Hypothermia

Hypothermia is a core body temperature under 35.0 degrees Celsius. It follows heat loss that exceeds heat production, or failure of thermoregulation. It is dangerous because cold slows cardiac conduction, depresses consciousness and breathing, impairs coagulation and can progress to ventricular fibrillation and asystole. A hypothermic patient is not declared dead until rewarmed.

Epidemiology and settings. It occurs with environmental exposure (cold, wet, wind, immersion, altitude), but also indoors in older adults, in people with impaired thermoregulation, and in sepsis, hypothyroidism and intoxication. In tropical highland and urban settings it is frequently missed because the ambient temperature is not low.

Heat balance. Heat is lost by radiation, conduction (wet clothing and cold surfaces), convection (wind) and evaporation. Wet clothing increases loss about 5 times, and cold water about 25 times.

Staging by clinical features (a core temperature is not always available).

• Stage I, mild (32 to 35 degrees Celsius): conscious and shivering.
• Stage II, moderate (28 to 32 degrees Celsius): impaired consciousness, shivering stops.
• Stage III, severe (24 to 28 degrees Celsius): unconscious, vital signs present.
• Stage IV (under 24 degrees Celsius): no vital signs.

Classification by cause. Exposure (primary) and secondary hypothermia from illness, drugs or injury. Accidental hypothermia is distinguished from therapeutic or induced hypothermia. Acute immersion, subacute exposure and chronic (days to weeks, often indoors in older adults) differ in the rate of cooling and in the associated metabolic state.

Pathophysiology by temperature.

• Around 35 degrees Celsius: shivering, tachycardia, vasoconstriction, cold diuresis.
• 32 degrees Celsius: confusion, slowed speech, bradycardia, atrial fibrillation.
• Under 30 degrees Celsius: coma, hypoventilation, hypotension, high risk of ventricular fibrillation, loss of reflexes.
• Under 28 degrees Celsius: fixed dilated pupils can occur with a preserved brain, and pulses may be undetectable.
• Cold depresses platelet and clotting enzyme function, so a coagulopathy exists despite normal laboratory tests run at 37 degrees Celsius.

History.

• Circumstances: exposure, immersion, avalanche burial, trauma, collapse at home, duration, clothing, wind, wet, altitude, rural or urban setting.
• Medical conditions: hypothyroidism, adrenal insufficiency, pituitary disease, diabetes, malnutrition, dementia, Parkinson disease, stroke, spinal cord injury, burns, skin disease, severe heart, kidney or liver disease.
• Alcohol and drugs: ethanol, benzodiazepines, opioids, barbiturates, phenothiazines, clonidine, antidepressants, antipsychotics, general anaesthesia, intravenous fluids and blood products.
• Prodrome of infection, vomiting, diarrhoea, poor intake, homelessness, social isolation, self harm, abuse or neglect.
• For immersion: time in the water and rescue time, and any submersion.

Examination.

• Core temperature with a low reading thermometer (oesophageal, rectal at 15 cm depth, bladder, or epitympanic when others unavailable). Surface and oral readings are unreliable.
• Airway, breathing and circulation. Check pulse and breathing for up to 60 seconds, because rates are very slow. Bradycardia, hypotension, slowed respiration, absent reflexes.
• Neurological state (confusion, slurred speech, ataxia, coma) and pupils.
• Skin: pallor, cold, mottling, frostbite, pressure sores, burns, trauma, needle marks, myxoedematous or dry skin, hair loss.
• Signs of causes: goitre scar, slow relaxing reflexes, hyperpigmentation, abdominal tenderness, focal neurology, features of sepsis.
• Paradoxical undressing and terminal burrowing are behaviours of severe cold.
• Rectal examination only after the airway and circulation are secure, with minimal handling.

Red flags.

• Core temperature under 32 degrees Celsius or absent shivering.
• Altered consciousness, bradycardia under 40 beats/minute, hypotension.
• Arrhythmia (atrial fibrillation with slow ventricular rate, ventricular ectopy), J wave on ECG.
• Trauma, immersion or submersion, avalanche burial, suspected sepsis, hypoglycaemia, hypothyroid or adrenal crisis.
• Pulselessness or apparent death.
• Child or neonate, pregnancy.
• Suspected abuse, neglect or self harm.

Environmental. Cold exposure, cold water immersion, avalanche burial, urban exposure in homeless people, prolonged lie after a fall, wet clothing, inadequate heating in older adults and poverty.

