Peter H. Hackett
Mark B. Rabold
Acclimatizationto. High. Altitude Ventilation
Effects, on. Exercise
Sleepat, High, .Altitude
Acute.. Moyn.tain...Sickness (AMS) Neurologic. . Syndromes. . of . High. . .Altitude HighAltitude. Cerebral. Edema Cereb.royascM!ar..S,yndromes ..o,f.,Alt,it,yde HighAltitude. Pulmonary. .Edema Peripheral. .Edema
High Altitude Retinopathy
HighAltitude. Pharyngitis.. and .Bronchitis
Ultraviolet .Keratitis., .(Snow., Blindness)
Illnesses., Aggravated. by, High, .Altitude Chro,nic...Lung..Dis,eas,e
Arteriosclerotic, ..Heart, .Disease..(ASHP)
Sickle. Cell ..Disease
Many millions of visitors annually visit the mountainous areas of the western United States at altitudes over 8000 feet (2440 meters). 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 increasingly likely to encounter persons ill with a high altitude illness or suffering an untoward effect of altitude on a preexisting condition. 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, which are covered elsewhere in this text.
High altitude is a hypoxic environment. Because the concentration of oxygen in the troposphere remains constant at 21 percent, the partial pressure of oxygen decreases as a function of the barometric pressure. In Denver (1610 m), air pressure is 17 percent less than at sea level and therefore contains 17 percent less oxygen. The air of Aspen, Colorado (2438 m) has 26 percent less oxygen, and the barometric pressure on top of Mt. Everest is merely one-third that of sea level. Paul Bert, in his classic experiments of the late nineteenth century, showed that supplemental oxygen prevented symptoms of altitude illness during hypobaric exposure and concluded that hypoxia, not hypobaria, was responsible for illness.
For the purposes of discussion, altitude may be divided into stages according to physiologic effects. Intermediate altitude, 5000 to 8000 feet (1500 to 2440 meters), produces decreased exercise performance and increased alveolar ventilation, without major impairment in arterial oxygen transport. Medical sickness due to altitude is rare in this range, although mild illness may develop in individuals with decreased cardiopulmonary reserve. High altitude, 8000 to 14,000 feet (2440 to 4270 meters), is associated with a decrease in arterial oxygen saturation, and hypoxemia may occur during exercise and sleep. Most cases of medical problems associated with altitude occur in this range because of the availability of overnight tourist facilities located at these heights. Very high altitude, 14,000 to 18,000 feet (4260 to 5490 meters), is difficult to find 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, over 18,000 feet (5490 meters), is available to only mountain climbers and accompanied by severe hypoxemia and hypocapnia. At this height, progressive physiologic deterioration eventually outstrips acclimatization and sustained human habitation is impossible.
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