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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.

The incidence of decompression sickness (DCS) in space appears to be less than that predicted to occur on the basis of ground based altitude chamber trials. Our current work uses six hours of chair rest adynamia and likewise produces fewer gas bubbles when compared to standing in a cross over study. Mild exercise during oxygen prebreathe is also very efficacious in reducing DCS and bubble formation (measured by Doppler ultrasound). The effect is postulated to be the result of the alteration in the populations of tissue micronuclei such that the radii are reduced. Surface tension then becomes too great for bubble growth from the existing inert gas partial pressures.