DISORDERS CAUSED BY ALTITUDE - Medicine for Mountaineering and Other Wilderness Activities - page 333

CHAPTER 24
DISORDERS CAUSED BY ALTITUDE
Charles S. Houston, M.D.
Ken Zafren, M.D.
Colin K. Grissom, M.D.
Principal Contributors
For centuries travelers returning from high mountains have reported unpleasant symptoms,
even fatalities, and have ascribed them to poisonous shrubs, emanations from ores—even
the breath of dragons. Only one hundred years ago was the real cause shown to be lack of
oxygen, which had been isolated a century earlier.
Although 21 percent of air is oxygen, no matter what pressure it is under, the weight
of the overlying atmosphere—atmospheric or barometric pressure—decreases as altitude
aboveEarth'ssurfaceincreases.Whenapersongoeshigher,fewermoleculesofoxygen—or
any of the other gases that comprise air—are available in the atmosphere. At 18,000 feet
(5500 m) the atmospheric pressure and the pressure of oxygen in the air are approximately
half those at sea level. On top of Mount Everest (29,035 feet or 8850 m) atmospheric pres-
sure and the amount of oxygen available are one-third of those at sea level. Because the
atmosphere is flattened at the poles by the centrifugal effect of Earth's rotation, the atmo-
sphere is thinner and the atmospheric pressure is lower nearer the poles, which makes the
physiologic altitude higher. For example, on Denali (Mount McKinley) in Alaska, baromet-
ric pressure equal to half that at sea level occurs at about 16,000 feet (4900 m), rather than
at 18,000 feet (5500 m) as it does in more equatorial mountain ranges.
Lack of oxygen, or hypoxia, is the primary cause of problems at high altitude known as
mountain or altitude sickness. The manner in which the body takes in oxygen, circulates it
throughout the body, and gets rid of carbon dioxide is described in Chapter 18: Respiratory
Disorders .
The rate of ascent is the most important determinant of whether an individual develops
mountain sickness. The faster the ascent, the more likely is illness to develop. (Going up
veryfastinanunpressurizedaircraft,aballoon,oradecompressionchamberproducesacute
hypoxia, which causes problems quite different from mountain sickness.)
Significantdifferencesbetweenindividualsexist,andascheduleofascentthatsuitsmost
membersofagroupmaybetoofastforsome.Onlysomeofthesedifferencesareknown,but
among them are recently recognized genetic variations. These differences are inherent and
have nothing to do with an individual's physical condition, determination, or courage. The
effects of hypoxia are also influenced by factors such as cold (hypothermia,) low blood sug-