Altitude Decompression Sickness Risk and Physical Activity During Exposure.

INTRODUCTION: Earlier research described a linear relationship between the highest 1 min of oxygen consumption (Vo2) during a recurring physical activity and incidence of decompression sickness (DCS) during research chamber exposures to high altitude. The current effort was designed to determine if that relationship holds true at a lower altitude. METHODS: Male subjects (20) were exposed without prebreathe to 22,500 ft (6858 m; 314 mmHg; 6.1 psi) for 4 h while seated, nonambulatory the entire time, with echo-imaging at 16-min intervals (Non-Amb Echo), breathing 100% oxygen. Average highest 1 min of Vo2 and level of activity was determined. Results during Non-Amb Echo were compared with earlier research data acquired under identical conditions except for higher levels of activity. RESULTS: No DCS was reported or observed and no venous gas emboli were observed. Combined with earlier data, a strong linear relationship (r > 0.99) was observed between DCS incidence and level of activity. DISCUSSION: These results suggest physiological envelopes might be expanded or prebreathe time reduced for some high-altitude aircraft operations that involve very low levels of physical activity. They may also help to explain the higher DCS risk for inside observers vs. trainees during altitude chamber training. The data imply potential for update of altitude DCS risk prediction models by adjustment with quantified level of activity during exposure.

Recollection of hypoxia symptoms between training events.

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.

Aerospace medicine at Brooks AFB, TX: hail and farewell.

With the impending termination of USAF operations at Brooks Air Force Base (AFB) in San Antonio, TX, it is time to consider its historic role in Aerospace Medicine. The base was established in 1917 as a flight training center for the U.S. Army Air Service and in 1926 became home to its School of Aviation Medicine. The school moved to San Antonio's Randolph Field in 1931, but in 1959 it returned to Brooks where it occupied new facilities to support its role as a national center for U.S. Air Force aerospace medicine, including teaching, clinical medicine, and research. The mission was then expanded to encompass support of U.S. military and civilian space programs. With the abrupt termination of the military space program in 1969, research at Brooks focused on clinical aviation medicine and support of advanced military aircraft while continuing close cooperation with NASA in support of orbital spaceflight and the journey to the Moon. Reorganization in the 1990s assigned all research functions at Brooks to the Human Systems Division and its successors, leaving to USAFSAM the missions related to clinical work and teaching. In 2002 the USAF and the city of San Antonio implemented shared operation of Brooks as a "City-Base" in the hope of deflecting threatened closure. Nevertheless, under continuing pressure to consolidate military facilities in the United States, the 2005 Base Closure and Realignment Commission ordered Brooks closed by 2011, with its aerospace medicine functions relocated to new facilities at Wright-Patterson AFB in Dayton, OH.

Fifty years of decompression sickness research at Brooks AFB, TX: 1960-2010.

INTRODUCTION, FACILITIES, AND METHODS: Decompression sickness (DCS) occurring in hypobaric environments related to aviation or spaceflight was a major focus of research at Brooks AFB/City-Base, TX, throughout the period 1960-2010. Multiple hypobaric chambers and extensive support facilities were built for research on altitude DCS using both human subjects and animal models. Areas of study included symptomatology, incidence, prediction, and prevention of DCS. High-altitude aviation, spacecraft atmospheres, and pressure suits were evaluated with various decompression and prebreathing schedules to reduce DCS risk. FACTORS AFFECTING DCS INCIDENCE: The results from these efforts were recorded in an extensive Altitude DCS Research Database which served as a resource for developing reports and exploring relationships of various parameters such as altitude, time at altitude, prebreathe time, and mode of activity while decompressed. PREVENTION AND PREDICTION OF DCS: Individual susceptibility to DCS was also evaluated in an effort to tailor preventive measures and predict susceptibility. Completion of the 26 human-use protocols provided information which was incorporated into NASA and USAF operational practices to reduce DCS risk. DOCUMENTATION: DCS researchers working at Brooks throughout this period produced 177 papers documenting results of thousands of subject-exposures and other experiments. An Altitude DCS Risk Assessment Computer Model was fielded in 2005. This review centers on the results of research at Brooks and notes questions about operational DCS risk that have not yet been answered.

