Perioperative Hypothermia

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Perioperative Hypothermia. Karim Rafaat, MD. Introduction. The human thermoregulatory system usually maintains core body temperature within 0.2℃ of 37℃ - PowerPoint PPT Presentation

Transcript of Perioperative Hypothermia

Mild perioperative hypothermia

PerioperativeHypothermiaKarim Rafaat, MD IntroductionThe human thermoregulatory system usually maintains core body temperature within 0.2 of 37Perioperative hypothermia is common because of the inhibition of thermoregulation induced by anesthesia and the patient`s exposure to cool environmentHypothermia complications:Shiveringprolonged drug effect, coagulopathy surgical wound infectionmorbid cardiac event Normal thermoregulationProcessing of thermoregulatory information: afferent input central control efferent responsesNormal thermoregulationAfferent input: cold signal-A fiberwarm signal-C fiber

Each of the following contribute 20% of the total thermal input: Hypothalamusother parts of brainskin surfacespinal corddeep abdominal and thoracic tissues Normal thermoregulationPrimary thermoregulatory control centerHypothalamus

Control of autonomic responses is 80% determined by thermal input from core structures

In contrast, behavior response may depend more on skin temperature Normal thermoregulationThe inter-threshold range (core temperatures that do not trigger autonomic thermoregulatory responses) is only 0.2

Each thermoregulatory response can be characterized by a threshold, gain, maximal response intensity

Behavior is the most effective responseNormal thermoregulationMajor autonomic defenses against heat: 1. sweating 2.cutaneous vasodilation

Major autonomic defenses against cold: 1.cutaneous vasoconstriction 2.nonshivering thermogenesis 3.shivering Normal Thermoregulation

Normal thermoregulationVasoconstriction occurs in AV shunts located primarily in fingers and toes, mediated by -adrenergic sympathetic nerves

Non-shivering thermogenesis is important in infants, but not in adults (brown fat)

Shivering is an involuntary muscle activity that increases metabolic rate 2-3 times Thermoregulation during general anesthesiaGeneral anesthesia removes a pts ability to regulate body temperature through behavior, so that autonomic defenses alone are available to respond to changes in temperature

Anesthetics inhibit thermoregulation in a dose-dependent manner and inhibit vasoconstriction and shivering about 2-3 times more than they restrict sweating

Inter-threshold range is increased from 0.2 to 4 (20 times), so anesthetized pts are poikilothermic - with body temperatures determined by the environment Thermoregulation during general anesthesiaThe gain and maximal response intensity of sweating and vasodilation are well preserved when volatile anesthetics are given

However volatile anesthetics reduces the gain of AV-shunt vasoconstriction, without altering the maximal response intensity

Nonshivering thermogenesis dosen`t occur in anesthetized adults

General anesthesia decreases the shivering threshold far more than the vasoconstriction threshold Anesthesia Impairs Regulation

Inadvertent hypothermia during general anesthesiaInadvertent hypothermia during general anesthesia is by far the most common perioperative thermal disturbance (due to impaired thermoregulation and cold environment)

Patterns of intraoperative hypothermiaPhase I: Initial rapid decrease Phase II : Slow linear reduction Phase III:Thermal plateau

Patterns of intraoperative hypothermiaInitial rapid decrease

heat redistribution

decreases 0.5-1.5 during 1st hr

Tonic thermoregulatory vasoconstriction that maintains a temperature gradient between the core and periphery of 2-4 is broken

The loss of heat from the body to environment is little

Heat redistribution decreases core temperature, but mean body temperature and body heat content remain unchanged

Patterns of intraoperative hypothermia2. Slow linear reduction

decreases in a slow linear fashion for 2-3hrs

Simply because heat loss >metabolic heat production

90% heat loss through skin surface by radiation and convection

Patterns of intraoperative hypothermia3. Thermal plateau

After 3-5 hrs, core temperature stops decreasing

It may simply reflect a steady state of heat loss=heat production

If a pt is sufficiently hypothermic, plateau phase means activation of vasoconstriction to reestablish the normal core-to-peripheral temperature gradient

Temperature plateau due to vasoconstriction is not a thermal steady state and body heat content continues to decrease even though temperature remains constant Regional AnesthesiaRegional anesthesia impairs both central and peripheral thermoregulation

