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Supersaturation, Bubble Growth and

Decompression Sickness

All body tissues contain dissolved oxygen, carbon dioxide and nitrogen. In a person who is equilibrated with air at sea level, the dissolved nitrogen partial pressure is about 79 percent of the one atmosphere of pressure, or 0.79 atmosphere (atm). If we fly in a commercial airliner at a cabin altitude of 8,000 feet (the maximum cruising cabin altitude according to Federal Aviation Administration regulations), the barometric pressure decreases from 1 atm at sea level to 0.74 atm.

Under these conditions, a state of supersaturation is said to exist in our tissues where the nitrogen tissue tension (0.79 atm) exceeds the barometric pressure (0.74 atm) by 0.05 atm (0.79– 0.74). Most of the nitrogen from supersaturated tissues is carried to the lungs in the circulation (that’s good), but some may diffuse into small bubbles known as gas nuclei (that’s potentially bad). These bubbles don’t grow large enough to cause DCS below a threshold altitude of about 16,000-18,000 feet. If a person was suddenly taken from sea level to a pressure equivalent of 30,000 feet altitude (0.29 atm, the pressure in an astronaut’s space suit), the super-saturation would be much larger, 0.50 atm (0.79–0.29), and mas-sive bubble growth would occur. Under these conditions DCS would be almost inevitable. Astronauts avoid this problem by breathing 100 percent oxygen before space walks (extravehicular activity, or EVA) to wash out much of their tissue nitrogen and reduce the supersaturation.