Oxygen consumption at altitude as a risk factor for altitude decompression sickness.

INTRODUCTION: The existence of a general influence of exercise on the incidence of decompression sickness (DCS) has been known for more than a half-century. However, quantification of the effect has not been done for several reasons, including isolation of exercise as the only variable. The DCS database at Brooks City-Base, TX, contains detailed physiologic information on over 3000 altitude exposures. The purpose of this study was to measure Vo2 during the activities performed during those exposures to retrospectively determine if Vo2, a quantifiable index of exercise intensity, was related to the level of reported DCS. METHODS: Ground-level activity was designed to duplicate the standardized activity during the altitude exposures. Breath-by-breath Vo2 was determined for each activity using a COSMED metabolic measurement system. Comparison of the Vo2 during four levels of activity performed under otherwise comparable conditions allowed a determination of correlation between Vo2 and DCS risk observed during the altitude exposures. RESULTS AND DISCUSSION: Four previous altitude exposure profiles at 8992 m to 9144 m (29,500 to 30,000 ft; 231 to 226 mmHg) for 4 h following a 1-h prebreathe resulted in 38-86% DCS. This study provided the Vo2 of activities during those studies. The correlation between DCS incidence and the highest 1-min Vo2 of activity was 0.89. CONCLUSION: The highest 1-min Vo2 showed a high correlation with level of DCS risk. Future exposures involving lower levels of activity could provide data that would allow improvement in modeling of DCS risk.

Air break during preoxygenation and risk of altitude decompression sickness.

INTRODUCTION: To reduce the risk of decompression sickness (DCS), current USAF U-2 operations require a 1-h preoxygenation (PreOx). An interruption of oxygen breathing with air breathing currently requires significant extension of the PreOx time. The purpose of this study was to evaluate the relationship between air breaks during PreOx and subsequent DCS and venous gas emboli (VGE) incidence, and to determine safe air break limits for operational activities. METHODS: Volunteers performed 30 min of PreOx, followed by either a 10-min, 20-min, or 60-min air break, then completed another 30 min of PreOx, and began a 4-h altitude chamber exposure to 9144 m (30,000 ft). Subjects were monitored for VGE using echocardiography. Altitude exposure was terminated if DCS symptoms developed. Control data (uninterrupted 60-min PreOx) to compare against air break data were taken from the AFRL DCS database. RESULTS: At 1 h of altitude exposure, DCS rates were significantly higher in all three break in prebreathe (BiP) profiles compared to control (40%, 45%, and 47% vs. 24%). At 2 h, the 20-min and 60-min BiP DCS rates remained higher than control (70% and 69% vs. 52%), but no differences were found at 4 h. No differences in VGE rates were found between the BiP profiles and control. DISCUSSION: Increased DCS risk in the BiP profiles is likely due to tissue renitrogenation during air breaks not totally compensated for by the remaining PreOx following the air breaks. Air breaks of 10 min or more occurring in the middle of 1 h of PreOx may significantly increase DCS risk during the first 2 h of exposure to 9144 m when compared to uninterrupted PreOx exposures.

Human health and performance for long-duration spaceflight.

Future long-duration spaceflights are now being planned to the Moon and Mars as a part of the "Vision for Space Exploration" program initiated by NASA in 2004. This report describes the design reference missions for the International Space Station, Lunar Base, and eventually a Mars Expedition. There is a need to develop more stringent preflight medical screening for crewmembers to minimize risk factors for diseases which cannot be effectively treated in flight. Since funding for space life sciences research and development has been eliminated to fund program development, these missions will be enabled by countermeasures much like those currently in use aboard the International Space Station. Artificial gravity using centrifugation in a rotating spacecraft has been suggested repeatedly as a "universal countermeasure" against deconditioning in microgravity and could be an option if other countermeasures are found to be ineffective. However, the greatest medical unknown in interplanetary flight may be the effects of radiation exposure. In addition, a Mars expedition would lead to a far greater level of isolation and psychological stress than any space mission attempted previously; because of this, psychiatric decompensation remains a risk. Historically, mortality and morbidity related to illness and injury have accounted for more failures and delays in new exploration than have defective transportation systems. The medical care system on a future Mars expedition will need to be autonomous and self-sufficient due to the extremely long separation from definitive medical care. This capability could be expanded by the presence of a physician in the crew and including simple, low-technology surgical capability.

