Hypothermia

See Heat Illness & Heat Stroke for the opposite end of thermoregulatory failure.

Core temperature <35°C. Severity is graded by core temperature:

Severity	Core temperature
Mild	32–35°C
Moderate	28–32°C
Severe	<28°C

Predisposing factors

Increased heat loss — cold water immersion, environmental exposure, vasodilation (e.g. alcohol), skin conditions affecting thermoregulation (burns, toxic epidermal necrolysis, erythroderma).

Decreased heat production — extremes of age (elderly, neonates), hypoglycaemia, muscle relaxants.

Clinical features by stage

Mild (32–35°C) — increased metabolic rate, maximal shivering, amnesia/ dysarthria/ataxia/apathy, normal blood pressure, “cold diuresis” (peripheral vasoconstriction shunts blood centrally, transiently raising BP and diuresis).

Moderate (28–32°C) — shivering stops, stupor, atrial fibrillation/other dysrhythmias, progressive loss of consciousness/pulse/respiratory rate.

Severe (<28°C) — progressive paralysis and loss of reflexes, pulmonary oedema, hypotension/bradycardia, fixed dilated pupils (the patient may appear dead), VF risk peaks at ~22°C, asystole risk at ~18°C.

Examination and investigations

• Measure core temperature with a rectal or bladder probe — standard thermometers often don’t read below 34°C, and tympanic thermometers aren’t validated at low temperatures either.
• Blood glucose (BSL) — hypoglycaemia both predisposes to and complicates hypothermia.

Management

Mild hypothermia — passive warming: remove wet clothing, dry the patient, cover with blankets in a warm dry environment, give warm caloric fluids to drink. Correct dehydration with warmed IV fluids and hypoglycaemia with IV dextrose.

Resuscitation principles

• Hypothermia is neuroprotective — continue CPR for longer than usual before considering cessation.
• Drugs and cardioversion are unlikely to be effective until core temperature is >30°C.
• Don’t diagnose death in a cold patient — the guiding principle is “not dead until warm and dead” (>30°C).
• CPR ratio 30:2; double the usual dosing intervals for drugs between 30–35°C; don’t give adrenaline below 30°C.

From the Emergency Care Institute NSW hypothermia clinical tool: https://www.aci.health.nsw.gov.au/networks/eci/clinical/clinical-resources/clinical-tools/environmental-health/hypothermia