Hypothermia is a core temperature of <35°C
(<95°F). Although most commonly seen in cold climates, it may
develop without exposure to extreme environmental conditions. Hypothermia
is not uncommon in temperate regions and may even develop indoors
during the summer. In the U.S., an average of 700 people die of
hypothermia each year. Half of those who die of hypothermia are
>65 years of age.1
Body temperature may fall due to heat loss by conduction, convection,
radiation, or evaporation. Conduction is the transfer
of heat by direct contact down a temperature gradient, such as from
a warm body to the cold environment. When immersed in water, the
body loses heat rapidly, which results in a swift decline in body
temperature, because the thermal conductivity of water is approximately
30 times that of air. Convection is the transfer
of heat by the actual movement of heated material, such as the wind’s
disrupting the layer of warm air surrounding the body. Convective
heat loss increases markedly in windy conditions. Heat also may
be lost by radiation to the environment (primarily
from noninsulated body areas) and by evaporation of water.
Evaporation of the water contained in exhaled, water-saturated air
occurs over a wide range of ambient temperatures and may be prevented
by inhalation of warmed, humidified air.
Opposing the loss of body heat are the mechanisms of heat conservation
and gain. In general, these are controlled by the hypothalamus;
thus hypothalamic dysfunction may cause impairment in temperature
homeostasis. Heat is conserved by peripheral vasoconstriction and,
importantly, by behavioral responses. If behavioral responses such
as putting on clothing or coming indoors from a cold environment
are impaired for any reason (e.g., dementia, drug intoxication,
or trauma), the risk of hypothermia is increased.
Heat gain is effected by shivering and by nonshivering thermogenesis.
The nonshivering component of heat production consists of an increase
in metabolic rate brought about by increased output from the thyroid
and adrenal glands.
The most important causes of hypothermia are listed in Table 203-1. Accidental (environmental) hypothermia
can be divided into immersion and nonimmersion cold exposure. Exposure
to cold environmental conditions may lead to hypothermia even in healthy
individuals, especially in wind and rain, and cold swimming water.
Inadequate clothing and physical exhaustion contribute to the loss
of body heat. The high thermal conductivity of water leads to the
rapid development of hypothermia during immersion. The rate of heat
loss is determined by water temperature, and immersion in any water
colder than 16°C to 21°C (60.8°F to 69.8°F) can lead to severe hypothermia.
Table 203-1 Causes
of Hypothermia |Favorite Table|Download (.pdf)
Table 203-1 Causes
|Accidental (environmental) exposure|
|Hypothalamic and central nervous system dysfunction|
|Acute incapacitating illness|
|Massive fluid or blood ...|