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To predict altitude decompression sickness (DCS) risk with any degree of accuracy, one must weigh variables such as prebreathe time, rate of ascent/ descent, time at altitude, altitude, mixed breathing gas (dependent upon altitude), and profiles with multiple ascents and descents. The length of research chamber exposures is fixed. Therefore, risk assessment is based on DCS incidence after this fixed period at simulated altitude. From an operational standpoint, variable time at altitude complicates any predictive capability, although a computer model to handle all of these variables is in development. In the interim, a retrospective study from the Armstrong Laboratory Decompression Sickness Research Database has produced risk curves which can be used to predict DCS or venous gas emboli (VGE) incidence as a function of time at various altitudes. We limited the data to: (1) zero-prebreathe exposures to less than 20,000 ft breathing 50% O2, 50% N2; (2) zero-prebreathe exposures to less than 20,000 ft breathing 100% O2; and (3) 1-h prebreathe exposures to greater than 20,000 ft breathing 100% 02. Using the curves, one can select a time/altitude of exposure and estimate the DCS and VGE percentage. Decompression sickness, Venous gas emboli, Prebreathe, Latency.
It is important to understand the risk of serious hypobaric decompression sickness (DCS) to develop procedures and treatment responses to mitigate the risk. Since it is not ethical to conduct prospective tests about serious DCS with humans, the necessary information was gathered from 73 published reports. We hypothesize that a 4-hr 100% oxygen (O2) prebreathe results in a very low risk of serious DCS, and test this through analysis. We evaluated 258 tests containing information from 79,366 exposures in altitude chambers. Serious DCS was documented in 918 men during the tests. A risk function analysis with maximum likelihood optimization was performed to identify significant explanatory variables, and to create a predictive model for the probability of serious DCS [P(serious DCS)]. Useful variables were Tissue Ratio, the planned time spent at altitude (Talt), and whether or not repetitive exercise was performed at altitude. Tissue Ratio is P1N2/P2, where P1N2 is calculated (N2) pressure in a compartment with a 180-min half-time for N2 pressure just before ascent, and P2 is ambient pressure after ascent. A prebreathe and decompression profile Shuttle astronauts use for extravehicular activity (EVA) includes a 4-hr prebreathe with 100% O2, an ascent to P2=4.3 lb per sq. in. absolute, and a Talt=6 hr. The P(serious DCS) is: 0.0014 (0.00096-0.00196, 95% confidence interval) with exercise and 0.00025 (0.00016-0.00035) without exercise. Given 100 Shuttle EVAs to date and no report of serious DCS, the true risk is less than 0.03 with 95% confidence (Binomial Theorem). It is problematic to estimate the risk of serious DCS since it appears infrequently, even if the estimate is based on thousands of altitude chamber exposures. The true risk to astronauts may lie between the extremes of the confidence intervals since the contribution of other factors, particularly exercise, to the risk of serious DCS during EVA is unknown. A simple model that only accounts for four important variables in retrospective data is still helpful to increase our understanding about the risk of serious DCS.
High altitude exposure in aircraft hypobaric chambers and with extravehicular- activity (EVA) in space results in an inherent risk of altitude decompression sickness (DCS). In the past general guidelines for safer altitude exposures have been developed through costly time-consuming studies each specific to unique scenarios of altitude exposure. Rapidly changing technology in aircraft design and mission requirements demand improved capabilities in predicting DCS risk during mission planning and execution.
The zero prebreathe altitude threshold for developing 5% decompression sickness (DCS) symptoms in men has been reported to be 6248 meters (20,500 ft). However, such an altitude threshold when 1 hour of oxygen prebreathe is used has not been well-documented and was the primary purpose of this study. The 51 male subjects were exposed to 9144 meters (30,000 ft), 8382 meters (27,500 ft), 7620 meters (25,000 ft), and/or 6858 meters (22,500 ft) for 8 hours. They were monitored for symptoms of DCS and venous gas emboli (VGE). The results showed that DCS symptom incidence after 4 hours of exposure decreased with exposure altitude from 87% at 9144 meters to 26% at 6858 meters. VGE were lower during the 4-hour, 6858-meter exposures (32%) than at the higher altitudes (76%-85%). The symptom incidences during the first 4 hours of exposure were lower at 6858 meters and 7620 meters following a 1-hour 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 hours of exposure. The overall results show that the altitude threshold for 5% DCS symptoms is below 6858 meters after 1 hour of prebreathe. However, during 6858-meter and 7620-meter exposures, a 1-hour 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 minutes of exposure. Use of 4-hour versus 8-hour exposures does not appear to underestimate DCS risk at or above 7620 meters. The data from this study also provided possible insight regarding the effects of exercise at 9144 meters on DCS incidence. The 87% DCS at 9144 meters during this study was higher than during other exposures to 9144 meters for the same duration, also with mild exercise and 1 hour of prebreathe, perhaps because the exercises used in this experiment involved more stress on the lower body than the mild exercises used in most of the authors' experiments. 7.
