Protocol Updated 13/11/23. Text still to be re-checked.

What is hypothermia and why is it Important?

The normal adult body temperature is about 37°C.  Hypothermia is defined as a fall in core body temperature to 35°C or less. However, in the presence of trauma, even a fall down to 36°C is associated with increased mortality (death rate), so every effort must be made to keep all trauma casualties warm.

Hypothermia occurs when the mechanisms to prevent heat loss (described below) are no longer sufficient to prevent a fall in temperature below the level required for normal metabolism and bodily function. From a purely body temperature point of view, a casualty’s chance of survival is directly related to their core body temperature. The lower the body temperature, the higher the mortality. Although the lowest temperature from which someone has recovered is 13.7°C, few people who get that cold make a full recovery.

Overview of the Physiological Effects of Hypothermia

As the body temperature drops, metabolic requirements fall significantly. The conscious level, heart rate, respiratory rate and breath size all decrease and the heart rhythm may become irregular. These changes make signs of life difficult, if not impossible, to detect and in severe cases, can result in living casualties appearing to be dead. Unless there are other injuries incompatible with life, casualties without detectable signs of life who have severe hypothermia (see below for stages) should be managed as alive. This has given rise to the guiding principle – “not dead until warm and dead” and is discussed further below.

Whilst reduced core temperature is an overall threat to life, it does offer some protection from hypoxia. Brain metabolism decreases as the temperature falls below 35°C, so that when cold, it requires less glucose and oxygen and can survive prolonged periods with a reduced blood supply without sustaining significant damage. This includes periods of cardiac arrest, which would prove fatal in a normothermic (normal temperature) individual. As a guide, a normothermic brain will only survive 3-4 minutes cardiac arrest. By contrast, at 27°C, in a young person, the brain will tolerate about 15 minutes and at 14°C, about 40 minutes. The figures for a healthy person in their 60’s will only be about half those, and even shorter in people with circulation problems or a history of stroke.

How is Body Heat Lost to the Environment?

Understanding how heat is lost from the body will enable the rescuer to better appreciate how to care for a casualty, and also how to stay safer on the hills themselves. There are four mechanisms:

• Radiation - the main route in a normal environment. It occurs when the ambient temperature is less than the body i.e. there must be a temperature gradient.  Heat loss by radiation can be reduced with a space blanket. However, if the casualty is already very hypothermic, a space blanket will be ineffective as there is very little body heat to reflect back.

• Conduction – transfer of heat from the body by direct contact with other objects. Water conducts heat away from the body 25 times faster than air so wet clothing is particularly dangerous. This emphasises the importance of staying dry and explains why a wet person cools off so quickly.

• Convection – the body heats air in contact with it. This air then moves away and is replaced by colder air, which in turn is heated up and then moves away. Thus, convection is the main mechanism of heat loss in windy conditions. A wind speed of 8 mph removes four times more heat than a wind of 4 mph. Heat loss by convection is reduced by insulation. Note that wool and synthetic insulation provides effective insulation when wet, unlike down, which does not.

• Evaporation – this applies to fluids on the body surface (sweat, rain, etc.). This also illustrates the importance of staying dry.
   

Body Mechanisms to Prevent Hypothermia

The brain senses when the body temperature starts to drop and immediately initiates a series of responses that are intended to prevent a further fall and restore the temperature back to normal. These responses are behavioural (e.g. seeking shelter, putting on extra clothing, increasing exercise, and taking in sufficient food) and physiological (e.g. shivering, vasoconstriction). Exercise produces much more heat than shivering, particularly when large muscles are working. However, we cool off more quickly when we stop after exercise unless we are adequately protected.

Hypothermia in Mountain Rescue

In mountain rescue, hypothermia can be primary i.e. the only problem, or secondary to other injuries and diseases. It can occur at any time of the year, although severe hypothermia is more likely to occur as a primary condition during the colder months.  Poor clothing, alcohol consumption, hypoglycaemia (low blood sugar) and exposure to water and wind increase the risk of it occurring at other times of the year.

Almost all casualties in the Lake District will be at least mildly hypothermic by the time we arrive if they have been sitting still on the hill side, particularly in winter, or in the wind and rain. Therefore, they should all be assessed for hypothermia, including measuring their temperature, and reassessed prior to hand over to another agency.

Practical Assessment of Hypothermia 

Who is at risk?

Hypothermia is particularly likely in certain situations so expect it if any of the following are present:

• Adverse weather conditions (cold, wet, windy)

• Night time

• Higher altitude. Hypothermia can occur at any altitude, but the higher the location, the lower the ambient temperature, the higher the wind speed, and the more energy that will have been consumed to get up that far.

