Hypothermia - "The Big Chill"

Small veterinary patients commonly lose heat when anesthetized but the negative impact of this is greatly underestimated. Maintaining body temperature within a narrow range is important for cardiac function, metabolism, normal enzyme activity, nerve conduction, and haemostasis.

Thermal balance
Homeothermy, a balance between heat loss and heat gain involves complex sensing mechanisms that drive the mechanisms controlling heat loss or gain in the correct direction. Heat gains can be obligatory or facultative. Obligatory gains occur independently of thermoregulation and include heat from basal metabolism, eating and exercise. Facultative gains act to restore thermal balance and the most important source is from shivering.

Three-quarters of heat loss occurs from the body surface and the remainder is lost from the respiratory tract. Losses occur through convection (transfer of heat from the animal to the air), conduction (transfer of heat from the animal to a surface that is cooler), evaporation (heat dissipated by evaporation of moisture from wet skin or the respiratory tract) and radiation (exchange of heat between the body and objects in the environment). Temperature sensors exist centrally (hypothalamus, spinal cord, brain stem, abdominal organs and skeletal muscles) and peripherally (warm and cold receptors in the skin). The hypothalamus acts by integrating thermal input and controlling effector organs; in many ways acting as a thermostat.

Anaesthesia and thermoregulation
When an animal is anesthetized many factors interrupt normal thermoregulation. Anaesthesia abolishes behavioural responses (the animal cannot seek out a warm environment), reduces metabolic rate, alters hypothalamic function, reduces muscle tone and effector responses (shivering). In addition theatre environments and surgical procedures impose large thermal stresses on patients.

Under general anaesthesia there is wider range of core

temperature where the animal does not respond to maintain normothermia. Vasoconstriction can occur in nesthetized patients and although it may slow down the rate of heat loss it has a negative effect on tissue perfusion. However this reflex will be counteracted if vasodilating agents such as acetylpromazine or isoflurane are used. [...]