R1iv: Additional – The effects of GA and Neuraxial Anaesthesia on Thermoregulation

Definition: Thermoregulation = the tight control of core body temperature despite fluctuating ambient temperatures


  • Causes core hypothermia
  • Widens interthreshold range
  • Reduces Cold Threshold 2.5C
  • Increases Warm Threshold 1.3C
  • Core temp at which thermoregulatory threshold triggers VC is agent & dose dependant
  • Reduced Cold Response
    • Heat loss 2 late VC
    • Reduced heat production
      • GA reduces BMR 30%
      • Non shivering thermogenesis abolished
      • Reduced threshold for shivering
    • Hypothermia ensues in 3 stages

Phase I: Internal Redistribution of Body Heat

  • Rapid 1-2 degree fall in first hour of sedation
  • VD by TIVA/volatiles → heat redistributed from core → mass heat loss by Radiation
  • GA → widens interthreshold range and delays activation of compensatory VC → heat redistributed from core to peripheries

Phase II: Heat Loss > Heat Production

  • Linear decline by 1 degrees over 2-3hrs by
    • Radiation: Loss of VC = increased heat loss by radiation
    • Conduction: heat loss by direct contact of patient w bed, probes, soiling
    • Convection: Loss of errrector pili increases heat loss by convection, no wind currents in ICU therefore heat conservation by hospital gowns, but frequent exposure for examination, procedures & nursing checks
    • Evaporation: insensible heat losses from skin & lungs, but loss of sweating mechanism
    • Behaviour: inability to shiver and increase BME, seek warmth, clothing etc
    • Equipment: indwelling catheters, ECMO, CRRT all increase heat loss by conduction, convection and radiation
  • Overall Heat loss >> heat production

Phase III: Plateau

  • Thermal steady state
  • Heat production = heat loss
  • Patient becomes sufficiently hypothermic to activate compensatory VC


Definition:  Injection of LA around nerves to blunt responses to pain, allowing patient to remain conscious

  • Neural blockade blocks efferent limb of thermoregulatory response in lower half of body → VC, shivering, sweating
  • Hypothermia develops → 2 phases, less marked

Phase I: Internal Redistribution

  • Loss of VC
  • Heat moves from core → peripheries
  • 1C drop in 30mins

Phase II: Heat Loss > Heat Production

  • Increase in peripheral temp = increased gradient for heat loss
  • Combined w other factors contributing to heat loss during surgery
  • Not as marked cf GA as BMR drop is less
  • Rate of linear decline not as steep

No Plateau Phase

  • Even if core temp falls below VC threshold, there is no efferent response as nerves are blocked
  • Heat continues to be redistributed to peripheries and temperature drops throughout surgery

Combined Neuraxial/GA

  • Greatest risk of hypothermia
  • Increase Interthreshold Range
  • Decrease Core Temp
  • Loss of VC
  • Absence of Plateau and at even lower temp
  • Close monitoring requires as high risk of severe hypothermia