State in which constant amount of molecules jump from the liquid phase to gas phase and visa versa.
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
- 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.
Defined by: Ali Kazemi, MD