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This ground-based study compared the risk of microbubbles during decompression under simulated space extravehicular activities (EVA) after three days of six-degree head-down bed rest with three days of ambulatory control. Test subjects were exposed to a pressure of 44.8 kPa (6.5 psi), breathed 100% oxygen, and exercised at reduced pressure either in the supine (during experimental) or upright (control) position. Circulating microbubbles were monitored by a precordial Doppler ultrasound device, and were found in 52% (12/23) of control and 26% (6/23) of experimental exposures. Survival analysis using Cox proportional hazards regression showed that there was 0.22 times (95% confidence interval=0.07-0.68) reduction in the risk of high grade microbubbles after bed rest, compared to controls (p=0.004). This finding is of importance in evaluating the risk of DCS during EVA.

To date, 59 man-EVA's have been conducted in the Shuttle Program with minimum physiological problems or limitations. The physiological requirements for life support in the Shuttle EVA include pressure, gas composition, inspired CO2 pressure, heat- removal capability, in-suit water replacement, and caloric replacement. These requirements and their basis in verification testing or analysis are reviewed. The operational measures are identified. The suit pressure in combination with a gas composition of at least 92 percent assures that sufficient O2 pressure is available to the crewmember. The nominal suit pressure of 4.3 psi±0.1 psi was maintained during all 59 man-EVA's. The contingency suit pressure was never required to be used. The suit pressure in combination with the cabin pressure and pre-EVA denitrogenation procedures minimize the risk of altitude decompression sickness. There has been no incidence of decompression sickness during Shuttle EVA.

Extravehicular Activity (EVA) is an important part of the U.S. space program. The argument that EVA's are unnecessary or too dangerous has been refuted by the successes of contingency and planned EVA's. The recent successes in satellite maintenance and retrieval have demonstrated EVA's to be useful, practical, and safe. During the Space Station Program, crewmembers will be expected to perform more frequent EVA's. As in the past, the physiological factors must be integrated with operational and engineering considerations to achieve a safe, effective system. In past programs, we have been concerned with factors including metabolic work rates and extensive prebreathing methods to rid the body of bends-inducing nitrogen. In the Space Station Program, we are presented with frequent and varied EVA tasks which require that the hardware withstand repeated use, and that physiological limits not be exceeded.