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Decompression Sickness

Decompression Sickness


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.


According to the Divers Alert Network (DAN), 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 gas embolism (AGE). A study of the United States military community in Okinawa, where tens of thousands of sport and military dives are made each year, identified 84 DCS and 10 AGE cases in 198995, including nine deaths. This translated into estimates of one case in every 7,400 dives and one death in every 76,900 dives. DCS symptoms can be quite mild, however, and many cases certainly go unnoticed by divers.

At times the terminology adopted by writers on DCS can be confusing. Some substitute the term decompression illness (DCI) for DCS. Others treat DCI as a label encompassing both DCS and AGE. An older term for DCS is caisson disease, coined in the nineteenth century when it was discovered that bridge construction crews working at the bottom of lakes and rivers in large pressurized enclosures (caissons) were experiencing joint pain (a typical DCS symptom) 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 he or she would at sea level. Instead of being exhaled, however, the extra nitrogen safely dissolves into the tissues, where it remains until the diver begins his or her return to the surface (under some circumstances the extra nitrogen can cause nitrogen narcosis, but that condition is distinct from DCS). On the way up, decompression occurs (in other words, the water pressure drops), and with the change 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, however, 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 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 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; these pauses are called decompression stops, and are carefully calibrated. DCS 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 as well because their bodies undergo further decompression. This is true even when flying in commercial aircraft. Many travelers are unaware that to save money on fuel 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 setting for everyone but recent divers. 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 and other 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, including the blood, bones, nerves, and muscles, many kinds of symptoms are possible. Symptoms can appear minutes after a diver surfaces, and in about 80% of cases do so within eight hours. Pain is often the only symptom; this is sometimes called the bends, although many people incorrectly use that term as a synonym for DCS itself. The pain, which ranges from mild to severe, is usually limited to the joints, but can be felt anywhere. Severe itching (pruritis), skin rashes, and skin mottling (cutis marmorata) 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 1 and type 2 symptoms.


Diagnosis requires taking a medical history (questioning the patient about his or her health and recent activities) and conducting a physical examination.


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 and allows the nitrogen to safely diffuse out of the tissues. Hyperbaric chamber facilities exist throughout 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. DAN maintains a list of facilities and a 24-hour hotline that can provide advice on handling DCS and other diving emergencies.


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


Gas embolism The presence of a gas bubble in the bloodstream that obstructs circulation.

Hyperbaric chamber A sealed compartment in which air pressure is gradually increased and then gradually decreased, allowing nitrogen bubbles to shrink and the nitrogen to safely diffuse out of body tissue.

Lymphatic vessels Vessels that carry a fluid called lymph from the tissues to the bloodstream.

Nitrogen narcosis Also called "rapture of the deep," the condition is caused by increased nitrogen pressure at depth and is characterized by symptoms similar to alcohol intoxication.


The obvious way to minimize the risk of falling victim to 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.



American College of Hyperbaric Medicine. PO Box 25914-130, Houston, Texas 77265. (713) 528-0657.

Divers Alert Network. The Peter B. Bennett Center, 6 West Colony Place, Durham, NC 27705. (800) 446-2671.

Undersea and Hyperbaric Medical Society. 10531 Metropolitan Ave., Kensington, MD 20895. (301) 942-2980.

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decompression sickness

decompression sickness may be defined as the illness associated with the formation, following a reduction in ambient pressure, of bubbles from gases dissolved in the body.

The clinical syndrome of decompression sickness was recognized in divers and compressed air workers in the middle of the nineteenth century, but the first description of the similar condition induced by exposure to altitude was not made until 1930, and altitude decompression sickness was not widely acknowledged until the late 1930s.

Bubble formation

Man's tissues at sea level have dissolved gas tensions in equilibrium with the atmosphere. At a raised environmental pressure, there is further uptake of gases from the lungs into solution by the blood and tissues until equilibrium is achieved with the new raised partial pressures of the respiratory gases — quickly by some tissues, and more slowly by others such as cartilage. On decompression these dissolved gases are excreted through the lungs until a new equilibrium is achieved at the new pressure. When the rate of decompression exceeds that at which the excess gases dissolved in the tissues can be excreted, gas may come out of solution and form bubbles.

In diving, therefore, the longer the exposure, and the higher the pressure, the more slowly a diver must return to the surface, to avoid decompression illness, which includes also the consequences of barotrauma — damage by expansion of gases in body cavities.

