decompression sickness

decompression sickness is what divers may be subject to if they rise too rapidly to the surface at the end of a dive, and is popularly known as ‘the bends’. To avoid it all divers have to follow a procedure as they rise to the surface when they finish diving, and even amateur divers breathing air need to be aware of what is involved.

Air is a mixture of oxygen and nitrogen, which pass from the lungs into the bloodstream and on to the body tissues. The oxygen is used in the body's metabolic processes, while the nitrogen remains dormant. However, as divers breathe air at ambient pressure—that is, the pressure of the depth of water surrounding them—nitrogen enters the body tissues and increases as they go deeper. The amount that has entered the tissues prior to the final ascent depends on the diver's blood supply—poorly supplied fatty tissue takes up nitrogen much more slowly than well-supplied major body organs—the maximum depth of the dive, and the time spent underwater.

The rate of ascent at the end of any dive must therefore be controlled, so that all the nitrogen in the various body tissues can move back into the bloodstream and be safely eliminated from the body. Should the ascent be too fast then nitrogen bubbles, much like those appearing in a bottle of soda water when the top is suddenly removed, may appear in any part of the body. These lead to a variety of symptoms, including rashes, itches, pain in the joints, and paralysis, which may take up to twelve hours or more to develop after the diver has surfaced. In extreme cases bubbles may form in the bloodstream, reach the heart, and cause death in a short time.

All decompression sickness symptoms should be treated as potentially dangerous and the victim removed to the nearest recompression facility as quickly as possible. Here, under medical supervision, they will be put into a recompression chamber where the pressure is increased to reduce the size of the nitrogen bubbles and relieve the symptoms. They will then be therapeutically decompressed which, hopefully, avoids any longer term effects.

At the beginning of the 20th century Professor Haldane developed stage decompression tables to avoid decompression sickness. These give a ‘bottom time’ for various depths: the deeper the dive, the shorter the bottom time. Dives made inside this time allow the diver to rise directly to the surface, but still following a maximum rate of ascent specified in the tables. If however, the diver exceeds the bottom time then, following the tables, he must rise to a specified safe depth where he makes a stop for a specified time. This allows the nitrogen level in his body to drop to a level where it is safe for him to ascend to a shallower depth where he again makes a stop, and so on in stages until he can safely surface. The stops become longer as the diver gets shallower, and even after surfacing he still has a residual nitrogen level in his body that takes around 24 hours to clear completely. Should he make a repeat dive during this period, this nitrogen must be taken into account. While repeat dive tables exist, it is now more common to use a wrist-mounted ‘personal’ decompression computer (DC). This remembers, and accounts for, nitrogen levels, and indicates the length of decompression that must be performed, and when a diver can move on to a shallower stop or to the surface. Like the original tables, they still specify a maximum rate of ascent which must be adhered to.

Decompression sickness is best avoided by learning to dive on a recognized diving course run by qualified instructors who will fully explain decompression procedures.

Peter Dick