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Altitude physiology typically focuses on people above 2500 m; ∼8000 ft. Altitudes above that are sometimes subdivided into very high (3500–5500 m; ∼11,500–18,000 ft) and extreme (>5500 m; >18,000 ft). An estimated 40 million people travel each year to altitudes >2500 m (∼8000 ft),1 and as many or more travel to altitude for leisure and sports, and work in mines, military or border operations, and the like. Altitude medicine considers the clinical disorders associated with acclimatization by the travelers, workers and migrants, and with adaptation by people with lifetimes or populations with millennia of residence (an estimated 83 million people).2
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With a hurried ascent, many (∼80%) will report a transient headache (high-altitude headache or [HAH]), and some will develop one of three forms of acute high-altitude illness: acute mountain sickness (AMS) and HAH, high-altitude cerebral edema (HACE), and high-altitude pulmonary edema (HAPE).3,4 AMS (see Table 92-1) and HAH are annoying and interfere with activity and work, however, HACE and HAPE can be fatal with mortality rates approaching 30%.5–7 Among some residents, chronic mountain sickness (CMS) and right ventricular hypertrophy develop over months to years of residence at altitude. Birth weights are generally lower and the rate of small-for-gestational-age babies and congenital heart defects are higher than that in lowland populations.4
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Hypoxemia (FIO2 equivalent to ∼17% O2 at 2500 m, down to ∼8% O2 at the summit of Everest) causes the physiologic responses and illnesses. Altitude-related exposure to cold and extreme exercise may also contribute to illness. Other environmental features may include UV radiation, trauma, and infection that are not covered in this chapter. Finally, ...