Perioperative Heat Balance

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Perioperative Heat Balance

• KyungHee Medical Center
• Department of Anesthesiology
• Shin Kwang Il

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Hypothermia

• Anesthesia and surgery ? Thermal perturbation
• Anesthesia impair thermoregulatory control
• A cool operating room
• Surgery promote excessive heat loss
• Inhibition of thermoregulatory defenses than cold
  exposure per se
• Core hypothermia
• Distribution of body heat than heat production
  and heat loss

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1. Heat Production and Distribution

• Tissue temperature - heat content of tissue about
  0.83 kcal/kg/ºc
• Heat production
• Proportion to metabolic rates
• Energy generation by a chemical reation
• Substrates glucose, protein and fat
• Products carbon dioxide and water
• Glucose 4.1kcal/kg
• Protein 4.1kcal/kg
• Fat 9.3kcal/kg

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1. Heat Production and Distribution

• Thermal compartment
• Core thermal compartment
• Well-perfused tissue
• Fast distribution of heat
• Consist of the trunk and head
• Comprises 5060 of the body mass
• Peripheral thermal compartment
• Consist of arms and legs
• 24 ? less than core temperature
• Core-to-peripheral temperature gradients
• in warm or vasodilation - lower
• in vasoconstriction higher

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1. Heat Production and Distribution

• Heat flow and generation
• Core tissue rapid distribution of heat
• Peripheral tissue slow distribution of heat
• Core-to-peripheral flow of heat
• Mediated by blood-borne convection and conduction
• Convective component
• large and rapid flow of heat
• big axial vessels
• Conductive component
• slower radial flow of heat
• warm tissue ? cooler tissue

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1. Heat Production and Distribution

• Convective component
• Large and relative rapid flow of heat
• by the big axial vessels
• by peripheral blood flow
• Heat exchange between adjacent AV
• Core-to-peripheral temperature gradient
• Conductive component
• Slower radial flow of heat by the diffusion
  coefficient
• Warm tissue ? cooler tissue
• Insulation fat (3X) gt muscle

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1. Heat Production and Distribution

• Deposition of heat into peripheral tissue
• Augmented by local tissue metabolism and
  diminished regional cutaneous heat loss
• Dissipate all metabolic heat ? maintain thermal
  steady state
• About 95 of this heat ? skin surface
• Remaining ? lost via the respiratory track
• In the absence of sweating
• 10 of cutaneous heat loss is evaporative in
  adults
• Sweating 10 times in dry state
• The upper chest and face are most sensitive to
  temp.
• Heat loss is roughly propotional to surface area
  over the entire body

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2. General Anesthesia

• General anesthesia 13 hypothermic
• Depend on the type and dose of anesthesia
• amount of surgical exposure
• ambient temperature
• A characteristic pattern of Hypothermia
• 1st hour core temp. - 11.5 ? decrease
• 23 hours slower, linear decrease
• Finally a plateau phase - constant core temp.

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1) Redistribution

• Core thermal compartment
• Well-perfused tissue of the trunk and head
  relatively high temp.
• Peripheral tissue
• 24 ? cooler than the trunk and head
• Normal core-to-peripheral temp. gradient
• Maintained by the tonic thermoregulatory
• vasoconstriction of A-V shunts in fingers
  and toes

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1) Redistribution

• (1) Mechanism
• General anesthesia vasodilation via 2
  mechanisms
• Reduces the vasoconstriction threshold
• Cause direct(peripheral) vasodilation(anesthetics)
  
• This internal redistribution of body heat
• Decrease core temp
• Increase peripheral temp

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1) Redistribution

• (1) Mechanism
• A quantitative study(in volunteers)
• After 1h of anesthesia
• Core temp decreased 1.6 ºc
• redistribution 81
• Subsequent 2h of anestheia
• Core temp decreased on additional 1.1 ºc
• redistribution only 43
• During the first 3h of anesthesia
• Redistribution contributed 65 to the total
  decrease in core temperature
• Core-to-peripheral redistribution - primary cause
  of hypothermia during initial phase of anesthesia

