Hypothermia leading to cardiac arrest and death is relatively uncommon in Aotearoa New Zealand. In the medical profession, it’s called ‘accidental hypothermia’ to distinguish it from ‘therapeutic hypothermia’, which is used to treat certain medical conditions. Reference to hypothermia in this article is the former condition.

Although death from hypothermia is uncommon, hypothermia accompanying injury and illness is common in the outdoors. Even if hypothermia leads to cardiac arrest, however, there is a strong chance of survival – provided certain management and treatments are applied during rescue.

Accidental hypothermia is defined as the unintentional decrease in core body temperature (CBT) to below 35°C in an otherwise healthy individual, as a result of a cold environment. The condition is exacerbated by the exhaustion and low energy reserves often encountered in the backcountry, through high-energy outdoor pursuits such as tramping, mountaineering, skiing and adventure racing.

Hypothermia has a protective metabolic effect, mainly through the reduction of oxygen demand (6% reduction for every drop in CBT of 1°C) which explains why prolonged resuscitation is often successful.

Prevention is best

Be aware of the conditions, especially cold and wind, and how they are affecting your group. If you are feeling low in energy or lightheaded, then chances are others in your party will be feeling the same. Be vigilant for early symptoms of hypothermia, which may include slurred or slow speech, stumbling, a lack of co-ordination, apathy and (paradoxically) feeling warm (in extreme cases this leads to ‘paradoxical undressing’). Ensure you and your party have adequate thermal and windproof clothing, keep well hydrated and regularly eat high-energy food. People who have recently been unwell (such as having the flu) or who are less fit, will be more susceptible to hypothermia.

Safety first; no more accidents

Remember that if one person in the party has hypothermia then it is likely others will too, especially if the group has to stop to administer aid. Ensure all of the party are protected from the environment, especially wind, and have adequate food and drink (preferably warm). Erecting a temporary shelter may be necessary.

Handle gently

Hypothermia patients are prone to lethal heart arrhythmias caused by rough handling. So minimise movement. Measuring CBT In the past, trying to measure the victim’s core body temperature has been a fixation in field treatments. However, most standard thermometers cannot accurately measure low CBT, and taking measurments can be extremely challenging in the outdoors. A much more useful tool is the ‘Swiss Staging System’, which can be used without knowing the CBT. Think of it as a hypothermia ‘severity prompter’ reflecting worsening condition, from alert and shivering, to not alert and not shivering.

The Swiss Staging System for Management of Accidental Hypothermia

StageClinical SymptomsBCTTreatment
IConscious, shivering35 to 32°CWarm environment, clothing, warm sweet drinks, active movement
IIImpaired consciousness, not shivering<32 to 28°CCardiac monitoring, minimal movement, horizontal position, insulation, active external rewarming, minimally invasive rewarming
IIIUnconscious, not shivering, vital signs present<28 to 24°CII plus airway, CPB, ECMO
IVNo vital signs<24°CII & III, CPR, ?drug therapy, ?defibrillation
CPB – cardiopulmonary bypass, ECMO – extracorporeal membrane oxygenation, CPR – cardiopulmonary resuscitation

Prevent further heat loss

Establish shelter on the lee side of the wind – using a tent, bivvy bag, snow cave trench or dome. The wind chill effect, especially combined with wet conditions, significantly increases cooling. Once in the shelter, remove all the patient’s wet clothing, including underwear. If the patient is severe, consider cutting clothing off to minimise movement. Insulate the patient (and rescuers) from further cooling: using insulation from the ground and dry warm clothing, especially on the head and hands.

Commence rewarming

Keep the patient as horizontal as possible, especially if rescued from a cold water immersion situation; the drop in blood pressure caused by being upright can be fatal. Shivering, if present, is the body’s way of very effectively promoting heat production. Commence passive and active rewarming. If the patient is conscious, give them warm, sweet drinks, but NOT alcohol. Wrap heat packs against their armpits, groin and the back of their neck. Warm, humid air is effective (boil a billy in tent vestibule, being extra careful about fire and carbon monoxide poisoning).

Use a warm rescuer to provide body-tobody contact with the patient in a sleeping or bivvy bag. Exhaled air from the rescuer in the vicinity of the patient’s nose and mouth will provide relatively warmed and humidified air with adequate oxygen content.


If hypothermia has caused a cardiac arrest, full recovery is possible (because of the hypothermic protective effect) even after prolonged resuscitation lasting hours. If no signs of life are present, commence cardio pulmonary resuscitation (CPR). The Wilderness Medicine Society recognises this is impractical in a field rescue setting and recommends an intermittent alternating sequence of CPR, consisting of five minutes continuous CPR and then up to five minutes without CPR, until such time as continuous chest compressions can be resumed.

Use a defibrillator if one is available and a paramedic or other medical professional recommends its use. Remember, however, that if it does not have a result after several shocks, it is likely to be ineffective. Advanced life support drug therapy and airway management also have limitations.

Not starting or terminating resuscitation – how to make this decision?

Patients should not be considered dead until they are ‘warm and dead’ noting that apparent rigor mortis and fixed dilated pupils can be present in patients with severe hypothermia.

Unless the patient shows obvious signs of death – definite rigor mortis, or other injuries incompatible with life, such as being frozen solid, or buried under an avalanche with more than 30 minutes of airway obstruction – then resuscitation should be commenced and continued until they can be transported to a centre where aggressive rewarming is possible.

Have a plan

Plan ahead for evacuation. This relies on leadership and coordination between the

rescue, pre-hospital, transport and hospital teams to avoid delays at each transition point.

Don’t attempt self-rescue, i.e. make the victim walk, even with a person who has seemingly recovered from severe hypothermia. Their reserves will be non-existent and there are numerous cases where this has been attempted with dire consequences.

Hypothermia-caused cardiac arrest is a completely reversible condition. While true cases of death caused solely by hypothermia are uncommon here, there is plenty of case literature from overseas, particularly colder countries, which guide our management and treatment. If all phases of hypothermia management and treatment are co-ordinated and timely, astounding saves can be achieved.

(FMC Bulletin 206)