Endocrine and metabolic. Hypothyroidism with myxoedema coma, adrenal insufficiency and adrenal crisis, hypopituitarism, hypoglycaemia, diabetic ketoacidosis, malnutrition, cachexia, uraemia, hepatic failure, hyponatraemia.

Infective. Sepsis (especially in older adults and neonates), pneumonia, meningitis, urinary tract infection, peritonitis, necrotising soft tissue infection.

Drugs and toxins. Ethanol, benzodiazepines, opioids, barbiturates, phenothiazines, antidepressants, clonidine, beta blockers, lithium, general anaesthesia and neuromuscular blockers, cold intravenous fluids and blood, carbon monoxide poisoning.

Neurological and spinal. Stroke, subarachnoid haemorrhage, hypothalamic lesion, Wernicke encephalopathy, Parkinson disease, dementia, spinal cord injury, head injury, tumour.

Cardiovascular and other. Cardiogenic shock, heart failure, major haemorrhage and trauma, burns, extensive skin disease (psoriasis, erythroderma), pancreatitis, hypoperfusion, immobility.

Mimics and iatrogenic. Inaccurate temperature measurement, therapeutic hypothermia, hypothermia after surgery and cardiopulmonary bypass.

1. Measure core temperature accurately and repeatedly. Start continuous cardiac monitoring.
2. ECG: bradycardia, atrial fibrillation, prolonged intervals, J (Osborn) waves, ventricular arrhythmia. Shivering artefact can mimic fibrillation.
3. Bedside tests: glucose, oxygen saturation, blood gas, urine output.
4. Laboratory: full blood count, urea and electrolytes (potassium, sodium, calcium), creatinine, glucose, lactate, blood gas (interpret without temperature correction), coagulation, creatine kinase, lipase or amylase, liver function tests, thyroid stimulating hormone and free thyroxine, random cortisol, blood cultures, urinalysis, toxicology and alcohol level. Take all tests before and after rewarming where possible.
5. Chest radiograph (aspiration, pneumonia, cardiogenic oedema). CT head for focal signs or trauma. Further imaging for trauma, pancreatitis or sepsis source.
6. Interpretation.

• Haemoconcentration and coagulopathy may be present; platelet count can fall with a rise in lactate.
• Hypokalaemia is common; in an avalanche burial with cardiac arrest, a serum potassium over 12 mmol/L indicates a non survivable arrest (cell death).
• Amylase and lipase are often elevated; creatine kinase may be markedly raised.
• Hypothyroid causes show a low free thyroxine and high thyroid stimulating hormone, but non thyroidal illness can distort tests.
• Cortisol is low or inappropriately normal in adrenal crisis.

1. Laboratory machines run at 37 degrees Celsius, so reported values can be misleading. Use uncorrected blood gas values for management.

Limits. Peripheral pulses are unreliable. Pupils and the apparent absence of vital signs do not exclude survival. Routine ECG filters may suppress J waves. Tympanic thermometers underestimate core temperature in a cold ear canal.

General principles. Handle gently and keep horizontal, because rough movement can precipitate ventricular fibrillation. Stop further heat loss, remove wet clothing by cutting, insulate with blankets and a vapour barrier, and shelter from wind. Give warmed, humidified oxygen. Establish intravenous access and monitor the ECG.

Mild hypothermia (32 to 35 degrees Celsius, shivering). Passive rewarming in a warm environment with insulation, dry clothing, warm sweet drinks and food if awake and able to swallow. Active external rewarming with forced warm air or heating blankets (40 to 43 degrees Celsius) is preferred in older adults, those with chronic illness, and when rewarming is slow. Expected rate is about 0.5 to 2 degrees Celsius/hour.

Moderate to severe hypothermia (under 32 degrees Celsius).

• Active external rewarming of the trunk first (forced air, heated blankets), with core rewarming by warmed intravenous fluid (0.9% sodium chloride at 38 to 42 degrees Celsius, 500 to 1000 mL boluses, titrated to haemodynamics) and warmed humidified oxygen.
• Avoid active rewarming of the limbs before the trunk, and avoid hot baths, because peripheral vasodilation can cause core temperature afterdrop and rewarming shock.
• Hypotension requires cautious volume replacement because cold diuresis causes hypovolaemia; avoid overload. Use vasopressors cautiously.
• Treat hypoglycaemia with glucose 10% 150 to 200 mL intravenously. Treat hypokalaemia cautiously, and avoid hyperkalaemia after rewarming.
• Antibiotics for suspected sepsis (ceftriaxone 2 g intravenously once daily as a starting empirical option), adjusted to source and local patterns.
• Thiamine 250 to 500 mg intravenously three times daily when Wernicke encephalopathy is possible (alcohol, malnutrition).
• Intubation is done gently when needed for airway protection; ventricular fibrillation may be triggered by instrumentation.
• Extracorporeal rewarming (extracorporeal membrane oxygenation or cardiopulmonary bypass) is the preferred method for severe hypothermia with haemodynamic instability or cardiac arrest, and body cavity lavage is an option where it is unavailable. Transfer to a centre with this capacity early.

