Travel Reference
In-Depth Information
Thebarometric pressure onthesummit ofEverest (29,035feet or8850m),asmeasured
directly in 1981, is 253 mm Hg or one-third sea level atmospheric pressure. That pressure
is 17 mm Hg higher than had been predicted, apparently due to the greater thickness of
the atmosphere near the equator. Mount Everest is located at 27° 59' N latitude, about the
same latitude as Tampa, Florida. The pressure may vary by the equivalent of 100 to 300
feet(30to90m)duetoweather-relatedchangesinatmosphericpressure,whichmeansthat
on days when the barometric pressure is high, the summit is physiologically a few hundred
feetlower.Inthewinter,thebarometricpressureislowerandthesummitisphysiologically
several hundred feet higher. On the summit of Everest, during the spring and fall climbing
seasons, climbers have an arterial oxygen pressure of about 28 to 32 mm Hg, approxim-
ately one-third of that at sea level. The arterial carbon dioxide pressure is about 10 to 13
mm Hg (normal at sea level is 35 to 45 mm Hg for adult males).
ALTITUDE ILLNESS
Most people can avoid getting serious altitude illness. A few simple measures prevent it in
most healthy individuals, and prudent individuals should experience no more than minor,
temporary discomfort. Some persons with specific conditions are extremely susceptible to
altitude illness and need to take special precautions to go safely to altitude (
Chapter 25:
Acute Mountain Sickness
Acute mountain sickness (AMS), the most common disorder that afflicts those who go
toohightoofast,resultsfromtheeffectsofloweredatmosphericoxygenonthebrain.Brain
cells are probably damaged only by extreme hypoxia, and changes in the circulation of the
brain and the accumulation of fluid (edema) are thought to be responsible for the symp-
toms of AMS. Low oxygen pressure dilates cerebral blood vessels, which occurs despite
the constricting effect of low carbon dioxide levels resulting from increased ventilation.
The symptoms of mild or moderate AMS are quite realistically described as those of a
bad hangover: headache, nausea, sometimes vomiting. The severity of symptoms depends
upon the rate of ascent, the altitude reached, and individual susceptibility. Symptoms usu-
allystarttwelve totwentyfourhoursafterarrival andbegintosubsidebythethirdday.The
headache is throbbing, tends to be at the back of the head, and is worse on awakening in
the morning. Dizziness, fatigue, loss of appetite, disturbed sleep, and general malaise are
common. The individual feels miserable.
One normal response to ascent is a temporary increase in urine output, but in AMS the
output is usually decreased and the person retains fluid. Broadly speaking, those people
who have the least increase in ventilation and the lowest oxygen levels retain the most wa-
ter and are the sickest. One attractive but oversimplified explanation for the symptoms of
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