Normal Thermoregulation

Many factors predispose an animal to develop heat stroke which are 1) lack of acclimatization to the environment, 2) excessive humidity, 3) water deprivation or dehydration, 4) drug administration, 5) obesity, 6) underlying cardiovascular disease, 7) exercise, 8) central nervous system disease and 9) previous episodes of heat stroke. Brachycephalic breeds and dogs with upper airway disease such as laryngeal paralysis are extremely susceptible to the development of heat stroke.

Body heat is produced by three main processes: basal metabolism, muscular activity, and the assimilation of food known as oxidation. Body heat is dissipated by several means. In humans, radiation of infrared heat waves account for the majority of heat loss from the body. Another common method of cooling is evaporation of water or sweat, which results in heat removal from the body. In dogs, the primary normal cooling mechanisms are evaporation and conduction rather than sweating. Evaporation occurs mainly through panting, as water is lost through the moist mucous membranes of the upper respiratory tract. Panting is an effective way of cooling but requires respiratory muscle activity and that in itself generates heat. Panting can become ineffective in dissipating heat in conditions of high environmental humidity when evaporation is reduced. It is also ineffective in conditions involving defects in the upper airway (i.e. laryngeal paralysis), therefore impairing breathing and increasing the work of respiration.

Heat loss through conduction is another common method of removing heat from the body. Conduction is the exchange of heat between two objects in direct contact with one another. Animals often lie down on cool surfaces allowing the relatively hairless skin surface of the ventral abdomen to lose heat by transfer of heat from the animal to the surface. Heat loss through conduction is aided by the development of peripheral vasodilation in warm environments, although, this reflex vasodilatation can be impaired by conditions that result in poor perfusion, such as underlyng cardiac disease, dehydration, or hypovolemia. A very small amount of heat is lost in association with excretion of feces and urine.

The thermoregulatory center is located in the preoptic regions of the anterior hypothalamus. Thermoregulation is simply the balance between heat loss and production. Normal homeostasis operates to keep the body temperature within a very narrow range, called the set point. This set point acts as a "trigger" for the body to respond in a manner, which activates physiologic processes that influence temperature elevation or decrease. For example, when an animal's body temperature decreases below the set point, heat-producing mechanisms are activated to raise the temperature -shivering, increased voluntary activity, increased catecholamine secretion, cutaneous vasoconstriction, postural changes, piloerection, and an increase in thyroxine production. When the temperature is elevated, some opposite signs occur - cutaneous vasodilation, increased respiration, panting, anorexia, and sweating.

Hyperthermia can be described as a pyrogenic or non-pyrogenic elevation in body temperature. Fever, a pyrogenic hyperthermia, is characterized by an increased body temperature due to fully functional thermoregulatory mechanisms in response to systemic inflammation. Pyrogens act on the anterior hypothalamus to raise the set point to a higher level. Non-pyrogenic hyperthermia occurs when the heat-dissipating mechanisms cannot compensate for the heat-producing mechanisms leading to an increase in body temperature above the set point.

Heat stroke can also be categorized as exertional or non-exertional. Exertional heat stroke is more likely to occur in late spring or early summer before dogs have the chance to become acclimatized to the high environmental temperatures. Dogs with obesity, laryngeal paralysis, tracheal collapse, eversion of laryngeal saccules, elongation of the soft palate, or brachycephalic conformation are at high risk for exertional hyperthermia. Heat stroke also occurs in working and athletic dogs that overexert themselves in hot, humid environments. Non-exertional heat stroke most commonly develops when dogs are confined outdoors in an overheated enclosure, such as an automobile. Tragically, this scenario is also the cause of numerous deaths in newborn children annually. Non-exertional heat stroke also occurs in animals that are chained outdoors in the hot sun, especially when they are deprived of water and or shade.