Department of O UTCOMES R ESEARCH

Thermoregulation & Heat Balance Thermoregulation during anesthesia Temperature monitoring Consequences of hypothermia Maintaining normothermia Providing the evidence for evidence-based medicine © The Department of Outcomes Research receives funding from various companies who produce warming and/or temperature monitoring systems. Dr. Sessler has no personal financial interest related to this presentation. And he drives a 12-year-old car.

Normal Thermoregulation

Anesthesia Impairs Regulation

Hypothermia During Anesthesia

Redistribution Hypothermia

Intraoperative Heat Transfer

Hypothermia During Anesthesia

Summary: General Anesthesia Central thermoregulatory inhibition Little effect on warm defenses Dose-dependent increase in interthreshold range Intraoperative hypothermia Redistribution of heat (initial decrease) loss exceeding heat production (slow linear decrease) Core-temperature plateau with sufficient hypothermia

Central Inhibition During Neuraxial Anesthesia

Reduced Shivering Gain & Maximum Control Epidural

Behavioral Inhibition

Summary: Neuraxial Anesthesia Central regulatory inhibition Increased inter-threshold range Peripheral sympathetic and motor block Reduced shivering gain and max intensity Behavioral inhibition Hypothermia fails to trigger cold sensation Hypothermia Comparable to general anesthesia Worst when general and regional combined

Temperature Monitoring Core Sites Pulmonary artery Distal esophagus Nasopharynx Tympanic membrane thermocouple Other generally-reliable sites Mouth Axilla Bladder Sub-optimal Forehead skin Infrared “tympanic” Infrared “temporal artery” Rectal Anesth Analg 2008

Complications of Mild Hypothermia Many! Well documented Prospective randomized trials 1-2°C hypothermia Effects on many different systems Most patients at risk for at least one complication

Myocardial Outcomes: Frank, et al.

Blood Loss 20% less blood loss per °C

Transfusion Requirement 22% less blood Transfusion per °C

Wound Infections

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

Duration of Vecuronium

Recovery Duration Time (min)

Thermal Discomfort

Summary: Consequences of Hypothermia Benefits Improves neurologic outcomes after cardiac arrest Improves neurologic outcomes after neonatal asphyxia Major complications Increases morbid myocardial outcomes Promotes bleeding and increases transfusion requirement Increases risk of wound infections and prolonges hospitalization Other complications Decreased drug metabolism Prolonged recovery duration Thermal discomfort

Insulating Covers

More Layers Do Not Help Much

Forced-Air vs. Circulating-Water

Over-body Resistive Warming Negishi, A&A 2003 Röder, BJA 2011

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 80% of heat loss is from anterior skin surface Cooling prevented by warming solutions Type of warmer usually unimportant

Surgical Care Improvement Project (SCIP) Patients included (denominator) Surgical procedure General or neuraxial anesthesia ≥60 minutes Not having documented intentional hypothermia Criteria (numerator), any one of: Active intraoperative over-body warming Core temp ≥36°C within 30 min before anesth end time Core temp ≥36°C within 15 min after anesth end time Comments “Core temperature” sites and devices undefined A similar “pay-for-reporting” measure effective Jan 2010

The Rule: Monitor and Warm Monitor core temperature General anesthesia >30 minutes Large procedures under neuraxial anesthesia Maintain normothermia: core temp ≥36°C Forced-air heating Best combination of efficacy, cost, and safety But any method(s) okay

Department of O UTCOMES R ESEARCH

Potential Benefits of Mild Hypothermia Improves neurologic outcome after cardiac arrest Bernard, et al. Hypothermia after cardiac arrest study group Now recommended by European and American Heart Associations Number needed to treat: ≈6 Hypothermia recommended by International Liaison Committee Improves neurologic outcome in asphyxiated neonates Shankaren, et al. Gluckman, et al. Eicher, et al. Number needed to treat: ≈6 No benefit in major human trials Brain trauma in adults (Clifton, et al.) or children (Hutchison, et al.) Anurysm surgery: Todd, et al. Acute myocardial infarction: Dixon, et al

Prewarming Prevents Hypothermia