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1) Redistribution

• (2) Major factors influencing magnitude
• The extent of redistribution hypothermia
• Patient's initial body heat content
• Core temperature normal even in warm environ.
• Redistribution magnitude is limited when
  peripheral and core temp. are similar
• Body morphology
• Obese pts lt normal weight pt
• The amount of redistribution
• Systemic heat loss, cool or large surgical
  incision

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2) Linear Phase

• 2nd portion of hypothermic curve
• Slow, linear decrease core temp.
• Heat loss exceed metabolic heat production
• Metabolic rate 1540 reduce by general Anes.
• Cutaneous heat loss
• Radiation, conduction, convection, evaporation
• Conductive loss - linearly
• Infants and children, large op. - great loss
• Warm environment - reduced hypothermia

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2) Linear Phase

• (1) Radiation
• Usually contribute most
• Transfer of heat via photons
• The amount of heat loss by radiation a
  function of the emissivity of the two surface
• Emissivity defines an object's ability to absorb
  and emit heat.
• Black bodies absorb and emit heat perfectly
• an emissivity of one,
  mirror 0
• Human skin (all colors) near 0.95 for infrared
  light.

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2) Linear Phase

• (2) Conduction and convection
• Conduction direct transfer of heat
• Proportional to surface temp. difference and
  insulation.
• Convection facilitated (or forced) conduction.
• Proportional to square root of air velocity
• Second most important source of heat loss during
  anesthesia and surgery
• Dominant laminar flow of units

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2) Linear Phase

• (3) Evaporation
• Heat of vaporization of water 0.58 kcal/g
• Skin 5 of basal metabolic rate host
• Respiration 10 ? of basal metabolic rate
• Surgical incision large ? ?
• Preparation solution
• Alcohol-based sol.gtwater-based sol.

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3) Core Temperature Plateau

• Final phase of the typical intra op. hypothermia
  curve
• After 24h of anesthesia and surgery
• Constant core temperature
• Passive and actively maintained

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3) Core Temperature Plateau

• (1) Passive plateau
• Metabolic heat production heat loss
• Thermoregulatory defences(-)
• During anesthesia and surgery
• Anesthesia metabolic heat production ?
• High heat loss cool op. room
• cool IV and
  irrigating fluid
• surgical
  incisions evaporation and radiation loss
• Behavioral components not available
• autonomic response impared.
• ? A passive plateau rarely develop
• Each degree core temp. reduce heat loss roughly
  by 10
• Slower decrease metabolic heat production (i.e
  6/ ºc )
• Most common effective insulted small op.

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3) Core Temperature Plateau

• (2) Active plateau
• by trigger thermoregulatory vasoconstriction.
• 3435? core temp trigger thermoregulatory
  vasoconstriction (typical concent. of most
  anesthetics)
• Vasoconstriction sl. reduce cutaneous heat loss
  but an important influence on distribution of
  body heat
• Body heat by metabolic active organ in core
  thermal compart.
• Tonic vasoconstriction constrain of heat in core
  compart.
• 3? or 4? core-to-peripheral temp. gradient.
• Revasoconstriction restrict further flow on
  heat.
• ? Core temp. plateau.
• Body heat content and mean body temp.?

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3) Core Temperature Plateau

• (3) Limb tourniquets
• A special case of core temperature plateau
• ? Intense vasoconstriction - no blood
• Not exchange blood or heat
• Not escape metabolic heat from core
• Remain relatively warm (core thermal
  compart.)
• Inflate - help to prevent hypothermia
• Release - precipitous hypothermia (after drop)

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4) Pediatric Patients

• Infant
• Larger head surface
• Thin skull and scalp
• Larger surface area
• ? Larger heat loss
• The linear hypothermic phase rapid
• ? High surface-area-to-weight ratio
• Respiratory lossessimilar to adults

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3. Neuraxial Anesthesia

• 1) Redistribution
• Major initial cause of hypothermia
• Central inhibit thermoregulatory control
• Peripheral block of sympathetic and motor NN.
• Prevent thermoregulatory vasoconstriction and
  shivering
• Typically restricted to the legs
• Core temp. decrease by restributionhalf of
  general anesthesia
• 1st hour most important cause of core
  hypothermia
• Core temp. subsequently decreases linearly