• Been out on the hills for a long time (which means they will have depleted energy reserves)

• Been unable to move from their current location because of injury, no lighting, crag fast, etc.

• Insufficient food

• Very young or very old

• Inadequate clothing

• Inadequate protective equipment e.g. no bag or tent to shelter in

• Reduced conscious level, particularly in the absence of trauma
  
  

Assessment – History

Consider the following:

• The ambient temperature

• Wind chill

• Clothing/shelter

• Length of exposure

• Have they been insulated from the ground

• Have they become wet

• Time since they stopped exercising

• Exhaustion

• When and what they last ate or drank. This is not for SAMPLE, but will give an indication of how depleted their energy reserves will have become
  
  

Assessment – Clinical Features

Signs that hypothermia is only mild are:

• Fully alert

• Actively shivering

• Able to walk

Indications in general demeanour and behaviour that things have deteriorated significantly are:

• Impaired consciousness, particularly in the absence of trauma is a warning sign

• Difficulty speaking, slow thinking and amnesia

• Paradoxical undressing (the casualty feels hot and tries to take their clothes off)

• Terminal burrowing (the casualty will enter small, enclosed spaces to shelter)

Physical signs on examination:

• Shivering may be present or absent

• Cold and pale skin

• Decreased body temperature (see below)

• Slow and or laboured movement, stumbling pace, inability to use hands

• Decreased heart rate

• Decreased respiratory rate

• Decreased blood pressure
  
  

Measuring temperature and using the ear thermometer

Why do we need to know the temperature in hypothermia?

There are three main reasons:

1. To help identify the cause of reduced conscious level. If the body temperature is 32°C or more and there is no trauma, then the cause of reduced conscious level is not hypothermia. So you need to look elsewhere e.g. hypoglycaemia, stroke, etc.

2. The risk of cardiac arrest increases as the temperature falls below 29°C.

3. In pure hypothermia, asystole (flat line on the ECG which means no heart activity) does not tend to occur until the temperature is <24°C. Therefore if the temperature is higher e.g. 28°C and asystole is present, the casualty may have had a hypoxic cardiac arrest (i.e. lack of oxygen e.g. myocardial infarction; avalanche; drowning), and therefore may not benefit from prolonged resuscitation attempts. The Alpine practice in casualties with asystole is to look for evidence that significant hypothermia was probably present before cardiac arrest occurred e.g. lightly dressed casualty (in our area, an example would be a Fell runner).
   
   

Which temperature is important?

In hypothermia, the most important temperature reading is also the hardest one to measure. We need the core body temperature, which essentially is the temperature around the heart, since this governs whether or not a cardiac arrest is likely to occur. In hospital, this is done by inserting a temperature probe either into the oesophagus, or alternatively along a major vein and passing it into the heart. Clearly, these are impossible in MR so we have to use an alternative site. The only thermometer that has been shown to be accurate in studies is a special type of ear thermometer, but this is not available for prehospital use in an easily-portable form. Therefore, we use an infrared ear thermometer, as this is the least inaccurate of the non-invasive methods available. It will read down to 20°C, and if used correctly, it will read to within ±2°C, which is accurate enough for our requirements.

For information, the following are either too inaccurate in hypothermia or cannot be easily used in a prehospital situation:

• Armpit (axilla)

• Rectal

• Oral (although we initially trialled this method, there are no published validated studies that indicate that oral temperature is accurate enough, which is why we have abandoned its use).

Measuring body temperature using the Braun Thermoscan Pro 6000 infrared ear thermometer

Severity of Hypothermia

In order to decide upon the appropriate clinical management and where the casualty should be evacuated to, we need to determine the severity of the hypothermia. This is done using the ICAR MEDCOM scale (International Commission for Alpine Rescue), which is used internationally. It has also recently been revised slightly. The letters ‘HT’ mean ‘Hypothermia’.

MREW Hypothermia Protocol

There is one protocol to guide management and separate Explanatory Notes.

Essentials of Management

In practice, and for simplicity, there are only two categories of hypothermia we need to worry about:

• Mild hypothermia in a low-risk casualty

• Everything else

The management is summarised on page 30 of Casualty Care Revision in Mountain Rescue.

ASSESS THE SEVERITY OF HYPOTHERMIA

Base this predominantly on the symptoms and signs:

• Shivering/not shivering

• Conscious level

• Pulse rate

• Presence of trauma

• Temperature

Checking for signs of life in the apparently dead person
People with severe hypothermia may be unconscious, and be breathing very slowly and have a very slow pulse e.g. 4 breaths per minute; irregular and shallow, with a pulse rate of <10/min. You can use the Propaq to determine the presence of cardiac activity using ECG.

