The effect of staged decompression while breathing 100% oxygen on attitudedecompression sickness

Introduction: Space Shuttle extravehicular activity (EVA) requires decompre ssion from sea level pressure (14.7 psia) to a 4.3 psia (30,300 ft) pressur e suit. The transition currently involves altering the shuttle atmosphere t o allow shirt-sleeve denitrogenation to occur during a 12 to 36-h staged de compression (SD) at 10.2 psia (9,800 ft) with an oxygen-enriched breathing gas (26.5% oxygen, 73.5% nitrogen). The denitrogenation provides protection from decompression sickness (DCS) during EVA in a 4.3 psia pressure suit. Our goal was to determine the highest altitude at which SD while breathing 100% oxygen (SD100) could provide effective protection from development of DCS symptoms after further decompression to 29,500 ft (4.5 psia). Methods: There were 30 male subjects exposed to at least 6 of 11 conditions in rando m order on successive months to 29,500 ft for 4 h while performing mild exe rcise and being monitored for venous gas emboli (VGE) with an echo-imaging system. The subjects received 15 min of ground-level (GL) preoxygenation an d an additional 60 or 120 min of SD100 at one of four altitudes between 8,0 00 ft (10.9 psia) and 18,000 ft (7.3 psia). Control exposures followed a 75 - or 135-min ground-level preoxygenation. Results: During SD100, one case o f DCS occurred at 18,000 Ft, but not at lower staging altitudes. Higher lev els of VGE were observed during SD100 at 18,000 ft than during SD100 at any lower altitude. Conclusion: Staged decompression at 16,000 ft and below re sults in decompression risk during subsequent decompression to 29,500 it si milar to that following equivalent periods of ground-level preoxygenation.