Hypothermia

Hypothermia occurs in many settings.

At home

Hypothermia occurs in cold climates when there is poor heating, inadequate clothing and poor nutrition. Depressant drugs (e.g. hypnotics), alcohol, hypothyroidism or intercurrent illness also contribute. Hypothermia is commonly seen in the poor and elderly, the latter having a diminished ability to sense cold and often loss of insulating fat. Infants and neonates become hypothermic rapidly at room temperature because of their relatively large surface area and lack of subcutaneous fat.

During exposure to extremes of temperature

Hypothermia is a prominent cause of death in climbers, skiers, polar travellers and in wartime. Wet, cold conditions with wind chill, physical exhaustion, injuries and inadequate clothing are contributory.

Following immersion in cold water

Dangerous hypothermia can develop after 1 hour's immersion at water temperatures of 15-20°C. Below 12°C limbs rapidly become numb and paralysed. Recovery takes some hours after rescue.

Clinical features

Mild hypothermia (core 32-35°C) causes shivering and initially quite intense cold. Though alert the subject may not act appropriately to rewarm (e.g. huddling, extra clothing or exercise). As core temperature falls below 32°C, severe hypothermia causes impaired judgement (including awareness of cold), drowsiness and coma. Death follows, usually from ventricular fibrillation.

Diagnosis

If a thermometer (low-reading) is available, diagnosis is straightforward. If not, a rapid clinical assessment is reliable. A person who feels icy to touch - the abdomen, groin and axillae - is hypothermic. If clammy and uncooperative, sleepy or in a coma, core temperature is almost certainly below 32°C - a medical emergency.

Sequelae

Pulse rate and volume fall; respiration becomes shallow and slow. Muscle stiffness develops and tendon reflexes become sluggish. Systemic blood pressure falls. As coma ensues, pupillary and other brainstem reflexes are lost (pupils are fixed and may be dilated in severe hypothermia).

Metabolic changes are variable, either metabolic acidosis or alkalosis developing. Arterial oxygen tension may appear normal since it is measured at room temperature, but the measurement is falsely high as arterial Po2 falls 7% for each degree Celsius (C) fall in temperature.

Ventricular arrhythmias (tachycardia/fibrillation) or asystole is the usual cause of death. 'J' waves - rounded waves above the isoelectric line immediately after the QRS complex - are pathognomonic of hypothermia. Prolongation of PR, QT intervals and QRS complex occurs.

Principles of management

Rewarm gradually.
Correct metabolic abnormalities.
Anticipate and treat dysrhythmias.
Check for hypothyroidism.

If the patient is awake, with core temperature above 32°C, place them in a warm room, use a 'space blanket', and give warm fluids orally. Outdoors, add extra dry clothing, huddle together and use a warmed sleeping bag. Rewarming may take several hours. Avoid alcohol: it may add to confusion, boost confidence factitiously, cause peripheral vasodilatation (and further heat loss), or precipitate hypoglycaemia.

Severe hypothermia

In severe hypothermia, people look dead. Always exclude hypothermia before diagnosing brain death. Warm gradually, aiming at a 1°C per hour increase in core temperature. Cover with a 'space blanket' and place in a warm room. Direct surface heat from an electric blanket is also helpful. Treat any underlying condition promptly, e.g. sepsis. Monitor all vital functions. Correct dysrhythmias. Drug screening is essential.

Give warm i.v. fluids slowly. Correct metabolic abnormalities. Hypothyroidism, if present, should be treated with triiodothyronine 10 μg i.v. 8-hourly. Various methods of artificial rewarming exist - inhaled humidified air, gastric or peritoneal lavage, and haemodialysis. These are rarely used.

Prevention

Hypothermia prevention is especially necessary in the elderly. Improved home heating and insulation, central heating in bedrooms and electric blankets are helpful. Finance is often needed. Supervision should be given during cold spells; warm food and extra blankets must be provided.