If the ECG shows regular similar-shaped squiggles, this means that the heart is working so do NOT start CPR, regardless of how slow the pulse rate is. Assume that the casualty is still alive, regardless of whether or not you can feel a pulse or detect that they are breathing. In extreme hypothermia, the brain can survive with a very slow heart rate.

If the breathing is sustained, this means that blood is reaching the brain and THEY ARE ALIVE

DO NOT BE TEMPTED TO START CPR

If you start CPR, you will actually cause the heart to stop and any blood flow to the brain will immediately stop, making the patient totally dependent on the quality of your CPR.

If in doubt, unless there clearly are injuries that are incompatible with life, they are not dead until warm and dead!

DECIDE WHICH GROUP THE CASUALTY IS IN

Mild Hypothermia in a basically well person (low risk)

• Normal conscious level

• Shivering

• Pulse steady and in normal range

• No trauma

All other cases (medium to high risk)

Even if the temperature is in the ‘mild’ range, the presence of any of these factors will increase the risk by reducing physiological reserve. These casualties need more careful handling

• Reduced conscious level / unconscious

• Shivering minimally or stopped

• Abnormal respiratory and pulse rates

• Trauma

• Temperature <32°C

• Unable to care for themselves

• History of inadequate food intake

• Older people (possibly >60, depending on fitness; certainly >70y)

• Presence of concurrent medical conditions e.g. diabetes

• Children (the smaller they are, the more vulnerable they are)

MODERATE & SEVERE (HT Stages II/III/IV) are life-threatening, and the colder they are, the more this is so.

Follow the MREW Severe Hypothermia Protocol. There is a copy in the Cas Card folder and in the AutoPulse battery rucksack.

The main points are summarised here.

Issues

• The casualty is still alive

• They have a significant level of hypothermia

• If the temperature falls to below 30°C, they cannot rewarm themselves, and are at increasing risk of a cardiac arrest

• All casualties in this category should go to hospital, even if there is no trauma

 Cardiac arrest in severe hypothermia

Important

Packaging and minimising further heat loss

When body temperature is below 30°C, you must balance the risks of removing the clothing (increased cooling during removal or triggering an arrhythmia if the body is handled roughly) against leaving wet clothing in place. Expert opinion suggests it is safer to leave wet clothes in place until the casualty is in hospital where it is warm.

If you do decide to replace wet clothes with dry, do not do it on an open hillside. Only consider it in a group shelter with four or more rescuers and after the temperature inside has warmed up and stabilised. Do not undress in the conventional sense but carefully cut off the clothes, removing them a bit at a time rather than all at once to minimise further cooling and reduce movement of the casualty. Insulate with dry clothing.

Heat packs will reduce further cooling. They are not to start rewarming in the field.

Wrap in an occlusive sheet (we are using the ‘Blizzard’ Blanket) and seal around the openings, ensuring access where necessary. Once the atmosphere inside the enclosure is fully saturated with water vapour, no more heat can be lost by evaporation from the skin, which is the major route of heat loss in this situation.

A reflective blanket has little additional value over a heavy duty plastic bag as virtually none of the heat loss is by radiation because the skin temperature is so low. However, a reflective blanket may be of value when heat packs are in place by helping to keep the heat from those inside the enclosure.

Finally wrap in the Cas Bag.
 

How to use heat pads

• Expose to air for 15 minutes to allow them to warm. For expediency, this can be done as you leave the vehicle/during the approach.

• Place heat pads around the chest and armpits only, not the limbs or the back. Do not worry about, or attempt to rewarm, the peripheries. Warming the limbs can have undesirable effects including triggering an arrhythmia.

• Do NOT place heat pads directly on the skin. They will cause a burn in hypothermic patients. Place them on top of a layer of clothes. For example, if laid on top of a wet base layer, this will warm up over about 15 minutes or so and the heat will pass to the patient.

Casualty handling and positioning

• Handle the casualty extremely carefully as a sudden shock can precipitate a cardiac arrest.

• Avoid raising the legs and arms. If the legs are raised, the sudden influx of cold blood from the legs into the core will cool the heart and can cause life-threatening arrhythmias.

• Keep the casualty horizontal. If they are to be winched into a helicopter, insist that they are winched on a stretcher to ensure they are kept horizontal. If horizontal is not possible, slightly head up is marginally safer than head down because with head down, cold blood will return from the legs and further cool the heart.
   

