Hypothermia

Hypothermia is defined as a core temperature of <35°C (<36°C in cases of trauma).

In a cold environment, our body will begin to shiver in order to prevent a drop in core temperature. Therefore, a shivering casualty may have a core temperature of >35°C (non-hypothermic) but if they are losing heat faster than it can be produced, this may be described as cold stress. The body will attempt to compensate through shivering and vasoconstriction.

There are three stages to hypothermia, which are defined by a core body temperature. However, pre-hospital in the outdoors, taking an accurate temperature can be difficult and therefore the assessment should be performed by symptoms rather than temperature.

Stage I – Mild (35-32°C)

• Concious
• Shivering
• Pale + Cold
• High HR + RR

 

Stage II – Moderate (32-28°C)

• Unable to care for themselves
• Impaired mental state
• Shivering may have stopped

 

Stage III – Severe (<28°C)

• Not shivering
• Unresponsive or severely reduced consciousness
• Vital signs present but faint, slow HR + RR

When moving through the stage of hypothermia everything speeds up, then slows down, then stops. This is Stage I “Compensating”, Stage II “Decompensating”, Stage III “Failing”. This is often simply described as moving through the continuum of:

• Stumbles
• Mumbles
• Fumbles
• Stumbles

To describe the decreasing level of consciousness and capability as hypothermia worsens.

Shivering is a compensation method the body can use to generate heat, however it will consume glucose and therefore hypothermia and hypoglycaemia may present together. As temperature decreases, the bodies standard functions become less effective – for example, blood clotting is less effective (coagulopathy). In cases of trauma this can be very significant due to the “Trauma Triad” (hypothermia > Coagulopathy > Acidosis).

When the core temperature is very low (moderate or severe hypothermia), any movement of the casualty can lead to cardiac arrest. This is due to hypothermia leading to cardiac irritability and rough movements therefore may precipitate an arrhythmia. Therefore all casualty handling must be as gentle as possible.

If the casualty is only mildly hypothermic they can be walked off, but in cases of moderate hypothermia you should have a low threshold for deciding to stretcher the casualty. In particular, you should be concerned with the potential for an “after drop”.

An “after drop” occurs where the casualties core body temperature is continuing to decrease (for example due to wet clothing) and they begin to walk off, however during this extraction their core temperature falls low enough to cause a cardiac arrest.

Management of hypothermic patients should be focused on preventing further heat loss, preventing cardiac arrest due to casualty handing, transferring the patient to hospital for rewarming. All casualties should be laid down immediately. Movement of limbs should be kept to a minimum. If necessary to remove wet clothing, consider cutting them off to reduce unnecessary limb movement. Utilise dry clothing, a vapour barrier, heat packs, and a survival shelter wherever possible. Heat packs should be applied on the trunk or under the arm pits, but not under the casualty or directly toughing the skin. 

In the event of hypothermic cardiac arrest, CPR should be started as soon as possible and should be continuous. Where this is not possible, for example due to difficult terrain or no mechanical CPR device being available, then intermittent CPR may be used:

• If the arrest was witnessed, CRP can be delayed by up to 10 minutes if necessary to allow time to move the casualty to a safer location, before starting intermittent CPR.
• If the arrest was not witnessed, perform 10 minutes of continuous CPR before performing intermittent CPR.

In both cases continuous CPR should be performed. As a last resort, in hypothermic cardiac arrest, in order to allow casualty extraction, intermittent CPR can be conducted as 5 minutes of CPR followed by a 5 minute gap – repeated until extraction is complete. This is only for hypothermic cardiac arrest and should not be used for another other arrest reason. Continuous CPR should be continued as soon as is possible.

In severe hypothermia the casualty may have no signs of life. However, there have been several cases of extremely low core temperature from which the casualty has fully recovered. For example, in 2011, a 7 year old girl had a core temperature of 13°C but was successfully rewarmed and resuscitated in hospital.

In all cases of severe hypothermia, the casualty should be sent to a hospital with an ECMO capability. This is extra-corporeal membrane-oxygenation and allows the casualty to be rewarmed by warming and oxygenating the blood outside of the body.