Those combinations of depth and duration of dive that allow the diver a direct return to atmospheric pressure without incurring the overt manifestations of decompression illness form the ‘no-stop curve’. Very approximately, he may surface directly after unlimited time at slightly less than 10 m, 60 min at 18 m, and 20 min at 30 m; at the extreme, experience from submarine escape suggests a safe duration of only about 30 sec at around 180 m. Increasing depths and durations beyond this fuzzy curve represent an increasing risk of decompression illness and require the use of a slow decompression profile to minimize bubble formation and its consequences. For many years the presence of bubbles in the bloodstream was thought to be synonymous with acute decompression sickness, until bubbles were detected by the use of ultrasound in divers who developed no subsequent manifestations. Thus another aspect in the causation of decompression sickness is the response of the individual to the presence of bubbles and their effects.

The causes of decompression sickness, the dissolved-gas variety of decompression illness, include omission of the accepted decompression stops, the use of decompression tables or of a personal diving computer based on inappropriate mathematical models — and, perhaps most commonly, individual variation. Many factors affect individual susceptibility, such as dehydration, adaptation to decompression, in-water exercise, and water temperature.

Decompression illness can occur even in those who have followed all the rules meticulously. The causes of its most severe form, neurological decompression illness from gas embolism, are the same as those of pulmonary barotrauma, but this neurological condition can arise with no evidence of lung injury. Thus the diagnosis of the underlying cause of a case of decompression illness can be difficult but, as the treatment procedures are virtually the same, a general diagnosis of decompression illness is all that is needed in this emergency.

Decompression sickness can be a hazard also when ascending to altitude in an aircraft. Some of the nitrogen normally dissolved at ground level is excreted in the expired gas, but the rate at which it can be excreted through the lungs is slow relative to the rate at which the environmental pressure is reduced in flight; some tissues therefore eventually become supersaturated with nitrogen. Under certain circumstances this supersaturation gives rise to the formation of bubbles of gas within the tissues and blood. The bubbles are carried in the blood to the right side of the heart and thence to the lungs. Bubbles can be detected in the chambers of the right side of the heart after exposure for several hours to altitudes of the order of 15 000–18 000 feet, but symptoms of decompression sickness do not occur until the altitude exceeds 18 000 feet. The incidence and intensity of the symptoms increase with the altitude and the duration of the exposure. The incidence of severe symptoms necessitating immediate descent varies from 0.7% after 1 hour at 28 000 feet to 45% after 4 hours' exposure to 35 000 feet.

Symptoms and signs

The commonest symptom of decompression sickness is a ‘bend’, which is a deep seated pain in or near a limb joint, probably caused by bubbles in the ligaments around the joint. Itching, tingling, and ‘formication’ (sensation as of crawling insects) are also common symptoms due to bubbles forming in the skin. Much less common but more serious are respiratory disturbances, ‘the chokes’, comprising a tightness in the chest, an inspiratory ‘snatch,’ and coughing. These symptoms are believed to be due to bubbles being carried from the periphery to the lungs. Other serious symptoms of decompression sickness include visual disturbances and severe headaches, and sensory disturbances. Less frequently, weakness or paralysis of a limb or limbs, or unconsciousness, may occur. Decompression sickness can give rise to circulatory collapse. Very rarely, recovery does not occur and death supervenes.


For divers and compressed air workers, prevention is based upon adherence to a slow decompression procedure designed to permit the safe excretion of dissolved gases. The printed Decompression Tables available and the online personal decompression computers used by some divers are all derived from the concepts of Professor J. S. Haldane nearly a century ago. These predictions, although validated by the experience of large populations, do not always hold for every dive. Illness still occurs even when everything appears to have been done correctly.

For aircrew, there are three ways of avoiding or preventing decompression sickness at altitude. The most satisfactory way is to avoid exposure to altitudes at which it occurs. Provided that an individual has not been exposed to breathing air at pressures greater than one atmosphere in the 24 hours prior to the ascent, decompression sickness does not occur at altitudes below 18 000 feet. Since the susceptibility to decompression sickness varies considerably from one person to another it is possible to select for high-altitude flights those who are relatively resistant to the condition. This approach was widely used during World War II. The third method of prevention is to remove the nitrogen from the tissues of the body by breathing 100% oxygen prior to the ascent — a procedure known as pre-oxygenation or denitrogenation. Complete protection requires that 100% oxygen is breathed for at least 4 hours before ascent, but even 30 min pre-oxygenation is effective for ascents up to 45 000 feet provided that the duration of exposure at more than 18 000 feet does not exceed about 20 min.

