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