hypothermia has been reported during marathons, on the mountains, in caving and canoeing, at work on land and at sea, and in and under water. It may occur, even during the summer, among the elderly and the very young, in the operating theatre, and in association with some medical disorders. Geographically hypothermia has in fact been recorded from most parts of the world even at low altitudes in the Sahara and from tropical Kampala.

Thermally the body can be divided into zones. The temperature of the ‘core’, including the vital organs, heart, and brain, is kept stable over a surprising range of environmental conditions. The ‘shell’ is superficial, and its size and temperature varies considerably. Hypothermia is defined as a core temperature below 35°C. However, hypothermia is not a diagnosis, but a sign that changes must have occurred in the body, which vary with the circumstances which have led to cooling. During cold exposure the body ‘burns’ carbohydrate to generate heat and the amount of fuel left depends on the rate of cooling. Also fluid is lost through increased urine output, and there are complex movements of fluids in the body, related in magnitude to the duration of exposure to cold, which reverse during rewarming. There are therefore different types of hypothermia.(i) In ‘immersion’ hypothermia the cold stress is so great that the body's heat-generating capacity is overwhelmed and the core temperature is forced down. Because energy reserves are available, the person will have very little difficulty in rewarming once removed from the severe environment. Falling into cold water is the commonest accidental cause, and hypothermia deliberately induced for medical reasons is of this type.(ii) In ‘exhaustion’ hypothermia the body maintains the temperature until energy sources are exhausted. Then, since the heat-generating ability is reduced, even relatively mild cold exposure may cause continued cooling. Thermal protection must therefore take account of every avenue of heat loss, because even small variations of available heat may make the difference between life and death. This is the type most commonly found in mountaineers or hill walkers.(iii) With ‘urban’ hypothermia, the cold has been mild but prolonged. The core temperature remains normal (35°C or above) possibly for days or weeks, with massive fluid shifts. The temperature eventually drifts down, or faster cooling is precipitated by some other factor such as a fall. This is the most usual type found in the elderly or in association with malnutrition.(iv) ‘Submersion’ hypothermia occurs in people who have been totally submerged in very cold water. Recovery has been known even after 15–60 min (typical submersion survival is about 3 min). Survival, more likely in children, depends on resuscitation being started immediately on rescue.

The case history distinguishes the different types, though they may overlap. A climber in a snowstorm disabled by a broken leg will cool as rapidly as if immersed, because the shock of the injury increases the rate of heat loss, and the fracture prevents heat generation from voluntary activity or shivering. A deep diver may suffer ‘immersion’ hypothermia, even in a dry pressure chamber, because of the tremendous heat transfer capacity of the compressed oxyhelium gas mixture which he is breathing. A swimmer lost overboard in relatively warm water is a candidate for ‘exhaustion’ hypothermia. A middle-aged man or a child with severe malnutrition is likely to develop ‘urban’ hypothermia, whereas a fit 70-year-old out walking in the hills would be liable to ‘exhaustion’ hypothermia.

When a person is in a situation where hypothermia is imminent there is a great temptation to continue to move in order both to keep generating heat from the activity and to escape the situation. This may not be the best option for survival, since the activity increases the rate of heat loss and aggravates exhaustion. The best prospect of survival at sea is not to try to swim but to stay with a capsized boat. Similarly in the hills in bad weather the macho response of trying to battle a way out has resulted in many deaths, whereas those who ‘go to ground’ — taking shelter until the weather improves — usually survive.

A variety of signs and symptoms of hypothermia have been described (see table). However there is great individual variation. For example, loss of consciousness may occur at a core temperature as high as 33°C, but in one case consciousness was still present at a rectal temperature of 24.3°C, and other causes of unconsciousness may confuse the issue. Shivering is considered to cease usually at 30°C — but shivering has been recorded at a core temperature of 24°C. At the other extreme some experimental subjects can cool without shivering and many mountain rescue cases never shiver.

Signs and symptoms in hypothermia

Core temperature °C
37.6	‘Normal’ rectal temperature.
37	‘Normal’ oral temperature.
36	Increased metabolic rate to balance heat loss. Breathing and pulse faster.
35	Shivering at a maximum.
	Reflexes exaggerated; speech disordered; thinking slowed.
34	Usually responsive; normal blood pressure.
	Lower limit compatible with continued exercise.
33–31	Retrograde amnesia; consciousness clouded; blood pressure low; pupils dilated;
	shivering usually ceased.
30–28	Progressive loss of consciousness; increased muscular rigidity;
	slow pulse and respiration; irregular heart beat.
	Susceptible to ventricular fibrillation if heart mechanically irritated.
27	Voluntary movement lost.
26	Pupillary light reflex and deep tendon and skin reflexes lost.
	Victims seldom conscious.
24–21	100% mortality in shipwreck victims in World War II
20	Heart standstill.
17	No electrical activity in brain.
14.4	Lowest known accidental hypothermic patient with recovery.
9	Lowest artificially cooled hypothermic patient with recovery.
4	Monkeys revived successfully.
1–7	Rats and hamsters revived successfully.

Diagnosis requires measurement of core temperature, usually rectal, but since this route may not be practical in the field, the person should be treated as a ‘cold casualty’ if the armpit feels ‘as cold as marble’ to the rescuer's hand. In hypothermia the diagnosis of other accompanying conditions is difficult. Slurred speech, staggering, incoordination or a change in personality may be due to hypothermia and not necessarily brain damage. In hypothermia the reflexes are affected and there is stiffness of the muscles; there are changes in the electrical and mechanical functions of the heart; and the lungs may show clinical and X-ray features similar to pneumonia, though these clear on rewarming. It is therefore important that the patient should be restored to normal body temperature before any diagnosis is made or any irrevocable treatment started.

If the heart stops, the lack of circulation, and therefore of oxygen supply to the brain would ordinarily cause permanent brain damage in about three minutes. Although hypothermia can give some protection for the brain by prolonging the time before oxygen lack causes permanent damage, it is cardiac function that is most important for survival. As the heart cools it becomes more susceptible to ventricular fibrillation (VF) (an uncoordinated electrical activity of the heart which produces no actual pumping of blood). This may be triggered by mechanical irritation (which may be as mild as rolling a patient for bedmaking), by hypoxia of the heart muscle, or by rapid changes in pH or electrolytes in the blood, or in temperature gradients within the heart muscle. Inappropriate rewarming techniques add to the hazard.

Rescue and care

Profound accidental hypothermia can be very difficult to distinguish from death. The only certain diagnosis of death in hypothermia is failure to recover on rewarming. However hypothermia is seldom present in isolation. Victims may also be injured or have some illness, possibly cold-related. In water, drowning may precede or follow cooling. If neither heart nor breathing activity can be detected and there is evidence that the person was alive within the previous two hours, the rescuers should start resuscitation, though only if this can be continued until the casualty has been rewarmed or has reached hospital: otherwise members of the rescue team will become exhausted and may then become hypothermic casualties themselves. Also, the heart may occasionally still be beating and providing some circulation, even when this cannot be detected; the mechanical effects of active resuscitation attempts may trigger ventricular fibrillation, and the patient would then be in a worse state and needing continuing resuscitation.

Evan L. Lloyd

Bibliography

Lloyd, E. L. (1986). Hypothermia and cold stress. Croom Helm, London.

See also cold exposure; near-drowning; survival at sea; temperature regulation.