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accumulation of fluid in the interstitial space increases the distance across which oxygen must diffuse to reach the cells and thus further compromises performance ability.

Acute Mountain Sickness (AMS)

Acute mountain sickness (AMS) has been recognized for centuries, and occurs in most individuals to some degree at altitudes of 11,500 ft (3,505 m) or more. About 25 percent of tourists develop signs of AMS at elevations of only 8,900 to 10,000 ft (2,712 to 3,048 m), the severity and incidence depending upon the rate of ascent (Hackett et al., 1989). A severe bilateral headache is the primary symptom, but AMS also produces extreme fatigue, shortness of breath, and sleep disturbances. The headaches are accompanied by sharp increases in cerebral blood flow velocities, as measured by Doppler techniques. Cerebral symptoms of AMS may be secondary to an initiation of brain edema (Hackett et al., 1989).

Intestinal disturbances, with vomiting, are aggravated by expanding intestinal gas, and gas trapped in nasal sinuses can be particularly painful. AMS severely reduces appetite. The ability to carry out various functions is markedly impaired. AMS is usually self-limiting within 3 to 7 days of exposure.

Strangely, some individuals show no symptoms of AMS, whereas those who suffer AMS once are likely to experience it every time they go to high altitudes. Freedom from AMS seems to be linked to the ability to diurese quickly, and diuretics have been used as therapy. The drugs most widely used to treat AMS are acetazolamide (Diamox), a carbonic anhydrase inhibitor, and adrenocorticoids (especially Decadron) (Hackett et al., 1989). Acetazolamide stimulates breathing and prevents nocturnal periodic respiration; it also stimulates renal bicarbonate excretion to balance the respiratory alkalosis caused by hyperventilation. Acetazolamide is effective at 125 mg given up to twice daily, beginning with the onset of symptoms (Hackett et al., 1989). Adrenocorticoid therapy should be reserved for those seriously ill with AMS due to its failure to promote acclimatization.

Subacute Mountain Sickness

A new syndrome, termed subacute mountain sickness, was observed in healthy young soldiers who had spent several months at extremely high altitudes of approximately 22,000 ft (6,706 m) (Anand et al., 1990). As described by Anand and Chandrashekhar (see Chapter 18 in this volume), the syndrome appeared to be one of severe systemic and congestive heart failure without pulmonary hypertension. About 20 percent of the group developed



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