Heatstroke is a life-threatening illness triggered in dogs and cats by a combination of increased heat production, environmental heat, and inadequate heat dissipation. The body’s thermoregulatory mechanisms fail, leading to cellular damage and death. In early stage hyperthermia, the pet experiences increased cardiac output and decreased peripheral vascular resistance. As hyperthermia advances, blood pressure and cardiac output decrease when the body can no longer compensate and without treatment, perfusion to vital organs decreases, resulting in widespread organ damage.
Thermal injury becomes widespread, involving neuronal tissue, cardiac myocytes, hepatocytes, renal parenchymal and tubular cells, and the gastrointestinal tract. Decreased organ perfusion, enzyme dysfunction, and severance of oxidative phosphorylation result in decreased aerobic glycolysis and an increased tissue-oxygen debt. The combination contributes to increased lactate production and lactic acidosis, which may occur as quickly as 3 to 4 hours after the initial heat-induced injury.
Hyperthermia may also lead to neuron damage, neuronal death, and cerebral edema; altered mentation is a common clinical sign. Severe central nervous system depression, seizures, coma, and death may be seen as the illness progresses.
During exposure to high ambient temperatures, the heat load increases faster than the animal can dissipate heat from the body.
Large- and giant-breed dogs are at increased risk for heatstroke, as well as animals that have predisposing conditions such as obesity, laryngeal paralysis, tracheal collapse, and thick coats, or are brachycephalic.1,2
In human medicine, a continuum of heat illness ranging from the least to most severe occurs. Heat cramps characterized by muscle spasms that result from the depletion of sodium and chloride would be on one side of the continuum. Fatigue, weakness, muscle tremors, vomiting, and diarrhea are representative of heat exhaustion. Severe central nervous system (CNS) disturbance associated with multiple organ dysfunction is the result of heatstroke.3
Heatstroke happens quickly, especially in poorly ventilated environments such as inside a car with the windows closed, even on a moderately hot day. Temperatures inside a closed car in direct sun may exceed 120° F (48° C) in less than 20 minutes and death may occur in less than one hour.4
Therefore, time is of the essence and veterinary team members must be familiar with the clinical signs and treatment options so they can intervene quickly from the outset. Education should emphasize the importance of instituting cooling measures when the animal is found, before he or she is presented to the practice.