Decompression sickness during simulated extravehicular activity: ambulation vs. non-ambulation.

BACKGROUND: Extravehicular activity (EVA) is required from the International Space Station on a regular basis. Because of the weightless environment during EVA, physical activity is performed using mostly upper-body movements since the lower body is anchored for stability. The adynamic model (restricted lower-body activity; non-ambulation) was designed to simulate this environment during earthbound studies of decompression sickness (DCS) risk. DCS symptoms during ambulatory (walking) and non-ambulatory high altitude exposure activity were compared. The objective was to determine if symptom incidences during ambulatory and non-ambulatory exposures are comparable and provide analogous estimates of risk under otherwise identical conditions. METHODS: A retrospective analysis was accomplished on DCS symptoms from 2010 ambulatory and 330 non-ambulatory exposures. RESULTS: There was no significant difference between the overall incidence of DCS or joint-pain DCS in the ambulatory (49% and 40%) vs. the non-ambulatory exposures (53% and 36%; p > 0.1). DCS involving joint pain only in the lower body was higher during ambulatory exposures (28%) than non-ambulatory exposures (18%; p < 0.01). Non-ambulatory exposures terminated more frequently with non-joint-pain DCS (17%) or upper-body-only joint pain (18%) as compared with ambulatory exposures, 9% and 11% (p < 0.01), respectively. DISCUSSION: These findings show that lower-body, weight-bearing activity shifts the incidence of joint-pain DCS from the upper body to the lower body without altering the total incidence of DCS or joint-pain DCS. CONCLUSIONS: Use of data from previous and future subject exposures involving ambulatory activity while decompressed appears to be a valid analogue of non-ambulatory activity in determining DCS risk during simulated EVA studies.

Partial pressure of nitrogen in breathing mixtures and risk of altitude decompression sickness.

BACKGROUND: Many aircraft oxygen systems do not deliver 100% O2. Inert gases can be present at various levels. The purpose of this study was to determine the effect of these inert gas levels on decompression sickness (DCS). METHODS: Subjects were exposed for 4 h to 5486 m (18,000 ft) with zero prebreathe, using either mild (Test A) or strenuous exercise (Test B), and breathing 60%N2/40%O2. Test C used a breathing mixture of 40%N2/60%O2 at 6858 m (22,500 ft) with zero prebreathe and mild exercise. Test D investigated a breathing mixture of 2.8%N2/4.2%argon/93%O2 with 4 h exposures to 7620 m (25,000 ft), mild exercise, and 90 min of preoxygenation. The controls were from previous studies using similar conditions and 100% O2. RESULTS: The DCS risk for Tests A and B and the Control for B was 7%; the Control for Test A was 0% (n.s.). Breathing the 40%N2/60%O2 mixture (Test C) resulted in 43% DCS compared with 53% DCS with 100% O2 (n.s.). When the 2.8%N2/4.2%argon/93%O2 mixture was used, the results showed 25% DCS compared with 31% DCS with 100% O2 (n.s.). CONCLUSIONS: The increased nitrogen and argon levels in the breathing gas while at altitudes of 5486 m to 7620 m did not increase DCS risk. These results support the concept of using the partial pressure gradient of inert gases instead of the percentage of N2 or argon in a breathing gas mixture to determine the risk of DCS during altitude exposure.

Depressurization in military aircraft: rates, rapidity, and health effects for 1055 incidents.

