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Millions of people annually visit mountainous areas of the western U.S. at altitudes of >2440 m (>8000 ft). In addition, tens of thousands travel to high-altitude regions in other parts of the world. Adventure travel to mountainous regions is booming.1 Physicians working or traveling in or near these locations are likely to encounter high-altitude illness or preexisting conditions that are exacerbated by altitude. Although the focus of this chapter is hypoxia-related problems, patients in the mountain environment also may require care for associated illnesses such as hypothermia, frostbite, trauma, ultraviolet keratitis, dehydration, and lightning injury.

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High altitude [>2440 m (>8000 ft)] is a hypoxic environment. Because the concentration of oxygen in the troposphere remains constant at 21%, the partial pressure of oxygen (Po2) decreases as a function of the barometric pressure. In Denver at 1610 m (5280 ft), air pressure is 17% less than at sea level, and therefore the air contains 17% less oxygen. The air of Aspen, Colorado, 2440 m (8000 ft), has 26% less oxygen, and the barometric pressure on top of Mount Everest is only one third that of sea level. Oxygen supplementation prevents symptoms of altitude illness during hypobaric exposure, and therefore hypoxia, not hypobaria per se, is responsible for illness.

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Altitude may be divided into stages of ascension according to physiologic effects. Intermediate altitude, 1520 to 2440 m (5000 to 8000 ft), produces decreased exercise performance and increased alveolar ventilation without major impairment in arterial oxygen transport. Acute mountain sickness (AMS) occurs at and above 2130 to 2440 m (7000 to 8000 ft) and sometimes at lower altitudes in particularly susceptible individuals. Patients who have hypoxic cardiovascular and pulmonary diseases such as chronic obstructive pulmonary disease (COPD) or congestive heart failure (CHF) at low altitude may become more symptomatic in this range of altitudes. High altitude, 2440 to 4270 m (8000 to 14,000 ft), is associated with decreased arterial oxygen saturation (Sao2), and marked hypoxemia may occur during exercise and sleep. Most cases of altitude-related medical problems occur in this elevation range, because of the availability of overnight tourist facilities located at these heights. Very high altitude, 4270 to 5490 m (14,000 to 18,000 ft) is uncommon in the U.S. but is encountered by visitors to the mountainous regions of South America and the Himalayas. Abrupt ascent can be dangerous, and a period of acclimatization is required to prevent illness. Extreme altitude, >5490 m (>18,000 ft) is experienced only by mountain climbers and is accompanied by severe hypoxemia and hypocapnia. At this height, progressive physiologic deterioration eventually outstrips acclimatization, and sustained human habitation is impossible. Because hypoxemia is maximal during sleep, the sleeping altitude is the critical altitude to consider.

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Acutely hypoxic individuals become dizzy, faint, and rapidly unconscious if hypoxic stress is sufficient (Sao2 <65%). Captain Hawthorne Gray, in an attempt to set the record for highest hot air balloon flight in 1927, lost consciousness and died when his balloon rose to >12,200 m (>40,000 ft). However, individuals given days to weeks to acclimatize can tolerate surprising ...

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