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Hyperthermia is a state in which
body core temperature rises as a result of heat generation and absorption
exceeding heat loss. Given that body temperature reflects the balance
between heat gain and heat loss, hyperthermia is found most frequently
under conditions such as exercise in which heat production is increased
or in subjects such as infants or patients with dehydration who
lack the ability to increase their heat loss in response to environmental
heat. Thus individuals who engage in athletic activities and infants
are at particular risk for heat-related illness. Heat stress has been
recognized as a cause of illness for more than 2000 years. It is
responsible for increases in population death rates during hot times
of the year in areas exposed to particularly high environmental
temperatures,1 and increases morbidity related
to other diseases. Heatstroke, the most serious form of heat-related
injury, is the second most common cause of athletics-related death
after head injury.
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It is important to differentiate fever, where the body temperature
set point is elevated as a result of the influence of pyrogenic
cytokines (see Chapters 198 and 227),
from hyperthermia, where the temperature is elevated above normal
because the heat-dissipating homeostatic mechanisms activated at
temperatures above the set point are overwhelmed. In febrile patients,
homeostatic mechanisms including vasoconstriction and shivering
are activated to raise body temperature by increasing heat production
and decreasing heat dissipation until a new set point is reached
whereas in hyperthermic patients, homeostatic mechanisms attempt
to lower body temperature.
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Metabolic activity leads to heat production. If no heat were
lost to the environment, resting heat production would raise body
temperature by approximately 1°C/hr. This increase would be
even greater during exercise, when heat production may increase
as much as 4-fold. Heat may also be absorbed from the environment
by radiation from the sun or the ground, or by convection when the
air temperature exceeds body temperature.
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Heat production and absorption are balanced by heat loss via
four mechanisms. Conduction carries heat between
a body and a contacting surface along a temperature gradient. This
is the primary mechanism of cooling during ice or water immersion. Convection transfers
heat from the body surface to or from a gas or fluid circulating around
the body. With conduction, heat transfer stops when the contacting
surface temperature reaches body temperature, whereas with convection,
circulation of fresh gas or fluid around the body preserves the
temperature gradient between the body and the circulating gas or
fluid thus increasing the efficiency of heat transfer. Radiation transfers
heat from a warmer to a colder body via electromagnetic waves. Evaporation removes
heat by promoting a phase transition from liquid to a gas. Of these
four mechanisms, radiation is the principal mechanism of heat elimination
in temperate environments. Heat loss by convection and radiation
are increased by cutaneous vasodilation, a neurally mediated response
to hyperthermia. As either heat production or environmental temperature increases,
evaporation of sweat becomes the ...