The vaporizing chamber may contain a wick to improve vaporization. Nonetheless, the gas in the chamber is unlikely to be fully saturated with anesthetic. The concentration leaving the vaporizing chamber is, therefore, less than the saturated vapor pressure and is usually unknown. The concentration of anesthetic emerging from the vaporizer is, therefore, also not known. Also, higher gas flow rates will tend to lower the concentration further since the carrier gas spends less time in the vaporizing chamber and less anesthetic is vaporized.
Changes in temperature
No compensation is made for the effect of changes in temperature on SVP, so the output of such vaporizers is highly dependent upon the ambient temperature:
|Effect of changes in temperature on the output concentration of a simple halothane vaporizer calibrated at 20 oC. |
The delivered concentration will also be reduced over time, since loss of the latent heat of vaporization causes the liquid to cool and lowers the saturated vapor pressure. This is most noticeable in the case of ether vaporizers, where ice may form on the outside of the vaporization chamber due to the extreme cold which develops owing to evaporation.
Because of these causes of variability in their output, simple vaporizers cannot generally be provided with a scale indicating the concentration of anesthetic being delivered. Most merely have some qualitative scale such as Off to On, or 0 to 10. Adjustments to the vaporizer can only be made on the basis of the response of the patient as judged by changes in the plane of anesthesia.