Accidental Hypothermia

Accidental hypothermia is a core temperature below 35 °C (95 °F) that nobody planned. ELSO 2025 sorts it into three stages: mild (35–32 °C), moderate (32–28 °C) and severe (below 28 °C). The original Swiss system used four bands instead: 35–32, 32–28, 28–24 and below 24 °C. Weather, water immersion, avalanche burial and indoor exposure can all cause it. The patients you see most are older adults with several comorbidities.

Cold attacks the heart's electrical system and its pump. Dysrhythmias appear, cardiac output falls and the heart can stop. Movement, airway management and other rescue procedures can trigger that arrest. Once hypothermic cardiac arrest (HCA) starts, outcome depends on ECLS access and the HOPE inputs, not on temperature alone.

• Risk of arrest: the colder the core, the higher the risk. Older patients and patients with comorbidities carry more of it.
• Staging systems: ELSO 2025 counts three temperature bands. The 2021 Revised Swiss System (ICAR MedCom) grades HT I to HT IV on responsiveness (AVPU), vital signs and arrest risk. The two systems are not interchangeable. Do not convert one into the other.
• Clinical course: a cold patient with stable circulation can be rewarmed by conventional methods. Unstable patients need extracorporeal rewarming.

Presentation by stage

• Mild (35–32 °C): mental status normal or mildly impaired.
• Moderate (32–28 °C): unconscious, or conscious with altered mental status.
• Severe (below 28 °C): unconscious, with a high risk of dysrhythmia and cardiac arrest.

Red Flags

• Unresponsive or altered mental status with cold exposure.
• Systolic blood pressure below 90 mmHg: the ELSO and ERC instability threshold, with weak validation (see Pitfalls).
• Any ventricular arrhythmia, or cardiac arrest.
• Pulseless patient with core temperature below 30 °C: treat as a possible HCA and assess for ECLS rewarming.

Scores: HOPE was derived in adults (Pasquier 2018, in the references). It estimates survival to hospital discharge after ECLS for hypothermic cardiac arrest. Its inputs are age, sex, mechanism, CPR duration, serum potassium and core temperature. A 2025 retrospective cohort applied it to 46 adults. ELSO 2025 lists an online calculator.

Every level follows the same pathway. What changes is the set of actions below.

1. Handle gently: careful movement may prevent hypothermic cardiac arrest, so limit lifting and rough handling.
2. Start CPR early: high-quality CPR is essential. Mechanical compressions earn their place when CPR may run long. Intermittent CPR is an option in awkward or hazardous spots, while moving the patient, or when there are too few rescuers.
3. Pads on the chest: put defibrillator pads on early, so a shockable rhythm gets treated without delay and you can watch the rhythm. Shivering and movement artifact can look like VF.
4. Remove wet clothing and insulate: strip off wet clothing and wrap the patient in a vapor barrier and insulation to stop heat loss. Keep them horizontal to limit cold-core circulation shifts, and move them as little as possible.
5. Check glucose: run a point-of-care glucose and treat any hypoglycemia. Look for toxic or alcohol contributors.
6. Mild stage only: an alert patient who can swallow may get warm sweet drinks, and active movement may help. Do not give oral fluids when mental status is altered.
7. Warm in transport: forced-air warming in transport is commonly recommended when you have it (see the WMS 2019 guideline in the references).

Doses vary by protocol; follow local protocols and medical direction.

• Monitoring (Paramedic/RN/CCT): cardiac monitor and pads on early; core temperature by esophageal probe where available.
• IV or IO access: warmed crystalloid at 38–42 °C is an adjunct only, not a sole rewarming method. See Lactated Ringer's.
• Oxygen: heated, humidified oxygen is an adjunct to other rewarming methods, not the only one. See oxygen.
• Cardiac arrest, ventricular fibrillation or pulseless VT: ventricular arrhythmias and asystole may not respond until core temperature is above 30 °C. ERC 2025 guidance: up to 3 shocks for VF or pulseless VT, then delay further shocks until core temperature is above 30 °C. Shock number and drug timing in hypothermic arrest vary between guidelines and local protocols; follow local protocol and medical direction. See defibrillation.
• Epinephrine (Paramedic/RN/CCT): core temperature sets the rule (ERC 2025 guidance). Below 30 °C, withhold epinephrine, because it accumulates; give 1 mg IV/IO once core temperature reaches 30 °C, unless ECPR is imminent. Between 30 and 35 °C, give 1 mg every 6–10 min instead of every 3–5 min. At 35 °C or higher, use the normothermic interval of every 3–5 min; ERC does not set a separate interval at this temperature. See epinephrine IV.
• Antiarrhythmics: for a shockable rhythm, amiodarone dosing and timing in hypothermia are protocol dependent; follow local protocol and medical direction. See amiodarone.
• Airway (Paramedic/RN/CCT): airway management can trigger arrest. Plan the move and the airway together.

