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Saturated vapour pressure

Created: 29/9/2004
Updated: 15/4/2008

The most important factor governing vaporiser design is the saturated vapour pressure (SVP) of the anaesthetic. SVP is a measure of the volatility of the liquid anaesthetic in the carrier gas: after equilibration between the carrier gas and the liquid anaesthetic, the concentration of highly volatile anaesthetics (e.g. isoflurane) in the gas will be higher than that of poorly volatile anaesthetics.

Anaesthetics with a high SVP will require a smaller proportion of the total gas flowing through the vaporiser to pass through the vaporising chamber to produce a given concentration than will anaesthetics with a low SVP. The following table shows the SVP of some anaesthetics at 20 degrees C and the proportion of the total gas flow required to pass through the vaporising chamber to produce an ouput concentration of 1% at a barometric pressure of 760 mmHg:

SVP @ 20 degrees C (mmHg)
Chamber flow/Total flow
243  (32 kPa)
3.1 %
250  (33 kPa)
3.0 %
175  (23 kPa)
4.3 %

It follows that it can be extremely dangerous to deliver anaesthetics from vaporisers for which they were not designed.

SVP and temperature

Saturated vapour pressure varies as a function of temperature:

Unless some means of compensation is employed, the vaporiser output will increase as temperature rises, and vice versa.

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