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Thermoregulation and Heat Balance Thermoregulation During General Anesthesia Consequences of Hypothermia Maintaining Normothermia

Normal Thermoregulation

Anesthesia Impairs Thermoregulation

Hypothermia During Anesthesia

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

Intraoperative Heat Transfer Evaporation Conduction Convection Radiation

Hypothermia During Anesthesia

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 thermocouples Other generally-reliable sites Mouth Axilla Bladder Sub-optimal Infrared “tympanic” Rectal

Benefits of Mild Hypothermia Cerebral ischemia and hypoxemia 1-3°C provides marked protection in animals Improves outcomes from cardiac arrest Brain trauma Marion, et al.: 62% vs. 38% “good outcome” Clifton, et al.: No benefit Malignant hyperthermia 2-3°C delays or prevents triggering Decreases severity once triggered Cardiac arrest Two studies showing improved outcome Acute myocardial infarction Convincing evidence in animals Equivocal human results

Frank, et al., JAMA, 1997 Myocardial Outcomes: n=300 NormothermicHypothermicP Core Temp (°C) 36.7 ± ± 0.1<0.001 Myocardial Events (%) That hypothermia causes myocardial ischemia was confirmed by Nesher, et al, Ann Thorac Surg, 2001

Coagulopathy: n=60, Hip Arthroplasty Three subsequent studies found that hypothermia increases blood loss (Kurz, Winkler, Widman) whereas one did not (Johansson).

Surgical Wound Infections: n=200 Three-fold reduction in infection risk from local or systemic warming confirmed by Melling, et al., Lancet, 2001

Duration of Vecuronium

Recovery Duration

Thermal Discomfort

Summary: Consequences Benefits Protects against cerebral ischemia Decreases triggering and severity of MH Major complications Myocardial ischemia and morbid myocardial outcomes Bleeding and increased transfusion requirement Surgical wound infections and prolonged hospitalization Other complications Decreased drug metabolism Prolonged recovery duration Thermal discomfort

Insulating Covers

More Layers Do Not Help Much

Forced-Air vs. Circulating-Water

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

The Rule: Maintain Normothermia Normal body temperature is °C Temperatures less than 36°C considered hypothermia Standard of care is to maintain normothermia Less effective: Reduce heat loss High ambient temperature Passive insulation Fluid warming More effective: Forced-air heating

Outcomes Research™ Medical Research to Guide Clinical Decisions ©