Authors: Tim Harris, Jonathan M Jones / Editors: Adrian Boyle, Gavin Lloyd / Reviewer: Rebecca Ford, Tim Harris / Code: ELP6, EnvC3, RP3, SLO3 / Published: 30/01/2023



The temperate climate of the UK means that severe hypothermia leading to cardiovascular instability and cardiac arrest is rare. Despite this it is important to understand the patho-physiology and treatment of this emergency as the prognosis, particularly in the young and previously healthy can be remarkably good. Achieving a good outcome requires aggressive and prolonged input from a multi-disciplinary team.

Definition and Epidemiology

The Body’s Core Temperature

The human body has evolved to work within a narrow, carefully maintained, core temperature range. As hypothermia develops, many organ systems stop working properly. The physiological effects of hypothermia mean that the management of cardiac arrest requires an altered approach. Remember, too, that hypothermia can be protective – greatly prolonged resuscitation efforts may be justified.

The image shows classifications of hypothermia. Hypothermia is defined as a core temperature below 35 °C, with mild hypothermia classified as 32-35°C, moderate 30-32°C and severe disease below 30°C. It is frequently associated with submersion injury and drowning (see forthcoming session).

The treatment of hypothermia depends on symptoms and temperature.

The Most Vulnerable

Two groups of patients are seen – winter sports and wilderness enthusiasts and urban poor. Factors contributing to the latter group include alcohol, drugs, malnutrition and poor socioeconomic conditions [2]. Deaths in the UK are rare and occur predominantly in the elderly, more often in women than men [3]. In 2017 there were 34 deaths in Scotland that involved hypothermia

This session concentrates on patients with actual or impending cardiopulmonary arrest. The treatment of mild-moderate hypothermia and localised cold injuries are dealt with elsewhere in the curriculum.

Body Cooling

Body temperature is normally tightly regulated but is less well-controlled in the very young, and impaired by age, disease, injury and alcohol/drugs.

As the body cools, the metabolic rate falls and neural transmission is inhibited [4]. Multiple derangements occur including:

  • Depressed myocardial contractility
  • Leftwards shift of the oxygen dissociation curve (leading to reduced availability of oxygen at the tissue level)
  • Vasoconstriction
  • Ventilation-perfusion mismatch
  • Increased blood viscosity

All of these reduce tissue oxygenation [5].

Initial Physiological Changes

Initially, sympathetic drive increases heart rate and respiratory rate, and heat is produced via shivering. At around 30°C, this process ceases and ventilation, heart rate, blood pressure and cardiac output fall.

Intravascular volume falls due to cold diuresis and fluid shifts into the extravascular space.

Conduction Abnormalities

Sinus bradycardia develops and can be followed by AF. Below 32°C, ventricular arrhythmias including ventricular fibrillation (VF) may occur. Finally, asystole [6] results. Note Osborn waves are another EGC abnormality.

Note that malignant arrhythmias are unlikely to be hypothermia-induced at temperatures above 32°C – consider alternative causes such as acute coronary syndrome (ACS). The rhythm strips below show the different stages of hypothermia.

Cardiac Arrest

Cardiac arrest may occur as a direct result of hypothermia, or hypothermia may occur secondary to cardiac arrest. Primary hypothermic cardiac arrest usually requires severe hypothermia.

Because hypothermia protects the vital organs, including the brain, the prognosis is better than in non-hypothermic arrest, especially if the initial insult did not involve asphyxia – a condition in which insufficient, or no, oxygen and carbon dioxide are exchanged on a ventilatory basis – caused by choking or drowning or electric shock or poison gas. There are many documented cases of survival after long periods of circulatory arrest [7-9].

‘You’re not dead until you’re warm and dead’.

It is important to note that these survivors are usually young fit individuals.

The lowest recorded neurologically intact survival following hypothermic cardiac arrest (in this case associated with immersion) is 13.7°C [9].


As with any critically ill patient, assessment and management will occur simultaneously and follow an ABC approach.

In determining whether hypothermia is playing a significant role in your patient’s presentation what environmental and patient specific factors should you consider?

In determining whether hypothermia is playing a significant role in your patient’s presentation consider:

  • Where they were found
  • The ambient temperature and weather conditions
  • The patient’s clothing
  • The patient’s age
  • Co-morbid conditions and state of nutrition
  • Alcohol and drug use

Primary or Secondary Problem?

It is important (but difficult) to try to decide whether hypothermia is the primary problem or secondary to another pathology. If they are simply cold because they have been lying in a cold environment following a cardiac arrest from another cause it is doubtful aggressive treatment of hypothermia is going to make much difference.

Getting as much information as possible about the circumstances is essential.

Furthermore, it is important to bear in mind that while hypothermic arrest is survivable, being frozen is not. If the body is frozen (for example the chest is so stiff that cardiopulmonary resuscitation (CPR) cannot be carried out or there is ice in the pharynx) resuscitation attempts are futile [10]. Unsurvivable injuries should always stop resuscitation attempts. A potassium > 12 mmol/L makes survival extremely unlikely.

