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Decompression sickness, DCS, the bends or caisson disease is an injury caused by bubbles of gas forming in the tissues of the body after breathing gas at high pressure.

DCS is a diving injury that affects SCUBA divers. Risk of DCS by diving long or deep without slowly ascending and making the decompression stops needed to eliminate the inert gases normally, although the specific risk factors are not well understood. Some divers seem more susceptible than others under identical conditions.

The bends is a slang term describing a specific symptom of decompression sickness: when a bubble forms in a joint, typically the elbow, pain is experienced from that joint. The pain may be reduced by bending the joint to find a more comfortable position. There are other terms describing other symptoms, such as, the "chokes", the "niggles" and the "staggers".

Air embolism and DCS have very similar symptoms and treatment because they are both the result of gas bubbles in the body. In a diving context, the two are often called decompression illness.

People flying in unpressurised aircraft at high altitude, such as stowaways in unpressurised parts of the aircraft or passengers after failure of the cabin pressure vessel, can suffer from decompression sickness.

Cause

Two linked factors contribute to DCS, although the complete relationship of causes is not fully understood:

1. deep or long dives : inert gases in breathing gases, such as nitrogen and helium, are adsorbed into the tissues of the body in high concentrations (Henry's Law) when breathed at high pressure.
2. fast ascents : reducing the ambient pressure, as happens during the ascent, causes the adsorbed gases to come back out of solution, and form "micro bubbles" in the blood. Those bubbles will safely leave the body through the lungs if the ascent is slow enough that the volume of bubbles does not rise too high.

The physiologist J.S. Haldane studied this problem in the early 20th century, eventually devising the method of staged, gradual decompression, whereby the pressure on the diver is released slowly enough that the nitrogen comes gradually out of solution without leaing to DCS. Bubbles form after every dive: slow ascent and decompression stops simply reduce the volume and number of the bubbles to a level at which there in no injury to the diver.

Repeated cases of decompression sickness can lead to the death of cells in long-bones and brittle bones. Severe cases can lead to death because large bubbles can impede the flow of oxygen-rich blood to the brain, central nervous system and other vital organs.

Avoiding decompression sickness

Decompression tables and dive computers have been developed that help the diver choose the depth and duration of decompression stops for a particular dive profile at depth.

Avoiding decompression sickness is not an exact science. Accidents can occur after relatively shallow and short dives. To reduce the risks, divers should avoid long and deep dives and should ascend slowly. Also, dives requiring decompression stops and dives with less than a 16 hour interval since the previous dive increase the risk of DCS. There are many additional risk factors, such as, age, obesity, fatigue, use of alcohol, dehydration and Patent foramen ovale.

History

An alternative name is caisson disease; this name comes from the 19th century, when large engineering excavations (bridges, tunnels) required the work to be done in caissons under pressure to keep water from flooding the excavations. This was a major factor during construction of Eads Bridge, when 13 workers died from what was then a mysterious illness, and later during construction of the Brooklyn Bridge, where it incapacitated the project leader Washington Roebling.

Helium

Nitrogen is not the only breathing gas that causes DCS. Gas mixtures such as trimix and heliox include Helium, which also causes decompression sickness.

If a whole dive were carried out on a helium-rich breathing gas, the decompression stops required would be much deeper and much longer than those of an air dive. Helium decompression can be accelerated by breathing Nitrox or pure oxygen, if in very shallow water, during the decompression phase of the dive.

Symptoms

Symptoms of DCS range from skin rashes, extreme fatigue, joint pain, visual disturbances, balance disturbances, breathing difficulties, lack of strength, numbness, paralysis, unconsciousness and death.

Treatment

Recompression is the only lasting treatment of DCS. Normally this is carried out in a recompression chamber. A high risk alternative is in-water recompression.

Oxygen first aid treatment is useful for suspected DCS casualties or divers who have made fast ascents or missed decompression stops. Most fully closed-circuit rebreathers can deliver sustained, high concentrations of oxygen- rich breathing gas and could be used as an alternative to pure, open cicuit oxygen.