Hypothermia is common in patients given spinal or epidural anesthetics

ThermoregulationAll thermoregulatory responses are neurally mediated

Spinal and epidural anesthetics disrupt nerve conduction to more than half the body

The peripheral inhibition of thermoregulatory defense is a major cause of hypothermia during RAControlEpidural

RA also impairs the central control of thermoregulationThe regulatory system incorrectly judges the skin temperature in blocked areas to be abnormally high

It fools the regulatory system into tolerating core temperatures that are genuinely lower than normal without triggering a response Heat Balance and Shivering Initial hypothermia (Phase I)

Redistribution of heat from core to periphery

Primarily caused by peripheral inhibition of tonic thermoregulatory vasoconstriction

Although the vasodilatation of AV shunts is restricted to the lower body, the mass of the legs is sufficient to produce substantial core hypothermia Subsequent hypothermia (Phase II)

Loss of heat exceeds production

Patients given SA or EA cannot reestablish core-temperature equilibrium because peripheral vasoconstriction remains impaired

Hypothermia tends to progress throughout surgery

ShiveringOccurs during spinal and epidural anesthesia

Disturbs patients and care givers but produces relatively little heat because it is restricted to the small-muscle mass cephalad to the block

Treated by warming surface of skin or administration of clonidine / meperidineTemperature MonitoringCore SitesPulmonary arteryDistal esophagusNasopharynxTympanic membrane thermocoupleOther generally-reliable sitesMouthAxillaBladderSub-optimalForehead skinInfrared tympanicInfrared temporal arteryRectal

Anesth Analg 2008Potential Benefits of Mild HypothermiaImproves neurologic outcome after cardiac arrestBernard, et al.Hypothermia after cardiac arrest study groupNow recommended by European and American Heart AssociationsNumber needed to treat: 6Hypothermia recommended by International Liaison Committee

Improves neurologic outcome in asphyxiated neonatesShankaren, et al.Gluckman, et al.Eicher, et al.Number needed to treat: 6

No benefit in major human trialsBrain trauma in adults (Clifton, et al.) or children (Hutchison, et al.)Anurysm surgery: Todd, et al.Acute myocardial infarction: Dixon, et alComplications of Mild HypothermiaMany!

Well documentedProspective randomized trials1-2C hypothermia

Effects on many different systemsMost patients at risk for at least one complicationWound infection---the most common serious complication due to Impaired immune function decreased cutaneous blood flow protein wasting decreased synthesis of collagenComplications of Mild HypothermiaWound Infections: Melling, et al.Normothermia is more effective than antibiotics!

CoagulopathyHypothermia reduces platelet function and decreases the activation of the coagulation cascade

From in vitro studies, it increased the loss of blood and the need for allogenic transfusion during elective primary hip arthroplasty

Blood Loss

20% less blood loss per CTransfusion Requirement

22% less blood Transfusion per CMyocardial Outcomes: Frank, et al.

Drug metabolismMild hypothermia decreases the metabolism of most drugs

Propofol ---during constant infusion, plasma conc. is 30 percent greater than normal

Atracurium---a 3 reduction in core temp. increase the duration of muscle relaxation by 60 percent

Significantly prolongs the postoperative recovery period Duration of Vecuronium

Recovery Duration

Time (min) Thermal comfortPatients feel cold in postoperative period, sometimes rating it worse than surgical pain

Shivering occurs in ~40 percent of unwarmed patients who are recovery from GA

Summary: Consequences of HypothermiaBenefitsImproves neurologic outcomes after cardiac arrestImproves neurologic outcomes after neonatal asphyxiaMajor complicationsIncreases morbid myocardial outcomesPromotes bleeding and increases transfusion requirementIncreases risk of wound infections and prolonges hospitalizationOther complicationsDecreased drug metabolismProlonged recovery durationThermal discomfort Treating and Preventing Intraoperative HypothermiaPreventing redistribution hypothermia

The initial reduction in core temperature is difficult to treat because it result from redistribution of heat

Prevent by skin-surface warming Peripheral heat content Temperature gradient Redistribution of heat Prewarming Prevents Hypothermia

Airway heating and humidificationLess than 10% of metabolic heat is lost through respiratory route

Passive or active airway heating and humidification contribute little to thermal managementFluid WarmingCooling by intravenous fluids0.25C per liter crystalloid at ambient temperature0.25C per unit of blood from refrigerator

Fluid warming does not prevent hypothermia!Most core cooling from redistribution80% of heat loss