Accidental Hypothermia and Rewarming: The Cold Patient in the ICU

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Updated July 2026  |  More ICU clinical guides →

Accidental hypothermia — an unintentional drop in core body temperature below 35 °C — turns ordinary physiology upside down. The heart slows and becomes electrically irritable, the coagulation system fails in the cold, and the patient can look dead while the brain is actually being protected by the very cold that endangers the heart. The nurse's job is to measure a true core temperature, rewarm at a controlled rate, and anticipate the two hazards that kill during recovery: rewarming arrhythmias and afterdrop. This guide covers staging, the physiology behind the dangers, the rewarming ladder, and the resuscitation rules that are unique to cold patients. Follow your facility's hypothermia protocol; the numbers here are typical reference ranges to frame the concepts.

Staging: measure a true core temperature

Peripheral and oral temperatures are unreliable in the cold — you need a true core reading, typically esophageal (in an intubated patient), bladder, or rectal, using a probe that reads to low temperatures. A standard clinical thermometer that bottoms out at 34 °C will not tell you how cold the patient actually is. Staging combines the number with the clinical picture:

Note the loss of shivering as the patient cools — shivering is the body's own heat generator, and when it stops, the patient can no longer rewarm without help. The classic ECG finding is the Osborn (J) wave, a hump at the QRS–ST junction, along with bradycardia and prolonged intervals. Cold also causes a diuresis, so many patients are volume-depleted despite looking puffy.

Why the cold heart is so dangerous

Below roughly 30 °C the myocardium becomes electrically unstable and exquisitely sensitive to handling. Rough movement, a jostled central line, or an abrupt position change can tip it into ventricular fibrillation — and a fibrillating cold heart is notoriously resistant to defibrillation and drugs until it is rewarmed. This is why cold patients are moved gently, why non-essential stimulation is minimized, and why some protocols limit shocks and hold repeat drug dosing until the core temperature rises. The same cold that threatens the heart, however, dramatically lowers the brain's oxygen demand, which is why prolonged resuscitation and remarkable neurologic recoveries are possible.

The rewarming ladder

Rewarming escalates with severity. The goal is a controlled rise, not the fastest possible one, because too-rapid or uneven rewarming worsens afterdrop and instability.

Passive external rewarming

For mild hypothermia in a patient who still shivers: remove wet clothing, dry the skin, and insulate with warm blankets in a warm room. The patient generates their own heat and rewarms slowly. Warm, sweet oral fluids help if the patient can safely swallow.

Active external rewarming

Forced-air warming blankets, radiant warmers, and warmed IV fluids add heat from outside. A subtlety: warming the extremities first can worsen afterdrop by opening peripheral vessels and dumping cold, acidemic blood back to the core — many protocols warm the trunk preferentially. Give warmed IV fluids (many cold patients are volume-depleted) and monitor continuously.

Active internal (core) rewarming

For moderate-to-severe cases: warmed, humidified oxygen, warmed IV fluids, and warm-fluid lavage of body cavities (bladder, and in some centers pleural or peritoneal). These add heat centrally, which is safer than peripheral-first warming.

Extracorporeal rewarming

For cardiac arrest or severe instability, extracorporeal life support (ECMO or cardiopulmonary bypass) is the definitive rewarming method — it warms the blood directly, supports the circulation, and provides the fastest controlled temperature rise. This is why a severely hypothermic arrest is ideally routed to a center with ECLS capability.

Afterdrop and the electrolyte swings

Afterdrop is a paradoxical continued fall in core temperature after rewarming begins, as cold blood pooled in the periphery circulates back to the core. Core-first rewarming strategies and gentle handling limit it. As the patient rewarms, watch electrolytes closely — potassium can shift, and a markedly elevated potassium in an arrested cold patient is a marker of poor prognosis. Rewarming also unmasks relative hypovolemia as vessels dilate, so blood pressure can drop and fluids are often needed. Because cold impairs the clotting cascade, coagulopathy improves as temperature normalizes — another reason warmth is treatment, not just comfort.

Where to go from here

Pair this with the unstable bradycardia guide for managing the slow cold heart, the cardioversion vs. defibrillation guide for the electrical decisions in cold arrest, and the hyperkalemia emergency guide for the potassium shifts that appear during rewarming.