Hypothermia is defined as a fall in the core (i.e. rectal) temperature to below 35°C. It is frequently lethal when the core temperature falls below 32°C. Frostbite is local cold injury that occurs when tissue freezes.
Hypothermia occurs in a variety of clinical settings: IN THE HOME ENVIRONMENT. Hypothermia may occur in cold climates when there is poor heating, inadequate clothing and poor nutrition. Depressant drugs (e.g. hypnotics), alcohol, hypothyroidism or intercurren illness may also contribute. Hypothermia is commonly seen in the poor and elderly, the latter having a diminished ability to feel cold and often a decrease in the insulating fat layer. Infants and neonates become hypothermic very rapidly at normal room temperature because of their relatively large surface area and lack of subcutaneous fat.
DURING EXPOSURE TO EXTREMES OF TEMPERATURE OUTSIDE. Hypothermia is a prominent cause of death in climbers, skiers, Arctic and Antarctic travellers and in wartime. Wet, cold conditions and windchill, physical exhaustion and inadequate clothing are common contributory factors.
FOLLOWING IMMERSION IN COLD WATER. Dangerous hypothermia can develop after several hours’ immersion at temperatures of IS-20°C. Below 12°C the patient’s limbs become anaesthetized and paralysed and take some hours to recover after the patient is rescued.
Mild hypothermia (32-35°C) causes shivering and initially a feeling of intense cold. The subject is alert and usually takes appropriate action to rewarm, e.g. huddling, extra clothing or exercise. As the core temperature falls, severe hypothermia (below 32°C) initially causes impairment of judgement (including awareness of the cold) and later leads to altered consciousness and coma. Death follows, usually from ventricular fibrillation.
If a thermometer is available (which must be low reading), the diagnosis is straightforward. If not, a rapid clinical assessment should be made. The hypothermic patient feels cold to the touch-the abdomen, groin and axillae are cold and clammy. If consciousness is impaired (i.e. if the patient is uncooperative, sleepy or in a coma), the core temperature is almost certainly below 32°C; this is a medical emergency.
The pulse rate and volume fall, and respiration becomes shallow and slow. Muscle stiffness develops and the tendon reflexes are depressed. The systemic blood pressure falls. As coma ensues, the pupillary and other brain-stem reflexes are lost (the pupils are fixed and may be dilated in severe hypothermia).
Metabolic changes are variable, with either metabolic acidosis or alkalosis occurring. Arterial oxygen tension readings may appear normal since they are measured at room temperature, but these measurements are falsely high as the arterial P02 falls 7% per °C fall in temperature. Ventricular arrhythmias (tachycardia and fibrillation) or asystole are the usual cause of death and may occur during treatment. ‘J’ waves-rounded waves above the isoelectric line immediately after the QRS complex-are pathognomonic of hypothermia. Prolongation of the PR interval, QT interval and QRS complex also occur.
The principles of management of this serious emergency are to rewarm the patient gradually while correcting metabolic abnormalities and treating cardiac arrhythmias. Hypothyroidism must always be looked for. If the patient is awake, with a temperature above 32°C, rewarming can be achieved by placing the patient in a warm room, using ‘space blankets’, and giving warm fluids orally. Outdoors, the same result can be achieved by adding extra clothing, huddling with the subject, and using a warmed sleeping bag. Rewarming may take several hours. Alcohol should be avoided-it may add to confusion, boost confidence factitiously, cause peripheral vasodilatation (and further heat loss) or precipitate hypoglycaemia.
In severe hypothermia, the patient may appear dead. (Hypothermia should always be excluded before brain death is diagnosed.) Warming should take place gradually, aiming at an increase in temperature of 1°C per hour. The patient should be covered with a ‘space blanket’ and placed in a warm room. Direct surface heat from an electric blanket is also helpful. Any underlying condition should be treated promptly. Drug overdose should always be excluded.
Warmed intravenous fluids are given slowly and metabolic disturbances are corrected. Hypothyroidism, if present, should be reated with triiodothyronine 10 ILg i.v.
8-hourly. Various methods of artificial rewarming have been suggested-warm humidified air by inhalation, gastric or peritoneal lavage, or haemodialysis-but in practice these are rarely used. The cardiac rhythm should be monitored and arrhythmias corrected. Careful monitoring of all vital functions is required; appropriate treatment and intensive care are given as necessary.
Prevention of hypothermia is particularly important inthe elderly, who should be advised to try to improve general heating and insulation in the house. Heat should be provided in the bedrooms, and the use of safe electric blankets advised. Financial help will be needed by many patients. Constant supervision should be given during cold spells, when warm food and extra blankets must be provided.
The formation of ice crystals in the skin and superficial tissues begins when the temperature there falls to -3°C; ambient temperatures generally have to be below -6°C for this to occur.
Frostbitten tissue is pale, greyish and initially doughy to the touch. Later it freezes hard, when it looks (and feels) like meat taken from a deep freeze. This condition may occur when working or exercising in low temperatures and typically develops without the patient’s knowledge. Hands and feet that have ‘lost their feeling’ are an important feature when the temperature is below -5°C, as frostbite may then develop insidiously.
The frostbitten patient should, if possible, be transported (or walk, even on frostbitten feet) to a place of safety before treatment commences. Warming using the body heat of a companion or by immersion in water at 39- 42°C should be continued until obvious thawing occurs. This may be painful. Blisters will form within several days and, depending on the degree of frostbite, a blackened carapace or shell develops as the blisters regress or burst. Dry, non-adherent dressings and strict aseptic precautions are essential. Frostbitten tissues are anaesthetized and are at risk from infection and further trauma. Recovery takes place over many weeks. Surgery may be required, but should be avoided in the early stages, as it is difficult to predict the eventual amount of recovery.