A comparison of the "high-altitude" and "high-pressure" syndromes of decompression sickness.

Decompression sickness is an illness which occurs in divers and caisson workers on return to normal atmospheric pressure after working at very high pressures, and in airmen on reaching very low pressures at great altitude. The disease seen after exposure to high pressure is described and compared with that seen on exposure to great altitude. Mild cases show little difference, but serious ones vary sufficiently to justify division of the disease into two distinct syndromes, high-pressure and high-altitude decompression sickness. Both syndromes are caused by the formation of gas bubbles in the blood and body fluids when the tension of dissolved gases becomes sufficiently greater than that of the surrounding atmosphere. These bubbles are composed of a mixture of gases, but their formation is determined mainly by the degree of supersaturation with nitrogen. The diver must first absorb excess nitrogen on exposure to pressure, and it is the subsequent release of this excess on return to the surface which causes decompression sickness; since his stay at pressure is limited, bubbles arise mainly from tissues which can absorb the excess gas readily. In the airman, on the other hand, all tissues are completely saturated with nitrogen before he is exposed to low pressure, and bubbles may arise equally well from any tissue. Nitrogen is much more soluble in fat than in water, so that fatty tissues can form a large reservoir of dissolved nitrogen which may later form a source of bubbles; but the blood supply of fat is poor and nitrogen diffuses slowly through it, hence although the potential capacity of the fatty tissues for excess nitrogen is great, they can only absorb it slowly. Obesity is therefore a major hazard in high-altitude decompression sickness, but relatively unimportant in the high-pressure syndrome. Liability to the airman's sickness increases markedly with age, but the incidence of the diver's syndrome is affected both much less and later in life. Decompression also involves great changes in the working atmosphere, and the effects of these and their possible influence on decompression sickness are discussed.