Hypothermia

Hypothermia is defined as a core temperature of less than 35°C. Some authorities classify this further into mild, moderate or severe:

Mild 32-35°C

Moderate 30-32°C

Severe <30°C

Variations in the temperatures used to define these categories exist between authorities/countries and this reflects the arbitrary nature of the definition and continuum of this problem rather than it being three distinct patterns of illness.

As the core temperature falls, numerous compensation mechanisms will try to counteract this. The patient will be peripherally vasoconstricted in an attempt to maintain their core temperature. This relative increase in the core circulating volume may generate a diuresis. The patient may also be shivering, this reaction is modulated by the posterior hypothalamus and spinal cord as an action to generate heat from skeletal muscle movement. It is important to note that the shivering response is absent at lower temperatures and so its absence does not rule out hypothermia. Shivering increases muscular work and also oxygen demand. this combined with the extra workload for the myocardium due to peripheral vasoconstriction (with increased cardiovascular afterload) may lead to myocardial ischaemia in susceptible individuals.

As the patient’s temperature falls, there are increasing impacts on other body systems, with deterioration until cardiac arrest.

Features of Hypothermia

Key features include³:

Temperature	Feature
35	Maximum shivering thermogenesis
34	Amnesia and dysarthria
33	Ataxia
31	Shivering stops
30	Atrial fibrillation and other dysrhythmias
29	Reduced conscious level, bradycardia and bradypnoea
28	Prone to ventricular fibrillation
27	Reflexes reduced
26	Major acid base disturbances
24	Profound hypotension
22	Ventricular fibrillation
18	Asystole
14.2	Lowest recorded hypothermia survival in infant
13.7	Lowest recorded hypothermia survival in adult

 

Treatment of Hypothermia

The first step is to remove the patient from the cold environment, remove any wet clothes and gently dry the skin. Wet clothes should be cut off rather than stripped to minimise patient movement. This should be done in conjunction with a ABCDE assessment, and any necessary immediate treatments. High flow oxygen should be administered via a non-rebreathe mask, and the patient rewarmed.

For mild hypothermia, it may be adequate to remove wet clothes, dry the patient and simply wrap the patient up/dress them in warm clothes. It is particularly important to ensure that their head is covered as this is an important area for heat loss.

For more marked hypothermia, it may be appropriate to actively warm the patient using a warm water bath. Clearly this is only an option if the patient has a normal level of consciousness and no other ABCDE issues requiring treatment.

During the rewarming process it is important to monitor the heart rate, blood pressure and conscious level, as marked vasodilatation may occur.

It is important that any patient with hypothermia is observed closely in case a rapid deterioration in their condition occurs.

It is also important that the patient is handled gently, since inappropriate stimulation (particularly of the airway) may cause the patient to deteriorate into ventricular fibrillation.

Any patient with all but the mildest hypothermia, should be transferred as soon as possible to an appropriate emergency department.

Hypothermia and Cardiac Arrest

As the human body cools, the demand for oxygen falls, and hypothermia can produce a neuroprotective effect, meaning that even after prolonged cardiac arrest a good neurological outcome may be possible. For this reason, resuscitation attempts should be continued until the patient has been warmed.

The usual resuscitation principles should be applied, but particular attention should be given to the following:

The airway must be open but extreme care should be exercised during airway manoeuvres, particularly the use of adjuncts as airway stimulation may trigger ventricular fibrillation.

The hypothermic myocardium may not respond to cardiac resuscitation drugs and defibrillation. Poor peripheral circulation and reduced drug metabolism may also lead to peripheral pooling of toxic levels of these drugs.

In the event of ventricular fibrillation or pulseless ventricular tachycardia, three shocks should be delivered as per the standard protocol irrespective of the temperature. If, however, the patient remains in VF/VT after these three shocks, further shocks should be withheld until the patient’s core temperature is above 30°C.

Clearly accurate temperature measurement in the pre-hospital setting is difficult, and clinical judgement will need to be used.