Effects of Altitude-Related Hypoxia on Aircrews in Aircraft With Unpressurized Cabins

Transcription

1 MILITARY MEDICINE, 176, 1:79, 2011 Effects of Altitude-Related Hypoxia on Aircrews in Aircraft With Unpressurized Cabins Shuji Nishi, PhD ABSTRACT Introduction: Generally, hypoxia at less than 10,000 ft (3,048 m) has no apparent effect on aircrews. Nevertheless, several hypoxic incidents have been reported in flights below 10,000 ft. A recently introduced pulse oximeter using finger probes allows accurate monitoring of oxygen saturation ( ) in the aeromedical environment. Using such a pulse oximeter, in-flight levels were evaluated in aircrew in unpressurized aircraft. In addition, career inflight hypoxic experiences were surveyed. Methods: In-flight was measured in aircrews operating UH-60J helicopters at up to 13,000 ft, and 338 aircrew members operating unpressurized cabin aircraft were surveyed concerning possible in-flight hypoxic experiences. Results: In aircrews operating UH-60J helicopters, decreased significantly at altitudes over 5,000 ft, most markedly at 13,000 ft (vs. ground level). The survey identified three aircrew members with experiences suggesting hypoxemia at below 5,000 ft. Conclusions: Careful attention should be paid to the possibility of hypoxia in aircrews operating unpressurized cabin aircraft. INTRODUCTION Generally, the hypoxia present at less than 10,000 ft (3,048 m) above mean sea level is said to have no apparent effect on aircrews. In texts and handbooks on aviation medicine, altitudes up to 10,000 ft are considered an acceptable compromise for safe flight, with aircrews able to operate below this altitude without supplemental oxygen. 1,2 In military helicopters without a pressurized cabin, pilots not provided with supplemental oxygen are allowed to fly at up to 10,000 ft (or at up to 13,000 ft for 2 hr) by the Japan Air Self-Defense Force (JASDF). 3 In the United States Air Force (USAF), pilots without supplemental oxygen are allowed to fly continuously at up to 10,000 ft, or at up to 12,500 ft for 1 hr, or at up to 14,000 ft for 30 min. 2 However, several hypoxic incidents have been reported in flights at below 10,000 ft. Cable, who examined reports made to the Directorate of Flying Safety of the Australian Defense Force concerning incidents of hypoxia occurring for the period , reported that 4 of the 27 incidents occurred at altitudes of less than 10,000 ft. 4 Smith, who surveyed the common symptoms of hypoxia in Australian Army helicopter aircrew operating at altitudes of up to 10,000 ft, reported that 40 of the 53 crew members experienced symptoms indicative of in-flight hypoxia. 5 Pulse oximeters are able to measure oxyhemoglobin saturation ( ) from the skin noninvasively, and consequently pulse oximetry is in widespread use in field trials. Recently introduced devises using finger probes provide more accurate monitoring in the aeromedical environment. 6 Previously, we demonstrated that in hypobaric-chamber training, the mean measured by pulse oximetry in 8 subjects exposed to a simulated altitude of 8,000 ft decreased to 91%, with half of the subjects reporting a feeling of warmth. 7 However, little Aeromedical Laboratory, Japan Air Self-Defense Force , Sakaecho, Tachikawa, Tokyo , Japan. The views expressed in this article are those of the author and do not reflect the official policy or position of the JASDF. is known about the effects of hypobaric hypoxia on in-flight oxygen saturation in aircrews operating aircraft with unpressurized cabins. To address this issue, the present study measured in-flight in aircrews operating UH-60J helicopters without a pressurized cabin at altitudes of up to 13,000 ft. In addition, to access the relationship between the incidence of hypoxic experiences and flight altitude, 338 aircrew members operating aircraft with unpressurized cabins were surveyed concerning their career in-flight hypoxic experiences. The goal was to obtain a better understanding of the relation of inflight levels to hypoxic experiences. METHODS In-Flight Oxyhemoglobin Saturation and Heart Rate Ten healthy male aircrew members (4 pilots and 6 nonpilots, who were flight engineers, rescuers, and measuring technicians), aged yr (mean, 33 yr), were enrolled in this study. Written informed consent was obtained from each one before the study. All protocols followed the guidelines laid down by the institutional ethics committee. For measurements of and heart rate, the 10 subjects were divided into two flights. Measurements were made using pulse oximeters fitted with finger probes (Pulsox-3i; Konica Minolta, Tokyo, Japan), as in a previous report. 8 This was done in a briefing room before the flight test (pretest), at ground level (18 ft) in a UH-60J helicopter cabin, and at 5,000 ft, 8,000 ft, and 10,000 ft in a UH-60J helicopter gaining altitude at 500 1,000 ft/min ( Fig. 1 ). One helicopter then returned to the ground because of adverse weather conditions, but the five crew in the other helicopter were measured again at 5,000 ft, 8,000 ft, 10,000 ft, and 13,000 ft. At each altitude, measurements were made after at least 5 min for stabilization of and heart rate. For each crew, 50 min elapsed between the briefing-room and ground-level measurements, and min between those made at ground level and those made at MILITARY MEDICINE, Vol. 176, January

