Rewarming Techniques

The appropriate rewarming technique for a hypothermic patient depends on the severity of hypothermia.

There are four types of rewarming:

Passive rewarming

This principle allows the patient's own thermogenic mechanisms to rewarm them.
Patients should be moved from the cold environment and wet or cold clothes should be removed. They can then be covered by a blanket or sleeping bag and have their head covered to reduce heat loss.

This technique can be used for patients with mild hypothermia who can still generate heat by shivering.

Active external rewarming

This is where heat is added to the patient from an external source and is the treatment of choice in mild-to-moderate hypothermia patients, whose own thermoregulatory mechanisms are impaired.

It can be accomplished by a variety of methods, including heat packs, heat lamps, blankets, warm water immersion, warmed blankets, and forced air systems.

Patients must be monitored carefully for a presumptive afterdrop, caused by the return of cool peripheral blood to the central circulation.

Forced air systems are one of the most widely used techniques as they're easy to apply, allow for patient monitoring and seem to limit afterdrop.

Rates of warming with these systems approach 1.0-2.5oC/hour [2-3]. The Bair Hugger is a well-known example.

Active core rewarming

The simplest example of active core rewarming is the use of warmed intravenous fluids and warmed, humidified oxygen. Fluids can be warmed to 44oC.

Warmed humidified air/oxygen should also be heated to 42-44oC. This may require special equipment or modification to the existing heating circuit.

More invasive methods of active rewarming include cavity (gastric, bladder, peritoneal, pleural) lavage using warm fluids.

These methods achieve the most rapid rewarming rates but, due to their invasiveness and complexity, should be reserved for the most severe and refractory of cases including hypothermic cardiac arrest, the failure of more conservative techniques, frozen extremities and evidence of rhabdomyolyis in conjunction with other electrolyte abnormalities.

Extracorporeal blood rewarming

This aggressively rewarms the blood in the severely hypothermic patient who has been refractory to all other methods of rewarming.

There are several methods including haemodialysis, arteriovenous, veno-venous and cardio-pulmonary bypass.

The biggest advantage of this method is the speed at which the patient can be rewarmed by directly warming their blood.

Additional advantages include the continual delivery of oxygenated blood to the tissues, despite the absence of mechanical cardiac activity (cardiopulmonary bypass).

Extracorporeal rewarming should be considered for patients without perfusion, who have no documented contraindications to resuscitation, patients with severe hypothermia and those with completely frozen extremities.

There are no specific criteria for placing a patient on extracorporeal rewarming, but several centres reserve it for patients with a pH >6.5, a serum potassium <10 mmol/L and a core temperature >12oC.