Hypothermia, Heat Stroke and Drowning

In depth

The one-page summary above is what the PDF prints. These sections go further.

Cold physiology and the hypothermic heart

Accidental hypothermia is a core temperature below 35 °C. Immersion, wet clothing and wind speed cooling, and a buried avalanche victim can cool by up to 9 °C/h (Paal 2022). Primary hypothermia comes from exposure; secondary hypothermia accompanies sepsis, hypoglycemia, hypothyroidism, adrenal insufficiency, intoxication, stroke, trauma and burns, and a long lie on the floor can chill an older adult indoors.

Cold first brings vasoconstriction, tachycardia and shivering; a patient who still shivers is usually at 30 °C or above (Paal 2022). As cooling continues, shivering fades and metabolism and oxygen demand fall, which is why the brain tolerates low flow and hypothermic arrest can end in full recovery.

The heart slows and conduction lengthens (PR, QRS, QT), and J (Osborn) waves appear at the QRS–ST junction, growing as temperature falls. Atrial fibrillation is common and usually resolves with rewarming (Paal 2022). The cold myocardium is irritable, so movement, a central line guidewire or rough handling can set off VF. Cold alone can arrest the heart below about 30 °C in young healthy adults and below 32 °C in older or sicker ones (Paal 2022); arrest at a higher temperature has another cause.

• Kidney: cold diuresis produces dilute urine, hypovolemia and hemoconcentration, so rewarming unmasks a volume deficit.
• Blood: clotting enzymes and platelets slow, but PT and aPTT are run at 37 °C and understate the coagulopathy.
• Metabolism: glucose can be high or low; a very high potassium signals cell death or asphyxia; drugs clear slowly, so repeated doses accumulate.
• Monitoring: pulse oximetry often fails on cold fingers.

Staging, temperature measurement and triage

ERC 2025 still prints the original Swiss stages with estimated core temperatures. The Revised Swiss System (ICAR MedCom, Musi 2021) drops temperature and uses AVPU to estimate the risk of arrest: alert (low), verbal (moderate), pain or unresponsive with vital signs (high), no vital signs (hypothermic arrest), because trauma, asphyxia or intoxication can dull consciousness at any temperature. US texts and exams use the WMS 2019 bands: mild 35–32, moderate 32–28, severe below 28 °C.

ERC 2025 sends hypothermic patients at risk of imminent arrest (heart rate under 45, SBP under 90 mm Hg, ventricular arrhythmia or core below 30 °C) and every patient in arrest directly to an ECLS center; both ERC 2025 and AHA 2025 (2b) accept a transfer of up to about 6 h. AHA 2025 adds that ECLS rewarming may be reasonable below 28 °C even without arrest (2b), though in one prospective study such patients bled more without better survival.

Rewarming, afterdrop and rescue collapse

Afterdrop is the continued fall in core temperature after rescue, as heat flows from the core into cold peripheral tissue and cold blood returns from the limbs; walking or warming the limbs first add to it (Paal 2022). Rescue collapse is arrest during extrication or transfer from hypovolemia, dysrhythmia and, most often, sudden movement; it appears to double the risk of death in severe hypothermia (Paal 2022). Hence horizontal, gentle handling, no walking, and heat packs on the chest and back, never directly on skin.

VA-ECMO is preferred to cardiopulmonary bypass: it can continue after ROSC to support a stunned heart or ARDS, needs less anticoagulation and has been linked to higher survival (Paal 2022). Faster is not better: among 658 patients rewarmed on ECLS, rates under 5 °C/h carried 2.41 times the odds of neurologically intact survival, hence 1.5–5 °C/h (AHA 2025, 2b). Rewarming dilates a volume-depleted circulation, so expect hypotension and give warmed isotonic fluid; recheck potassium, glucose and rhythm often; and look for a secondary cause (Endocrine Emergencies covers myxedema coma).

