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Anesthesia Service and Equipment

Simple Vaporizers

These vaporizers consist of a vaporizing chamber, usually a glass bowl or bottle, containing liquid anesthetic and a tap controlling the proportion of gas flowing through it. Examples include the Ohio and Boyle's bottles, and the Stephens and Goldman vaporizers.

Boyle's bottle Ohio 8 Bottle
Stephens Universal Vaporizer Goldman vaporizer

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.

Time
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.

Accuracy
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.

Uses
The major advantage of simple vaporizers is their relatively low cost. Many are also designed to have a low internal resistance which allows them to be used as draw-over vaporizers.
   
Although simple vaporizers may in principle be used to vaporize any agent, the large difference in the saturated vapor pressure of different anesthetics presents some practical difficulties. For example, vaporizers intended for use with halothane or isoflurane are designed to be relatively inefficient in order to produce a maximum output of around 3%. If such vaporizers were to be used to vaporize methoxyflurane, it would be difficult to produce an adequate concentration due to the very much lower SVP of this agent. Conversely, were a methoxyflurane vaporizer (such as the Ohio bottle) used to vaporize isoflurane, lethally high concentrations might be produced.
   
The disadvantage of simple vaporizers is their unknown and variable output. This does not present a major problem when slowly-equilibrating agents (such as methoxyflurane or ether) are used and reasonable vigilance is employed. However, the rapid changes in depth of anesthesia which may be achieved with halothane and isoflurane may cause major difficulties to occur if their concentration is not known.

Precision vaporizers   

Comments on this article should be addressed to Dr Guy Watney
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