INTRODUCTION: Aircraft cabin depressurization is a rare event but one which demands attention because of the grave potential for aircrew incapacity in flight. The purpose of the current study was to determine rates of depressurization incidents for U.S. military aircraft, to examine their causes, and to evaluate the medical importance of these incidents. METHODS: The U.S. Navy and U.S. Air Force safety center databases were searched for decompression incidents during FY1981-FY2003. A total of 1055 incidents were analyzed as to the cause, speed of onset, and adverse health effects (hypoxia, barotrauma, DCS, or any combination of these). The causes of each incident were identified and classified by aircraft type. RESULTS: The number of incidents per airframe varied from 1 (in many airframes) to 276 in the T-38. The number of total hours flown ranged from 16,332 in the T-6 to 8,101,607 in the C-130. The number of sorties flown ranged from 8800 in the B-2 to 3,543,061 in the C-130. Of 35 common airframes, 30 showed rates between 0 and 20 incidents per million flying hours. Depressurization was "slow" in 83% of incidents. Of the 1055 incidents, only 350 (33.2%) involved adverse health effects. Hypoxia occurred in 221 incidents, DCS in 83, and barotrauma in 71. Only 4 (0.4%) resulted in a fatality. Of the 199 incidents involving hypoxia, 12 (6%) occurred below 4267 m (14,000 ft). CONCLUSION: Most reported military aircraft depressurization incidents are slow and do not affect aircrew health. Rates have decreased dramatically since the 1980s. Still, this study lends support to continuing hypobaric chamber training for military pilots.

Altitude decompression sickness susceptibility: influence of anthropometric and physiologic variables.

INTRODUCTION: There is considerable variability in individual susceptibility to altitude decompression sickness (DCS). The Air Force Research Laboratory Altitude DCS Research Database consists of extensive information on 2980 altitude exposures conducted with consistent procedures and endpoint criteria. We used this database to quantify the variation in susceptibility and determine if anthropometric and/or physiologic variables could be used to predict DCS risk. METHODS: There were 240 subjects who participated in at least 4 of 70 exposure profiles in which between 5 and 95% of all subjects tested developed DCS symptoms. A subject/study ratio (SSR) was calculated by dividing the DCS experienced by a subject during all their exposures by the DCS incidence for all subjects who participated in the identical exposures. The SSR was used to identify the relative susceptibility of subjects for use in analyzing possible relationships between DCS susceptibility and the variables of height, weight, body mass index, age, percent body fat, and aerobic capacity. RESULTS: The DCS incidence was 46.5% during 1879 subject-exposures by subjects exposed at least 4 times. A significant relationship existed between higher DCS susceptibility and the combination of lower aerobic capacity and greater weight (p < 0.05). DISCUSSION: Despite a correlation, less than 13% of the variation in DCS susceptibility was accounted for by the best combination of variables, weight and VO2max. CONCLUSION: Differences in DCS susceptibility cover a wide range and appear to be related to some anthropometric and physiologic variables. However, there was insufficient correlation to allow prediction of an individual's susceptibility.

Altitude decompression sickness between 6858 and 9144 m following a 1-h prebreathe.

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.

Central nervous system decompression sickness and venous gas emboli in hypobaric conditions.

INTRODUCTION: Altitude decompression sickness (DCS) that involves the central nervous system (CNS) is a rare but potentially serious condition. Identification of early symptoms and signs of this condition might improve treatment. METHODS: We studied data from 26 protocols carried out in our laboratory over the period 1983-2003; all were designed to provoke DCS in a substantial proportion of subjects. The data set included 2843 cases. We classified subject-exposures that resulted in DCS as: 1) neurological DCS of peripheral and/or central origin (NEURO); 2) a subset of those that involved only the CNS (CNS); and 3) all other cases, i.e., DCS cases that did not have a neurological component (OTHER). For each case, echo imaging data were used to document whether venous gas emboli (VGE) were present, and their level was classified as: 1) any level, i.e., Grade 1 or higher (VGE-1); and 2) high level, Grade 4 (VGE-4). RESULTS: There were 1108 cases of altitude DCS in the database; 218 were classified as NEURO and 49 of those as CNS. VGE-1 were recorded in 83.8% of OTHER compared with 58.7% of NEURO and 55.1% of CNS (both p < 0.001 compared with OTHER). The corresponding values for VGE-4 were 48.8%, 37.0%, and 34.7% (p < 0.001, compared to OTHER). Hyperbaric oxygen (HBO) was used to treat about half of the CNS cases, while all other cases were treated with 2 h breathing 100% oxygen at ground level. DISCUSSION: Since only about half of the rare cases of hypobaric CNS DCS cases were accompanied by any level of VGE, echo imaging for bubbles may have limited application for use as a predictor of such cases.

