Hypothermia, or subnormal body temperature, may be classified as primary or secondary.1 Primary hypothermia typically results from environmental exposure despite normal heat production by the body.2 Secondary hypothermia results from alterations in heat production because of illness, injury, or drugs.3,4 Understandably, secondary hypothermia may frequently influence morbidity and mortality in critically ill animals.
Photo by Greg Kindred
There are four basic mechanisms of heat loss5:
Convection transfers heat from the body surface to air moving past the animal.
Conduction transfers heat from the body surface to colder objects in contact with the skin.
Radiation is the exchange of heat between the body and objects in the environment that are not in contact with the skin, independent
of the temperature of the surrounding air.
Evaporation occurs when moisture in contact with skin or the respiratory tract dissipates into the air.
Heat production due to the body's various metabolic processes is directly proportionate to body mass, and, thus, cutaneous
heat loss is a function of body surface area.6 Small companion animals have higher surface-area-to-body-mass ratios that make them uniquely susceptible to heat loss. Additionally,
cachectic, debilitated, immobile, and critically ill patients have impaired thermoregulatory capabilities and may not be able
to retain or seek heat.
Receptors for cold and warm are distributed throughout the body. Cold signals traverse A-delta fibers, and signals from warmth
receptors are relayed through C fibers.7 Processing thermoregulatory information occurs through three pathways7: afferent thermal sensing from the periphery, central regulation in the hypothalamus, and efferent responses.
Given these three pathways, peripheral body temperatures are constantly fluctuating while the posterior hypothalamic thermoregulatory
center maintains a relatively constant core temperature.8 Cellular metabolism results in heat production by the body, and heat is lost from the body when core heat is transferred
through variably conductive tissues to the skin and is subsequently lost to the environment.6 Specifically, heat is transferred from the body's core to the skin through a multitude of blood vessels, including venous
plexuses and capillaries, with arteriovenous connections that are under the control of the autonomic nervous system.9,10 The rate of blood flow through these arteriovenous anastomoses varies depending on the degree of vasoconstriction or vasodilation
desired.9,10 Increased blood flow leads to increased heat loss, whereas decreased blood flow results in core heat conservation.5
As core body temperature dips below 94 F (34.4 C), thermoregulation is impaired, and animals characteristically cease to shiver
or seek heat.11 Peripheral vasodilation rather than vasoconstriction predominates, leading to continued core heat loss.12 Additionally, heat production decreases because of the decreased metabolic rate.4,6 Concurrently, severe hypothermia depresses the central nervous system, ultimately resulting in a hypothalamus that is less
responsive to hypothermia.6 Indeed, when the body core temperature drops below 88 F (31.1 C), thermoregulation ceases.5