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Thermoregulation & Heat Balance Thermoregulation During General Anesthesia Temperature Monitoring Consequences of Hypothermia Maintaining Normothermia

Normal Thermoregulation

Anesthesia Impairs Regulation

Hypothermia During Anesthesia

Redistribution Hypothermia Core 37°C Vasoconstricted Periphery 31-35°C Anesthesia Periphery 33-35°C Core 36°C Vasodilated

Redistribution Hypothermia

Intraoperative Heat Transfer Evaporation Conduction Convection Radiation

Vasoconstriction Constrains Heat

Summary: General Anesthesia Central thermoregulatory inhibition Dose-dependent increase in interthreshold range More impairment of cold than warm responses Intraoperative hypothermia Redistribution of heat (initial decrease) loss exceeding heat production (slow linear decrease) Core Temperature Plateau Decreased cutaneous heat loss; thermal steady-state Separation of core and peripheral compartments

Temperature Monitoring Core Sites Pulmonary artery Distal esophagus Nasopharynx Tympanic membrane thermocouple Other generally-reliable sites Mouth Axilla Bladder Forehead skin with 2°C compensation Sub-optimal Infrared “tympanic” Rectal

Benefits of Mild Hypothermia Cerebral ischemia and hypoxemia 1-3°C provides marked protection in animals No benefit in major human trials Brain trauma: Clifton, et al. Anurysm surgery: Todd, et al. Acute myocardial infarction: Dixon, et al. Improves neurologic outcome after cardiac arrest Bernard, et al. Hypothermia after cardiac arrest study group Improves neurologic outcome in asphyxiated neonates Shankaren, et al. Malignant hyperthermia 2-3°C delays or prevents triggering and decreases severity

Myocardial Outcomes: Frank, et al.

Perioperative Blood Loss 16% less blood loss (95% CI 4-28%, P = 0.009)

Wound Infections: Kurz, et al. Normothermia is more effective than antibiotics!

Duration of Vecuronium

Recovery Duration

Thermal Discomfort

Summary: Consequences Benefits Protects against cerebral ischemia (cardiac arrest) Decreases triggering and severity of MH Major complications Morbid myocardial outcomes Bleeding and increased transfusion requirement Wound infections and prolonged hospitalization Other complications Decreased drug metabolism Prolonged recovery duration Thermal discomfort

Insulating Covers

More Layers Do Not Help Much

Active Warming

Prewarming Prevents Hypothermia

Fluid Warming Cooling by intravenous fluids 0.25°C per liter crystalloid at ambient temperature 0.25°C per unit of blood from refrigerator Fluid warming does not prevent hypothermia! Most core cooling from redistribution 90% of heat loss is from anterior skin surface Cooling prevented by warming solutions Type of warmer usually unimportant Use high-flow systems for major trauma

The Rule: Monitor and Warm Monitor core temperature General anesthesia >30 minutes Large procedures under neuraxial anesthesia Maintain normothermia Core temperature ≥36°C Forced-air heating Best combination of efficacy, cost, and safety

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