Misfilled vaporizer: Output calc

Basic, Basic Sciences, Physics, Monitoring, & Devices

Equilibrium: State in which constant amount of molecules jump from the liquid phase to gas phase and visa versa.

Vapor pressure: The pressure exerted by the gas phase on the walls of the container at constant temp and equilibrium.

Saturated Vapor pressure (SVP):

  • When the gas phase on top of the liquid phase is saturated by the molecules of vapor (i.e. it contains as much vapor as it can handle).
  • Sevoflurane = 160 mmHg
  • Enflurane = 175 mmHg
  • Halothane = 243 mmHg
  • Isoflurane = 241 mmHg

Vapor concentration:

  • Volume % = (saturated partial pressure of volatile anesthetic/atm pressure) x 100

  • Sevoflurane = 160/760 = 21%

  • Enflurane = 175/760 = 23%

  • Halothane = 243/760 = 32%

  • Isoflurane = 241/760 = 32%

In variable bypass vaporizers:

  • SVP agent (mmHg) / Total pressure (mmHg) = Volume of volatile anesthetic / total volume leaving vaporizer = Volatile anesthetic (ml) / (carrier gas (ml) + Volatile anesthetic (ml)).
  • Thus, if the fresh gas flow is set to 1L and dial is set at 1%, 10 ml/min of sevoflurane gas must exit the vaporizer

If Vaporizer specific for agent with low SVP (Enflurane or Sevoflurane) is misplaced with an agent that has high SVP (halothane or isoflurane) then actual output concentration will be greater than the concentration indicated by dial.

58%

Answered correctly

2018

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54%

Answered correctly

2015

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Author
Ali Kazemi, MD