John Ernsting, and David Elliott


Bennett, P. B. and and Elliott, D. H. (1993). The physiology and medicine of diving, (4th edn). Saunders, London.
Ernsting, J. and and King, P. (1988). Aviation medicine, (2nd edn). Butterworth-Heinemann, Oxford.

See also diving; gases in the body; hyperbaric chamber; nitrogen; oxygen.

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decompression sickness

decompression sickness, physiological disorder caused by a rapid decrease in atmospheric pressure, resulting in the release of nitrogen bubbles into the body tissues. It is also known as caisson disease, altitude sickness, and the bends. It is an occupational hazard of persons who work under greatly increased atmospheric pressure below the surface of the earth (e.g., divers and laborers who work under compressed air) when their return to normal atmospheric pressure is made too quickly. When the body is subjected to high atmospheric pressure the respiratory gases are compressed and larger amounts are dissolved in the body tissues. During ascent from depths greater than 30 ft (9.1 m), these gases escape as the external pressure decreases. Airplane pilots who go rapidly from normal atmospheric pressure to high altitudes (low atmospheric pressure) in unpressurized aircraft or in aircraft with faulty pressurizing apparatus also encounter the disorder. The decrease in air pressure releases body nitrogen in the form of gas bubbles that block the small veins and arteries and collect in the tissues, cutting off the oxygen supply and causing nausea, vomiting, dizziness, pain in the joints and abdomen, paralysis, and other neurological symptoms. In severe cases there may be shock, total collapse, and, if treatment is not prompt, death. Persons who work under increased atmospheric pressure must make the ascent to normal atmospheric pressure gradually, often through pressurized chambers, a procedure that allows the nitrogen to be released slowly from the blood and expired from the lungs. Inhalation of pure oxygen aids in clearing nitrogen from the body. Those who suffer symptoms of decompression sickness at high altitudes (commonly called aeroembolism) experience relief on returning to an atmospheric pressure normal to them; this and oxygen inhalation will usually effect recovery.

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Decompression Sickness



Decompression sickness (DCS) is a condition that occurs when divers come back to the surface too quickly after being deep under water. It is caused by the formation of nitrogen bubbles in the blood stream and, in the worst cases, can cause death.


Decompression sickness is a relatively uncommon disorder among divers. Divers Alert Network (DAN), a worldwide organization devoted to safe-diving research and promotion, estimates that less than 1 percent of all divers experience the condition. A study conducted on Okinawa (an island in the Pacific Ocean near Japan) of military personnel who make tens of thousands of dives each year, found an average of one case of decompression sickness for every 7,400 divers and one death for every 76,900 dives. Mild cases may even go unnoticed by divers.

Decompression Sickness: Words to Know

Decompression stops:
Stops divers should make when returning to the surface to let the nitrogen in their blood dissolve safely out of their bodies. Charts developed by the U.S. Navy and other groups list the number of stops and the time to be spent at each stop.
Hyperbaric chamber:
A sealed compartment used to treat decompression sickness, in which pressure is first increased and then gradually decreased.
A tasteless, odorless gas that makes up four-fifths of Earth's atmosphere.

Decompression sickness is also known by other names, such as decompression illness and caisson (pronounced KAY-son) disease. DCS was called caisson disease in the nineteenth century because it occurred among construction workers who worked in caissons, building the supports for bridges at the bottom of lakes and rivers.


Air is primarily a mixture of two gases, oxygen and nitrogen. The oxygen we breathe in is used by cells in our bodies to metabolize (burn up) food. The nitrogen has no function in our bodies. Most of it is expelled from the body when we breathe out. Some of it is absorbed into our body tissues and our bloodstream.

As a person goes underwater, pressure increases on his or her body. The deeper one goes, the greater the pressure. This increased pressure forces more and more air into the body. All gases become more soluble (dissolve better) as pressure increases. Increased pressure causes no problem with oxygen, which is used up by the body. But it can cause problems with nitrogen. Deep under water, where the pressure is high, much more nitrogen dissolves in blood than it does at the surface.

