The Physiologic Basis of High-Altitude Diseases

Clinical Principles

Three major high-altitude diseases

• Acute mountain sickness (headache, lightheadedness, fatigue, insomnia, anorexia)

• High-altitude pulmonary edema (dyspnea, reduced exercise tolerance, cough, tachycardia, crepitations)

• High-altitude cerebral edema (confusion, ataxia, mood changes, coma, papilledema)

Other high-altitude conditions

• Chronic mountain sickness (severe polycythemia, headache, somnolence, fatigue, depression)

• Subacute mountain sickness (affects infants and adults; right-heart failure with peripheral edema)

• Retinal hemorrhage (common at extreme altitude but usually causes no visual impairment)

Physiologic Principles

Hypoxia of high altitude impairs physical performance, mental performance, and sleep.

In acclimatization, hyperventilation is the most important feature. Acclimatization reduces but does not abolish the effects of hypoxia.

Extreme altitude causes severe hypoxemia, respiratory alkalosis, and greatly reduced maximal oxygen consumption.

The mechanisms of acute mountain sickness and high-altitude cerebral edema are not fully understood, but brain swelling may be a feature. Acetazolamide reduces the incidence of acute mountain sickness.

The mechanism of high-altitude pulmonary edema is probably uneven hypoxic pulmonary vasoconstriction that exposes some capillaries to a high pressure, damaging their walls and leading to a high-permeability form of edema.