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.
Prevention
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
Bibliography
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.
