Hyperthermia

ISSN 2398-2942

Contributor(s) :

Introduction
Pathogenesis
Diagnosis
Treatment
Prevention
Outcomes
Further Reading

Synonym(s): afebrile hyperthermia, inadequate heat dissipation (heat stroke), exercise induced hyperthermia, pathological hyperthermia, malignant hyperthermia

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.

Cost considerations

Variable depending on severity and requirement for hospitalization.
Most patients can be cooled quickly but ongoing monitoring for secondary complications may be warranted.
Pathological hyperthermia may have costs associated with further diagnostics.

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).