DECOMPRESSION SAFETY
U. S. Navy Probabilistic Decompression Procedures
In spite of the difficulty in accurately predict-ing low DCS risk, USN93 has been an impor-tant yardstick for grading dive-profile severity and a useful tool for developing decompression procedures.177
Upon examining the results of their exper-imental trials, the U.S. Navy judged that the
no-D exposure limits that were acceptable corresponded to a PDCS of 230 DCS/104 dives. This became the “target” PDCS for dives with decompression times of 0 to 20 min.175 For decompression times of 20 to 60 min, the target PDCS was allowed to rise from 230 to 500 DCS/104 dives. A “sliding”
target was used because USN93 estimated much longer decompression times than the corresponding schedules in the approved U.S. Navy Standard Air Decompression Tables.115The Navy considered 60 min to be the longest acceptable time for in-water decompression, and dives with longer decompressions were listed as exceptional exposure. The target PDCS for exceptional exposure dives was 500 DCS/104 dives until the decompression time reached 180 min, after which the target increased from 500 to 1000 DCS/104 dives as the decompression time rose from 180 to 220 min.
The USN93 no-D exposure limits were longer than the Standard Air limits at 90 fsw (27 msw) and deeper and shorter than the Standard Air limits at 30, 35, and 40 fsw (9, 10.7, and 12 msw).177 USN93 decompres-sion schedules were substantially longer than the Standard Air schedules but with lower estimated PDCS.175 Because of com-plexity, the USN93 decompression algorithm did not lend itself to repetitive diving accord-ing to the familiar methods of the Standard Air Tables. An alternative method was devel-oped whereby every dive was assigned an A-to-Z “exit state” similar to the Repetitive Group of the Standard Air Tables, and a separate table of schedules was prepared for each exit state. The USN93 tables were as flexible as the Standard Air Tables but not as compact.
Ultimately, the USN93 tables were not accepted by the U.S. Navy as a replacement for the Standard Air Tables (Dr. E.D. Thalmann and Dr. E.T. Flynn, personal communication).
The Navy did not perceive a problem with the existing tables that needed to be fixed, and the new tables were thought to reduce capability because:
• Shallow no-D exposure limits were shorter and might restrict a ship’s husbandry diving.
• Dives that were formerly available for routine use were now classified as excep-tional exposure.
• Repetitive diving procedures were complex.
These views were largely determined by Master Divers—practitioners with strong grounding in tradition. Perhaps this is as it should be. New tactics, equipment, or revolu-tionary concepts (such as probability in diving) are historically slow to be accepted by the military, with good reason. Change is ex-pensive and time-consuming, and the conse-quences of being wrong can be catastrophic.
For those less wedded to practice and tra-dition, probability might be viewed differently given the uncertainty of the present U.S. Navy tables. In 1972 to 1973, only 4% of the 113,007 operational air dives conducted required decompression,156 and this fraction was less than 4.7% in 1990 to 1994 (Dr. E.T. Flynn, per-sonal communication). U.S. Navy dive trials found specific areas of concern:
• In tests of the 200-min no-D exposure limit at 40 fsw, two DCS incidents occurred (one joint pain, the other with cerebral findings and residual effects) in 91 trials (220 DCS/104dives).178
• Trials of Standard Air Decompression schedules resulted in four DCS incidents in 77 trials (519 DCS/104 dives)179 and sug-gested that some of the Standard Air Schedules would benefit from tripling the decompression time.174
• When DCS occurred operationally, the problem was often fixed by ad hoc reduc-tions of bottom time or increases in decompression time.114
Figure 7–26. Data collection progress for Project Dive Exploration.140Data represented include the number of divers in the database, the number of dives collected, the number of divers who underwent recompression for decompression sickness (DCS), and the DCS morbidity per 10,000 dives.
40,000
1995 1996 1997 1998 1999 2000 2001
Dives DCS
Dives Divers
DCS/10,000 dives DCS
Table 7–7.Comparison of repetitive dive decompression schedules for 20-min dives to depths of 150–200 fsw according to the U.S. Navy Standard Air Tables (Std), the USN93 Tables (’93), and the INA95 Tables First DiveSecond Dive USN Stops*INA Stops†PDCS/104USN Stops*INA Stops†PDCS/104 DStd’9330′20′DivesDTStd’9330′20′/104 150 (fsw)95015115150 (fsw)202116002094 16014150201051603816002586 170191502012717043165025102 180231502014818050NA030106 190282052512919060NA53592 2003720530153200NANA540103 * Total time of all stops (min). †30′stop on air (min), 20′stop on O2(min). NA, not allowed; PDCS, probability of decompression sickness. U.S. Navy Standard Air Table data from reference 115; USN93 data from reference 177; INA95 data from reference 149.
