What is hypothermia?Hypothermia occurs when the body’s core temperature drops below 35°C. A state of severe hypothermia is reached when body temperature drops below 30°C.
How does hypothermia occur?There are two main reasons for hypothermia:
- Accidental exposure to very low temperatures over a period of time, usually while pursuing outdoor activities. This is made worse if the weather is not only cold, but either wet or snowy as well. Immersion in cold water is also a common cause of hypothermia. After an accident outside (for example a motor vehicle accident) the patient may be exposed for a long period of time. Patients rapidly loose heat and this is worse if they have wet clothes or dressings on. Patients with burn wounds are particularly susceptible. Wet people lose heat more quickly as the thermal conductivity of water is 20-30 times that of air.
- Elderly people are particularly susceptible to hypothermia and can lose core temperature sufficiently to become hypothermic at relatively high ambient temperatures – between 10°C and 15°C. Conditions in the elderly which may predispose to hypothermia are malnutrition, alcohol abuse and low levels of thyroid hormones (hypothyroidism).
Other predisposing factors are certain drugs, kidney failure, heart failure and infections in general.
What are the symptoms and signs of hypothermia?A hallmark of hypothermia is overwhelming tiredness. This is accompanied by weakness and general lethargy which will prevent the person from seeking help or warming themselves. As the core temperature drops below 32°C, the hypothermic person becomes confused and eventually comatose.
The heart is adversely affected in hypothermia. As the body temperature drops, abnormal heart rhythms, such as ventricular fibrillation, may occur. Ventricular fibrillation is an irregular, uncoordinated quivering of the lower chambers of the heart. Such fibrillation prevents the heart from effectively pumping blood around the body. If body temperature continues to fall, the heart may stop beating altogether.
If a patient has had an accident, they will tend to bleed more from their wounds. Once this starts happening, it causes major problems. Shivering is one of the body’s ways of maintaining core temperature. Once body temperature falls to below 32°C, shivering stops.
At temperatures below 30°C, the person may well appear to be dead. The blood circulation to the skin shuts down, pulses are very difficult or impossible to feel, and breathing may be too shallow to notice. A person with hypothermia must never be assumed to be dead.
How is hypothermia diagnosed?The diagnosis of hypothermia rests on careful observation of the state of the person, and an accurate measurement of their body temperature. This can only be achieved by using a low-reading thermometer.
A knowledge of where the person was found and how long they had been in that situation is very useful, particularly if there are few signs of life. A basic rule is that no-one with suspected hypothermia can be pronounced dead until they are “warm and dead”.
Any person with suspected hypothermia should have their heart rhythm monitored. This should continue during the rewarming process.
Can hypothermia be prevented?People who spend time in the outdoors need to be aware of the risks of hypothermia in adverse weather conditions. Correct clothing is essential, as is knowledge of weather conditions so that shelter can be sought in time.
If a person is involved in an accident, keeping them warm while they are outside may well save their lives.
Elderly people are less likely to become hypothermic if they are well nourished and provided with a warm, dry environment. However, this is such a common condition in the elderly in cold conditions, that family and other carers need to be aware of the risks at all times and should pay special attention to the welfare of the elderly person during the winter.
How is hypothermia treated?In all patients:
- Remove wet clothes.
- Use blankets and insulating covers to protect against wind and wet and further heat loss. Ensure that the person is not lying on a cold, damp surface. If possible, the person should be carefully transferred onto a blanket. In the case of an accident, however, they should not be moved as the person may have injured their spine.
- Keep the person lying flat.
- Try not to move the person too much since this can precipitate abnormal heart rhythms.
- Measure, and keep monitoring, the core temperature.
Treatment differs according to the degree of hypothermia.
At temperatures between 36°C and 34°C, the person can be gently allowed to regain their body temperature by preventing further heat loss. This involves wrapping the person in blankets and offering hot drinks (coffee, tea and alcohol should be avoided). In these situations rapid rewarming can result in irreversibly low blood pressure, so is not recommended.
Moderate hypothermia (temperatures between 34°C and 30°C) requires passive rewarming by preventing further heat loss as well as active rewarming of the trunk. This can be achieved using electric or charcoal warming devices, hot water bottles, heating pads, heaters and warming beds.
Severe hypothermia (temperatures less than 30°C) is a medical emergency and requires active rewarming both externally and internally. There is controversy as to whether this should only be carried out in hospital or should be started at the scene of the accidental hypothermia. In most cases, if the emergency personnel have the equipment, they will start rewarming while the person is being transferred to hospital.
Active internal rewarming includes:
- Warm intravenous fluids at a temperature of 43°C.
- Warm, humidified oxygen at temperatures between 42°C and 46°C.
- The circulation of warm fluid through the abdominal cavity.
- Some centres use warmed tubes inserted into the oesophagus (swallowing tube). These are called oesophageal warming tubes.
Active internal rewarming should be continued until the core temperature has risen to at least 35°C and blood circulation has started again.
In some circumstances the person will have no pulse and will not be breathing. In these cases, cardiopulmonary resuscitation (CPR) is started at the same time as active internal rewarming. If the core temperature remains below 30°C, the normal medications used in CPR are not used until the person has been transferred to hospital. If ventricular fibrillation occurs, electrical defibrillation should be used to correct the abnormal heart rhythm.
Loss of heart beat (cardiac arrest) in someone who is severely hypothermic is more difficult to manage than in someone with a normal temperature. The heart is not as responsive to the normal cardiac drugs, or to defibrillation.
Rewarming a patient with injuries is extremely difficult. The best method is to prevent hypothermia where ever possible.
What is the outcome of hypothermia?Death from hypothermia usually occurs at temperatures below 30°C, although people with core temperatures as low as 24°C have been successfully rewarmed.
Complications such as gangrene of the hands and feet, pancreatitis, pneumonia and kidney failure can result from hypothermia. Nevertheless, particularly in cases of mild to moderate hypothermia, recovery is good once body temperature is normal again.
If a person has multiple injuries, they may bleed from the injuries as their blood does not clot in the normal manner.
When to see the doctorMedical attention must be sought immediately if someone appears to be suffering from hypothermia and is becoming lethargic and confused.
An elderly person with any of the above symptoms must be taken to an emergency facility immediately.
(Reviewed by Dr Jenny Edge, General Surgeon)