Hypothermia: Thresholds, Stages, and Clinical Data
Hypothermia stages: mild 32–35°C (shivering, confusion); moderate 28–32°C (lethargy, bradycardia); severe <28°C (arrhythmia risk); cardiac arrest below ~28°C. Lowest survived core temperature recorded: 13.7°C (Brown et al. 2012, NEJM).
| Measure | Value | Unit | Notes |
|---|---|---|---|
| Mild hypothermia threshold | 32–35 | °C core | Shivering, confusion, slurred speech; thermoregulation impaired but active |
| Moderate hypothermia threshold | 28–32 | °C core | Shivering stops, severe lethargy, bradycardia; cardiac arrhythmia risk begins |
| Severe hypothermia threshold | <28 | °C core | Ventricular fibrillation risk; apparent death; requires ECMO rewarming |
| Profound hypothermia | <20 | °C core | Cardiac standstill; appears dead; survival possible with ECMO |
| Lowest survived core temperature | 13.7 | °C | Brown 2012; child; ECMO rewarming; full neurological recovery |
| Time to hypothermia in cold water | 30 min in 10°C water | Significant core cooling for average adult; faster in thin, less muscular individuals | |
| Expected survival time in cold water | 1–3 hours | (depends on water temp) | At 10°C; 0°C: 30–90 min; 15°C: 2–6 hours; national safety guidelines |
Hypothermia is defined clinically as core body temperature below 35°C. It represents the failure of the thermoregulatory system to maintain normal core temperature. Understanding its stages is essential for cold exposure safety and emergency response.
Hypothermia Staging
| Stage | Core Temperature | Symptoms | Treatment |
|---|---|---|---|
| Mild | 32–35°C | Shivering, confusion, clumsiness | Passive rewarming; remove from cold |
| Moderate | 28–32°C | Shivering stops, stupor, bradycardia | Active external rewarming |
| Severe | <28°C | No shivering, arrhythmia, unconscious | ECMO; invasive rewarming |
| Profound | <20°C | Apparent death | ECMO or cardiopulmonary bypass |
Key clinical principle: “Not dead until warm and dead.” Hypothermic cardiac arrest may be reversible with aggressive rewarming. Resuscitation should not be abandoned until core temperature reaches ≥32°C.
Why 28°C Is the Critical Threshold
The heart’s electrical conduction system is exquisitely sensitive to cold:
| Core Temperature | Cardiac Effects |
|---|---|
| 35°C | Mild bradycardia; PR interval prolongation |
| 32°C | Arrhythmia risk begins; J (Osborn) wave on ECG |
| 28°C | Ventricular fibrillation risk high |
| <25°C | Asystole common; cardiac standstill |
The “Osborn wave” (J wave) on ECG is a pathognomonic sign of hypothermia — a positive deflection at the QRS-ST junction; correlates with core temperature.
Cold Water Survival Times
US Coast Guard and Royal National Lifeboat Institution (RNLI) guidance on cold water survival:
| Water Temperature | Expected Survival Time (without drysuit) |
|---|---|
| 0–2°C | < 30 min (cold incapacitation within 5–10 min) |
| 3–8°C | 30–90 min |
| 9–15°C | 1–6 hours |
| 16–21°C | 6–24 hours |
| >24°C | Survival time measured in days |
Note: these estimates assume the person is in the water passively. Swimming increases heat loss 30–50% and dramatically reduces survival time.
Cold Water Killing vs Drowning
The leading cause of cold water death is not hypothermia — it is cold incapacitation and cold shock drowning:
- Cold shock (0–3 min): Gasping, hyperventilation → inhale water → drowning before hypothermia
- Cold incapacitation (3–30 min): Swimming failure — muscles cool below functional temperature
- Hypothermia (>30 min): Core cooling reaches dangerous levels
Most cold water drowning deaths occur within the first 3–30 minutes — long before true hypothermia develops. This is why cold water swimming education emphasizes breath control over “lasting longer.”
The ECMO Rewarming Cases
Walpoth et al. (1997) and subsequent reports document miraculous survivors of profound hypothermia rewarmed by extracorporeal membrane oxygenation (ECMO):
- A child survived 13.7°C core temperature with full neurological recovery (Brown 2012)
- ECMO bypasses the hypothermic heart, warms the blood externally, returns it to the body
- Modern hypothermia protocol: if no signs of lethal injury, initiate ECMO regardless of apparent death