Altitude decompression sickness at 7620 m following prebreathe enhanced with exercise periods.

INTRODUCTION: Over 80% altitude decompression sickness (DCS) was reported during a 4-h exposure with mild exercise to 7620 m (25,000 ft) without prebreathe. Prebreathe for more than 1 h would be necessary to reduce the DCS risk below 40%. Use of a single period of exercise to enhance prebreathe effectiveness has been successfully tested and used during some U-2 operations. The current tests used multiple exercise sessions to enhance prebreathe (MEEP) as a means of improving denitrogenation efficiency. METHODS: Two MEEP profiles, 30 or 60 min, preceded 4-h exposures to 7620 m with mild, upper-body exercise while breathing 100% oxygen. Resting prebreathe controls were from published studies at the same laboratory. Both MEEP profiles involved 10 min of strenuous dual-cycle ergometry (75% of maximal oxygen uptake) at the beginning of prebreathe. After a 15-min rest period during the 60-min prebreathe, an additional 5 min of strenuous ergometry was performed. Mild exercise was performed during 15 of the last 20 min of both prebreathe profiles. RESULTS: The 60-min MEEP resulted in 25% DCS and the 30-min MEEP 40% DCS (N.S.). The 25% incidence of DCS following the 60-min MEEP profile was significantly less than the 63% DCS following an equal-time, resting prebreathe control. Following the 30-min MEEP, DCS incidence was not greater than the incidence following a 60-min, resting prebreathe control. There was a lower incidence of venous gas emboli during the MEEP exposures than during resting control exposures. CONCLUSION: Denitrogenation with multiple periods of exercise provides a shorter alternative to resting prebreathe for reducing DCS risk during exposure to 7620 m.

Decompression sickness risk model: development and validation by 150 prospective hypobaric exposures.

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.

Altitude decompression sickness symptom resolution during descent to ground level.

INTRODUCTION: Altitude decompression sickness (DCS) is a health risk associated with the conduct of high altitude airdrop operations, high altitude reconnaissance, future fighter operations, hypobaric chamber training, unpressurized flight, and extravehicular activity (EVA) in space. The treatment for DCS includes the provision of 100% oxygen (O2) at ground level (GLO) and/or hyperbaric oxygen therapy (HBO). In this paper we examine the effect of repressurization to ground level from hypobaric conditions on DCS symptoms. Timely recompression (descent at first recognition of any DCS symptom) may be a safe, effective treatment for the large majority of DCS symptoms. METHODS: Data from altitude chamber exposures recorded in the Air Force Research Laboratory (AFRL) Altitude DCS Database were reviewed to determine the level of recompression required for complete resolution of 1,699 observed symptoms. RESULTS: Of the 1,699 DCS symptoms reviewed, 66 (3.9%) resolved at altitude, 117 (6.9%) resolved at ground level, and 1,433 (84.3%) resolved during descent. Increasing the pressure by 138 mmHg from the altitude of exposure where symptoms occurred resolved roughly 50% of symptoms. Little resolution of symptoms was noted with recompressions of < 50 mmHg. The greatest rate of symptom resolution occurred with recompressions of 50-250 mmHg. CONCLUSION: These findings support the concept that descent and postflight, ground-level oxygen may be sufficient to relieve the majority of altitude DCS symptoms. HBO may be reserved for serious, recurring, delayed, or refractory symptoms. The findings also suggest a need for further study of DCS symptom resolution.