Airway management in severe hypothermia

• Start medium flow oxygen(6L/min)

• When evacuating by stretcher and the casualty is not breathing, it is only possible to breathe efficiently for the casualty if you insert an i-Gel and ventilate with a bag-valve.

• Squeeze the bag gently at a rate of about 6 breaths per minute.
   

Monitor for cardiac instability

• Connect the Propaq. Monitor continually in case a cardiac arrest occurs. It doesn’t matter whether you have been taught about ECG’s. The interpretation is very simple in these cases. If you get regular similar-shaped squiggles on the display e.g. at 20/min (the monitor displays the rate), you know you have got acceptable heart function. If it becomes a flat line or a haphazard wave form, that’s a cardiac arrest. Always do a snapshot so that you can show it to someone who can read ECGs.
   

CPR in severe hypothermic cardiac arrest

If you are sure that a cardiac arrest has occurred, the ideal management is to deliver continuous CPR until the casualty reaches hospital and is being rewarmed with the ECMO machine.

• If defibrillation is required, do not attempt it more than three times. This is because at temperatures below 30°C, the heart is usually resistant to defibrillation until the heart rewarms. Multiple attempts at defibrillation whilst the heart is cold will be ineffective and will injure the heart muscle.

• Call for the AutoPulse immediately. Only team members who have been trained to use it should do so. Everyone else must only do manual CPR.

DELAYING THE START OF CPR
If you witness a hypothermic cardiac arrest (e.g. the patient is connected to the Propaq monitor and you observe the ECG change from a normal-looking rhythm to an abnormal rhythm e.g. ventricular fibrillation or asystole (flat line), you do not have to start CPR immediately if delaying a few minutes would enable the casualty to be moved to a safer location e.g. out of a stream, off a ledge, or away from where an avalanche has just occurred (in case of a secondary avalanche). You have up to 5 minutes before you need to start CPR.

INTERRUPTIONS TO CPR
Once you have started CPR, the ideal is to continue it until the casualty reaches hospital and is being rewarmed with an ECMO machine. The best way to achieve this is with the AutoPulse. If the AutoPulse is not available or it breaks down, you will have to resort to manual CPR.

There may be occasions during an evacuation when it is physically impossible to deliver good quality manual CPR e.g. during a steep descent, when moving the casualty on the stretcher, etc. In these situations, it is permissible to interrupt CPR periodically. The aim will be to deliver at least 5 minutes uninterrupted manual CPR and then stop chest compressions for a period whilst you move further off the hill.

Note that the fact that CPR should be uninterrupted does not mean it has to be carried out by the same person (see below).

DELAYED AND INTERMITTENT CPR TIMES: ‘5 on; 5 off’

• For a witnessed hypothermic cardiac arrest, you can delay start of CPR by up to 5 mins

• Try to start CPR as soon as is practicable and do at least 5 mins uninterrupted CPR

• If necessary, CPR can be interrupted again for up to 5 minutes before it must be resumed.

• Resume continuous CPR as soon as feasible

Changing rescuers to ensure high-quality CPR

Delivering chest compressions is exhausting. Studies have shown that by two minutes, CPR efficiency, as shown by rate, regularity, and depth of compression, will have dropped dramatically. Therefore, change rescuer every 2 minutes. If someone is getting tired before the two-minute period has elapsed, change rescuer at the next opportunity i.e. when the next two rescue breath are delivered.

Referral To ECMO

Once in cardiac arrest, your chances of resuscitating a severely hypothermic casualty are vanishingly small. The casualty will need to be rewarmed before any type of cardiac rhythm can return. Specialist cardiac support will be required to rewarm them effectively and maximise their chance of survival.

ECMO Centres

• Wythenshawe Hospital, Manchester

• Royal Victoria Infirmary, Newcastle

• Alder Hey Children’s Hospital

These phone numbers are all included on the Hypothermia Protocol so you can contact them about direct admission.  ECMO is a complex and very expensive undertaking. When you call, you will be asked a series of questions as they try to ascertain whether the casualty is a good candidate for ECMO.

The ECMO referral form (below) has been developed to include the key questions they are likely to ask you and is carried in the hypothermia sac and AutoPulse battery sack so you can ensure you have the answers prepared when you call.

The video below is an amazing story about the Praesto Fjord incident in Denmark, in which seven teenagers were immersed in ice cold water following a boating incident. All were rewarmed with ECMO and all survived and made a good recovery. It reminds us why we do what we do. Seven young adults are alive because someone didn't give up just because they appeared to be dead.