Checking Cardiac Output

Hypothermia can cause a low volume low pressure cardiac output with a blood pressure un-recordable by non-invasive techniques. (The image shows suprasternal Doppler cardiac output monitoring.) [1]. To avoid missing a weak pulse, standard advanced life support (ALS) technique is modified, in that the pulse check should last 1 minute.

There is a role for invasive monitoring and non-invasive Doppler or ultrasound techniques in assessing cardiac output but do not let prolonged attempts stop CPR.

If there is any doubt about finding a pulse, start CPR as for a normothermic patient.

Core Temperature

Knowledge of core temperature confirms the diagnosis, directs future therapy and assesses the effectiveness of re-warming. Skin surface temperature measurement is of no use. Different sites of measurement may give different temperature readings and imply differing rates of re-warming – so it is sensible to select one site and use it consistently.

Rectal temperatures may demonstrate a lag during the re-warming phase of several degrees, which may lead to escalation of care when simpler re-warming strategies are in fact working [4]. Rectal temperatures can also be unreliable if there is faecal loading. Tympanic thermometers are usually accurate down to about 20°C but cannot provide continuous measurement. For these reasons, oesophageal or vascular temperature probes are recommended [4].

Blood Tests

Blood should be taken for arterial blood gas, full blood count, electrolytes/urea/creatinine, calcium, magnesium, coagulation studies and amylase [2].

A variety of abnormalities are possible:

  • Hypoglycaemia and obvious electrolyte disturbance should be treated [2].
  • Hypokalaemia is very commonly observed due to intracellular potassium shifts. Treat if severe but use caution as the potassium will increase with re-warming.
  • Hyperkalaemia my represent cell necrosis. Very high levels (>10 mmol/L) are predictive of death and contraindicate re-warming.
  • Elevated creatinine is also seen and may not be a true reflection of renal function.
  • A coagulopathy similar to disseminated intravascular coagulopathy is common.
  • Loss of intravascular fluid causes haemocrit to rise around 2% for each.1°C fall in core temperature. Lower than expected haemocrit may suggest blood loss.
  • Thrombocytopenia may occur due to marrow suppression or hepato-splenic sequestration [2].

Blood Gas Analysis

Blood gas analysis can cause some confusion in hypothermia. Blood gas machines will warm the sample to 37°C and report those values. In vivo, the values will be different – partly because of the increased solubility of gases as the temperature of a liquid falls.

It is possible to mathematically ‘correct’ the values to the patient’s actual body temperature but interpretation of these values is very difficult – partly because we have little idea of what the ‘normal’ values for hypothermia should be.

To avoid confusion and ensure consistency use the ‘uncorrected’ values: that is the values the machine reports after warming the blood to the standard 37°C.

Cardiac Arrest

Once cardiac arrest is confirmed:

  1. Apply basic life support (BLS) and ALS as described in the European Council Guidelines (ERC) guidelines [1].
  2. Re-warm the patient as fast as possible – re-warming is the best anti-arrhythmic and inotrope in this situation.
  3. Look for, and treat, co-morbidities and consider alternative diagnoses. Why are they hypothermic? Is there a significant pathological process other than hypothermia?Remember the 4Hs:
    • Hypoxia
    • Hypovolaemia
    • Hyperkalaemia, hypokalaemia, hypocalcaemia, acidaemia, and other metabolic disorders
    • Hypothermiaand 4Ts:
    • Tension pneumothorax
    • Tamponade
    • Toxic substances
    • Thromboembolism (pulmonary embolus/coronary thrombosis).
  4. Anticipate a prolonged resuscitation that could require significant multi-specialty input.

Changes to Standard Advanced Life Support

Both ALS and BLS should be instituted [2,4,8]. However, there are some important adjustments to the usual algorithm [1]:

Defibrillation and pacing

Defibrillation is less effective in hypothermia. For ventricular fibrillation/ventricular tachycardia (VF/VT) defibrillation may be tried up to three times but is then not tried until the temperature reaches 30°C.

Do not try pacing unless bradycardia persists when normothermia is reached. Sinus bradycardia may be a physiological response and is not treated specifically.


Normocapnia will be achieved at lower minute volumes than normal and hyperventilation risks cerebral hypoxia through reduction of cerebral blood flow [1-2,4,5]. Aim for a normal CO2 on ABG (not corrected for the patient’s temperature).


In a patient with a perfusing rhythm, intubation (or other rough handling of the patient) may precipitate VF, although the evidence for this is mainly animal-based and it is rare. One observational study of > 100 patients recorded no cases of intubation induced arrhythmia.

Resuscitation drugs

Drugs are often ineffective and will undergo reduced metabolism; so these are withheld below 30°C then given with twice the time interval between doses until either normothermia is approached or circulation restored. So, adrenaline would be given about every 8-10 minutes once the core temperature is above 30°C.