Probabilistic Decompression Procedures for Underwater Archeology
Probabilistic modeling was also used to develop decompression schedules for under-water archeology. In the late 1960s, the Institute of Nautical Archeology (INA) began using in-water oxygen decompression with the U.S. Navy Standard Air Tables during the excavation of ancient shipwrecks in the Mediterranean Sea on the unofficial recom-mendation of Dr. Robert Workman, then Senior Medical Officer at the U.S. Navy Experimental Diving Unit.149 Although un-documented, the success of this technique (supported by an on-site recompression chamber with medical personnel for manag-ing divmanag-ing injuries) led to a formal effort beginning in 1985 with orderly records of diving activity and, in 1988, to a series of probabilistic decompression schedules based on models.27,180,181 Methods for introducing new diving procedures in the field were adopted as outlined by Schreiner and Hamilton,182including:
• Approval of an Institutional Review Board
• Approval of a Decompression Monitoring Board
• Documentation involving written dive logs
• A recompression chamber and diving medical personnel on site
• Incremental introduction of the new procedures
The INA decompression schedules were for dives to a maximum depth of 200 fsw, with bottom times of up to 40 min and oxygen decompression at 20 fsw. There were two dives per day with a 5- to 6-hour surface interval. The diving season was June through September, with 6 dive days per week. The approach to acceptable DCS risk was empiri-cal and similar to that used by the U.S. Navy for USN93. Based on previous INA experi-ence, a target PDCS of 150 DCS/104dives was selected for the first dive and a target of 100 DCS/104 dives was selected for the second dive.
Table 7–7 compares the decompression schedules for 20 min dives to 150 to 200 fsw with the Standard Air schedules and the USN93 schedules. The divers breathed air during decompression at 40 and 30 fsw. All other decompression in the INA schedules occurred at 20 fsw while divers breathed 100% oxygen. In 1998, oxygen decompres-sion schedules were introduced for dives to
120 fsw with a bottom gas of 32% oxygen in nitrogen.150 Seven DCS incidents (3 DCS/104 dives) and no oxygen toxicity were reported for 26,274 dives using all INA schedules.149,150
CONCLUSIONS
Statistical methods used in probabilistic modeling are not wise in themselves and are simply data-fitting tools that compensate for ignorance regarding underlying mech-anisms. Bubble formation, inert gas exchange, and pathophysiology cannot be assumed to be identical in the brain, spinal cord, and limbs. This is why decompression modes should represent premorbid physiol-ogy as closely as possible and why under-standing this physiology has practical importance. Relating physiology to decom-pression safety is an epidemiologic problem associated with finding the probability of injury in the context of the individual, the environment, and the exposure. Much will be gained by formalizing operational and clinical methods and by applying analytical techniques used widely in science and medicine.
References
1. Harvey EN, Barnes DK, McElroy WD, et al: Bubble formation in animals. I: Physical factors. J Cell Comp Physiol 24:1–22, 1944.
2. Evans A, Walder DN: Significance of gas micronuclei in the aetiology of decompression sickness. Nature 222:251–252, 1969.
3. Vann RD, Grimstad J, Nielsen CH: Evidence for gas nuclei in decompressed rats. Undersea Biomed Res 7:107–112, 1980.
4. Daniels S, Eastaugh KC, Paton WDM, Smith EB:
Micronuclei and bubble formation: A quantitative study using the common shrimp, Crangon crangon.
In: Bachrach AJ, Matzen MM (eds): Proceedings of the Eighth Symposium on Underwater Physiology.
Bethesda, Md., Undersea and Hyperbaric Medical Society, 1984, pp 147–157.
5. Walder DN: Adaptation to decompression sickness in caisson work. In: Proceedings of the Third International Biometeorology Congress. Oxford, 1968, pp 350–359.
