DISORDERS CAUSED BY ALTITUDE - Medicine for Mountaineering and Other Wilderness Activities - page 339

Very soon blood carbon dioxide rises so high—and oxygen falls so low—that breathing
starts up again with ever-increasing depth until the blood carbon dioxide level decreases
and the blood oxygen level increases to more normal levels. At that point respirations tail
offagain.Periodicbreathingoftenbeginsasapersonreachesamoderateelevation,persists
or even becomes more marked with acclimatization, and may be an important cause of de-
terioration during prolonged stays at extreme altitude.
At 14,000 feet (4200 m), where resting oxygen saturation is about 86 percent in people
who are awake, periodic breathing, which is almost universal, may cause saturation to fall
as low 60 percent.
Acetazolamide almost completely eliminates the wide swings caused by periodic
breathing. At 14,000 feet (4200 m), for example, acetazolamide limits the lower level of
saturationduringsleeptoabout82percent.Sedativesandtranquilizersmakesleephypoxia
worse and should be avoided at high altitudes. On the other hand, a small dose of acetazol-
amide (62.5-125 mg taken by mouth at dinner time) is a simple and safe way to improve
sleep despite the increased urination it may cause.
Sleep hypoxia may account, in part, for the inability of many individuals to sleep well
at high altitude. It may also explain why headache and other symptoms of acute moun-
tain sickness are more severe in the morning hours and why both high-altitude pulmonary
edema (HAPE) and high-altitude cerebral edema (HACE) often become worse during the
night. Part of the beneficial effect acetazolamide has for acute mountain sickness probably
results from decreased sleep hypoxia.
Sleep hypoxia may cause disturbed sleep, resulting in sleep deprivation and a decrease
in physical working capacity during the day. This provides one physiologic explanation
for the wisdom behind the mountaineer's dictum “Climb high, sleep low.” Climbers have
noted better physical performance when low-flow oxygen has been used during sleep be-
cause this decreases the fall in arterial oxygen saturation. Chronic lack of sleep on a moun-
tain, like hypoxia, interferes with intellectual function, particularly at extreme altitude, and
increases the likelihood of mistakes.
Acclimatization
The evolution of short-term changes to longterm adjustments constitutes acclimatiza-
tion. Survival and effective functioning at 18,000 feet (5500 m), and the ability of some
personstoworkwithoutsupplementaloxygenashighas29,000feet(8800m),aredepend-
ent upon the ability to adjust or acclimatize to oxygen lack. Taken abruptly from sea level
to the summit of Everest, an unacclimatized person would have only five to ten minutes of
decreasingconsciousnessbeforelapsingintocomaanddyinginaboutthirtyminutes.Birds
can fly higher for much longer periods, but no mammals, including man, live permanently
above 17,500 feet (5300 m), suggesting that this is the upper limit to which they can accli-
matize.