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3. Neuraxial Anesthesia

• Redistribution during epidural anesthesia in
  volunteers
• 1st hour core temp.? 0.80.3?
• redistribution 89
• 20 Kcal trunk ?extremities
• 2hours core temp.? additional 0.40.3?
• redistribution 62
• during 3hour core temp.? 1.20.3?
• redistribution 80

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3. Neuraxial Anesthesia

• 2) Lack of an active core temp. plateau
• Neuraxial anesthesia
• Primary cause of hypothermia
• Peripheral nerve block
• Unblocked region vasoconstriction and shivering
  ? insufficient prevent further hypothermia
• Minor surgery no matter
• Large operation serious hypothermia

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3. Neuraxial Anesthesia

• 3) Neuraxial and general anesthesia
• Core temp? general anes.gtregional anes.
• Redistribution of body heat primary cause of
  hypothermia during 1st hour of neuraxial or
  general anesthesia
• Redistribution decreases core temp. gen. ane.
  2x
• Short proceduregreater hypothermia in general
  anes
• Loss less to redistribution and near normal
  metabolic heat production in reg. anes.
• Long, large operations
• General anes core temp. plateau
• Neuraxial anes continue hypothermia

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3. Neuraxial Anesthesia

• (1) Combined neuraxial and general anesthesia
• Great risk for perioperative hypothermia
• ? Initially rapid hypothermia (redistribution to
  all 4 extremities)
• During linear phase continue to cool
  relatively higher rate
• 3 additional factors
• Neuraxial reduces vasoconstriction threshold
• Epidural effect superimposed general anesthesia
  effect
• Later vasoconstriction
• Lower core temperature
• General anesthesia inhibits shivering
• Peripheral N. block prevents vasoconstriction in
  the legs
• - most important factor core temp. continue
  to decrease

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4. Return to Normothermia

• Surgical pt. hypothermia
• ? Thermoregulatory defenses impaired
• by general and regional
  anesthesia
• most sedatives
• Heat loss during surgery
• Ex.) 3? reduction in mean body temp.
• 175 Kcal in a 70-Kg pt.
• roughly the basal heat production for
  3hr.

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4. Return to Normothermia

• Full postop. recovery of protective
  thermoregulatory responses limited by residual
  volatile anesthetics or opioid (pain)
• Brain anesthetic concentrations decrease rapidly
• Reemergence of thermoregulatory responses
  (vasoconstriction and shivering)
• Decrease continuous heat loss
• Constrain metabolic heat to the thermal core.
• Increase metabolic heat production
• Core temp. increase toward normal value after
  anesthesia is discontinued

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4. Return to Normothermia

• Arterio-venous shunt vasoconstricton
• during postanesthetic recovery in
  hypothermic pts
• Shivering is common
• Postop. core temp. in hypothermic pts.
• Increase relatively slowly
• Often require 2 hr to normal values
• Rapid increase in anesthetized volunteers
• ? Prolonged hypothermia
• Magnitude of the heat debt
• Thermoregulatory compensation
• Not max. activate by residual volatile
  anesth.
• and opioid

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Summary

• Hypothermia during general anesthesia
• 3 phase
• Initial rapid reduction core temp
• Internal redistribution of body heat
• Redistribution from inhibit tonic
  vasoconstriction
• Linear decrease core temp.
• Determined by difference between heat loss and
  protection
• Core temperature plateau
• Thermoregulatory vasoconstriction
• Restrict core-to-peripheral flow of heat
• Normal core-to- peripheral temp. gradient

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Summary

• Spinal or epidural anesthesia
• Hypothermia
• Redistribution of body heat major initial cause
• Restricted to the leg half
• Linear hypothermia
• Continued
• Constriction is blocked peripherally
• Large operations serious hypothermia

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Summary

• Postperative return to normothermia
• Trigger normal thermoregulatory defenses
• Need 2-5h
• Residual anesthesia
• Opioid (for pain)
• Degree of hypothermia
• Age of the patient