BASIC MEDICAL CARE - Medicine for Mountaineering and Other Wilderness Activities - page 12

ensure enough is consumed. These measurements and estimates must be written down so
the individual's fluid needs can be calculated.
Urinevolumeandcoloraregoodindicatorsofaperson'sfluidbalancebuttendtoreflect
what has already happened. Measuring losses and gains as they occur provides more im-
mediate insight into the condition of someone with a fluid-losing disorder.
Dehydration at Altitude
Higher altitudes tend to cause dehydration, and this tendency becomes progressively
greater as the elevation increases and the environment becomes colder. Almost all trekkers
or climbers are dehydrated above 18,000 feet (5500 m) Some investigators have suggested
that the depression, impaired judgment, and other psychological and intellectual changes
that commonly occur at high altitudes—and have been blamed on hypoxia—may actually
be the result of dehydration.
The principal cause of dehydration at high altitude is the increased fluid loss associated
withmorerapidanddeeperbreathingofcoldair.Airiswarmedtobodytemperature andis
saturated with water as it passes through the upper air passages; it has a relative humidity
of 100 percent when it reaches the lungs. Since cold air contains essentially no moisture
(at 5°F or −15°C the vapor pressure of water is 1.24 mm Hg), it requires more water for
humidification. Most of this moisture, and the heat expended to warm the air to body tem-
perature, is lost when the air is exhaled. (Some of the water may be regained during ex-
piration by condensation in cool upper air passages, but mouth breathing bypasses the air
passageswheremostcondensationoccurs.)Inaddition,lossofheatthroughevaporationof
water and through warming inhaled cold air is a significant contributor to hypothermia at
high altitudes.
If individuals are not careful about managing clothing to minimize sweating, particu-
larly with the bulky clothing required to keep warm during periods of immobility at high
altitude, fluid loss from this source may not be held to the lowest levels possible.
Decreased fluid consumption often contributes to dehydration at high elevations. The
need to carry fuel and melt snow to obtain water for drinking or cooking, as well as the
dulling of the sensation of thirst that accompanies the loss of appetite, nausea, or even
vomiting of acute mountain sickness, both tend to reduce fluid intake.
Individuals who are active at high altitudes must consciously force themselves to drink
large volumes of fluid. Thirst alone is not a reliable indicator of the need for water.
Above 15,000 to 16,000 feet (4600 to 4900 m) fluid requirements often exceed 4 liters
per day. Urine color and volume indicate the adequacy of fluid intake. Darkly colored ur-
ine—orange instead of light yellow—and the absence of a need to void upon awakening
from a night's sleep are indicators of significant dehydration.
Fluid Replacement