2 FIGURE 1. Flight schedules for in-flight oxyhemoglobin saturation ( ) and heart rate measurement study in UH-60J helicopters. N, number of aircrew members. FIGURE 2. Oxyhemoglobin saturation (SPO2 ) data obtained in UH-60J flights. ** p < 0.01, * p < 0.05: indicated altitude vs. pretest. 10,000 ft. In the additional period of flight made by one crew, 17 min elapsed between the measurements made at 10,000 ft and 5,000 ft and 22 min between those made at 5,000 ft and 13,000 ft. Questionnaire About Hypoxic Experiences in Aircrews Operating Unpressurized Aircraft Three hundred thirty-eight JASDF aircrew members operating in helicopter rescue or transport squadrons were invited to participate in the survey. This survey examined the subjects entire career, during which they had recorded flight reports every flight. Moreover, they had had hypobaric-chamber training once every 3 yr and had experienced hypoxic symptoms while in the hypobaric chamber. Therefore, they vividly remembered their hypoxic experiences from their flight reports. The information collected was as follows: neurologic symptoms suggestive of hypoxia (a feeling of warmth, poor concentration, and diminished vision), 9,10 the altitude at which the incident occurred, the detailed flight schedule, and physical health status. Statistical Analysis Data are presented as mean ± SE. Data analysis was performed using SPSS (SPSS Japan Inc., Tokyo, Japan). Statistical differences were calculated for and heart rate data using a one-way ANOVA. When an overall difference was found, individual differences between a given altitude and the pretest situation were assessed using Dunnett s post hoc test. For hypoxic incidence, differences were assessed using a c 2 test. RESULTS In-Flight Oxyhemoglobin Saturation and Heart Rate The SPO2 of the 10 aircrew members was 97.4 ± 0.31% in the briefing room ( pretest value) and 97.1 ± 0.23% at ground level (18 ft) ( Fig. 2 ). The in-flight fell to 89.2 ± 0.63% at 10,000 ft. At each altitude (5,000; 8,000; and 10,000 ft), the was significantly less than in the briefing room ( p < 0.05 or p < 0.01 vs. pretest; Fig. 2 ). In the additional period of flight made by one group, the in-flight displayed partial recovery FIGURE 3. Heart rate data obtained in UH-60J flights. ** p < 0.01: indicated altitude vs. pretest. to 94.6 ± 0.4% at 5,000 ft, but fell to reach 81.4 ± 2.3% at 13,000 ft ( p < 0.05 or p < 0.01 vs. pretest; Fig. 2 ). In the same 10 subjects, heart rate increased from 69.3 ± 4.4 beat/min in the briefing room (pretest) to 85.6 ± 5.6 beat/min at ground level (18 ft), and was still high at 5,000 ft ( Fig. 3 ). Although it showed partial recovery to 79.5 ± 4.0 beat/min at 8,000 ft, it increased again to 83.3 ± 4.0 beat/min at 10,000 ft. In these data, there were significant increases (vs. pretest) at ground level (18 ft), 5,000 ft, and 10,000 ft (each, p < 0.01). In the additional flight made by one crew, heart rate showed partial recovery toward the pretest value at 10,000 ft. However, it increased again to reach 85.4 ± 1.9 beat/min at 13,000 ft ( p < 0.01 vs. pretest). Questionnaire About Hypoxic Experiences by Aircrews Operating Unpressurized Aircraft All aircrews (comprising 337 men and one woman) responded to the survey. There were 138 pilots (41%), 85 flight radiomen (25%), 63 rescuers (19%), 30 loadmasters (9%), 15 flight engineers (4%), and 7 unidentified crew members (2%). The incidence of experiences suggestive of hypoxemia was analyzed according to aircrew role, smoking, age, aircraft type, altitude, and neurologic symptoms. The incidence of such experiences did not differ significantly among the various aircrew roles (Table I ). Moreover, there was no significant relationship to smoking behavior. Regarding age, the incidence was apparently lower in younger subjects (in their 20s) than in older ones (30s, 40s, or 50s). Concerning aircraft type, the 80 MILITARY MEDICINE, Vol. 176, January 2011

