DISORDERS CAUSED BY ALTITUDE - Medicine for Mountaineering and Other Wilderness Activities - page 336

As individuals ascend to higher elevations, the body makes adjustments to sustain its
supply of oxygen. These alterations begin almost immediately but evolve over days into
changes that comprise the process of acclimatization.
Short-Term Changes
Increased Breathing
An early and important response to lack of oxygen is an increase in both the rate and
depthofbreathing.Thisnaturalandlogicalresponsebringsairdeeperintothelung,flushes
out the carbon dioxide and the oxygen-depleted air in the alveoli (air sacs of the lung), and
bringsthealveolaroxygenpressureclosertothatoftheoutsideatmosphere.Personswhose
breathing response to low inhaled oxygen (hypoxic ventilatory response) is brisk appear to
be less susceptible to mountain sicknesses. Those with a blunted response, which includes
some world-class distance runners, may be slightly more susceptible or may simply take
longer to adjust. However, increased depth and rate of breathing is the first and most im-
portant response to oxygen deficiency.
At sea level the work of breathing requires only about 5 percent of the oxygen used by
the body. At higher altitude the respiratory muscles must work harder and require a larger
share of the inhaled oxygen. Near the summit of Everest, climbers who are not using sup-
plemental oxygen are estimated to be consuming approximately two-thirds of their inhaled
oxygen just in the work of respiration.
Atveryhighaltitudes the lungsandlack ofoxygen,rather than the heart, fail tokeep up
with demand and limit the amount of work that can be done. The decreased ability to per-
formphysicalworkisproportionaltothealtitude.Therateofdeclineinmaximumexercise
capacityisapproximately3percentper1000feet(300m)ofelevationgain,butthedecline
is even faster at extreme altitude. Acclimatization improves the ability to work somewhat,
but even the most well-acclimatized persons cannot reach their sea-level work capacity.
Decreased Oxygen Saturation
At sea level the hemoglobin in arterial blood leaving the lungs carries almost its full ca-
pacity of oxygen and is 95 percent or more saturated. As altitude increases, the saturation
decreases proportionally. Resting arterial oxygen saturation at 15,000 feet (4500 m) is ap-
proximately 85 percent.
During exercise at sea level, arterial saturation remains normal, though it may fall
slightly with very strenuous effort, such as running a 440-yard (400-m) race. During ex-
ercise at high altitude, the lower alveolar oxygen pressure causes incomplete loading of
red blood cells with oxygen, and saturation falls dramatically ( Fig. 24-1 ) . The decrease is
proportional to the exercise level and altitude. Since the working muscles do not get as
much oxygen as they need, climbers take more frequent rests, allowing the saturation to