Millions of people annually visit mountainous areas of the western United States at altitudes of >2440 m (>8000 ft). In addition, tens of thousands travel to high-altitude regions in other parts of the world. 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 may require care for associated illnesses such as hypothermia (see Chapter 209, “Hypothermia”), frostbite (see Chapter 208, “Cold Injuries”), dehydration, and lightning injury (see Chapter 219, “Electrical and Lightning Injuries”).
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. Two locations in the U.S. western state of Colorado are illustrative. In Denver, at 1610 m (5280 ft), air pressure is 17% less than at sea level; Aspen, at 2440 m (8000 ft), has 26% less oxygen than sea level. At 5490 m (18,000 ft), there is half the oxygen available at sea level, whereas on top of Mount Everest (8848 m or 29,029 ft), there is only one third. Oxygen supplementation prevents symptoms of altitude illness during hypobaric exposure. Symptoms of acute mountain sickness (AMS) are due primarily to hypoxia, not hypobaria.
Altitude may be divided into stages 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. 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 limitations in ventilatory response (e.g., patients with some neuromuscular diseases or those with preexisting hypoxemia) may become more symptomatic in this range of altitude. High altitude, 2440 to 4270 m (8000 to 14,000 ft), is associated with decreased arterial oxygen saturation (SaO2); 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 (14000 to 18,000 ft), is uncommon in the United States, 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), experienced only by mountain climbers, 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, sleeping altitude is the critical driver of altitude illness.
PHYSIOLOGY AND PATHOPHYSIOLOGY OF ALTITUDE ACCLIMATIZATION