Chapter 40. Hypothermia
Hypothermia is defined as having a core temperature below 35°C. Clinically, it can be divided into three categories:
• Mild: 32–35°C
• Moderate: 30–32°C
• Severe: below 30°C
and into three groups according to circumstances:
• Immersion – patient has been in water
• Dry – patient is on dry land but has been exposed to low air temperatures
• Urban – such as elderly patients who fall in their home.
The underlying clinical effects are broadly the same for each group.
Heat loss
Heat loss occurs from the body via conduction, convection, radiation and evaporation.
The surface area over which heat is lost is important. Children have a larger surface area for their weight than do adults and hence tend to lose heat more quickly.
The temperature of the human body is normally regulated within strict limits around an average core temperature of 37°C.
Clinical recognition of hypothermia
Cases will be missed unless specific consideration is given to its possible presence. It should be remembered that where hypothermia has occurred in one member of a party, the others will also be at risk and further cases may occur unless corrective action is taken.
| Core temperature (°C) a | Clinical feature |
|---|---|
| 36 | Sensation of cold, stumbling, personality changes, mild confusion |
| 35 | Slurred speech, incoordination. Amnesia of events (on recovery) |
| 34 | Development of arrhythmias – typically atrial fibrillation |
| 33 | Shivering lost – replaced by muscular rigidity |
| 31 | Pupils become dilated. Loss of consciousness |
| 30 | Insulin ineffective. Risk of spontaneous ventricular fibrillation – often unable to defibrillate |
| 26 | Major acid-base disturbance |
| 24 | Significant hypotension |
| 23 | Apnoea |
| 18 | Asystole |
| aThese temperatures are a guide and will vary between individuals. |
|
History
The history frequently gives an indication of the likely presence of hypothermia and may well be the only indicator. The following questions should be asked:
• Has the patient been immobile for a prolonged period (after a fall or entrapment)?
• Is the patient wet (rain or immersion)?
• Are they adequately clothed for the conditions?
• Has the patient been exposed to the wind?
• Has the patient got open wounds, increasing heat loss?
Examination
There are few consistent signs other than the patient’s skin feeling cold.
If a temperature is taken, it should be performed with a low-reading thermometer. Axillary temperatures are unreliable, but will at least give an indication of the degree of hypothermia.
The best guide is a rectal temperature, which is usually not practical prehospital.
Clinical signs of hypothermia:
• Shivering – disappears around 33°C
• Pulse – initially raised, then falls (but other factors interfere, e.g. hypovolaemia and raised intracranial pressure). The pulse is difficult to feel and weak. It may be irregular owing to cold-induced arrhythmias
• Breathing – slow and shallow (although initially the rate may be raised)
• Breath – fruity, acetone smell due to incomplete metabolism
• Mental state – confusion through to unconsciousness (but other factors interfere, e.g. hypovolaemia and raised intracranial pressure).
The patient who is thought to be hypothermic must be examined for signs of injury or medical illness which may be masked by the effects of the hypothermia. Hypoglycaemia should be specifically excluded.
Treatment
The initial management of the patient is aimed at reducing further heat loss. The patient must therefore be provided with protection against the elements. This involves:
• Insulating them from the ground using a foam mat
• Removing and replacing wet clothes
• Covering the head
• Applying blankets (including a ‘space’ blanket)
• Providing a windproof outer layer
• Providing shelter
• Providing warm drinks if the patient is conscious (alcohol must be avoided as it causes peripheral vasodilation)
• Careful handling as sudden manoeuvres can precipitate cardiac arrhythmias. (insertion of an oral airway may precipitate bradyarrhythmias or cardiac arrest and should be carried out carefully)
• Administering oxygen (ideally warmed and humidified)
• Where possible, administering warmed intravenous fluids.
The main prehospital danger is that the patient may suffer a cardiac arrest. This is most likely to be due to ventricular fibrillation.
The protocols for treating arrests should be followed, but it must be appreciated that it may be impossible to defibrillate successfully if the core temperature is below 30°C.
The relative protective effect of severe hypothermia on the brain gives rise to the edict that ‘no one is presumed dead until they are warm and dead’. Thus, resuscitation should be continued until the patient is adequately rewarmed.
No one is dead unless they are warm and dead
No attempt should be made to actively warm the hypothermic patient prehospital by other means, such as hot-water bottles or heaters. This causes peripheral heating, with opening of the skin and splanchnic blood vessels, resulting in the washing out of metabolites that have built up in the hypoxic tissue. When these arrive at the heart they can induce fatal arrhythmias.
Careless handling of hypothermia victims may precipitate fatal arrhythmias
Active rewarming
Active rewarming involves actively reheating the patient. There are a number of methods used in hospital:
• Warm humidified oxygen
• Warmed intravenous fluids
• Thoracic cradle heating
• Peritoneal lavage
• Oesophageal warming
• Extracorporeal rewarming using cardiopulmonary bypass
• Immersion rewarming in a bath (only used in immersion hypothermia).
For further information, see Ch. 40 in Emergency Care: A Textbook for Paramedics.
