Department of OUTCOMES RESEARCH Thermoregulation Heat Balance Thermoregulation
Department of OUTCOMES RESEARCH
Thermoregulation & Heat Balance Thermoregulation during anesthesia Temperature monitoring Consequences of hypothermia Maintaining normothermia 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. Providing the evidence for evidence-based medicine©
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 OUTCOMES RESEARCH
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
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