3 TABLE I. Number of Aircrew Members Reporting Symptoms Suggesting Hypoxemia (Analyzed According to Aircrew Role, Smoking, Age, and Type of Aircraft) Symptoms Suggesting Hypoxemia N No. of Cases (%) c 2 Test Total (26.0%) Aircrew Role Pilot (29.0%) NS Rescuer (25.4%) Flight Engineer 15 4 (26.7%) Flight Radioman (21.2%) Loadmaster (33.3%) Unidentified 7 0 (0%) Smoking Nonsmoker (24.8%) NS Smoker (25.5%) Past Smoker (28.8%) Age 20s 62 7 (11.3%) P < s (29.2%) 40s (28.3%) 50s (33.3%) Aircraft Type UH-60J (21.7%) a P < 0.05 V (25.0%) UH-60J or V (23.1%) CH47J (37.5%) a N, number of aircrew members in each category. a P < 0.05 between indicated percentages. percentage incidence was significantly greater in type CH47J than in type UH-60J ( p < 0.05). When the altitude levels were divided into three groups (low altitude, under 5,000 ft; medium altitude, from 5,000 to 8,000 ft; and high altitude, above 8,000 ft), the incidence of neurologic experiences suggesting possible hypoxemia was only 0.9% (3 cases) at low altitude and 3.3% (11 cases) at medium altitude, but 25.4% (86 cases) at high altitude (Table II). The 3 cases showing symptoms at low altitude comprised 3 pilots who experienced a feeling of warmth, both poor concentration and diminished vision, or diminished vision. Apart from those 3 cases, a flight engineer reported toothache and a rescuer reported headache, in each case at low altitude. Of these, the headache symptom occurred at 500 ft after training for scuba diving and was possibly due to decompression sickness, and the informant with toothache was in the midst of a course of treatment on that tooth. Therefore, although mentioned here, these 2 cases were omitted from the survey data. In the 11 reports made concerning medium altitude, the symptoms mentioned were a feeling of warmth (25.0%), poor concentration (58.4%), and diminished vision (16.6%). In the 86 reports concerning high altitude, the most commonly mentioned symptoms were a feeling of warmth (60.0%), poor concentration (24.8%), and diminished vision (11.0%). In these reports of experiences at high altitude, four informants said they had requested a decrease in altitude. In one of these 4 cases, another pilot had taken over control of the aircraft. The flight altitude was recorded in 2 of the 4 cases (12,000 ft and 10,000 ft). DISCUSSION Although these are published reports of several hypoxic incidents in flights below 10,000 ft (see Introduction), there have been only a few studies of in-flight levels in aircrews operating aircraft with unpressurized cabins. The present data reveal that in aircrews operating UH-60J helicopters without a pressurized cabin, there were significant decreases in oxygen saturation at over 5,000 ft, most strikingly at 13,000 ft. Furthermore, in a survey of possible in-flight hypoxic experiences among 338 members of aircrews operating aircraft with unpressurized cabins, neurologic symptoms suggesting possible hypoxemia were reported at below 5,000 ft, although admittedly the incidence was low. These findings emphasize the importance of considering hypoxia as a risk in aircrews operating aircraft with unpressurized cabins even when they do not ascend to high altitudes. The SPO2 of normal healthy subjects at sea level is above 95% when measured using pulse oximeters. 7,11 13,14 When falls to 87 90%, hypoxic symptoms may appear. 13,14 Cottrell et al., who measured in-flight levels using pulse oximeters in 38 aircrew members in a pressurized aircraft, found that in-flight decreased from 95 99% at ground level to 80 93% at a cruising altitude of 6,000 8,850 ft. 11 Humphreys et al., who examined in-flight using pulse oximeters in 84 passengers (aged 1 78 yr) in commercial aircraft, reported in-flight levels of % at ground level, and 85 98% at a cruising altitude of 5,000 6,000 ft. 12 The above values for cruising altitudes of 5,000 8,850 ft are consistent with those reported here for in-flight values in aircrew members TABLE II. Symptoms Suggesting Hypoxemia in 338 Airmen at Various Altitudes Altitude No. of Reports ( n = 88) Feeling of Warmth ( n = 62) Symptoms Suggesting Hypoxemia a Poor Concentration ( n = 28) Diminished Vision ( n = 13) Total ,000 ft >5,000 and <8,000 ft ,000 ft N, number of aircrew members. a The number of reports is exceeded by the sum of the numbers for the various symptoms because of multiple answers. Other ( n = 9) MILITARY MEDICINE, Vol. 176, January

