I'm not the good doctor, heck I'm not even a doctor. Nonetheless, here's a short explanation of the decompression issues facing spacefarers that I found on an MIT server (
click here for the full article with photos and graphs):
Pressurization Issues
For a given shuttle mission, the requirements for cabin pressure and suit pressure are in conflict. A low suit pressure is optimum, as it allows for flexibility and mobility of the suit during EVA. A relatively higher pressure is desirable in the cabin, however, because pressures significantly below one atmosphere combined with increasing oxygen levels poses a threat of fire.
While this difference in pressure may solve some problems, it raises other physiological issues. Decompression sickness is a major concern when extra-vehicular activity is performed. When an astronaut enters the relatively low pressure environment of the EMU, a station of nitrogen supersaturation in the tissue occurs because of the lower ambient pressure. As a result, nitrogen bubbles are formed in the tissue and released into the blood stream. The bubbles tend to gather in joints, which has earned the symptoms the title of "the bends." The symptoms tend to occur either during or after an EVA, and in severe cases can be fatal.
To counter decompression sickness, the crewmembers, prior to EVA, must pre-breathe 100% oxygen. As shown by the figure, this is accomplished by wearing a mask while still inside the orbiter. Through this process, the nitrogen content in the astronauts' bodies is reduced to the point that the risk of decompression sickness is minimized. Pre-breathing time is a function of cabin pressure, as shown by the chart to the left. At a nominal shuttle cabin pressure of 101 kPa and suit pressure of 21 kPa, the pre-breathe time would be over 6 hours. This, however, is an unproductive use of the crewmembers' time, so other steps are taken.devote the necessary time to pre-breathing, however, so several steps are taken to minimize the pre-breathe.
The time required for pre-breathe can be related to the ratio of tissue nitrogen pressure to suit pressure, known as the R-factor. The chart above is for R=1.4, which is used by the United States space program. The Russians use an R-factor of 1.7. With this in mind, it should be noted that lower R factors have lower risk of decompression sickness. While higher pressure suits can reduce the pre-breathe time, American EVA's begin by lowering the shuttle cabin pressure to 70.3 kPa for several hours, as shown in the chart on the right. As can be seen, this allows all astronauts to continue normal activities in a shirt-sleeve environment. Approximately 40 minutes before beginning the EVA at the lower suit pressure, the astronauts breathing 100% oxygen in the suit.
Hope this helps answer your question.
