Definition and epidemiology
Hypothermia is defined as a core body temperature below 35°C (95°F). It is classified by severity:
| Stage | Core temp | Clinical state |
|---|---|---|
| Mild | 32-35°C | Shivering, tachycardia, confusion |
| Moderate | 28-32°C | Shivering stops, decreased consciousness, arrhythmia risk |
| Severe | <28°C | Coma, severe bradycardia, risk of cardiac arrest |
Hypothermia occurs worldwide but is more common in cold climates, among the elderly, homeless populations, and in wilderness or water-immersion settings. It is also an underrecognized contributor to trauma mortality (part of the "lethal triad" with coagulopathy and acidosis) and a cause of accidental death in intoxicated individuals.
- Core temperature <35°C defines hypothermia; stages are mild (32-35), moderate (28-32), severe (<28°C)
- Osborn (J) waves on ECG are classic for hypothermia
- "Not dead until warm and dead" - do not stop resuscitation based on exam alone in severe hypothermia
- Limit defibrillation to about 3 shocks until core temp exceeds ~30°C
- Handle patients gently - rough movement can precipitate ventricular fibrillation
- Paradoxical undressing is a classic finding in fatal hypothermia
- Extracorporeal rewarming (ECMO/bypass) is preferred for unstable severe hypothermia
- Alcohol intoxication is a major risk factor via vasodilation and impaired judgment
Etiology and risk factors
Hypothermia results from an imbalance between heat loss and heat production. Causes include:
- Environmental exposure - cold air, immersion in cold water (heat loss ~25x faster than air), inadequate clothing/shelter
- Impaired thermoregulation - extremes of age (infants, elderly), hypothyroidism, hypoglycemia, adrenal insufficiency
- Decreased heat production - malnutrition, immobility, neuromuscular disease
- Impaired behavioral response - alcohol or sedative intoxication, dementia, psychiatric illness, traumatic brain injury
- Iatrogenic - cold IV fluids, surgical exposure, massive transfusion
- Sepsis - can present with hypothermia instead of fever, particularly in elderly or immunocompromised patients
Alcohol is a classic risk factor: it causes peripheral vasodilation (increasing heat loss), impairs shivering, and clouds judgment about environmental risk.
Pathophysiology
Normal thermoregulation is controlled by the hypothermalic preoptic area, which balances heat production (shivering, non-shivering thermogenesis, behavioral change) against heat loss (radiation, conduction, convection, evaporation). When heat loss overwhelms compensatory mechanisms, core temperature falls progressively.
Physiologic cascade
- Mild stage: sympathetic activation causes tachycardia, tachypnea, shivering, and peripheral vasoconstriction to preserve core temperature
- Moderate stage: shivering ceases (glycogen depletion), consciousness declines, and the myocardium becomes increasingly irritable
- Severe stage: progressive bradycardia, hypoventilation, decreased oxygen demand, and risk of ventricular fibrillation or asystole, especially with rough handling or movement
Cold slows all enzymatic and metabolic processes, which paradoxically can be protective for the brain and organs during cardiac arrest (reduced oxygen demand), underlying the principle that hypothermic cardiac arrest victims deserve prolonged resuscitation efforts.
Clinical features
Presentation correlates with severity:
- Mild (32-35°C): shivering, cold diuresis, tachycardia, mild confusion, dysarthria
- Moderate (28-32°C): shivering stops, progressive CNS depression, muscle rigidity, arrhythmias (atrial fibrillation common), paradoxical undressing (confused patients removing clothing)
- Severe (<28°C): coma, fixed/dilated pupils, severe bradycardia, hypotension, pulmonary edema, apparent death (very weak or absent pulses and respirations)
"Paradoxical undressing" and "terminal burrowing" (hiding in confined spaces before death) are classic forensic findings in fatal hypothermia.
Diagnosis
Diagnosis requires a low-reading rectal, esophageal, or bladder thermometer, since standard thermometers often do not register below 34°C.
Key studies
- ECG: sinus bradycardia, Osborn (J) waves (positive deflection at the J point, classically in moderate-severe hypothermia), prolonged PR/QRS/QT intervals, risk of atrial fibrillation and ventricular fibrillation
- Basic labs: glucose (hypoglycemia common), electrolytes, renal function, coagulation studies (cold impairs coagulation enzymes, though standard coagulation tests run at 37°C may appear falsely normal)
- Arterial blood gas: interpret with caution, as cold shifts the oxyhemoglobin dissociation curve left (results are often not temperature-corrected in practice)
- Search for underlying cause: toxicology screen, TSH, cortisol, blood cultures if sepsis suspected, trauma survey
Cardiac monitoring is essential since rough handling of a hypothermic patient can precipitate ventricular fibrillation.
Differential diagnosis
- Sepsis - can cause hypothermia instead of fever, especially in elderly or neonates
- Hypothyroidism/myxedema coma - causes mild hypothermia with bradycardia and altered mental status
- Hypoglycemia - can mimic or coexist with hypothermia-induced confusion
- Drug overdose/intoxication - sedative-hypnotics, alcohol, opioids can cause both hypothermia and depressed consciousness
- CNS stroke or trauma - impaired thermoregulation and altered consciousness
- Adrenal insufficiency - hypotension and hypothermia
Always evaluate for a precipitating or masked illness, since hypothermia is often secondary to another underlying process.
Management
Management is guided by severity and the presence or absence of a perfusing rhythm.