6. Walder DN: The prevention of decompression sick-ness. In: Bennett PB, Elliott DH (eds): The Physiology and Medicine of Diving and Compressed Air Work, 2nd ed. London, Bailliere Tyndall, 1975, pp 456–470.
7. McDonough PM, Hemminsgsen EV: Bubble forma-tion in crabs induced by limb moforma-tions after decom-pression. J Appl Physiol 57:117–122, 1984.
8. McDonough PM, Hemmingsen EV: Bubble forma-tion in crustaceans following decompression from
hyperbaric gas exposures. J Appl Physiol Respir Environ Exerc Physiol 56:513–519, 1984.
9. McDonough PM, Hemmingsen EV: A direct test for the survival of gaseous nuclei in vivo. Aviat Space Environ Med 56:54–56, 1985.
10. Dervay JP, Powell MR, Butler B, Fife CE: The effect of exercise and rest duration on the generation of venous gas bubbles at altitude. Aviat Space Environ Med 73:22–27, 2002.
11. Vann RD, Gerth WA: Is the risk of DCS in micrograv-ity less than on Earth? Aviat Space Environ Med 68:621, Abstract 45, 1997.
12. Mitragotri S, Edwards D, Blankschtein D, Langer R:
A mechanistic study of ultrasonically-enhanced transdermal drug delivery. J Pharmacol Sci 84:697–706, 1995.
13. Lambertsen CJ, Idicula J, Cunningham J, Cowley JRM: Pathophysiology of Superficial Isobaric Counterdiffusion. Philadelphia, Institute for Environmental Medicine, 1980.
14. Lambertsen CJ, Idicula J: A new gas lesion in man, induced by “isobaric gas counterdiffusion.” J Appl Physiol 39:434–443, 1975.
15. Hills BA: Supersaturation by counterperfusion and diffusion of gases. J Appl Physiol 42:758–760, 1977.
16. Bennett PB: Discussion. In: Nashimoto I, Lanphier EH (eds): What is Bends? 43rd ed. Shimizu, Japan, Undersea and Hyperbaric Medicine Society, 1991, pp 107–109.
17. Rashbass C: The aetiology of itching on decom-pression. Report No. 7/57. Royal Naval Physiological Laboratory, 1957.
18. Lanphier EH: Discussion. In: Nashimoto I, Lanphier EH (eds): What is Bends? 43rd ed. Shimizu, Japan, Undersea and Hyperbaric Medicine Society, 1991, pp 107–109.
19. Farmer JC: Otological and paranasal sinus prob-lems in diving. In: Bennett PB, Elliott DH (eds): The Physiology and Medicine of Diving, 4th ed. London, WB Saunders, 1993, pp 267–300.
20. Doolette D, Mitchell S: A biophysical basis for inner ear decompression sickness. J Appl Physiol 94:2145–2150, 2003.
21. Behnke AR: The Harry G. Armstrong Lecture.
Decompression sickness: Advances and interpreta-tions. Aerosp Med 42:255–67, 1971.
22. Ferris E, Engle G: The clinical nature of high altitude decompression sickness. In: Fulton JF (ed): Decompression Sickness. Philadelphia, WB Saunders, 1951, pp 4–52.
23. Thomas S, Williams OL: High altitude joint pains:
Their radiographic aspects. Report No. 395. U.S.
NRC, C.A.M., 1944.
24. Thomas SF, Williams OL: High altitude joint pains (bends): Their roentgenographic aspects.
Radiology 44:259–261, 1945.
25. Webb JP, Ferris EB, Engel GL, et al: Radiographic studies of the knee during bends. Report No. 305.
U.S. NRC, C.A.M., 1944.
26. Gernhardt ML: Development and Evaluation of a decompression stress index based on tissue bubble dynamics. Ph.D. Dissertation, University of Pennsylvania, 1991.
27. Gerth WA, Vann RD: Probabilistic gas and bubble dynamics models of decompression sickness occurrence in air and nitrogen-oxygen diving.
Undersea Hyperbar Med 24:275–292, 1997.
28. Bason R, Yacavone D: Decompression sickness: U.S.
Navy altitude chamber experience 1 October 1981 to 30 September 1988. Aviat Space Environ Med 62:1180–1184, 1991.
29. Rush WL, Wirjosemito SA: The risk of developing decompression sickness during air travel following altitude chamber flight. Aviat Space Environ Med 61:1028–1031, 1990.
