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Department of OUTCOMES RESEARCH
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 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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