Introduction

• Hyperthermia: an elevation in core body temperature above accepted normal ranges secondary to an imbalance between heat production and heat loss.
• Distinguish from true fever (or pyrexia) - the body’s normal response to endogenous pyrogens (produced during  infection, inflammation or injury). It is associated with an increase in the ‘set point’ in the thermoregulatory center in the hypothalamus. 
• Pathophysiology: heat production exceeds heat loss.
• Signs: variable based on severity and duration but can include altered mentation, hyperdynamic state and tachypnea with progression to collapse, hypotension and evidence of multi-organ involvement. 
• Diagnosis: based on history, physical examination (NB temperature may be normal depending on owner care). Baseline blood tests may determine development of secondary organ injury or underlying disease process.
• Treatment: focuses on active cooling.
• Prevention: minimize environmental risk factors , exposure to triggers, control underlying disease.
• Prognosis: dependent on severity and duration and underlying disease.

Pathogenesis

Predisposing factors

Heat stroke

• Patient related:
  • Large dog breeds.
  • Obesity.
  • Brachycephalic breeds.
  • Males.
  • Dark coat colors.
  • Very young or old animals may be less able to increase heat dissipation.
• Environment related:
  • Hot environment.
  • High humidity.
  • Inadequate ventilation.
  • Close confinement.

Other

• Small breed dogs more likely to reach toxic dose of drug exposure.
• Dogs that scavenge are more likely to be exposed to intoxications.
• Certain breeds of dog are considered predisposed to malignant hyperthermia Malignant hyperthermia:
  • Greyhounds, Labrador Retrievers, Saint Bernards, Pointers, Dobermans, Border Collies, English Springer Spaniels.

Pathophysiology

• Information from peripheral and central thermoreceptors is normally conveyed to the hypothalamus in order to tightly regulate body temperature.
• If required, the hypothalamus can stimulate heat loss mechanisms, eg panting, vasodilation, behavioral and postural changes to aid heat dissipation.
• Hyperthermia results when physiological, pathological or pharmacological changes cause heat production to exceed heat loss.
• Temperatures above 41.6°C (106.88°F) can start to cause cellular damage secondary to relative hypoxemia Hypoxemia.
• Exertional heat stroke Heat stroke and malignant hyperthermia Malignant hyperthermia can also cause rhabdomyolysis Exertional rhabdomyolysis with myoglobinemia.

• In severe cases a lack of cellular oxygen can quickly lead to multiple organ dysfunction Multiple organ dysfunction syndrome (MODS) and failure:
  • Cardiac (leading to arrhythmias) Heart: dysrhythmia.
  • Acute kidney injury Kidney: acute kidney injury (AKI).  
  • Hepatic injury (risk of hypoglycemia  Hypoglycemia and development of hyperbilirubinemia).
  • Gastrointestinal tract injury (endotoxin absorption/hemorrhage).
  • Disseminated intravascular coagulation Disseminated intravascular coagulation (leading to thrombosis/bleeding).
  • Cerebral edema (confusion, delerium, obtundation, seizures, coma).

Diagnosis

Treatment

Prevention

Outcomes