Thermoregulation, Post-Arrest Care and Submersion

Accidental Hypothermia: Pathophysiology and Rewarming Strategies

Accidental hypothermia is categorized by core body temperature: mild hypothermia spans 32°C to 35°C (89.6°F to 95.0°F), moderate hypothermia ranges from 28°C to 32°C (82.4°F to 89.6°F), and severe hypothermia is defined as a core body temperature below 28°C (82.4°F). As core temperature drops below approximately 30°C to 32°C, the body loses its primary physiological heat-generation defense because shivering ceases. Electrocardiogram changes emerge predictably across cooling, including sinus bradycardia, PR and QT interval prolongation, and characteristic Osborn (J) waves. Hypothermia also triggers cold-induced diuresis secondary to peripheral vasoconstriction and an impaired renal tubular response to antidiuretic hormone.

A critical safety priority in severe hypothermia is gentle handling. An extremely cold myocardium is profoundly irritable, and rough movement or jarring transfers can trigger life-threatening ventricular arrhythmias, particularly ventricular fibrillation. Hypothermia also impairs both platelet aggregation and the enzymatic clotting cascade, resulting in clinical coagulopathy. Despite severe bleeding tendencies at the bedside, standard laboratory values for PT/INR and aPTT may appear deceptively normal because diagnostic assays are routinely warmed to 37°C in the laboratory.

Rewarming carries its own physiological hazards. Afterdrop is a further decline in core temperature during or after removal from the cold; heat transfer from the warmer core to colder tissues and, particularly with peripheral rewarming, the return of cold blood to the core can contribute. Nurses must continuously monitor for afterdrop and associated hypotension. Furthermore, as cells warm, potassium shifts out of the intracellular space and into the extracellular space, creating a risk for severe hyperkalemia. In hypothermic cardiac arrest, cardiopulmonary resuscitation and rewarming continue while following hypothermia-specific protocols, which may limit repeat defibrillation attempts and defer medications until the core temperature reaches approximately 30°C. Extracorporeal membrane oxygenation (ECMO) or cardiopulmonary bypass provides the most effective active internal rewarming method for severe hypothermic arrest.