Approach

Suspect hypothermia (core body temperature <95°F [<35°C]) based on the condition the patient is found in and/or the presence of risk factors.

Prehospital assessment

In a prehospital setting, use the four-stage original Swiss system to help estimate the patient’s core temperature at the scene (if this isn’t already available).[19]

Vital signs can be present even when core temperature is below 75°F (24°C).[44][45]​​​​ Stages of hypothermia are based on clinical signs (shivering, vital signs, level of consciousness) that roughly correlate to the patient’s core temperature and are used to guide management.[46]​​ However, it is important to note that factors such as trauma, central nervous system failure, and substance misuse and overdose may impair shivering and consciousness, independent of a patient’s core temperature.[19]

​Consider the revised Swiss system, which may simplify clinical staging in the field, if appropriate; the revised system incorporates risk of cardiac arrest as part of staging.[47]

See Cardiac arrest.

If the patient has been immersed in water, see Drowning.

History

Ask the prehospital team about the condition in which the patient was found. This may provide important clues to the diagnosis; for example, patients who are inappropriately dressed for a cold climate and have spent a long time outdoors or in a cold environment may be hypothermic.

Consider risk factors for hypothermia. These include:[2][16]​​[19][27]​​[31]​​​​[34][35][36][37][38]

  • General anesthetic use

  • Trauma

  • Drowning

  • Extremes of age

  • Immobility (e.g., due to illness or injury)

  • Substance misuse

  • Impaired cognition

  • Hypothyroidism

  • Stroke

  • Parkinson disease

  • Homelessness

  • Antipsychotic use

  • Gram-negative septicemia.

Physical exam including core temperature measurement

Examine and move the patient very carefully while you are assessing them. Keep the patient in a supine position if they have features of moderate or severe hypothermia (e.g., they have stopped shivering or have a reduced level of consciousness) as movement can precipitate ventricular fibrillation, especially if the patient’s temperature is <82°F (<28°C).[20][48]

Check for vital signs (including a carotid pulse) for up to 1 minute.[20][48]​​​ Measure and monitor vital signs as part of ongoing assessment, including: blood pressure; pulse rate; respiratory rate; and oxygen saturations. Be aware that vital signs may be very difficult to detect in a patient with hypothermia, especially in the prehospital setting; a very hypothermic patient may appear dead but still survive with resuscitation.[20]​​[45]​​[48]​​[49]​​​​ Other causes of cardiac arrest may need to be excluded; cardiac arrest is unlikely to be solely due to hypothermia unless the core temperature is less than 82°F (28°C).[19]

Patients often show signs of confusion or impaired judgment. Additionally, they may be shivering, have increased urinary frequency, and show signs of frostbite on their skin. See Frostbite.

Note that shivering will be absent once the patient’s core temperature drops below a certain level; the threshold varies between patients but is typically 82°F to 90°F (28°C to 32°C).[48]​ However, it is important to be aware that factors such as trauma, central nervous system failure, and substance misuse and overdose may impair shivering and consciousness, independent of a patient’s core temperature.

Look for any signs of the underlying cause of hypothermia. For example:[45]​​

  • Self-harm. Consider this particularly if the patient has a reduced level of consciousness or been immersed in water. Check for any signs of drug overdose or alcohol intoxication. See Overview of substance use disorders and overdose.

  • Acute illness (e.g., stroke) or injury that has resulted in the patient lying on the ground outdoors for a long period of time.

Clinical signs correlate approximately to the patient’s core temperature.[48]​ However, an individual patient’s response to hypothermia may vary considerably; clinical signs can only provide an estimate of core temperature. Urgent critical care support is required for any patient with severe hypothermia.

Core temperature

Do not use a standard clinical thermometer to measure core temperature. This may be inadequate as it will not measure temperatures below 94°F (34.4°C). Conventional mercury thermometers are also not recommended, owing to the risk of breakage and poisoning.

Where feasible (usually in hospital) the 2019 Wilderness Medical Society guidelines and the 2021 European Resuscitation Council guidelines recommend:

  • Preferably: an esophageal probe.[20][48]​ An esophageal probe correlates well with the temperature of the pulmonary artery and is the preferred method when available.[19][48]​ This is usually only possible in critically ill patients as readings must be obtained from the lower third of the esophagus when the airway is secured (i.e., tracheal tube or a supraglottic device with an esophageal channel in place).

