Investigations
1st investigations to order
core temperature measurement
Test
Measure and monitor the patient’s core temperature.
Check your local protocols. If the patient has severe hypothermia, common practice in hospital emergency departments in the UK is to use an oesophageal probe (a probe positioned in the lower third of the oesophagus) if the patient has a secured and protected airway (i.e., tracheal tube or a supraglottic device with an oesophageal channel in place), or to measure rectal temperature.
Bear in mind, however, that the 2021 European Resuscitation Council (ERC) guideline has de-emphasised measurement of rectal temperature (or bladder temperature, which may be used if the patient requires a bladder catheter) for patients with severe hypothermia because temperature at these sites lags behind core temperature.[24]
Never measure rectal temperature if the patient is in a cold environment because this method requires the patient to be further exposed, which will increase heat loss and potentially worsen the hypothermia.[4]
Where feasible (usually in hospital) the 2021 ERC and 2019 Wilderness Medical Society guidelines recommend:
Preferably: an oesophageal probe.[24][4] An oesophageal probe correlates well with the temperature of the pulmonary artery and is the preferred method when available.[3][4]
OR
Alternatively: a low-reading tympanic membrane thermistor-based thermometer (where the thermistor touches the tympanic membrane) if the patient is spontaneously breathing.[24][4] Thermistor-based thermometers may not be widely available in some countries, including the UK.
Do not use a standard clinical thermometer to measure core temperature. This may be inadequate because it will not measure temperatures below 34.4°C (94°F). Conventional mercury thermometers are no longer recommended, owing to the risk of breakage and poisoning.
If the preferred methods for measuring core temperature are not available (e.g., in a pre-hospital setting), suspect hypothermia from the history and setting, and assess whether the patient’s trunk feels cold.[24]
Practical tip
Cold water or snow in the patient’s ear canal can lead to localised cooling of the tympanic membrane, which may result in a falsely low core temperature reading.
Result
<35°C (<95°F)
12-lead ECG
Test
Monitor the ECG continuously.
This is essential for detecting arrhythmias, which may be fatal. Where possible, ECG monitoring should also be used to detect cardiac arrest.[4][24]
Arrhythmias can occur at any stage of hypothermia, and also during re-warming. 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.[5] With the exception of ventricular fibrillation, these changes are likely to improve without treatment as the patient’s core temperature increases.[4][24]
J waves (or Osborn waves) occur in most, but not all, patients.[27] However, they do not correlate well with temperature.[28]
[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].
Result
sinus bradycardia; atrial fibrillation; J wave or Osborn wave; ST elevation or depression; T wave inversion; prolonged PR, QT, and QTc interval; broad QRS complexes
blood glucose
Test
Bear in mind that 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 normoglycaemic because rebound hypoglycaemia may develop when normal insulin production resumes.
Treat hypoglycaemia promptly. Hypoglycaemia can stop shivering (because the central control of shivering is dependent on glucose), leading to subsequent heat loss.[29]
Result
may be elevated, often normal, sometimes low
blood gas
Test
Note that blood gases may show respiratory alkalosis, metabolic acidosis, or a mixture of both.
As core temperature decreases, respiration is depressed, resulting in hypoxaemia 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°C (1.8°F) drop in body temperature.
In general, use blood gas results without adjustment for temperature to guide treatment decisions.[30]
Be aware that there is debate in the literature about how to interpret blood gases in patients with hypothermia.[3][30] This is a complex issue, because blood gas analysers warm the blood sample to 37℃ (98.6℉) - higher than the patient’s temperature if they are hypothermic - and pH, PO2 and PCO2 all vary with temperature.[3][30] Therefore, interpretation of uncorrected blood gas results is a widely used approach because clinicians are more familiar with this.[30]
In practice, perform a venous rather than an arterial blood gas; a venous blood gas is associated with less risk than an arterial blood gas and gives adequate results for most patients.
Monitor blood gases to ensure resolution of hypoxia and normalisation of pH.
Result
respiratory alkalosis, metabolic acidosis, or a mixture of both
respiratory acidosis is suggested by pH <7.35 and PCO2 >40 mmHg
metabolic acidosis is suggested by pH <7.35, bicarbonate ≤24 mEq/L, and a normal PCO2, although it may be low with compensation
PaO2 may be low with severe hypothermia, and/or if there are pulmonary infiltrates or oedema
serum urea, electrolytes, and creatinine
Test
Hypokalaemia may occur as a result of the hypothermia or the associated treatment. Hyperkalaemia may occur during re-warming.
For a patient in cardiac arrest, hyperkalaemia can also indicate that hypoxia preceded hypothermia (e.g., if the patient was found in an avalanche).[4]
Hyperkalaemia is part of the HOPE (Hypothermia Outcome Prediction after ECLS re-warming for hypothermic arrested patients) score for prognostication of successful re-warming.[4][24] Initial serum potassium >12 mmol/L (12 mEq/L) is associated with irreversible death if the patient is in cardiac arrest.[3]
Renal function may be impaired due to dehydration, cold exposure, or rhabdomyolysis.
Result
hypokalaemia, hyperkalaemia
FBC
Test
Haemoglobin and haematocrit may be elevated due to haemoconcentration. Platelets and WBCs are abnormally low due to sequestration in the spleen.
Result
elevated haemoglobin and haematocrit, low WBC and platelet counts
clotting screen
Test
Prothrombin time (PT) and PTT are prolonged due to inhibition of enzymatic activity in both the intrinsic and the extrinsic coagulation cascade. The cause of coagulopathy is unknown.[31]
Result
elevated PT and PTT
chest x-ray
Test
A chest x-ray is particularly important if the patient has an altered level of consciousness.
It may show pulmonary oedema 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 our topic Foreign body aspiration.
Result
may be normal or may show pulmonary infiltrates, oedema, or foreign bodies
Investigations to consider
serum creatine kinase
Test
Order if the patient has not been immersed in water and therefore may have been lying on the ground for a long time, to check for rhabdomyolysis. See our topic Rhabdomyolysis.
Result
>5 times the upper limit of normal indicates rhabdomyolysis
myoglobin levels
Test
Order if the patient has not been immersed in water and therefore may have been lying on the ground for a long time, to check for rhabdomyolysis. See our topic Rhabdomyolysis.
Result
increased levels in blood and/or urine is an indicator of rhabdomyolysis
end-tidal CO₂
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