Cardiac arrest in hypothermia.

• Check for signs of life for up to 60 seconds. Start cardiopulmonary resuscitation if absent, and continue throughout rewarming.
• Ventricular fibrillation or pulseless ventricular tachycardia: up to 3 defibrillation attempts; further shocks and drugs are withheld until core temperature is over 30 degrees Celsius.
• Between 30 and 35 degrees Celsius, give adrenaline 1 mg intravenously at doubled intervals (every 6 to 10 minutes). Over 35 degrees Celsius use normal intervals.
• Resuscitation continues until the patient is rewarmed (over 32 degrees Celsius), unless there are obvious lethal injuries, an obstructed airway with asphyxia, chest wall incompressible, or a serum potassium over 12 mmol/L.
• Extracorporeal rewarming achieves survival with good neurological outcome even after prolonged arrest.

Cause directed treatment.

• Myxoedema coma: levothyroxine 200 to 400 micrograms intravenously as a loading dose, then 1.6 micrograms/kg daily (75% of the oral dose) intravenously, or liothyronine 5 to 20 micrograms intravenously loading then 2.5 to 10 micrograms every 8 hours; plus hydrocortisone 100 mg intravenously every 8 hours until adrenal insufficiency is excluded. Warm passively and slowly (active rewarming can precipitate vasodilatory hypotension). Watch for arrhythmia and ischaemia with thyroid hormone in older adults.
• Adrenal crisis: hydrocortisone 100 mg intravenously at once, then 200 mg over 24 hours by infusion or 50 mg every 6 hours, with 0.9% sodium chloride 1 litre over the first hour.
• Sepsis: antibiotics and source control directed at the infection.
• Drug toxicity: reverse where specific antidotes exist (naloxone for opioid toxicity, flumazenil only with great caution), and stop precipitating drugs.
• Trauma: remember the lethal triad of hypothermia, acidosis and coagulopathy, and warm early.

Rewarming targets. Continue active rewarming until core temperature is 35 to 36 degrees Celsius, then keep warm. Avoid overshoot and avoid fever.

Monitoring. Core temperature every 15 to 30 minutes, ECG, blood pressure, urine output (aim for 0.5 mL/kg/hour or more), glucose, potassium, lactate, coagulation and creatine kinase. Watch for rewarming complications: afterdrop, hypotension, arrhythmia, acute respiratory distress syndrome, pancreatitis, compartment syndrome and rhabdomyolysis.

Referral and escalation. Critical care for temperature under 32 degrees Celsius, haemodynamic instability, arrhythmia or coma. Early contact with an extracorporeal rewarming centre for any patient with severe hypothermia, cardiac arrest or instability. Endocrinology for suspected myxoedema coma or hypopituitarism. Social work and safeguarding services for homelessness, neglect, abuse or self harm.

Admission. All patients with moderate to severe hypothermia, any with significant comorbidity, sepsis, an unexplained cause, abnormal laboratory tests, or social vulnerability. Patients with mild, clearly accidental hypothermia who rewarm fully, have normal examination and tests, and a safe home may be discharged.

Safety netting. Return for confusion, drowsiness, new shortness of breath, black or bloody stool, persistent shivering, and recurrent exposure risk. Arrange heating, clothing and follow up.

Special groups. Older adults: indoor hypothermia with hypothyroidism, sepsis, drugs and poor nutrition; slow onset and few signs. Neonates and infants: rapidly lose heat; keep skin to skin and use a warmed environment. Trauma: warm early, treat bleeding and coagulopathy. Pregnancy: protect the mother first; the foetus tolerates less cold. Homeless and substance users: assess for intoxication, infection and social needs.

Immersion: after rescue, stay horizontal to avoid circulatory collapse. Frostbite: rapid rewarming of affected parts in water at 37 to 39 degrees Celsius after core rewarming, avoiding refreezing; ibuprofen 400 mg every 8 hours for analgesia.