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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 ...