30. Weien RW, Baumgartner N: Altitude decompression sickness: Hyperbaric therapy results in 528 cases.
Aviat Space Environ Med 61:833–836, 1990.
31. Rudge FW, Shafer MR: The effect of delay on treat-ment outcome in altitude-induced decompression sickness. Aviat Space Environ Med 62:687–690, 1991.
32. Lam TH, Yau KP: Manifestations and treatment of 793 cases of decompression sickness in a com-pressed air tunneling project in Hong Kong.
Undersea Biomed Res 15:377–388, 1988.
33. Hallenbeck JM, Andersen JC: Pathogenesis of the decompression disorders. In: Bennett PB, Elliott DH (eds): The Physiology and Medicine of Diving, 3rd ed. London, Bailliere Tindall, 1982, pp 435–460.
34. Helps SC, Parsons DW, Reilly PL, Gorman DF: The effect of gas emboli on rabbit cerebral blood flow.
Stroke 21:94–99, 1990.
35. Helps SC, Meyer-Witting M, Reilly PL, Gorman DF:
Increasing doses of intracarotid air and cerebral blood flow in rabbits. Stroke 21:1340–1345, 1990.
36. Helps SC, Gorman DF: Air embolism of the brain in rabbits pretreated with mechlorethamine. Stroke 22:351–354, 1991.
37. Martin JD, Thom SR: Vascular leukocyte sequestra-tion in decompression sickness and prophylactic hyperbaric oxygen therapy in rats. Aviat Space Environ Med 73:565–569, 2002.
38. Francis TJ, Pezeshkpour GH, Dutka AJ: Arterial gas embolism as a pathophysiologic mechanism for spinal cord decompression sickness. Undersea Biomed Res 16:439–451, 1989.
39. Francis TJR, Smith DJ (eds): Describing Decompression Illness. Bethesda, Md., Undersea and Hyperbaric Medical Society, 1991.
40. Hills BA, Butler BD: Size distribution of intravascu-lar air emboli produced by decompression.
Undersea Biomed Res 8:163–170, 1981.
41. Butler BD, Hills BA: The lung as a filter for microbubbles. J Appl Physiol 47:537–543, 1979.
42. Vik A, Jenssen BM, Brubakk AO: Paradoxical air embolism in pigs with a patent foramen ovale.
Undersea Biomed Res 19:361–374, 1992.
43. Vik A, Jenssen BM, Brubakk AO: Arterial gas bubbles after decompression in pigs with patent foramen ovale. Undersea Hyperbar Med 20:121–131, 1993.
44. Butler BD, Katz J: Vascular pressures and passage of gas emboli through the pulmonary circulation.
Undersea Biomed Res 15:203–209, 1988.
45. Optison: Human albumin microspheres. Injectable suspension octafluoropropane formulation. St.
Louis, Mallinkrodt, 1998.
46. Butler BD, Luehr S, Katz J: Venous gas embolism:
Time course of residual pulmonary intravascular bubbles. Undersea Biomed Res 16:21–29, 1989.
47. Vann R, Fawcett T, Currie M, et al: No-stop repeti-tive N2/O2 diving with surface interval O2 (SIO2):
Phase I. Undersea Biomed Res 19(Suppl):125, 1992.
48. Vann R, Dick A, Barry P: Doppler bubble measure-ments and decompression sickness. Undersea Biomed Res 9(Suppl):24, 1982.
49. Vann R: MK XV UBA decompression trials at Duke:
A summary report. Final report on ONR Contract N00014-77-C-0406.Office of Naval Research, Washington, D.C. 1982
50. Vann RD: Decompression sickness and venous gas emboli in helium and nitrogen diving. Undersea Biomed Res 30(Suppl):Abstract, 2003.
51. Thalmann ED: Refinement of 0.7 atm oxygen in nitrogen decompression tables. Undersea Biomed Res 8(Suppl):Abstract 8, 1981.
52. Thalmann ED: Phase II testing of decompression algorithms for use in the U.S. Navy underwater decompression computer. Report No. 1–84. Navy Experimental Diving Unit, 1984.
53. Thalmann ED: Air-N202 decompression computer algorithm development. Report No. 8–85. Navy Experimental Diving Unit, 1985.
54. Thalmann ED: Development of a decompression algorithm for constant 0.7 ATA oxygen partial pres-sure in helium diving. Report No. 1–85. Navy Experimental Diving Unit, 1986.