General principles
- Handle patients gently - rough movement can trigger ventricular fibrillation in moderate-severe hypothermia
- Remove wet clothing, insulate, and shield from wind
- Monitor cardiac rhythm continuously
- Correct hypoglycemia if present
Rewarming strategies
- Passive external rewarming - insulation and warm environment; adequate for mild hypothermia with intact shivering
- Active external rewarming - forced-air warming blankets, warm packs to trunk (not extremities first, to avoid "afterdrop" and peripheral vasodilation shock); used for moderate hypothermia
- Active internal (core) rewarming - warmed IV fluids, warm humidified oxygen, body cavity lavage (peritoneal, pleural), and for severe hypothermia with hemodynamic instability or cardiac arrest, extracorporeal rewarming (ECMO or cardiopulmonary bypass) is the preferred method where available
Cardiac arrest in hypothermia
- Continue CPR; hypothermic myocardium may be unresponsive to drugs and defibrillation until rewarmed
- Limit defibrillation attempts to 3 shocks until core temperature rises above approximately 30°C, per most resuscitation guidelines (ACLS/ERC), then resume standard protocols
- Withhold or space vasopressor doses since hypothermic metabolism slows drug clearance (practice varies by guideline)
- Do not declare death based on exam alone in severe hypothermia - the principle "not dead until warm and dead" guides prolonged resuscitation, particularly in witnessed submersion/cold exposure, since full neurologic recovery has occurred after prolonged CPR with extracorporeal rewarming
Treat any identified underlying cause (sepsis, hypoglycemia, hypothyroidism, overdose) concurrently.
Complications and prognosis
- Arrhythmias - atrial fibrillation, ventricular fibrillation, asystole
- Afterdrop phenomenon - further core temperature drop during rewarming due to peripheral vasodilation returning cold blood to the core
- Rewarming shock - hypotension from peripheral vasodilation outpacing volume status
- Coagulopathy and bleeding
- Acute kidney injury, rhabdomyolysis
- Frostbite/tissue loss with concurrent cold injury
- Compartment syndrome after rewarming of frozen limbs
Prognosis depends heavily on underlying cause, rate of cooling, and comorbidities. Rapid accidental hypothermia (e.g., cold-water immersion) carries a better neurologic prognosis than slow hypothermia with prolonged hypoxia, because rapid cooling protects the brain before cardiac arrest occurs. Survival with good neurologic outcome has been reported even after several hours of CPR when extracorporeal rewarming is used.
"HEAT ESCAPES" - Hypothyroidism, Elderly/extremes of age, Alcohol, Trauma, Environmental exposure, Sepsis, CNS dysfunction, Adrenal insufficiency, Pills/drugs, Extremes of exertion/exhaustion, Submersion
Practice questions
1. A 68-year-old man is found outdoors after several hours in freezing temperatures. He is confused, shivering has stopped, and his core temperature is 29°C. ECG shows sinus bradycardia with a positive deflection immediately after the QRS complex. Which of the following best describes this ECG finding?
2. A 45-year-old woman is pulled from a frozen lake after prolonged submersion. On arrival, she has no palpable pulse, no respirations, and fixed dilated pupils. Core temperature is 24°C. Which of the following is the most appropriate next step?
3. A 72-year-old woman with dementia is brought from a nursing home with altered mental status and a core temperature of 33°C. Point-of-care glucose is 58 mg/dL. Her family reports she has not been eating well. Which underlying condition should be specifically evaluated given this combination of findings?
Frequently asked questions
What causes hypothermia?
Hypothermia is caused by heat loss exceeding the body's heat production, most often from cold environmental exposure, cold-water immersion, or inadequate clothing. Risk is increased by alcohol or sedative use, extremes of age, hypothyroidism, hypoglycemia, trauma, and sepsis, all of which impair thermoregulation or heat production. Elderly and intoxicated individuals are especially vulnerable because of blunted shivering and impaired judgment about cold exposure.
What are the first symptoms of hypothermia?
The first symptoms are shivering, cold and pale skin, tachycardia, and mild confusion or clumsiness as core temperature drops below 35°C. As temperature falls further, shivering paradoxically stops, speech becomes slurred, and consciousness declines. Severe cases progress to coma, very slow heart rate, and barely detectable breathing or pulse.
How is hypothermia diagnosed?
Hypothermia is diagnosed by measuring core temperature with a low-reading rectal, esophageal, or bladder thermometer, since standard thermometers do not read accurately below 34°C. An ECG is obtained to check for Osborn (J) waves and arrhythmias, and labs assess glucose, electrolytes, and coagulation. Clinicians also search for an underlying cause such as sepsis, hypoglycemia, or intoxication.
Is hypothermia curable?
Yes, hypothermia is reversible with prompt rewarming and supportive care, and most mild to moderate cases recover fully. Severe hypothermia, including cases with cardiac arrest, can also have good outcomes with aggressive rewarming such as extracorporeal (ECMO/bypass) techniques, even after prolonged CPR. Outcomes depend on the degree and duration of cooling and any underlying illness or injury.
Hypothermia vs frostbite — what is the difference?
Hypothermia is a drop in core body temperature below 35°C affecting the whole body, while frostbite is localized freezing injury to skin and underlying tissue, usually in the extremities, nose, or ears. The two often occur together in cold exposure, but hypothermia is life-threatening systemically while frostbite primarily threatens the affected tissue. Management differs: hypothermia requires core rewarming, while frostbite requires careful rewarming of the specific body part and wound care.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: T68
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.