16 Hyperthermia and pyrexia
The thermoregulatory centre is located in the hypothalamus and controls core body temperature closely around the thermoregulatory set point. This is achieved through a balance between heat loss and heat gain. Heat loss occurs via evaporation, conduction, convection and radiation. In people, evaporative cooling is mainly by perspiration. In dogs, evaporative cooling is mainly through panting that brings large amounts of air into contact with the nasal and oral mucosa. Minimal perspiration occurs through the footpads. Heat dissipation also occurs via radiation and convection through the skin. Heat gain can be classified as endogenous production (from metabolic processes and exercise) or exogenous gain (from the environment). Thermoregulatory mechanisms are summarized in Box 16.1.
Marked elevations in core body temperature may have severe consequences, with cellular injury possible above 41.5°C. Multiple organs and body systems may be affected, and disseminated intravascular coagulation (DIC) and multiple organ dysfunction syndrome (MODS) are possible sequlae.
In dogs and cats, normal core (rectal) temperature is approximately 38.0–39.0°C, although some normal fluctuation outside this range is likely during the day. This range reflects the normal thermoregulatory set point. Core temperature may become elevated due to hyperthermia and/or pyrexia. An important distinction exists between these two processes that has important implications in terms of appropriate management.
In hyperthermia the thermoregulatory set point is unchanged and the hypothalamus attempts to return core temperature to normal limits. Clinically significant consequences occur when physiological attempts to cool the body become overwhelmed and active cooling is therefore appropriate. The most common causes of hyperthermia are heat stress (see Ch. 38) and severe or sustained seizure activity (see Ch. 24). Other causes include malignant hyperthermia, primary brain lesions and drug reactions.
Hyperthermia is identified most commonly in dogs and is a rare presentation in cats. In most (but not all) hyperthermic animals, the history is suggestive of this being the cause of temperature elevation and the diagnosis is usually relatively straightforward.