Decompression sickness (DCS) results when the body is exposed to rapid and significant decreases in atmospheric pressure. DCS is called "the bends" among scuba divers, who can be affected by the sickness if they resurface, or decompress, too quickly. Humans are affected by the illness when nitrogen, normally dissolved in the blood and body tissues, comes out of solution and forms bubbles as a result of the rapid decrease in pressure. This phenomenon can be compared to opening a can of pop; the dissolved carbon dioxide instantly forms bubbles in reaction to the sudden decrease in pressure caused by opening the can. Tiny nitrogen bubbles in the bloodstream and tissues can lead to a variety of symptoms ranging from numbness and tingling to joint pain or even death.
The presence of nitrogen bubbles in the blood can be monitored using Doppler ultrasound on the heart.
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Construction of the International Space Station (ISS) requires over 160 assembly and maintenance extravehicular activities (EVAs), also known as spacewalks. The space shuttle and the ISS are both pressurized to 14.7 psi, which is equivalent to the atmospheric pressure at sea level. When doing a spacewalk, astronauts wear either the American EVA Mobility Unit (EMU), which is pressurized to 4.3 psi (the same atmospheric pressure as at the summit of Mount Everest), or the Russian Orlan suit (about 5.6 psi). Rapid decompression from normal cabin pressure to a lower atmospheric pressure will result in DCS if appropriate countermeasures are not taken.
Breathing 100% pure oxygen before a sudden exposure to low atmospheric pressure decreases the risk of developing DCS by eliminating nitrogen from the tissues. A 12-hour decompression protocol using 100% oxygen prebreathe has successfully protected astronauts from DCS during EVAs from the space shuttle. Astronauts breathe 100% oxygen for one to two hours at normal cabin pressure, and then reduce the cabin pressure to 10.2 psi for 12 hours. Finally, after donning the EMU, astronauts breathe 100% oxygen for another hour before performing the EVA. For EVAs performed from the ISS rather than the space shuttle, the existing 12-hour protocol is being replaced with a more efficient yet equally effective method to prevent DCS.