  • Alternatively: a low-reading tympanic membrane thermistor-based thermometer (where the thermistor touches the tympanic membrane) if the patient is spontaneously breathing.[20][48]

Bladder catheter temperature sensors can be used in patients who require a urinary catheter, but bladder and rectal temperature lag behind core temperature and are only recommended for stable patients in a hospital setting.[19][20]​ Never measure rectal temperature if the patient is in a cold environment. This method requires the patient to be further exposed, which will increase heat loss and potentially worsen hypothermia.

Investigations

Investigations in the workup of accidental hypothermia are not diagnostic, but do help to guide acute management.

ECG

Continuous ECG monitoring is essential for detecting arrhythmias, which may be fatal. Where possible, ECG monitoring should also be used to detect cardiac arrest.[20][48]

Arrhythmias can occur at any stage of hypothermia, and also during rewarming. Initially, in mild hypothermia, the ECG may show tachycardia. In more severe cases of hypothermia, the ECG may show progressive sinus bradycardia, atrial or ventricular fibrillation, junctional rhythms, ST segment changes, T-wave inversion, prolongation of the QT interval, and eventually asystole.[11]​ With the exception of ventricular fibrillation, these changes are likely to improve without treatment as the patient’s core temperature increases.[20][48]

J waves (or Osborn waves) occur in most, but not all, patients.[50]​​ However, they do not correlate well with temperature.[51]

[Figure caption and citation for the preceding image starts]: A 12-lead ECG obtained from a hypothermic patient; note, Osborn waves (arrows), which have an extra deflection at the end of the QRS complexAydin M, Gursurer M, Bayraktaroglu T, et al. Tex Heart Inst J. 2005;32(1):105 [Citation ends].com.bmj.content.model.Caption@6010e69e

Laboratory tests

Initial investigations should include: arterial blood gas (ABG), blood glucose, and blood chemistries. Further tests are less useful for the acute assessment and management of hypothermia but generally include a complete blood count (CBC) and clotting screen.

  • An ABG may show respiratory alkalosis, metabolic acidosis, or a mixed picture. As core temperature decreases, respiration is depressed, resulting in hypoxemia and hypercapnia. A combined respiratory and metabolic acidosis occurs as a result of hypoventilation, retention of carbon dioxide, decreased bicarbonate, impaired hepatic metabolism of organic acid production (owing to impaired hepatic perfusion), and increased lactic acid production. It is important to note that blood pH rises by 0.015 for every 1.8°F (1°C) drop in body temperature. In general, use blood gas results without adjustment for temperature to guide treatment decisions.[52]

  • Glucose levels may be normal, high (owing to increased secretion of stress hormones - cortisol, growth hormones, and catecholamines - and reduced insulin secretion, together with increased peripheral resistance to insulin), or low (owing to cold-induced inhibition of hepatic glucose production). Monitor blood glucose even after the patient is normoglycemic because rebound hypoglycemia may develop when normal insulin production resumes. Treat hypoglycemia promptly. Hypoglycemia can stop shivering (because the central control of shivering is dependent on glucose), leading to subsequent heat loss.[53]

  • Renal function may be impaired due to dehydration, cold exposure, or rhabdomyolysis.

  • Hypokalemia may occur as a result of hypothermia or the associated treatment. Hyperkalemia may occur during rewarming. For a patient in cardiac arrest, hyperkalemia can also indicate that hypoxia preceded hypothermia (e.g., if the patient was found in an avalanche).[48]​ Initial serum potassium >12 mEq/L (>12 mmol/L) is associated with irreversible death if the patient is in cardiac arrest.[48]​ Serum potassium is part of the HOPE (Hypothermia Outcome Prediction after ECLS rewarming for hypothermic arrested patients) score for prognostication of successful rewarming.[20][48] [ Hypothermia outcome prediction after ECLS (HOPE) score Opens in new window ] ​

  • A CBC may show elevated hemoglobin and hematocrit, and low platelet and white blood cell counts.

  • Prothrombin time and partial thromboplastin time (PTT) tend to be prolonged, although the cause for this is unknown.[54]

Imaging

A chest x-ray is particularly important if patients have an altered level of consciousness. It may show pulmonary edema or infiltrates.

If the patient has been immersed in water, it may show inhaled foreign bodies, such as false teeth or debris from the water, which will need to be removed. See Foreign body aspiration.

Investigations to consider

Serum creatinine kinase and myoglobin levels should be checked for rhabdomyolysis if the patient may have been lying on the ground outdoors for a long time and they have not been immersed in water. See Rhabdomyolysis.

If vital signs are undetectable, use ultrasound and end-tidal CO₂, where possible, to confirm cardiac arrest.[20][48]

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