55. Behnke AR, Willmon TL: Gaseous and helium elimi-nation from the body during rest and exercise. Am J Physiol 131:619–626, 1941.
56. Momsen CB: Report on use of helium-oxygen mix-tures for diving. Report No. 2–42, AD728758. Navy Experimental Diving Unit, 1942.
57. Dunford R, Vann R, Gerth W, et al: The incidence of venous gas emboli in recreational diving. Undersea Hyperbar Med 27(Suppl):179, 2000.
58. Dunford R, Vann R, Gerth W, et al: The incidence of venous gas emboli in recreational divers. Undersea Hyperbar Med 2003, in press.
59. Gault KA, Tikuisis P, Nishi RY: Calibration of a bubble evolution model to observed bubble incidence in divers. Undersea Hyperbar Med 22:249–262, 1995.
60. Batson O: The function of the vertebral veins and their role in the spread of metastases. Ann Surg 112:138–149, 1940.
61. Haymaker W, Johnston A: Pathology of decompres-sion sickness. Milit Med 117:285–306, 1955.
62. Haymaker W: Decompression sickness. In:
Handbuch des speziellen pathologischen anatomie und histologie. Berlin, Springer Verlag, 1957, pp 1600–1672.
63. Lehner CE, Lanphier EH: Influence of pressure profile on DCS symptoms. In: Vann RD (ed): The Physiological Basis of Decompression, 38th ed.
Bethesda, Md., Undersea and Hyperbaric Medical Society, 1989, pp 299–326.
64. Waligora JM, Horrigan DJ, Conkin J: The effect of extended O2 pre-breathing on altitude decompres-sion sickness and venous gas bubbles. Aviat Space Environ Med 58:A10–A112, 1987.
65. Medical Research Council: Decompression sick-ness and aseptic necrosis of bone. Br J Industr Med 28:1–21, 1971.
66. Berghage TE: Decompression sickness during satu-ration dives. Undersea Biomed Res 3:387–398, 1976.
67. Smith KH, Stayton L: Hyperbaric decompression by means of bubble detection. Report No. NOOO1469-C-0402. Office of Naval Research, 1978.
68. Neuman TS, Hall DA, Linaweaver PG: Gas phase separation during decompression in man:
Ultrasound monitoring. Undersea Biomed Res 3:121–130, 1976.
69. Pilmanis AA: Ascent and silent bubbles. In: Lang MA, Egstrom GH (eds): Biomechanics of Safe Ascents Workshop. Cost Mesa, Cal., American Academy of Underwater Sciences, 1990, pp 65–71.
70. Uguccioni DM, Vann RD, Smith LR, et al: Effect of safety stops on venous gas emboli after no-stop diving. Undersea Hyperbar Med 22(Suppl):A53, 1995.
71. Rubenstein CJ: Role of decompression conditioning in the incidence of decompression sickness in deep diving. Report No. 12–68. Navy Experimental Diving Unit, 1968.
72. Eckenhoff RG, Hughes JS: Acclimatization to decom-pression stress. In: Bachrach AJ, Matzen MM (eds):
Underwater Physiology VIII, 8th ed. Bethesda, Md., Undersea Medical Society, 1984, pp 93–100.
73. Hempleman HV: Decompression procedures for deep, open sea operations. In: Lambertsen, CJ (ed):
Underwater Physiology III. Baltimore, Williams &
Wilkins, 1967, pp 255–266.
74. Elliott DH: Decompression—a hazard of underwa-ter sports. J R Coll Gen Pract 18:233–237, 1969.
75. Thalmann ED, Zumrick JL, Schwartz HJC, Butler FK Jr: Accommodation to decompression sickness in HeO2 divers. Undersea Biomed Res 11(Suppl):
Abstract 6, 1984.
76. Vann RD: Exercise and circulation in the formation and growth of bubbles. In: Brubakk A, Hemmingsen BB, Sundnes G (eds): Supersaturation and Bubble Formation in Fluids and Organisms. Trondheim, Norway, Royal Norwegian Society, 1989, pp 235–258.
77. Harvey EN, McElroy WD, Whiteley AH, et al: Bubble formation in animals. III: An analysis of gas tension and hydrostatic pressure in cats. J Cell Comp Physiol 24:117–132, 1944.