Hypothermic cardiac arrest: guidelines, ECLS and prognosis

Restraint rests on a cold heart that rarely converts, repeated shocks that injure it and drugs that accumulate (AHA 2025). ERC and WMS allow up to a minute for the pulse check. When extraction demands it, AHA 2025 and ERC accept delayed or intermittent CPR, below 28 °C at least 5 min on and up to 5 off, or up to 10 off below 20 °C (ERC 2021); ERC adds mechanical CPR for long transport.

The HOPE score (Pasquier 2018; 286 patients, 37% survival) estimates survival after ECLS rewarming from age, sex, mechanism (asphyxia or not), core temperature, potassium and CPR duration. It beat potassium alone (AUC 0.895 vs 0.774) and held up in external validation (AUC 0.825; a predicted survival under 10% had a 97% negative predictive value, Pasquier 2019). ERC bases ECLS decisions on HOPE with a 10% threshold; AHA 2025 finds prognostic scores reasonable (2a), naming HOPE and the ICE score (Saczkowski 2018), mentions a 5% HOPE cutoff for children and advises against predicting outcome from ETCO₂ (3: no benefit). Before HOPE, guidelines from 2001 to 2021 called potassium above 12 mmol/L a contraindication to ECLS, with a lower avalanche cutoff (8 mmol/L per Hall 2025; ERC 2021 cites 7).

Outcomes can be remarkable: 15 of 32 patients rewarmed on bypass after hypothermic arrest survived without disabling sequelae (Walpoth 1997), and a woman cooled to 13.7 °C recovered (Gilbert 2000). Asystole, unwitnessed arrest, fixed pupils, a very low temperature and long low-flow time are not by themselves reasons to stop (Paal 2022). Reasons not to start, or to stop:

• Lethal injuries, a frozen body or a chest too stiff to compress (ERC 2021).
• Irreversible signs such as fixed dependent lividity; rigidity alone is not reliable in the cold (Paal 2022).
• Avalanche burial over 60 min with a completely obstructed airway and asystole; burial under 60 min is an asphyxial arrest treated with standard ALS (ERC 2021).
• A valid do-not-resuscitate order, danger to rescuers, or a HOPE estimate under 10%.

Heat illness: types, risks and look-alikes

Heat stroke follows when heat gain outruns heat loss, which humidity, dehydration, equipment and poor sweating all limit. Direct heat injury combines with a systemic inflammatory response: endothelial injury, widespread thrombosis and bleeding (Bouchama 2022). The usual definition is a core above 40 °C with CNS dysfunction (WMS 2024; SCCM 2025; Bouchama 2022); NATA 2015 describes exertional heat stroke as typically above 40.5 °C. A temperature taken late or after cooling can be lower, so WMS 2024 treats on clinical suspicion.

Risk rises with poor acclimatization (about 80% is gained in 7 days of 1–2 h of moderate exertion in heat, WMS 2024), dehydration, recent illness, obesity, heavy equipment, and drugs that raise heat production or block sweating, such as stimulants, anticholinergics and MAOIs (Merck).

Heat exhaustion is the same exposure with intact mental status and a temperature that is usually normal or below 40 °C: weakness, headache, dizziness, nausea, cramps, sometimes syncope. Treat with shade, rest, cooling and about 1 L/h of oral rehydration solution; give IV saline, 1–2 L at about 500 mL/h (slower in older adults), if vomiting or no better after 30–60 min (Merck). Any confusion or ataxia makes it heat stroke.

• Exercise-associated hyponatremia: sodium under 135 mEq/L with confusion or seizures after long exertion and a near-normal rectal temperature; give 3% saline in 100 mL boluses, up to 3 at 10-min intervals, and no isotonic or hypotonic IV fluid (WMS 2019 update, via AAFP 2021).
• Also consider sepsis or meningitis, thyroid storm, sympathomimetic or anticholinergic toxicity, serotonin toxicity, NMS and malignant hyperthermia (Toxidromes and Antidotes sheet).

