Heat Stroke and Hypothermia

Homeothermic animals need to keep their core temperature within a set range to allow all body systems and reactions to work appropriately and prevent severe damage or even death. Several interlinked mechanisms, ultimately regulated by the thermoregulatory center in the hypothalamus, ensure that a constant temperature is maintained in the face of changing environmental and internal conditions. When these compensatory mechanisms are overwhelmed, body temperature goes over or under the set limits, and a series of deleterious effects can occur.

Both heat stroke and hypothermia are common emergency presentations, or can develop whilst the animal is in hospital (particularly hypothermia). A thorough understanding of the temperature preservation mechanisms and the pathophysiologic changes associated with overheating and hypothermia, as well as with rewarming and cooling, are necessary to understand the different therapeutic options, monitoring required and complications.

Heat stroke

Heat stroke is a condition of extreme hyperthermia resulting from the body’s inability to dissipate heat generated by metabolism, exercise, environmental conditions or a combination various or all of these. Core temperature has to be (or has been) above 41ºC (106ºF), and signs of neurological and other organ dysfunction are usually present. Heat stroke can be classified as classical (resulting from exposure to a hot and humid environment) or exertional (associated with intense physical exercise). A combination of both can frequently be encountered, particularly if there has been a lack of acclimatization (such as in working dogs at the beginning of the warm season, or if they move to a warmer area, or in a dog that has been frantically trying to escape from a locked car in the sun).

There does not appear to be a direct correlation between core temperature and severity of clinical signs or outcome, nor has a cut-off value for temperature at which prognosis clearly changes been identified. However, time to cooling does seem to have an effect on outcome, suggesting that longer times at a certain temperature might have more detrimental effects than shorter periods at a higher temperature.

Due to the paucity of information about heat stroke in small animals, most information is derived from human medicine, and also from experimental models. This information, however, should be interpreted with caution. The canine brain, for instance, has a much higher thermal resistance than the human brain. This may account for neurological signs often being the first to be present in people, and for the higher incidence of neurological damage that people seem to suffer when compared to our patients.  […]