78. Whitaker DM, Blinks LR, Berg WE, et al: Muscular activity and bubble formation in animals decom-pressed to simulated altitudes. J Gen Physiol 28:213–223, 1945.
79. McDonough PM, Hemmingsen EV: Swimming move-ments initiate bubble formation in fish decom-pressed from elevated gas pressures. Comp Biochem Physiol 81A:209–212, 1985.
80. Nishi RY, Eatock BC, Buckingham IP, Ridgewell BA:
Assessment of decompression profiles by ultra-sonic monitoring. Phase III: No-decompression diving. Report No. 82-R-38. Defense and Civil Institute of Environmental Medicine, 1982.
81. Hughes JS, Eckenhoff RG: Spinal cord decompres-sion sickness after standard U.S. Navy air decom-pression. Milit Med 151:166–168, 1986.
82. Piwinski SE, Mitchell RA, Goforth GA, et al: A blitz of bends: decompression sickness in four students after hypobaric chamber training. Aviat Space Environ Med 57:600–602, 1986.
83. Vann R, Gerth W: Six days of weight training did not increase susceptibility to mild altitude decompres-sion sickness. Undersea Hyperbar Med 20(Suppl):
106, 1993.
84. Broome JR, Dutka AJ, McNamee GA: Exercise con-ditioning reduces the risk of neurologic decom-pression illness in swine. Undersea Hyperbar Med 22:73–85, 1995.
85. Wisloff U, Brubakk AO: Aerobic endurance training reduces bubble formation and increases survival in rats exposed to hyperbaric pressure. J Physiol 537(Pt 2):607–611, 2001.
86. Rattner B, Gruenau S, Altland P: Cross-adaptive effects of cold, hypoxia, or physical training on decompression sickness in mice. J Appl Physiol 47:412–417, 1979.
87. Carturan D, Boussuges A, Burnet H, et al:
Circulating venous bubbles in recreational diving:
relationships with age, weight, maximal oxygen uptake and body fat percentage. Int J Sports Med 20:410–414, 1999.
88. Balke B: Rate of gaseous nitrogen elimination during rest and work in relation to the occurrence
of decompression sickness at high altitude. Project No. 21-1201-0014, Report No. 6. U.S. Air Force School of Aviation Medicine, 1954.
89. Vann RD, Gerth WA: Factors affecting tissue perfu-sion and the efficacy of astronaut denitrogenation for extravehicular activity. Final Report on NASA Contract NAG 9-134/4. Durham, N.C., F.G. Hall Hypo/Hyperbaric Center, Duke Medical Center, 1995.
90. Webb JT, Fischer MD, Heaps CL, Pilmanis AA:
Exercise-enhanced preoxygenation increases pro-tection from decompression sickness. Aviat Space Environ Med 67:618–624, 1996.
91. Waligora JM, Horrigan DJ, Conklin J, Hadley AT III:
Verification of an altitude decompression sickness prevention protocol for shuttle operations utilizing a 10.2 psi pressure stage. NASA JSC Report of Jan. 30, 1984.
92. Sowden LM, Kindwall EP, Francis TJ: The distribu-tion of limb pain in decompression sickness. Aviat Space Environ Med 67:74–80, 1996.
93. Nicogossian AE, Parker JF Jr: Space Physiology and Medicine: National Aeronautics and Space Administration. Report No. SP-447. NASA, 1982, pp 141–152.
94. Levy MN, Talbot JM: Cardiovascular decondition-ing of space flight. The Physiologist 26:297–303, 1983.
95. Gernhardt M, Conkin J, Foster P, et al: Design and testing of a two hour oxygen pre-breathe protocol for space walks from the International Space Station. Undersea Hyperbar Med 27(Suppl):
Abstract 11, 2000.
96. Butler B, Vann R, Nishi R, et al: Human trials of a two hour pre-breathe protocol for extravehicular activity. Aviat Space Environ Med 71:278, Abstract 44, 2000.
97. Balldin UI, Pilmanis AA, Webb JT: The effect of sim-ulated weightlessness on hypobaric decompres-sion sickness. Aviat Space Environ Med 73:773–778,
97. Balldin UI, Pilmanis AA, Webb JT: The effect of sim-ulated weightlessness on hypobaric decompres-sion sickness. Aviat Space Environ Med 73:773–778,