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Misfilled vaporizer: Output calc

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

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

When a liquid is in a closed space such as a vaporizer at a constant temperature, it will reach a state of equilibrium where a stable amount of molecules are in liquid form and a set amount are in gaseous form over the liquid. The pressure in the canister is the saturated vapor pressure.

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

The percent of a gas in a given volume equals the saturated vapor pressure divided by atmospheric pressure times 100.

Vapor concentration:

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

o Sevoflurane = 160/760 = 21%

o Enflurane = 175/760 = 23%

o Halothane = 243/760 = 32%

o Isoflurane = 241/760 = 32%

At atmospheric pressure, this is the percent of the air in the chamber that will be volatile anesthetic.

The purpose of a vaporizer is to administer a set amount of volatile anesthetic to a patient. In variable bypass vaporizers, some of the fresh gas flow is directed through the vaporizer to mix with the anesthetic gas. It then rejoins the remainder of the fresh gas flow and is administered to the patient. In order to administer more or less of the anesthetic gas to the patient, the ratio and amount of fresh gas flowing through the vaporizer can be adjusted.

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 someone were to accidentally put sevoflurane in an enflurane vaporizer, the amount of volatile anesthetic would be roughly the same because their saturated vapor pressures are roughly the same (160mmHg and 175mmHg respectively). The same is true for halothane and isoflurane. However, if one were to put halothane in a vaporizer programmed for sevoflurane which has a lower saturated vapor pressure, then the actual output of halothane will be higher than what is indicated by the dial.

Other References

  1. Boumphrey S, Marshall N. Understanding vaporizers. Continuing Education in Anaesthesia Critical Care & Pain. 2011;11:199-203. Link