Decompression sickness occurs when a person returns to the surface after being deep under water. When that happens, pressure on the person's body decreases. Nitrogen begins to come back out of the blood. If the person comes to the surface slowly, there is no problem. The nitrogen gas can escape from the blood slowly and be exhaled.

If the person comes up too quickly, however, a problem can develop. Nitrogen begins to form tiny bubbles as it escapes from the blood. The bubbles cannot be exhaled through the lungs. Instead, they can block blood vessels, push on nerves, and cause other disturbances in the body.

The amount of risk for DCS depends on the depth to which a person has gone under water. The deeper the dive, the greater the risk. To avoid decompression problems, divers may need to stop one or more times on their way back to the surface. Charts developed by the U.S. Navy and other groups list the number of stops and the time to be spent at each stop. When these directions are followed, nitrogen has time to escape from the bloodstream normally. Bubbles do not form, and DCS does not occur.

A number of factors can increase the risk of DCS for divers. For example, people who are overweight or who have recently had alcohol to drink are at greater risk for DCS. Also, people planning to fly or travel to high-altitude locations after diving are at increased risk for DCS because they experience further decompression at higher altitudes. Experts recommend that individuals wait between twelve to twenty-four hours after a dive before flying or traveling to high altitudes.


Nitrogen bubbles can affect any of the body's tissues, including nerves, bone, blood, and muscles. For that reason, DCS can cause a wide variety of symptoms. These symptoms usually appear almost as soon as the diver surfaces. In 80 percent of all cases, they do so within eight hours of the dive.

The most common symptom of DCS is pain, often referred to by the term "bends." This pain can range from mild to severe. It usually affects the joints, but can occur anywhere. Skin problems, such as itching and rashes, are other symptoms. In more serious cases, DCS can result in paralysis, brain damage, heart attacks, and death.


Decompression sickness is usually first diagnosed by observing the diver's symptoms. If further care is necessary, a doctor may conduct a physical examination and take a medical history.


Treatment of DCS involves reversing the conditions under which it first occurred. A person is placed into a hyperbaric (high pressure) chamber. Pressure is increased in the chamber, causing nitrogen gas bubbles to go back

into the bloodstream. The pressure in the chamber is then reduced slowly. Nitrogen gas escapes from the blood again, but not in the form of bubbles.

This treatment should be used even when a person's symptoms seem to have disappeared. Nitrogen bubbles may still be present in the blood, and symptoms may reappear at a later time.

Hyperbaric chambers are now available in many locations. DAN maintains a list of such facilities and provides a twenty-four-hour hotline that provides advice on DCS and other diving emergencies.


DCS patients who receive treatment in a hyperbaric chamber usually experience a full recovery quickly. If treatment is delayed, the prognosis is less predictable. People who have been treated even a few days later, however, may still receive benefits. A 1992 report from DAN said that half of all divers who were treated immediately after an incident experienced full recovery. Others, however, experienced numbness, tingling, and other symptoms for a few weeks, months, or even a lifetime.


Decompression sickness is an entirely preventable condition. Divers who follow rules of safe diving are unlikely to experience its effects. People who are overweight, have lung or heart problems, or are otherwise in poor health should not dive. Good divers should be aware of the need for decompression stops after a dive and follow standards developed for these stops.



Martin, Lawrence. Scuba Diving Explained: Questions and Answers on Physiology and Medical Aspects of Scuba Diving. Flagstaff, AZ: Best Publishing, 1997. [Online] (accessed May 22, 1998).


Clenney, Timothy L., and Lorenz F. Lassen, "Recreational Scuba Diving Injuries." American Family Physician (April 1996): pp. 1761+.


American College of Hyperbaric Medicine. P.O. Box 25914130, Houston, TX 77265. (713) 5285931.

Divers Alert Network (DAN). The Peter B. Bennett Center, 6 West Colony Place, Durham, NC 27705. (919) 6848111; (919) 6844326 (diving emergencies); (919) 6842948 (general information).

Undersea and Hyperbaric Medical Society. 10531 Metropolitan Avenue, Kensington, MD 20895. (301) 9422980.

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decompression sickness

decompression sickness n. see compressed air illness.

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decompression sickness

decompression sickness See bends

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