Shock, Sepsis & Multisystem · Topic 10 of 14

Heat Stroke & Accidental Hypothermia

Multisystem · environmental emergencies

Heat Stroke & Accidental Hypothermia

Heat stroke: immerse in cold water to below 39 °C within 30 minutes. Hypothermia: stage by core temperature, rewarm by stage, and send arrests for ECLS.

Key numbers

Heat stroke: > 40 °C plus confusion> 40 °C + CNS change

Cold-water immersion to < 39 °C< 39 °C within 30 min

Hypothermia: core ≤ 35 °C, staged≤ 35 °C

01 · Defined byHeat stroke: > 40 °C plus confusion

Heat stroke = core (rectal) temperature > 40 °C with CNS dysfunction — confusion, delirium, ataxia, seizures or coma; sweating may persist. CNS dysfunction separates it from heat exhaustion.

02 · DoCold-water immersion to < 39 °C

Cool at once, before or during transport: ice- or cold-water immersion is fastest (goal ≥ 0.155 °C/min), reaching < 39 °C within 30 minutes of recognition; stop active cooling near 39 °C to avoid overshoot.

03 · The numberHypothermia: core ≤ 35 °C, staged

Hypothermia = core temperature ≤ 35 °C, staged mild 32–35 °C (shivering, conscious), moderate 28–32 °C (impaired consciousness, shivering stops), severe 24–28 °C (unconscious), profound < 24 °C (apparent death).

Causes & risk factors

Why it happens

Look for secondary causes of hypothermiaCheck glucose, alcohol, TSH and free T4, and cortisol, and look for sepsis and occult trauma; failure to rewarm suggests hypothyroidism (myxedema) or adrenal insufficiency.

Presentation

What you'll see

Hypothermia ECG: Osborn J wavesHypothermia ECG shows J (Osborn) waves and prolonged PR, QRS and QT; rhythm progresses from sinus bradycardia to slow atrial fibrillation, then ventricular fibrillation or asystole.

Diagnosis & monitoring

Work-up

Measure core temperature with an esophageal probeDiagnose and stage with a core temperature — esophageal (or epitympanic) probe is most accurate; rectal and bladder readings lag during rewarming and oral or temporal thermometers are unreliable.

Do right now

In this order

First moves

Cold-water immersion to < 39 °C< 39 °C within 30 min

→

Rewarm by stage: passive, active external, core

→

Hypothermic arrest: CPR, rewarm with ECMO

Why each step

Cold-water immersion to < 39 °CCool at once, before or during transport: ice- or cold-water immersion is fastest (goal ≥ 0.155 °C/min), reaching < 39 °C within 30 minutes of recognition; stop active cooling near 39 °C to avoid overshoot.

Rewarm by stage: passive, active external, coreMild (32–35 °C) with shivering: passive rewarming. Below 32 °C or unstable: active rewarming — forced air to the trunk, IV fluids at 40–42 °C, humidified oxygen at 40–45 °C; severe: core rewarming or ECLS.

Hypothermic arrest: CPR, rewarm with ECMOHypothermic arrest: full CPR concurrent with rewarming and transfer to an ECLS center (ECMO preferred over bypass); defibrillate once, then defer further shocks — and reasonably epinephrine — until core temperature ≥ 30 °C.

Management

Orders

Benzodiazepine for shivering during coolingGive a benzodiazepine (diazepam or lorazepam) to blunt shivering during cooling; intubation with neuromuscular blockade may be needed in obtunded patients. Give isotonic saline boluses guided by BP and urine output.

Handle gently and keep horizontalMove the hypothermic patient gently and horizontally, without exertion — rough handling can trigger ventricular fibrillation, and afterdrop and rewarming collapse occur as cold, vasodilated periphery reconnects to the core.

Warm saline for cold-diuresis hypovolemiaHypothermic patients are hypovolemic from cold diuresis and capillary leak: give 500 mL–2 L of 0.9% saline warmed to 40–42 °C, then more as needed.

Red flags & don'ts

Avoid harm

No antipyretics or dantrolene in heat stroke avoidDo not give dantrolene, and avoid acetaminophen, NSAIDs or salicylates to lower temperature in heat stroke — they do not treat the underlying problem and antipyretics can add liver or kidney injury; cool physically.

Heat stroke injures every organ: check CK, coags complicationHeat stroke triggers multiorgan injury — rhabdomyolysis, AKI, liver injury, ARDS, DIC, hyperkalemia, hypoglycemia: order CBC, coagulation studies, electrolytes, renal and liver panels, CK and urinalysis, and admit to the ICU.

The rules

Know these

Potassium > 12 mmol/L predicts fatal outcomeBradycardia and hypotension from hypothermia alone need no aggressive treatment beyond rewarming; serum potassium > 12 mmol/L during resuscitation indicates a fatal outcome and can guide termination.

Classic or exertional heat stroke?

Commonly confused

Who	Older, sedentary adults; young children; chronic illness
Onset	2–3 days of heat-wave exposure
Skin	Usually hot and dry
Complications	Multiorgan dysfunction
Treatment	Rapid cooling — cold-water immersion

Commonly confused

Tell apart

Hyperthermia with delirium: rule out mimicsWhen the picture is atypical, consider mimics of heat stroke: sepsis or meningitis, neuroleptic malignant syndrome, serotonin syndrome, thyroid storm, stimulant or anticholinergic toxicity, status epilepticus, stroke.

More detail

Reference

Show 6 reference facts

Do not temperature-correct arterial blood gases in hypothermia.

Heated humidified oxygen adds about 1–2 °C/hour; closed thoracic lavage via 2 chest tubes and warm peritoneal lavage are efficient core-rewarming options where ECLS is unavailable.

In stable hypothermic patients, raising core temperature about 1 °C/hour is acceptable.

Cold IV fluids and cold-saline lavage are only adjuncts in heat stroke; prophylactic antibiotics and antiseizure drugs are not recommended outside research.

Heat stroke outcome depends most on the duration of hyperthermia and how fast cooling begins; neurologically intact survival after hypothermic arrest treated with ECLS is about 42–56%.

Stimulants (cocaine, amphetamines, PCP), MAO inhibitors and anticholinergic drugs raise heat stroke risk by increasing heat production or impairing sweating.

Pearl

Cool first, transport second for heat stroke; for hypothermic arrest, keep going — rewarm before deciding the patient is dead.

Sources · Merck Manual Professional — Heatstroke (Dec 2025) · Society of Critical Care Medicine — Guidelines for the Treatment of Heat Stroke (Barletta JF et al. Crit Care Med 2025;53:e490–e500) · Merck Manual Professional — Hypothermia (Aug 2026; cites WMS 2019, 2025 AHA special circumstances, ELSO 2025) · 2025 AHA Guidelines for CPR and ECC — Part 10: Adult and Pediatric Special Circumstances of Resuscitation, §17 Hypothermia · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).