Decompression sickness

Decompression sickness (DCS) is a dangerous and occasionally lethal conditioncaused by nitrogen bubbles that form in the blood and other tissues of scubadivers who surface too quickly.

According to the Divers Alert Network, a worldwide organization devoted to safe-diving research and promotion, less than 1% of divers fall victim to DCS or the rarer bubble problem called gas embolism, air embolism, or arterial gasembolism. A study of the U.S. military community in Okinawa, where tens of thousands of sport and military dives are made each year, identified 84 DCS and 10 arterial gas embolism cases in 1989-95, including 9 deaths. This translated into one case in every 7,400 dives and one death in every 76,900 dives. But DCS symptoms can be quite mild, and many cases go unnoticed.

The air we breathe is mostly a mixture of two gases, nitrogen (78%) and oxygen (21%). Unlike oxygen, nitrogen is not converted into other substances by the body. For this reason, most of the nitrogen we inhale is expelled when we exhale, but some is dissolved into the blood and other tissues. During a dive,however, the lungs take in more nitrogen than usual. This happens because the surrounding water pressure is greater than the air pressure at sea level (twice as great at 33 ft [10 m], for instance). As the water pressure increases, so does the pressure of the nitrogen in the compressed air inhaled by the diver. Because increased pressure causes an increase in gas density, the divertakes in more nitrogen with each breath than at sea level. But instead of being exhaled, the extra nitrogen safely dissolves into the tissues, where it remains until the diver begins returning to the surface (under some circumstances the extra nitrogen can cause nitrogen narcosis, but that condition is distinct from DCS). On the way up, the water pressure drops, and with this decompression the extra nitrogen gradually diffuses out of the tissues and is delivered by the bloodstream to the lungs, which expel it from the body. But if the diver surfaces too quickly, potentially dangerous nitrogen bubbles can form in the tissues and cause DCS. These bubbles can compress nerves, obstruct arteries, veins, and lymphatic vessels, and trigger harmful chemical reactionsin the blood. The precise reasons for bubble formation remain unclear.

How much extra nitrogen enters the tissues varies with the dive's depth and duration. Dive tables prepared by the U.S. Navy and other organizations specify how long most divers can safely remain at a particular depth. If the dive table limits are exceeded, the diver must pause on the way up to allow the nitrogen to diffuse into the bloodstream without forming bubbles. DCS can occur,however, even when a diver obeys safe-diving rules. These cases may involveas contributing factors, fatigue, obesity, dehydration, hypothermia, and recent alcohol use. As well, people who fly or travel to high-altitude locationswithout letting 12-24 hours pass after their last dive are at risk for DCS, because their bodies undergo further decompression. This is true even when flying in commercial aircraft. Many travelers are unaware that to save money thecabin pressure in commercial aircraft is set much lower than the pressure atsea level. Exactly how long a diver should wait before flying or traveling to a high-altitude location depends on how much diving he or she has done andother considerations. If there is uncertainty about the appropriate waiting period, the sensible course of action is to let the full 24 hours pass.

Because the nitrogen bubbles that cause DCS can affect any of the body's tissues, a wide range of symptoms is possible. Symptoms can appear minutes aftera diver surfaces, and in about 80% of cases do so within eight hours. Pain is often the only symptom. It ranges from mild to severe, and is usuallylimited to the joints but can be felt anywhere. Severe itching, skinrashes, and skin mottling are other possible symptoms. All of these are sometimes classified as manifestations of Type 1 or "mild" DCS. Type 2 or "serious" DCS can lead, among other things, to paralysis, brain damage, heart attacks, and death. Many DCS victims, however, experience both Type 1and Type 2 symptoms.

DCS is treated by giving the patient oxygen and placing him or her in a hyperbaric chamber, an enclosure in which the air pressure is first gradually increased and then gradually decreased. This shrinks the bubbles, allowing nitrogen to safely diffuse out of the tissues. Hyperbaric chamber facilities existthroughout the United States. No matter how mild one's symptoms may appear, immediate transportation to a facility is essential. Treatment is necessary even if the symptoms clear up before the facility is reached, because bubbles may still be in the bloodstream and pose a threat. The Divers Alert Network maintains a list of facilities and a 24-hour hotline that can provide advice onhandling DCS and other diving emergencies.

DCS sufferers who undergo chamber treatment within a few hours of first experiencing symptoms usually enjoy a full recovery. If treatment is delayed the consequences are less predictable, although many people have been helped evenafter several days have passed. A 1992 report on diving accidents indicated that full recovery following chamber treatment was immediate for about 50% ofdivers. Some people, however, suffer numbness, tingling, or other symptoms that last weeks, months, or even a lifetime. In the Okinawa study, 6 of the 94patients experienced "long-lasting" symptoms even after repeated chamber treatments.

The obvious way to minimize the risk of DCS is to follow the rules on safe diving and air travel after a dive. People who are obese, suffer from lung or heart problems, or are otherwise in poor health should not dive. And because the effect of nitrogen diffusion on the fetus remains unknown, diving while pregnant is not recommended.

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