07. Hypothermia

Definition

Hypothermia is a lower than normal core body temperature, which can further be classified as mild (32 – 35˚C), moderate (28-32˚C) or severe (<28˚C).

  • The gold standard for core body temperature is by use of an esophageal probe, though rectal and bladder probes are acceptable. Tympanic measurements should be noted with suspicion. Children, the elderly and malnourished individuals are at high risk for developing it.
  • It is unlikely that you will be called at night for new onset hypothermia.
  • Cardiac arrests (typically “found down” situations, not in-hospital arrests) cannot be “called” until the patient is >35˚C and resuscitative efforts cease or there is return of spontaneous circulation. (“You’re not dead until you’re warm and dead”).

Differential Diagnosis

  • Infections: severe sepsis especially in the elderly and already hospitalized patients.
  • Exposure: drowning, heavy alcohol use (impairs shivering and causes vasodilation), extreme cold temperatures, cold fluid infusions, dermatitis or burns.
  • Ingestion: medication overdoses especially phenothiazines, barbiturates, and alcohol.
  • Neurologic: spinal cord injury, central core temperature dysregulation from hypothalamic dysfunction, which can be present in head injury and alcohol use disorder.
  • Metabolic/Endocrine: hypothyroidism, adrenal insufficiency, hypopituitarism, hypoglycemia.
  • Miscellaneous: age extremes, malnutrition, fatigue.

Evaluation and Findings

  • Mild Hypothermia:
    • Initial increase in metabolic rate and shivering.
    • CV and Respiratory: tachycardia, hypertension, and tachypnea.
    • Neurologic: Impaired judgment, lethargy, confusion and loss of fine motor coordination.
    • GU: cold diuresis, bladder atony.
  • Moderate Hypothermia:
    • Neurologic: Pupillary dilation, loss of shivering, severe lethargy and confusion.
    • CV: Hypotension, bradycardia, prolonged QT, and arrhythmia is common (especially atrial fibrillation).
    • Respiratory: respiratory acidosis, hypoxemia, aspiration, atelectasis.
  • Severe Hypothermia:
    • CV/Respiratory: Severe bradycardia, cessation of cardiac activity or life-threatening arrhythmias (VF, VT), hypotension and decreased respiratory effort. The risk of VF rises as core temperature reaches 28˚. At 23˚, apnea is common. Asystole is known to occur at 20˚.
    • Neurologic: muscle rigidity, loss of consciousness, absent deep tendon reflexes and brainstem reflexes.
    • Hematologic: disseminated intravascular coagulation (DIC), bleeding.
  • ECG Findings:
    • Osborne Waves (J-wave): present in 80% of patients with hypothermia, and is characterized by a second positive deflection just following the QRS complex.  Often present in leads II, V3 and V4.
    • Prolonged PR, QRS, and QTc intervals can be seen.
  • General Principles of Evaluation:
    • Obtain a reliable core body temperature; rectal probe is often easiest.
    • Hypothermia evaluation should mimic sepsis workup, especially in already hospitalized patients (i.e. blood cultures, urinalysis, chest x-ray, additional imaging as clinically indicated; consider broad spectrum antibiotics).
    • 12-lead ECG.
    • Evaluate for trauma and remove wet clothes - these are especially important during initial evaluation in the emergency department.
    • Consider ordering creatinine kinase (CK) if the patient was found down to evaluate for rhabdomyolysis.

Management

  • Place patient on continuous monitoring by ECG due to high risk of cardiac instability.
  • Passive rewarming: Recommended in a mildly hypothermic patient, core temp > 32˚. Examples: warm environment, blankets, warm clothing, head cover.
  • Active external rewarming: Indicated in moderate hypothermia or mild hypothermia not responding to passive rewarming.
  • Examples: Bair Hugger, heating pads, warm water immersion.
  • Active internal rewarming: For severe hypothermia. Examples: warm IV fluids, peritoneal lavage, extracorporeal warming (i.e. dialysis).
  • Vasopressors for hypotension if needed but be careful for cardiac irritability and potential for VF/VT. Dopamine would have highest likelihood of ectopy and phenylephrine is likely to have the least risk.

Key Points

  • Treat hypothermia like sepsis in already hospitalized patients. Severely hypothermic patients are best managed in the ICU.
  • Cold hearts are irritable hearts; watch out for asystole, ventricular fibrillation and ventricular tachycardia. Risk for arrhythmia is especially high during rewarming.
  • Rapid core rewarming is the key. Peripheral warming causes vasodilation and hypotension.
  • Cold hearts have a poor response to cardioactive stimuli, especially those methods used in ACLS. In general, cardiac drugs and defibrillation are withheld until rewarming until at least 28˚ is achieved.
  • ACLS meds can be pro-arrhythmic at low temperatures.

Biem, J; Koehncke, N; Classen, D; Dosman, J. “Out of the cold: management of hypothermia and frostbite.” CMAJ 2003; 168(3).

Petrone, P; Asensio, J; Marini, C. “Management of accidental hypothermia and cold injury.” Current Problems in Surgery 2014: 51(417-431).