HYPOTHERMIA AND HYPERTHERMIA

1. What is hypothermia?

2. How can hypothermia occur in anesthetized animals?

3. What is fever? What are its beneficial effects?

4. Where is the need for fever.sensed?

5.

Reduction of the deep body temperature below normal in nonhibernating homeotherms is known as hypothermia; hyperthermia is the reverse.

Hypothermia

Hypothermia can readily occur during central nervous system anesthesia because the hypothalamic response to cold blood is depressed. It normally occurs as a result of prolonged exposure to cold, coupled with an inability of the heat-conserving and heat-generating mechanisms to keep pace. Tolerance to lowered body temperatures varies among species. In dogs, death can occur when the rectal temperature approximates 25 °C (77 °F). Hypothermia in any animal can become life threatening unless environmental conditions improve or external heat is provided.

It is important to monitor body temperature during and after procedures requiring anesthesia because of the depressed hypothalamic response. External heat sources are often fitted to surgical tables for the maintenance of body temperature. When these are used, there must be assurance that local injury to skin does not occur. When animals do not recover quickly after anesthesia (e.g., pentobarbital anesthesia), monitoring body temperature and provision of external heat is extremely important.

Fever

Fever is an elevation of deep body temperature that is brought on by microorganism-caused disease. Fever is usually beneficial because immunologic mechanisms are accelerated and the high temperature induced is detrimental to the microorganisms, but it can be damaging if allowed to go too high. In fever, the set point of the hypothalamus is elevated and the body senses that the blood is too cold, so heat-conserving and heat-generating mechanisms are recruited. Shivering and a feeling of coolness are characteristics of beginning fever. Fever is generally self-limiting; maximum temperatures of 41 °C (106 °F) can be approached.

Heat Stroke and Impaired Evaporation

Hyperthermia exclusive of fever can be associated with heat stroke. In this condition, heat production exceeds the evaporative capacity of the environment and occurs when the humidity is high. Hyperthermia can also develop when the evaporative mechanisms become impaired as a result of loss of body fluid or reduced blood volume. Antipyretic drugs (effective against fever) are ineffective in reducing the body temperature in heat stroke and impaired evaporation conditions, and relief is obtained only by whole-body cooling.

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