II. EQUIPMENT, INSTRUMENTATION AND TECHNOLOGY

PATIENT WARMING EQUIPMENT

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FLUID / BLOOD WARMERS

FORCED AIR WARMING DEVICES

HEAT AND MOISTURE EXCHANGER (HME)

RADIANT WARMERS

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Fluid / Blood Warmers

Temperature regulation:

  • Afferent input
    • Heat and warmth receptors travel through unmyelinated C fibers
    • Cold receptors travel along A-delta nerve fibers

 

  • Central control
      • Hypothalamus- primary thermoregulatory control center receives sensory input via spinothalamic tracts in the anterior spinal cord

 

  • Efferent response
    • Increase metabolic heat production (autonomic)
    • Heat- sweating and cutaneous vasodilation
    • Cold- shivering, vasoconstriction, and alpha stimulation
      • SHIVERING INCREASING HEAT PRODUCTION BY 50-100%
      • SHIVERING ALSO INCREASES OXYGEN CONSUMPTION BY UP TO 400% (MYOCARDIAL ISCHEMIA RISK)
      • INFANTS CANNOT SHIVER SO THEY RELY ON NONSHIVERING THERMOGENESIS (MEDIATED BY BETA3 RECEPTORS/BROWN FAT)
    • Alter heat loss (behavioral)
      • Irrelevant under anesthesia

 

  • Both general and neuraxial anesthesia impair thermoregulatory control.
    • Volatile anesthetics, propofol, and opioids are examples of anesthetics that effect temperature.
    • Midazolam has little effect.
    • Neuraxial anesthesia prevents pain from hot/cold, reduces vasoconstriction and shivering, and vasodilation and sweating.

 

  • Heat loss
    •  Radiation
      • About 40% of heat loss (most)
      • Body surface exposed to environment
      • Infants at higher risk

 

    • Convection
      • 30% (2nd most)
      • Heat loss to the air that immediately surrounds the body
      • Clothing/drapes help decrease heat loss
      •  
    • Evaporation
      •  8-10%
      • Sweating is the main pathway, but also occurs from open body cavities

 

    •  Conduction
      • Direct contact of body tissues with a colder material
      • Examples: operating table and infusion of fluids

 

  • Perioperative hypothermia is a core body temperature less than 36o

 

  • Complications of hypothermia:
    •  Coagulopathy
      • Decreases thromboxane A3 release (platelet aggregation impairment)
    • Wound infection
      • Vasoconstriction decrease oxygen to wound
      • Immune system decreases
      • MILD INTRAOPERATIVE HYPOTHERMIA TRIPLES THE RISK OF SURGICAL WOUND INFECTION IN COLORECTAL SURGERY
    • Myocardial injury
    • Drug metabolism slows
        •  

 

  • Hypothermia benefits:
    • Protects against cerebral ischemia
        •  

 

  • Hyperthermia
    • More dangerous than hypothermia at similar deviations from the norm.
      • Causes discomfort
      • Increases metabolic demand
      • Increases cardiovascular stress
      • Risk of coagulopathies, renal/liver dysfunction, neuropathies, and seizures

 

    • More commonly seen in:
      • Longer procedures
      • Head/neck (most of the body is covered)
      • Infants/children (sweating less effective)
      • Allergic reactions
      • Malignant hyperthermia
      • Infection (PACU)
          •  

 

  • Temperature management
    • Increase OR temperature
    • Heat and moisture exchanging filters (HMEs)
    • Warming IV fluids- not effective in raising core temperature; however, large volumes of cold IV administration can lead to hypothermia

FLUID/BLOOD WARMERS

  • Ranger system
    • Disposable set is inserted into a warming unit
    • Externally heats the IV line
    • Blood warmers are not needed for platelets or cryoprecipitate because this may render them less effective.

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Forced Air Warming Devices

  • Hot water mattresses (e.g. Kimberly-Clark)
    • Under the patient prevent heat loss by conduction. This can, however, lead to thermal damage and necrosis. These are more effective and safer when placed over the patients.

 

  • Carbon fiber and carbon polymer heating mattresses (like VitaHEAT UB3)
    • Has better thermal transfer and provides pressure relief. This helps to prevent heat loss from conduction.

 

  • Forced-air warming device (e.g. created by Arizant, more than 25 different blankets available)
    • Air is blown over an electrical element to warm the air to 35-46o
    • This air passes over the patient within a “quilted” blanket
    • Prevent heat loss from radiation while transferring heat to the patient by convection.
    • Can increase core body temperature 0.75oC per hour
    • Superior to the water mattresses and thermal pads

 

  • Invasive perioperative system (e.g. Thermogard XP)
    • Precise and rapid
    • Warms and cools
    • Reserved for trauma, critically ill, major surgeries, and CABG
    • Inserted through any central vein
    • Circulates saline within a closed-loop, multi-balloon catheter, cooling and warming the patient from the core as blood passes over each balloon/
    • Exchanges heat without infusing saline

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Heat and Moisture Exchanger

  • Consist of an outer casing with an inlet and an outlet with a partial barrier in between.

 

  • Placed between the Y-piece and ETT/LMA

 

  • HMEs increase resistance and dead space and can cause lower readings in end-tidal monitoring when the sampling port is between the HME and anesthesia machine

 

  • The sampling port is still recommended to be placed on the machine side to prevent contamination and excess water.

 

HMEs can increase resistance between 0.7-3.8 cm H2O but can be overcome during mechanical ventilation with the addition of 5-10 cm H2O inspiratory pressure.

  • Devices should be changed every 24 hours but can maintain moisture for up to 7 days; can become obstructed with sputum.

 

  • Less than 10% of heat is lost through the respiratory tract

 

  • Heat loss is minimal with low flow and semiclosed circuits

 

  • More effective in infants and children than in adults because of the high ratio of minute volume to surface area; however, the added dead space makes selection more difficult. There are no HMEs approved for neonates less 3 kg.

 

  • Heat and moister exchanger filters
    • HMEs that prevent both airborne and water pathogens from passage either direction.
  • Active humidifiers
    • Neonates, copious secretions, and hypothermic patients benefit greatly from active humidifiers versus an HME.
    • Classified as:
      • Bubble
      • Passover
      • Counter-flow
      • Inline vaporizer
      • More expensive than HMEs; risk of overheating or electrical malfunction; risk of water aspiration from tubing.
    • Inspired gas must be heated to 37-40oC to be more efficient than an HME

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Radiant Warmers

Radiant warmers are electric heaters that transfer heat using infrared radiation

  • Requires no intermediate conductor or convector
  • Infrared energy passes directly from the source to the receiver
  • Used mainly in children and trauma

COOLING

  • Uncovering the patient
  • Bags of ice on the groin, neck, and axilla
  • Cavity lavage is more effective; e.g. iced fluids into stomach
  • NG and bladder lavage are effective
  • Some convective warmers can cool as well

REFERENCE: Ehrenwerth J, Eisenkraft J, Berry J, eds. Anesthesia Equipment: Principles and Applications. 3rd edition, pages 187-191, 287-300 Elsevier; 2020.

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