Rewarming of a person with hypothermia

The approach to rewarming depends on a number of factors—including available equipment, ability to monitor a patient, ability to carefully control the rate of rewarming and the need to avoid delaying retrieval.

Passive rewarming aims to avoid further cooling, rather than to increase core body temperature. It is the primary method of rewarming outside of a hospital environment because:

• materials are generally more available than those required for active rewarming
• monitoring equipment that is required to ensure a safe rate of active rewarming is often not available in the field.

Passive rewarming techniques include:

• placing the patient in a warm, wind-free environment insulated from the cold
• removing any wet clothing and drying the patient, if possible; do not delay evacuation or resuscitation to dry the patient
• insulating the patient in a multilayered ‘burrito style’ wrap; use clothing, quilts, sleeping bags, bubble wrap; this is still effective when applied over the top of wet clothes
• adding a vapour barrier using a material such as bubble wrap, a tarpaulin, a plastic sheet, reflective blanket, rubbish bag with holes cut for the face; place it inside the insulation layer if the patient is wet or outside the insulation to protect it from a wet environment
• allowing the patient to shiver
• providing warm, caloric fluids to drink if the patient is alert and oral intake is tolerated.

Active rewarming aims to increase core body temperature at a safe rate of 1 to 2ºC per hour; this limits the risk of afterdrop (further core cooling caused by the return of cool acidaemic blood to the core from the peripheral circulation). Afterdrop is associated with an increased risk of cardiac arrhythmias, hypotension and electrolyte abnormalitiesDow, 2019. Electrolyte concentrations can change rapidly and unpredictably with rewarming, and should be monitored closely.

Active external warming techniques include:

• warmed blankets or heating pads; apply large heating pads to the upper torso if possible (axillae, chest, back, groin in that order); it is important to check heated skin every 20 to 30 minutes to assess for excess reddening
• forced warm air devices (eg Bair Hugger, Warm Touch).

Do not use small chemical heat packs for rewarming because they are ineffective and can cause thermal burns.

Avoid placing the person in a warm bath or shower because it risks precipitating collapse.

Active internal rewarming may be required for more severe cases of hypothermia, and includes:

• warmed intravenous fluids, ideally at 40 to 42ºC (a dedicated fluid warmer should be used if available but, as a simple alternative, fluid bags stored in a blanket warmer are adequate; fluid should never be heated in a microwave)
• warmed humidified oxygen
• extracorporeal blood rewarming, including cardiopulmonary bypass (requires advanced hospital facility support).

Warming of peripheries to reduce localised cold injury can be considered in the field for people with mild hypothermia, but it should be deferred until core body temperature has been corrected in those with moderate or severe hypothermia, as it can cause afterdrop.