Introduction
General cooling, or hypothermia, is a fall of the core body temperature below 35 °C, caused by heat loss exceeding heat production. It is a systemic condition, distinct from local cold injury such as frostbite (topic 39). In surgery it matters both as an emergency in exposed patients and as an unplanned complication during anaesthesia and operations.
Definition and classification
Hypothermia is defined by core temperature (rectal, oesophageal or bladder), not by skin temperature. Clinically it is graded in three degrees:
- Accidental hypothermia: exposure to cold, immersion, trauma, sepsis, hypoglycaemia, hypothyroidism, alcohol or sedative intoxication, and old age.
- Induced hypothermia: deliberate cooling, for example during cardiac surgery or neuroprotection, and unplanned cooling during anaesthesia.
Pathophysiology
Body heat is produced by metabolism and lost by radiation, conduction, convection, evaporation and respiration. The hypothalamus keeps the balance through peripheral vasoconstriction and shivering thermogenesis. Hypothermia develops when heat loss overwhelms these defences, when thermoregulation is impaired (drugs, anaesthesia, brain injury, hypothyroidism), or when heat production fails (malnutrition, sepsis, hypoglycaemia).
As the core cools, the effects spread across all systems:
- Metabolic rate falls by roughly 5–7 % per degree, so oxygen consumption and drug clearance decrease.
- Cardiac conduction slows, which predisposes to bradyarrhythmias, atrial fibrillation and ventricular fibrillation, especially with rough handling of the patient.
- Cold-induced diuresis reduces circulating volume, and peripheral vasoconstriction masks shock.
- Enzyme and coagulation cascades slow, causing coagulopathy and platelet dysfunction.
- Respiratory drive falls, and the cold airways and lungs are prone to infection.