4 operating a UH-60J helicopter without a pressurized cabin at 5,000 ft and 8,000 ft. It is well known that in aircrews, heart rate can be used as an index of psychophysiological workload, and that it increases during take-off and landing. 17 Not surprisingly, it is higher during difficult flight tasks than during easy tasks. 16 In the present study, significantly increased heart rates were found at take-off, at 5,000 ft and 10,000 ft in the first flight, and at 13,000 ft in the additional period of flight. Although the increase in heart rate at take-off may be caused by the psychological workload, the increases detected at 10,000 ft in the first flight and at 13,000 ft in the additional flight may have been due to the decreases in that occurred during the flights. In the present survey, the incidence of apparent hypoxic experiences did not differ significantly among the various aircrew roles. In contrast, Smith, who employed an anonymous survey to examine hypoxic symptoms during helicopter operations at below 10,000 ft, found that both the number of symptoms suggesting hypoxemia and their incidence were greater in loadmasters than in pilots. 5 Accordingly, he suggested that nonpilot aircrew members, such as loadmasters, may be more likely to experience hypoxia because of their greater physical activity levels. In line with that idea, Thornton et al. reported that during routine sorties, the workloads of loadmasters increased almost 1.5-fold (compared to the resting state), and that they were greater than those of pilots. 18,19 Regarding the effect of performing mild physical exercise on, a recent study noted a decrease in of 1% at ground level, a decrease of 4.3% at 7,000 ft, and a decrease of 5.5% at 9,000 ft (all values vs. the resting value at ground level). 10 On the basis of those data, the more physically active crew members might have been expected to experience more symptoms suggestive of hypoxemia. Regarding the age of the informant and the aircraft type, the incidence of symptoms suggesting hypoxemia was found to be significantly higher in aircrew members in their 30s or above and in those flying in CH47J craft than in younger subjects and those flying in UH-60J craft, respectively. Possible reasons for these different incidences between age groups and between aircraft types could include age-related differences in flight duties and differences in missions between the two types of aircraft. The most commonly mentioned neurologic symptoms in the present study were a feeling of warmth, poor concentration, and diminished vision. This is consistent with findings reported previously. 5,20,21 Even at altitudes below 5,000 ft, the present respondents reported a small number of neurologic experiences suggestive of hypoxemia. On that basis, it would be advisable for aircrews operating aircraft with unpressurized cabins to learn to recognize their own symptoms of possible hypoxia (by repeated hypoxic training and teaching). The present study had several methodological limitations. In the study of in-flight oxyhemoglobin saturation and heart rate, the study design was limited by the small number of aircrew members, although the standard errors in the data were small. Furthermore, hyperventilation has been reported to increase independently of changes in arterial oxygen tension. 22 In survey studies, neurologic hypoxic symptoms may be indistinguishable from symptoms due to other hazards affecting helicopter crews, such as sleep deprivation, dehydration, sun glare, cold, heat, and/or noise. Therefore, the findings of such surveys need to be interpreted with caution. Although the present study should be considered preliminary, it does emphasize the point that in helicopter aircrews operating aircraft with unpressurized cabins, the potential exists for symptoms due to hypoxia. CONCLUSIONS (1) In aircrews operating UH-60J helicopters without a pressurized cabin, in-flight decreased significantly at altitudes over 5,000 ft. (2) Symptoms suggestive of in-flight hypoxia may be uncommon at altitudes below 5,000 ft, although in the present survey 3 of 338 aircrew members reported such experiences at those altitudes during the course of their careers. (3) Although additional data are required, attention should be paid to the possibility of hypoxic symptoms affecting aircrews operating aircraft with unpressurized cabins. ACKNOWLEDGMENTS This work was supported by the Aeromedical Laboratory, Japan Air Self- Defense Force. REFERENCES 1. Department of Defense : Joint service specification guide. Crew systems oxygen systems handbook, pp 5 7. JSSG, Wright Patterson AFB, OH. 2. Headquarters Air Force Flight Standards Agency (AFFSA) : Air Force Instruction, , Vol 3, 2006 ; 42. Washington, DC, United States Air Force, Air Staff Office of the Japan Air Self-Defense Force (JASDF) : Flying Operations, General Flight Rules. Kujitatsu No. 28. (in Japanese )., JASDF, November, Shinjyuku Tokyo, Japan. 4. Cable GG : In-flight hypoxia incidents in military aircraft: causes and implications for training. Aviat Space Environ Med 2003 ; 74: Smith A : Hypoxia symptoms reported during helicopter operations below 10,000 ft: a retrospective survey. 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