Workup for Hypothermia
The workup for hypothermia centers on accurate core temperature measurement using esophageal, bladder, or rectal thermometry, followed by systematic assessment of multi-organ dysfunction and identification of underlying precipitants. 1, 2
Core Temperature Measurement
Accurate core temperature measurement is the cornerstone of diagnosis and must be obtained with a low-reading thermometer capable of measuring below 35°C. 3, 4
- Preferred methods include esophageal, bladder (urinary), or rectal thermometry, as peripheral measurements (tympanic, axillary, skin) are unreliable and can underestimate severity by up to 1°C 5, 2
- Pulmonary artery catheter temperature is most accurate (within 0.1°C of true core) but rarely practical in acute settings 5
- Oral and tympanic measurements are acceptable only when core methods are unavailable, but can be influenced by external factors 5
- Monitor temperature every 5 minutes in moderate-severe hypothermia (<32°C) and every 15 minutes in mild cases 1, 2
Critical pitfall: Standard clinical thermometers do not measure below 34°C—you must use a low-reading thermometer or you will miss severe hypothermia entirely 4
Classification by Core Temperature
Once measured, classify severity to guide workup intensity 1, 3:
- Cold stress: 35-37°C
- Mild hypothermia: 32-35°C
- Moderate hypothermia: 28-32°C
- Severe hypothermia: <28°C
- Profound hypothermia: <24°C
Laboratory Evaluation
Obtain the following labs immediately to assess organ dysfunction and identify precipitants:
Essential Studies
- Cortisol level to evaluate for adrenal insufficiency—recommended in all hypothermic patients 1, 2
- Coagulation studies (PT/PTT): hypothermia impairs platelet function at 33-37°C and clotting factors below 33°C, with PTT increasing from 36 seconds at 37°C to 57 seconds at 28°C 5
- Arterial blood gas to assess metabolic acidosis and ventilation status 5
- Electrolytes including potassium (risk of hyperkalemia with tissue damage) 6
- Creatine kinase to detect rhabdomyolysis 3
- Blood glucose to identify hypoglycemia as cause or consequence 6
- Thyroid function tests (TSH, free T4) to evaluate for myxedema coma 6
- Blood cultures if infection suspected as precipitant 6
- Toxicology screen if exposure suspected 6
Hematologic Considerations
- Recognize that coagulopathy may not be apparent on standard lab testing performed at 37°C—functional platelet and clotting factor activity is temperature-dependent and worsens progressively below 33°C 5
Cardiovascular Assessment
Obtain 12-lead ECG to identify characteristic changes and arrhythmias:
- Mild hypothermia (<36°C): Expect sinus tachycardia, hypertension, increased cardiac output from sympathetic activation 5
- Moderate hypothermia (34°C): Look for impaired diastolic relaxation 5
- Severe hypothermia (28°C): Bradycardia, prolonged PR interval, Osborne (J) waves, T-wave inversions 5
- Critical hypothermia (25°C): High risk of ventricular fibrillation 5
Handle the patient gently during examination—rough movement can precipitate ventricular fibrillation in severe hypothermia 7
Neurologic Assessment
Document mental status carefully, as it correlates with severity:
- Mild hypothermia: Confusion, uncoordination, shivering present 5
- Moderate hypothermia (32°C): Reduced airway reflexes, aspiration risk 5
- Severe hypothermia (30°C): Somnolence to coma expected 5
- Below 27°C: Loss of deep tendon reflexes and pupillary reflexes 5
Critical caveat: Brain death cannot be diagnosed until the patient is rewarmed to at least 34°C, as severe hypothermia mimics brain death 5
Important exception: Rare cases exist where patients remain alert despite severe hypothermia by temperature criteria—always base treatment decisions on measured core temperature, not clinical presentation alone 3
Respiratory Evaluation
- Assess respiratory rate: Mild hypothermia causes tachypnea and decreased PaCO2; severe hypothermia (32°C) causes medullary depression with decreased minute ventilation 5
- Monitor for increased secretions and atelectasis in moderate-severe cases 5
- Recognize increased pulmonary vascular resistance and V/Q mismatch 5
Renal Function
- Expect cold-induced diuresis initially from increased cardiac output and renal blood flow 5
- Monitor urine output: GFR decreases to 50% of normal at 30°C, though urine output remains until 20°C 5
Identification of Underlying Cause
Systematically evaluate for precipitants:
- Environmental exposure (most common in accidental hypothermia) 6
- Endocrine: Hypothyroidism, adrenal insufficiency, hypopituitarism 6
- Toxins: Alcohol, sedatives, opioids 6
- Infections: Sepsis, particularly in elderly 6
- Metabolic: Hypoglycemia, diabetic ketoacidosis 6
- CNS dysfunction: Stroke, trauma, Wernicke's encephalopathy 6
- Trauma: Consider in all cases, as hypothermia is frequently overlooked (documented in only 38% of trauma admissions at one major center) 2