Cooling: methods, speed, stopping and drugs

Outcome depends on how long the core stays above about 40 °C, so when a tub is ready cooling starts where the patient collapses: cool first, transport second (NATA 2015). Immerse to the neck in stirred ice water, keep the head and airway clear, and follow a rectal or esophageal temperature. NATA counts no deaths among more than 2,000 exertional heat stroke patients immersed within 30 min; at the Falmouth Road Race all 274 survived, cooled at a mean 0.22 °C/min (Demartini 2015).

Guidelines differ on the water: AHA 2025 and SCCM 2025 prefer ice water at 1–5 °C (SCCM also 9–12 °C), NATA 2015 uses 1.7–15 °C and ERC 2021 1–26 °C. Aim for at least 0.15 °C/min (AHA 2025, for sympathomimetic hyperthermia too; SCCM 2025 says 0.155) and a core below 39 °C within 30 min of recognition (SCCM 2025). Stop active cooling at 38.5–39 °C to avoid overshoot: AHA 2025 stops at 38.6 °C (2a), NATA at 38.9 °C, and the Chinese consensus at 38.5 °C before holding 37.0–38.5 °C. Watch for rebound, and treat shivering and agitation with benzodiazepines (Merck).

Antipyretics lower a raised hypothalamic set point, which heat stroke does not have, and aspirin and acetaminophen can worsen its coagulopathy and liver injury (Bouchama 2007); SCCM 2025 advises against routine antipyretics and limits prophylactic antibiotics and antiseizure drugs to research. Dantrolene did not shorten cooling (67.9 vs 69 min) in a 52-patient randomized trial (Bouchama 1991), and SCCM 2025 recommends against it (strong). Volume needs are modest: about 1 L (0.5–2.5 L) of crystalloid restored filling pressures without overload; the shock is mainly distributive and myocardial failure is rare (Bouchama 2007).

Heat stroke in the ICU: organ injury and nursing care

• Neuro: seizures and cerebral edema; mentation that clears as the core falls is the best early sign, but survivors can keep neurologic deficits (Bouchama 2022).
• Muscle and kidney: CK above 1,000 U/L or 5 times normal marks rhabdomyolysis; aim for urine output of 200–300 mL/h while the kidneys work; oliguric AKI follows 25–35% of exertional cases (Chinese consensus 2020).
• Liver: AST, ALT and LDH rise within hours and peak at day 3–4; fulminant failure is rare (about 5%), so trend INR, bilirubin and glucose and involve a transplant center if it does (Chinese consensus 2020).
• Blood: DIC in about 45% (Chinese consensus 2020); check platelets, PT/INR, fibrinogen and D-dimer at least daily, and treat oozing from line sites as a warning.
• Metabolic: hypoglycemia and hyperkalemia from muscle breakdown (Merck); continuous ECG.
• Lungs and circulation: aspiration and ARDS; vasoactive drugs in the first 24 h were linked to higher mortality in observational data, so restore volume first (Bouchama 2007).

Liver and clotting injury peak days after cooling, so monitoring continues past the first normal temperature; prevention is acclimatization and drinking to thirst (WMS 2024).

Drowning: mechanism, resuscitation and hospital care

Drowning is respiratory impairment from submersion or immersion in liquid; WMS 2024 retires near, dry, secondary and delayed drowning. The injury is hypoxia: breath-holding ends in aspiration, laryngospasm may follow, and aspirated water causes pneumonitis, surfactant loss, atelectasis and shunt (Merck). Respiratory arrest progresses to bradycardia, PEA and asystole; shockable rhythms make up only 2–12% (AHA 2025). Fresh and salt water behave alike (Merck); cold, trauma, alcohol, seizures or heart disease matter more.

• Breaths matter: compression-only CPR is for bystanders who cannot give them (WMS 2024), and CPR with breaths starts before the AED is applied (AHA 2025, class 1).
• No abdominal thrusts or attempts to drain the lungs: they delay ventilation and provoke vomiting (WMS 2024; Merck).
• Spinal motion restriction only with diving, high-impact trauma or signs of injury: 11 of 2,244 submersion victims (0.5%) had neck injuries, all after diving, crashes or falls (Watson 2001).
• Cold water adds immersion hypothermia: follow the hypothermia rules once it is recognized (ERC 2025).

