinical observations and testing to detect mechanisms in addition to hypocapnic vasoconstriction., Methods: The experiment was designed to investigate factors limiting physiological performance at altitude and the pathophysiology of acute mountain sickness. A retrospective analysis was made comparing the three cases of transient neurological disorder at high altitude (TNDHA) with the five subjects who had no neurological symptoms., Results: Analysis of clinical and blood parameters showed no difference between cases and controls. The cases showed no neurological sequelae following the experiment and were normal on cardiac imaging. However, one case had a history of migraine in his youth, leading us to hypothesize that segmental vasoconstriction was a factor. In another case, gas bubbles were detected in the pulmonary artery by transthoracic echocardiography when he was symptomatic, suggesting that gas emboli may have played a role. All three cases shared a possible triggering factor in that each experienced hyperventilation alternating with straining against a closed glottis shortly before the onset of symptoms., Conclusion: Mechanisms other than hypocapnic vasoconstriction in hypoxia may be causal factors of TNDHA. The existence of triggering factors and evidence of a possible embolic mechanism should be further explored.

Background: Primary hypertensives who are acutely exposed to hypoxic hypoxia show an enhanced reactivity of arterial chemoreceptors as well as an exaggerated response of the sympathetic nervous system. Since these phenomena could influence their ability to tolerate sustained hypoxic hypoxia, this study was performed to determine whether persons predisposed to hypertension have a normal tolerance of simulated high altitude., Methods: Subjects were 18 young men with a family history of hypertension (sons of hypertensives, SOHT) whose BP values were in the upper normal or borderline hypertensive range. Controls were 15 young men without parental hypertension (sons of normotensives, SONT) who had normal BP values. Each subject underwent both a control and an altitude experiment. The latter consisted of an 8-h exposure to hypobaric hypoxia (equivalent to 4200 m) while resting supine in an altitude chamber. Fluids were administered by mouth and by intravenous line to produce sustained diuresis. Variables measured included heart rate, BP, respiratory rate, O2 saturation, urine flow rate, and sodium excretion., Results: All subjects tolerated the control experiment and all SONT also completed altitude exposure. However, 8 of 18 SOHT developed antidiuresis and had to leave the chamber early due to symptoms of mild acute mountain sickness. Compared with SONT, SOHT exhibited more stable cardiorespiratory parameters at altitude., Conclusions: The data support the hypothesis that borderline hypertensives have stronger cardiorespiratory responses to altitude than controls, a response that is compatible with higher excitability of their arterial chemoreceptors. However, their altitude tolerance is reduced even at rest, probably because of the renal effects of an exaggerated response in the sympathetic nervous system.

Background: Exposure to high +Gz forces may induce arm pain, which has been hypothesized to be caused by pressure-induced overdistension of local blood vessels. The purpose of the present investigation was to study the pressure-distension relation of veins and arteries in the human arm and the relation between arm pain and distension of local vessels., Methods: Increased distending pressures (DP) in the vasculature of the arm were accomplished by placing the subject (n = 8) in a pressure chamber with one arm positioned through a port in the chamber door, and increasing chamber pressure to +180 mm Hg in a stepwise manner. Diameters in the brachial artery and in the brachial, radial and cephalic veins were measured by ultrasonography. Changes in forearm volume were estimated from measurement of tissue impedance. Perceived pain was rated using a 10-point scale., Results: Arm pain increased with pressure to a maximum rating of 8.5 (= median; range: 4-10). Increasing DP from 30 to 180 mm Hg resulted in a steady increase (p < 0.05) in venous diameter which varied from 12 +/- 8% (mean +/- SD) in the brachial vein to 23 +/- 14% in the radial vein. Inthe brachial vein diameter increases were most pronounced at the sites of the venous valves. Arterial diameter was unchanged up to a DP of about 200 mm Hg (calculated as diastolic arterial pressure + applied chamber pressure), but then increased by 32 +/- 9% (p < 0.001). Forearm impedance dropped with increasing pressure (delta = 23 +/- 5%; p < 0.01); the rate of change was non-linear with a faster change at the highest DP which may indicate pressure distension of precapillary resistance vessels., Conclusions: Elevation of pressure in arm vessels to levels that may occur in pilots flying high-performance aircraft results in distension not only of veins but also of arteries and probably of smaller precapillary vessels. Therefore, and because these changes coincide with the development of severe arm pain, local overdistension of blood vessels remains a plausible cause of G-induced arm pain.

Background: Psychological data from environments that are considered as analog to space was collected from 68 subjects; 18 in hyperbaric chambers, 16 in polar expeditions, and 34 on Arctic stations. The objective was to identify psychological reactions expected in different phases of spaceflights and examine personality variables associated with superior adaptation., Method: Helmreich Personality Characteristic Inventory was administered before the isolation. Adaptation to the environment was assessed through a questionnaire once a week., Results: Crews in hyperbaric chambers indicated a steady increase in coping over the isolation. Polar expeditions members reported high aggressiveness and anxiety in the first quarter and an increase in homesickness over time. A personality characterized by strong expressiveness and instrumentality ("the right stuff") predicted superior adaptation in hyperbaric chambers., Conclusion: The data suggest that isolation in hyperbaric chambers and polar expeditions should be considered as models for different aspects of the space environment.

Cardiovascular deconditioning (CD) has been reported to occur within 24-48 h of exposure to 4, 11, or 31 ATA environment and following decompression to sea level pressure. The CD was indicated by orthostatic intolerance, exaggerated cardiovascular responses to a passive tilt, an elevated resting heart rate and a reduced stroke volume postdive. In this dive, one of the New Seatopia series, we used a non-syncope criterion, the cardiovascular index of deconditioning (CID; Bungo MW, Johnson PJ Jr. Aviat Space Environ Med 1983; 54:1001), to evaluate CD in 3 male subjects. The CID sums the changes in heart rate and blood pressure in response to orthostatic stress. An elevated CID indicates CD. We used a passive 70 degrees head up tilt as the orthostatic stress. The CID was measured before and after a bout of underwater exercise at predive, during the early, mid, and late exposure of the 7-d 31 ATA, and after the dive. The CID and circulatory responses to tilt were similar before and after the exercise. The CID increased (p < 0.05) from the predive value of 20 +/- 1.6 to 25 +/- 0.9 on the 2nd day, to 25 +/- 0.8 on the 4th day at 31 ATA, indicating the presence of CD at the early and mid periods of hyperbaric exposure. However, CID was indifferent (18 +/- 0.6) from the predive on the 7th day at 31 ATA. The increased CID corresponded to decreases in plasma volume during the early and mid periods of 31 ATA exposure.(ABSTRACT TRUNCATED AT 250 WORDS)