Chest compressions

Hypothermia causes muscular stiffness: chest compressions may be harder work than normal. Make sure that the individual performing chest compressions is swapped frequently. There are several case reports about the successful use of mechanical compression devices in hypothermic cardiac arrest.

Passive Measures

Re-warming can be passive or active (internal or external). In a patient with a perfusing rhythm, passive measures may be sufficient. In patients in cardiac arrest, active internal re-warming is mandatory.

For passive re-warming to be effective it’s necessary to:

  • Ensure a warm environment
  • Cover the patient with a blanket (forced air warming ‘blanket’)
  • Cover the patient’s head (theatres will usually stock insulated elasticated hats)
  • Remove wet clothing and dry the patient.

Active Measures: Cardiopulmonary Bypass

In a hypothermic arrest, the therapeutic manoeuvre of choice is cardiopulmonary bypass (CPB) [4,5]. This may be instituted using femoro-femoral or aortic-right atrial bypass. The former is more suited to the emergency department (ED) environment as it is easier and faster to establish and prevents further heat loss, which will result from opening the chest [5]. If femoro-femoral bypass is established, chest compressions should generally be continued to ensure acceptable flow through the cardio-pulmonary circulation.

Temperature increases of 1-2°C every 5 minutes may be achieved. Bypass should be instituted for patients in cardiac arrest, patients with haemodynamic instability and a core temperature below 32°C, frozen extremities and rhabdomyolysis with hyperkalaemia [5].

Note the size of the cardiopulmonary bypass machine in the image shown. It may be easier to take the patient to theatre than the machine to Resus.

Active Measures: Alternatives to Cardiopulmonary Bypass

CPB simply will not be available to a lot of departments. If it is not an option (or while you are waiting for CPB) you should:

Start forced-air warming ‘blanket’ (e.g. Bair Hugger®)

Warm IV fluids

Warmed IV fluids will not do too much to increase core temperature but they will certainly prevent any further drop that might result from using room temperature infusions. Large volumes of fluid may be required.

Warm inspired air to 40-46°C

This requires a heater humidifier which if not available in the ED, should be obtained from the ICU. If unavailable, ensure that a heat and moisture exchange filter is in place.

Try gastric, bladder, peritoneal and/or pleural lavage

Use warmed Hartmann’s solution for gastric lavage via an NG tube or bladder lavage with a three-way catheter. Use warmed peritoneal dialysate (4L) via a peritoneal catheter. Leave the fluid in place for 10-15 minutes to allow heat exchange before draining out and replacing with fresh warm fluid. Pleural lavage requires an apical and basal chest drain on each side. 2L of warmed Hartmann’s is infused into each hemithorax via the apical drain and then removed through the basal drain about 10-15 minutes later.

Use a high volume renal haemofilter

A high volume renal haemofilter may be a readily available tool in smaller hospitals as most intensive care units have the facility for renal support. It may be technically difficult in a patient with no blood pressure but if flow can be established the machine will generally allow heating of the blood as it flows through the circuit. The kit is also fairly easily moved to the patient. Intravascular temperature management devices exist too and some case reports have highlighted there use in re-warming hypothermic patients. The rate at which they achieve re-warming in severe hypothermia is unclear.

Target Temperature

If you get a return of spontaneous circulation (ROSC), the ERC guidelines suggest re-warming to mild hypothermia of 32–34°C and then applying mild therapeutic hypothermia as for non-hypothermic cardiopulmonary arrests complicated by coma [1].

Try and avoid over-shooting the mark. Aggressive re-warming can be discontinued when cardio-vascular stability is achieved.

Re-Warming Shock

The act of re-warming may precipitate cardiovascular collapse (‘re-warming shock’) and rhythm change.

The reasons are not completely understood but include electrolyte shifts, temperature fluctuation, intravascular depletion and drug therapy [4]. This will, of course, not be an issue if bypass is used to re-warm. Patients should be adequately volume loaded to maximise cardiac output during the re-warming process. In the absence of cardiac output monitoring aim for a central venous pressure (CVP) of 8-12 cm H2O.

Morbidity and Mortality

With CPB, survival rates in arrested patients approach 50%, even with a time to put bypass in place of around an hour [2,11,12]. The best figures are obtained from units dealing with cold mountain regions, as the population in question are younger and fitter than the urban population which typify the UK case mix. However, reasonable figures have also been reported from urban populations [12].

Common problems in the survivors include adult acute respiratory distress syndrome (see radiograph shown), acute renal failure and pneumonia [5]. The majority of the survivors can be expected to have good neurological outcome [5]. The majority of deaths are due to failure to wean the patients from bypass.


Patients with a potassium level over 12 mmol/L or severe traumatic injuries will not benefit from bypass. Similarly, patients with pre-existing cardiopulmonary, renal or neurological disorders require careful selection as they have a poorer prognosis [4,5].

In primary hypothermic cardiac arrest, death should not be confirmed until:

  • The patient has been re-warmed


  • Other unsurvivable injuries have been identified


  • Re-warming has failed despite all available measures.
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