Decompression Sickness Health Article

Definition

Decompression sickness (DCS) is a dangerous and occasionally lethal condition caused by nitrogen bubbles that form in the blood and other tissues of scuba divers who surface too quickly. It also occurs in the blood of tunnelers or miners who work in conditions of increased pressure and return to normal atmospheric pressure too quickly.

Description

According to the Divers Alert Network (DAN), a worldwide organization devoted to safe-diving research and promotion, fewer than 1% of divers fall victim to DCS or the rarer bubble problem called gas embolism, air embolism, or arterial gas embolism (AGE). 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 cases of decompression sickness and 10 instances of arterial gas embolism between 1989 and 1995. These numbers included 9 deaths. Translated, this provides an estimated incidence of one case of decompression sickness in every 7,400 dives and one death in every 76,900 dives. Since symptoms of decompression sickness can be quite mild, many cases go unnoticed by divers; thus this estimate is probably understated.

Different terminology is used in discussing decompression sickness and it can be confusing. Sometimes the term illness is used instead of sickness. Others treat decompression illness as a term that encompasses both decompression sickness and arterial gas embolism. An older term for decompression sickness is caisson disease. This term was coined in the nineteenth century, when bridge construction crews working at the bottom of lakes and rivers in large pressurized enclosures (caissons) were found to experience joint pain (a typical symptom of decompression sickness on returning to the surface).

Causes and symptoms

The air we breathe is mostly a mixture of two gases, nitrogen (78%) and oxygen (21%). Unlike oxygen, nitrogen is a biologically inert gas, meaning that it is not metabolized (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 diver takes in more nitrogen with each breath than would occur at sea level. But instead of being exhaled, the extra nitrogen safely dissolves into the tissues, where it remains until the diver begins the return to the surface. Under some circumstances, the extra dissolved nitrogen can cause nitrogen narcosis, although this condition is different from decompression sickness. On the way up, decompression occurs; in other words, the water pressure drops. With the decrease in pressure, the extra nitrogen gradually diffuses out of the tissues and is delivered by the bloodstream to the lungs, which expel it from the body. If the diver surfaces too quickly, potentially dangerous nitrogen bubbles can form in the tissues and cause decompression sickness. These bubbles can compress nerves, obstruct arteries, veins, and lymphatic vessels, and trigger harmful chemical reactions in the blood. The precise reasons for bubble formation remain unclear.

How much extra nitrogen enters the tissues varies with the dive's depth and the duration of the dive. 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 out of tissues and into the bloodstream without forming bubbles. These pauses are called decompression stops and are carefully calibrated. Decompression sickness can occur, however, even when a diver obeys safe-diving rules. In such cases, the predisposing factors include fatigue, obesity, dehydration, hypothermia, and recent alcohol use. People who fly or travel to high-altitude locations without 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, since the cabin pressure in commercial aircraft is set much lower than the pressure at sea level. At 30,000 ft (9,144 m), for instance, cabin pressure is usually equivalent to the pressure at 7,000–8,000 ft (2,133–2,438 m) above sea level, a safe level for everyone except people who have recently been diving. Exactly how long a diver should wait before flying or traveling to a high-altitude location depends on how much time has been spent diving. If there is uncertainty about the appropriate waiting period, the sensible course of action is to let a full 24 hours pass.

Since the nitrogen bubbles that cause decompression sickness can affect all body tissues, many different symptoms are possible. Symptoms can appear minutes after a diver returns to the surface. The appearance of symptoms occurs within eight hours in about 80% of cases. Pain is often the only symptom. This is sometimes called "the bends," although many people incorrectly use that term as a synonym for decompression sickness. Pain, which ranges from mild to severe, is usually limited to the joints, but can be felt anywhere. Severe itching (pruritus), skin rashes, and skin mottling (cutis marmorata) are other relatively common symptoms. All of these are sometimes classified as manifestations of Type 1 or "mild" decompression sickness. Type 2 or "serious" decompression sickness can result in paralysis, brain damage, heart attack, and death. Many persons with decompression sickness experience both Type 1 and Type 2 symptoms.