Estimating the risk of decompression sickness (DCS) in aircraft operations remains a challenge, making the reduction of this risk through the development of operationally acceptable denitrogenation schedules difficult. In addition, the medical recommendations which are promulgated are often not supported by rigorous evaluation of the available data, but are instead arrived at by negotiation with the aircraft operations community, are adapted from other similar aircraft operations, or are based upon the opinion of the local medical community. We present a systematic approach for defining DCS risk in aircraft operations by analyzing the data available for a specific aircraft, flight profile, and aviator population. Once the risk of DCS in a particular aircraft operation is known, appropriate steps can be taken to reduce this risk to a level acceptable to the applicable aviation community. Using this technique will allow any aviation medical community to arrive at the best estimate of DCS risk for its specific mission and aviator population and will allow systematic reevaluation of the decisions regarding DCS risk reduction when additional data are available.
The additional decrease in ambient pressure which occurs when a compressed air diver flies in an aircraft within a short time after diving may be sufficient to precipitate decompression sickness, even though the dive itself was in accordance with the U.S. Navy decompression tables. The current practice by both military and civilian divers of using air transportation after compressed air diving suggests the need for specific instructions regarding the decompression required before flying after diving. In order to quantitate the importance of this problem, an experiment was designed in which large dogs were exposed to compressed air for 7 hours at their 'no-bends' pressure threshold as determined after the method of Reeves and Beckman. After pressurization, the animals were decompressed within 2-3 minutes to sea level. A sea level decompression interval of 1, 3, 6, or 12 hours was given prior to further decompression to a simulated altitude of 10,000 feet. The incidence of decompression sickness at altitude was 92.9% for the 1 hour surface decompression interval, 30% for the 3 hour interval, 27.8% for the 6 hour interval and 0% for the 12 hour interval. From these large animal studies it may be postulated that a surface decompression interval of at least 12 hours should be allowed before flying after compressed air diving of a depth and duration to require the use of diving tables.
We conducted 25 altitude chamber decompression exposure profiles incorporating both genders in a prospective attempt to clarify the role of gender in DCS susceptibility. METHODS. The 291 human subjects were exposed (961 subject-exposures) to simulated altitude for up to 8 h, using zero to 4 h of preoxygenation. Subjects breathed 100% oxygen, rested or performed mild or strenuous exercise while decompressed, and were monitored for precordial venous gas emboli (VUE) and DCS symptoms. RESULTS. No differences (P--0.24) in DCS incidence were observed between males (49.5%) and females (45.3%). Higher DCS incidence (P
Exposure to 35,000 ft without preoxygenation, breathing 100% oxygen prior to decompression, can result in severe decompression sickness (DCS). Exercise while decompressed increases the incidence and severity of symptoms. Clarification of the level of activity versus time to symptom onset is needed to refine recommendations for current operations requiring 35,000-ft exposures. Currently, the USAF limits these operations to 30 min following 75 min of preoxygenation. The objective of this study was to determine the effect of exercise intensity on DCS incidence and severity at 35,000 ft. Following 75 or 90 min of ground-level preoxygenation, 54 male and 38 female subjects were exposed to 35,000 ft for 3 hours while performing strenuous exercise, mild exercise, or seated rest. The subjects were monitored for venous gas emboli (VGE) with an echo-imaging system and observed for signs and symptoms of DCS. Results. Exposures involving strenuous and mild exercise resulted in higher incidence (P
The cabin altitude experienced by U-2 pilots during high altitude reconnaissance missions is approximately 30,000 feet. Increasing the duration of preoxygenation (i.e., breathing 100% oxygen prior to decompression to reduce the incidence of decompression sickness (DCS)), increases denitrogenation, but is impractical due to mission time constraints and crew duty limitations. Use of exercise-enhanced preoxygenation instead of increasing the duration of preoxygenation to provide better protection from DCS has been successfully demonstrated in the laboratory. This report describes the first operational use of this new procedure. A U-2 pilot, who had previously reported serious DCS symptoms resulting in mission aborts during two of his first twenty-five high-altitude flights, volunteered to operationally try the exercise-enhanced preoxygenation procedure. His next 28 U-2 high-altitude flights incorporated moderate aerobic upper and lower body exercise at a controlled rate during the first 10 minutes of a 90-minute preoxygenation. The total preoxygenation time was of the same duration as accomplished prior to his last aborted mission. The pilot reported no DCS symptoms during the subsequent 28 high-altitude flights. An operational trial of exercise-enhanced preoxygenation has been shown to be feasible under operational constraints and, to date, has been successful.
The altitude threshold for decompression sickness (DCS) symptoms has been variously described as being 18,000 ft (5,487 m) to above 25,000 ft (7,620 m). Safety and efficiency of aerospace operations require more precise determination of the DCS threshold. One hundred fifteen male human-subjects were exposed to simulated altitude (11 at 11,500 ft; 10 at 15,000 ft; 8 at 16,500 ft; 10 at 18,100 ft; 10 at 19,800 ft; 20 at 21,200 ft; 20 at 22,500 ft; 10 at 23,800 ft, and 16 at 25,000 ft) for up to 4 h. All breathed 100% oxygen beginning with ascent. Subjects were monitored for precordial venous gas emboli (VGE) and DOS symptoms. Probit curves representing altitude versus incidence of DCS symptoms and VGE allowed estimation of respective risk. VGE were first observed at 15,000 ft with increasing incidence at higher altitudes; over 50% at 21,200 ft and at least 70% at 22,500 ft and above. DCS symptoms were first reported at 21,200 ft with an incidence of 5%. At 22,500 ft, the DOS incidence climbed to 60%. The 5% threshold for zero-preoxygenation altitude DCS symptoms is at 21,000 ft and an abrupt increase in DCS symptom incidence with increased altitude.