Hypothermia

Overview

•Core temperature <35°C; standard thermometers only read to 35°C - always use a low-reading thermometer (rectal or oesophageal) for true core temperature

•Primary - environmental (cold water immersion, avalanche, exposure)

•Secondary - underlying cause impairs thermoregulation: sepsis, hypothyroidism, alcohol/drug intoxication, hypoglycaemia, major trauma

Severity of hypothermia

Feature	Mild (~32-35°C)	Moderate (~28-32°C)	Severe (<28°C)
Shivering	Present, vigorous	Decreasing	Absent (<30°C)
Consciousness	Confused, ataxia	Stupor	Coma
Cardiac	Tachycardia initially	Bradycardia, J waves	VF risk, arrest

Presentation

•Confusion / reduced GCS - most common presenting feature; progresses to stupor then coma

•Shivering - vigorous in mild; absent below ~30°C (critical threshold)

•Bradycardia and hypotension - cardiac output falls with temperature

•Respiratory depression - rate slows progressively; apnoeas in severe hypothermia

•Paradoxical undressing - peripheral vasodilation from central thermoregulatory failure causes sensation of warmth; patient removes clothing

•Fixed dilated pupils - can occur in severe hypothermia; does NOT confirm irreversible brain injury

🚨

Fixed dilated pupils, absent pulse, and apnoea do NOT confirm death in a hypothermic patient - all are reversible with rewarming. Never pronounce death until the patient is warm and still in cardiac arrest ('not dead until warm and dead').

Investigations

•Core temperature - rectal or oesophageal low-reading thermometer (axillary/oral unreliable)

•ECG - J (Osborn) waves, prolonged PR/QRS/QT, bradyarrhythmias

•Blood glucose - hypoglycaemia is both a cause and consequence; check in every patient

•Serum potassium - K+ >12 mmol/L indicates non-survivable cell death in non-traumatic hypothermia

•ABG - initial respiratory alkalosis; progresses to mixed acidosis

•FBC, U&E, coagulation, CXR, toxicology screen, blood cultures - assess for complications and secondary causes

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J (Osborn) waves are the pathognomonic ECG finding - positive deflection at the J-point, best seen in leads II and V5-V6. Prominence increases as temperature falls.

Management

•All patients: remove from cold environment, remove wet clothing, cover with dry blankets, warmed humidified oxygen, gentle handling (physical disturbance can precipitate VF in moderate-severe hypothermia)

•Mild hypothermia: passive external rewarming - insulation, dry warm environment, warm sweet oral fluids if conscious and able to swallow

•Moderate-severe hypothermia: active external rewarming - warming torso first (not limbs, to reduce afterdrop risk); warm IV fluids

•Severe/haemodynamically unstable: active internal core rewarming - warm bladder irrigation, pleural lavage, ECMO or cardiopulmonary bypass for refractory cardiac arrest

•Treat underlying causes in parallel: IV dextrose for hypoglycaemia, antibiotics for sepsis, IV levothyroxine or liothyronine for myxoedema coma

⚠️

Afterdrop - as rewarming causes peripheral vasodilation, cold blood returns to the core causing a further fall in core temperature before it rises, which can precipitate VF. Warm the torso first and avoid limb massage.

Complications

Key complications

VF - risk sharply rises below 28°C

Afterdrop - during rewarming

Aspiration pneumonia

Pulmonary oedema

DIC - impaired platelet and clotting factor function

AKI - hypoperfusion, rhabdomyolysis

Rhabdomyolysis - monitor CK

Pancreatitis - check amylase

Prognosis

•Full neurological recovery possible even after prolonged cardiac arrest in severe hypothermia if aggressive rewarming achieved (especially with ECMO)

•Serum K+ >12 mmol/L on admission = marker of massive cell death, indicates non-survivable injury in non-traumatic hypothermia

•Worse prognosis: secondary hypothermia, prolonged hypoxia before cooling, elderly with comorbidities