In hospital, alert patients with mild to moderate hypoxemia can be supported with CPAP or noninvasive ventilation (WMS 2024), with caution if vomiting or confused. Intubate for falling consciousness or failing gas exchange and ventilate with PEEP and lung-protective volumes; bronchospasm and pneumonitis are common and ARDS can follow. Give antibiotics only after grossly contaminated water or proven infection, and no glucocorticoids (Merck).

A patient with only a mild cough and normal lung sounds may leave the scene with someone to watch them for 6 h; abnormal lung sounds, severe cough, frothy sputum, altered mentation or hypotension need hospital care (WMS 2024), and mildly symptomatic patients are observed 4–6 h (Merck). Submersion time dominates prognosis: in 24 cohort studies, submersion of 5–6 min or less favored a good outcome (RR 2.90) and more than 25 min was invariably fatal, while age, water temperature and witnessed status showed no clear effect (Quan 2016). WMS 2024, after Tipton and Golden (2011), finds stopping reasonable after a known submersion over 30 min in water above 6 °C, or 90 min in colder water.

Frostbite in depth

Frostbite is tissue freezing: ice forms between cells and damages membranes, and on thawing endothelial injury and microvascular thrombosis extend the damage, so refreezing after a thaw is the worst sequence (WMS 2024). Depth shows after rewarming: clear blisters suggest superficial injury, hemorrhagic blisters and cyanotic, numb tissue deep injury; the Cauchy grades score how far up the digits it reaches.

• Treat hypothermia, remove wet or tight clothing and rings, and avoid rubbing or dry heat, which burns numb tissue.
• Thaw only when refreezing can be prevented: circulating water at 37–39 °C for about 30 min, until the tissue is soft and pliable (WMS 2024); give IV opioids, since thawing hurts.
• Ibuprofen 12 mg/kg/day in two doses in the field, up to 2,400 mg/day in four doses; topical aloe vera; elevate and pad (WMS 2024).
• Drain clear blisters selectively, for example by needle aspiration; leave hemorrhagic blisters intact (WMS 2024).

Iloprost, a prostacyclin analog, became the first FDA-approved frostbite drug in February 2024, based on the 47-patient Cauchy (2011) trial: bone scans predicting amputation were abnormal in 0% with iloprost alone, 19% with iloprost plus tPA and 60% with buflomedil (label). Watch for hypotension, headache, flushing and palpitations. Hyperbaric oxygen is not recommended for lack of evidence, and amputation waits for clear demarcation, with examination and imaging setting the margins (WMS 2024).

Exam pearls

• Bands: 35–32 °C shivering, 32–28 impaired consciousness, below 28 unconscious; shivering means 30 °C or more.
• J waves, bradycardia, long intervals and AF are cold signs; rough handling triggers VF.
• Afterdrop: keep the patient flat and still, and warm the trunk before the limbs.
• Below 30 °C: one shock (AHA 2025) or up to 3 (ERC), then no shocks or epinephrine until 30 °C; at 30–35 °C epinephrine every 6–10 min (ERC).
• VA-ECMO rewarms arrested patients at 1.5–5 °C/h (AHA 2025); HOPE 10% or more selects them; potassium above 12 mmol/L was the old futility line.
• Heat stroke is a core above 40 °C plus CNS dysfunction; heat exhaustion keeps mentation intact.
• Cool first, transport second: ice-water immersion at 0.15 °C/min or faster, stop at 38.5–39 °C; no antipyretics, no dantrolene.
• Drowning: ventilation first (5 breaths in ERC), no abdominal thrusts, spinal precautions only with a mechanism.
• Submersion over 25 min is almost always fatal; search limits are 30 min in water above 6 °C, 90 min below.
• Frostbite: rewarm in 37–39 °C water; tPA within 24 h or iloprost within 72 h for deep injury; amputate late.