• ECLS for HCA: indicated when core temperature is below 30 °C and HOPE is 10% or higher. Femoro-femoral VA-ECMO is preferred. Keep doing CPR until ECLS is established. Do not let CT delay cannulation or rewarming. See ECMO.
• ECLS evaluation for unstable patients without arrest: consider it when systolic pressure is below 90 mmHg (the ELSO and ERC instability threshold, weakly validated) or a ventricular arrhythmia is present. Eligibility should not hang on core temperature or stage alone.
• Conventional rewarming (stable patients): warm IV fluids, warm humidified oxygen and in-hospital methods such as bladder or gastric lavage and peritoneal or thoracic lavage, as local resources allow. Unstable patients need an ECLS evaluation.
• Rewarming rate: up to 5 °C per hour to 30 °C or return of spontaneous circulation, whichever comes first. Keep the water heater at no more than 10 °C above venous inflow, to a maximum of 42 °C, with an outlet gradient of 4 °C or less.
• Anticoagulation: anticoagulate once circulation is stable. Reduce or stop it with excessive bleeding or signs of trauma.
• Weaning: weaning may be attempted once core temperature is above 35 °C with improved ventricular function and stable hemodynamics.
• After rewarming: manage post-arrest care as in post-cardiac-arrest care.

• Destination: transfer early to an ECLS-capable hypothermia center when the patient is in arrest, unstable or at risk of arrest. Core temperature alone does not make the transfer call. HOPE, the patient's condition and ECLS availability make it. ERC 2025 sends the patient to an ECLS centre without delay for these risk factors for imminent arrest: heart rate below 45/min, systolic pressure below 90 mmHg, ventricular arrhythmia, or core temperature below 30 °C.
• Resuscitation in transit: a long transport with CPR still running is no reason to withhold resuscitation. For prolonged CPR, use a mechanical compression device. This page does not cover stop rules. Follow local protocol and medical control for them. If no ECLS centre is reachable in a reasonable time (ERC 2025 cites about 6 hours), start non-extracorporeal rewarming.
• Warming in transit: use forced-air warming if you have it. Keep the patient insulated and monitor esophageal temperature where available.
• Handoff: time of onset, CPR start time and duration, rhythm, core temperature, potassium, blood pressure and the HOPE inputs (age, sex, mechanism, CPR duration, potassium, core temperature).
• Equipment: defibrillator pads, esophageal temperature probe, mechanical CPR device and warmed fluids (ELSO and WMS guidance).

• Pediatrics: HOPE was derived in adults and has only been applied in adult cohorts. For children, use pediatric ECLS center protocols, including the cannulation approach. Drug doses are weight-based. See weight-based dosing and pediatric cardiac arrest.
• Pregnancy: the sources reviewed do not establish a specific adjustment for accidental hypothermia in pregnancy. Use standard resuscitation and local obstetric protocol.
• Older Adults: age 70 or older, an unwitnessed arrest, a long low-flow time and a low end-tidal CO₂ are not contraindications to ECLS for HCA. Older patients with comorbidities face a higher risk of arrest as core temperature falls.
• Patients on Anticoagulants: hypothermic coagulopathy raises the bleeding risk. With signs of trauma, consider ECLS rewarming with minimal or no anticoagulation.

• Error: stopping CPR just because the patient is cold. HCA may be reversible. If the patient is a candidate, continue CPR until ECLS is established.
• Error: applying normothermic ECLS criteria to a hypothermic arrest. ELSO 2025 says those criteria should not be used for hypothermic patients.
• Error: making the ECLS call from core temperature alone. Use the HOPE score and the patient's condition.
• Error: estimating core temperature by feel, or reading it from a non-core thermometer. Field estimates are imprecise, so use an esophageal probe where possible (Revised Swiss System).
• Error: rewarming too fast. Going above 5 °C per hour, or running a water heater above 42 °C, risks neurologic complications and hemolysis.
• Error: handling and airway work with no warning. Movement and airway management can trigger arrest. Plan the move and the intubation together.
• Error: using the normothermic epinephrine interval below 35 °C, giving epinephrine below 30 °C (ERC 2025 withholds it), or treating a systolic pressure of 90 mmHg as a proven ECLS trigger. Between 30 and 35 °C, lengthen the interval to 6–10 min, per ERC 2025. SBP below 90 mmHg is the ELSO and ERC instability threshold. A 2025 multicenter retrospective study of non-ECLS rewarming found SBP 90 mmHg was the best-performing cutoff for death (sensitivity 0.74, specificity 0.70). Treat it as a prognostic marker, not a proven ECLS trigger. A hypotensive patient should go to an ECLS center.