Safe Inner Ear Gas Tensions for Switch from Helium to Air Breathing During Decompression

Abstract : Inner ear decompression sickness (DCS) has followed deep breathing gas switches to air (or other nitrogen-oxygen mixtures) during decompression from deep heliox dives. This study investigates the safety of heliox-to-air breathing gas switches at moderate depths. Divers immersed in 80 F (27 C) water were compressed to 150, 170, or 190 fsw (561, 622, 683 kPa) breathing 79% helium-21% oxygen or to 220 fsw (775 kPa) breathing 84% helium-16% oxygen for a 60-minute bottom time- a time estimated to allow 99% equilibration of inner ear gas tensions with breathing gas. Divers were then decompressed at 30 fsw/min to 70 fsw (316 kPa) from 150 fsw, or to 100 fsw (408kPa) from the other depths. At 70 or 100 fsw, divers exited the water and breathed chamber air for 60 minutes. Following this air stop, divers were decompressed breathing 50% nitrogen-50% oxygen and 100% oxygen. Profiles were tested in order of increasing maximum depth. Each profile was accepted if 25 man-dives were free from inner ear or other central nervous system DCS during the air stop- a design chosen to detect greater than 0.2% probability of central nervous system DCS with approximately 3% significance and 96% power. There was no diagnosed DCS during or following any of the 104 man-dives completed. On the 220 fsw dive profile, one diver complained of fullness in the ear at the air stop that resolved during subsequent decompression and one diver complained of fleeting, mild shoulder pain (niggles) during decompression from 100 fsw. Equilibration of the inner ear with 651 kPa inspired helium followed by no-stop decompression to 100 fsw and switch to air breathing appears to have a low risk of inner ear DCS.