What are the differences between passive and active rewarming in treating hypothermia?

Passive and Active Rewarming in Hypothermia

Passive rewarming relies on the patient's intrinsic heat-generating mechanisms to counteract heat loss, while active rewarming applies external or internal heat sources to directly increase core temperature—with passive methods appropriate for mild hypothermia and active methods required for moderate to severe cases.

Passive Rewarming

Passive rewarming allows the body's natural thermoregulatory mechanisms to restore temperature without applying external heat sources. 1

Mechanism and Rate

• Natural rewarming occurs at approximately 1.2°C/h, which can increase to 3.6°C/h if shivering is present 1
• The technique depends entirely on endogenous heat production through metabolic processes and shivering thermogenesis 1

Specific Techniques

• Remove all wet clothing immediately to prevent evaporative heat loss 1
• Move the patient to a warm environment and shield from wind 2
• Cover with at least two dry insulating blankets 2
• Insulate the patient from cold ground surfaces 2
• Cover the head and neck, as significant heat loss occurs from these areas 2
• Increase ambient environmental temperature 1

Clinical Application and Limitations

• Passive rewarming is most appropriate for mild hypothermia (32-35°C) 1, 2
• Evidence shows passive rewarming may actually cause a decrease in core temperature during transport in trauma patients, with one study demonstrating only active rewarming produced an increase in core temperature (+0.74°C) 1
• However, in mild hypothermia, passive rewarming can achieve similar temperature increases (0.9°C) as active methods 1

Active Rewarming

Active rewarming applies heat directly to the patient through external or internal methods and is required for moderate to severe hypothermia. 1

Active External Rewarming

Indications and Methods

• Used for moderate hypothermia (28-32°C) in patients without cardiac comorbidities 1, 2
• Techniques include radiant warmers, electric blankets, forced warm air blankets, heating pads, and chemical heat blankets 1, 3

Efficacy

• Forced-air warming blankets increase rewarming rates to approximately 2.4°C/h compared to 1.4°C/h with passive blankets alone 3
• Active resistive heating during prehospital transport produces higher temperature levels (+0.8°C vs. -0.4°C) and reduces patient pain and anxiety 1
• Even in severe hypothermia below 30°C, forced-air warmers can effectively rewarm patients to above 35°C, with mean rewarming rates of 1.7°C/h 1, 4

Important Caveat

• In severe hypothermia, peripheral vasoconstriction limits the efficacy of external rewarming, though it can still be considered 1

Active Internal (Core) Rewarming

Indications

• Used for moderate to severe hypothermia (<32°C), particularly when external methods are insufficient 1
• Required for severe hypothermia (<28°C) 2

Methods and Advantages

• Ventilation with humidified, warmed oxygen 1, 3
• Warmed intravenous fluids 1
• Peritoneal lavage with warmed fluids 1, 2
• Extracorporeal modalities including dialysis, cardiopulmonary bypass (CPB), continuous arteriovenous rewarming (CAVR), and heparin-free extracorporeal life support 1
• Intravascular balloon-catheter rewarming systems (mean rewarming rate 1.5°C/h) 5

Key Advantage

• Internal rewarming restores temperature to normal levels faster than surface methods and is associated with more rapid normalization of cardiac output 1

Critical Treatment Targets and Monitoring

Temperature Goals

• Target a minimum core temperature of 36°C before considering the patient stable 2, 3
• Cease rewarming after reaching 37°C, as higher temperatures are associated with poor outcomes and increased mortality 2, 3

Temperature Measurement

• Use oral or esophageal probes for accurate core temperature monitoring 2, 3
• Tympanic infrared probes serve as acceptable alternatives when oral measurement is not feasible 2, 3
• Avoid axillary measurements as they consistently read 1.5-1.9°C below actual core temperature 2, 3
• Monitor core temperature every 5-15 minutes depending on severity 2, 6

Critical Pitfalls to Avoid

• Do not use cold IV fluid boluses for treating accidental hypothermia—this is only indicated for therapeutic hypothermia 3
• Do not rewarm above 37°C, as this worsens outcomes 2, 3
• Handle severely hypothermic patients extremely gently to avoid triggering cardiac arrhythmias 2, 3
• Monitor continuously for rewarming complications including cardiac arrhythmias, coagulopathy, electrolyte abnormalities, hyperglycemia, and rebound hyperthermia 2, 3
• Be aware that hypothermia is frequently overlooked, with one major trauma center documenting temperature in only 38% of trauma admissions 6