Cold Weather Blues: Hypothermia

Accidental hypothermia is a systemic emergency marked by progressive neurologic depression, cold diuresis, and escalating cardiac instability as core temperature falls. Management is driven more by symptoms than by the thermometer, and severe cases can deteriorate into refractory ventricular fibrillation with minimal movement.

Hypothermia Recognition and Physiology

• Primary versus secondary causes: Not all hypothermia comes from outdoor exposure; sepsis, intoxication, hypothyroidism, adrenal insufficiency, shock, and malnutrition can all impair thermoregulation and change the workup.
• Symptom based severity: Clinical status matters more than the exact core temperature: shivering and intact mentation suggest milder disease, while lethargy, coma, and loss of shivering mark dangerous progression. We lay out the bedside staging in the episode.
• Shivering physiology shift: Physiology rises early with cold stress, but around 30 C shivering stops and metabolism falls, removing the patient's main endogenous heat source and accelerating decompensation.
• Cold diuresis and hypovolemia: Hypothermic patients are often intravascularly depleted from cold-induced diuresis, plasma shift, and poor intake, which helps explain hypotension and the need for volume repletion.
• Cardiac irritability threshold: Below 30 C, the myocardium becomes exquisitely prone to refractory ventricular fibrillation, triggered by movement, acidosis, hypocarbia, or hypoxia.

ED Management and Cardiac Arrest

• Immediate ED priorities: Start with rewarming, a STAT fingerstick glucose, ECG, and basic labs including coagulation studies; if the cause is unclear, broaden quickly to endocrine, infectious, and toxicologic testing.
• Electrocardiographic warning signs: Expect early tachycardia followed by bradycardia that is often atropine-refractory, with PR, QRS, and QT prolongation; Osborn waves may appear below 30 C.
• Rewarming strategy tiers: Passive external, active surface, warmed IV fluids, heated humidification, cavity lavage, and ECMO all have roles, but the choice depends on instability and available resources. We get into the escalation logic in the chapter.
• Medication and shock caution: Drug metabolism slows in severe hypothermia, so repeat doses can accumulate; mild hypotension often improves with rewarming, while vasopressors are reserved for selected patients.
• Defibrillation and pulse checks: Serial shocks for VT or VF are often low yield until rewarming advances, and peripheral pulses can be misleading enough that ultrasound or an arterial line may be more trustworthy.
• Termination of resuscitation: Hypothermic arrest can still have good neurologic outcomes, so resuscitation generally continues until meaningful rewarming, with a very high potassium level serving as a grave prognostic marker.

References: 

1. Paal P, Pasquier M, Darocha T, et al. Accidental Hypothermia: 2021 Update. Int J Environ Res Public Health. 2022;19(1):501. Published 2022 Jan 3.  PMID: 35010760
2. Dow J, Giesbrecht GG, Danzl DF, et al. Wilderness Medical Society Clinical Practice Guidelines for the Out-of-Hospital Evaluation and Treatment of Accidental Hypothermia: 2019 Update. Wilderness Environ Med. 2019;30(4S):S47-S69. PMID: 31740369