Physics of vaporisers

An Ohmeda Isotec 5 vaporiser filled correctly with isoflurane has a 1 L/min fresh gas flow delivered to it at sea level and 20 °C. The control dial is set such that the splitting ratio is 5%.What is the resulting concentration of isoflurane at the outlet of the vaporiser?

a) 0.8%

b) 1.6%

c) 2.4%

d) 3.2%

e) 5.0%

Answer: c

Explanation

There is a temptation to oversimplify calculations with respect to vaporisers. This can

cause errors that are significant given the small concentrations of volatile agent that are clinically useful. The saturated vapour pressure of isoflurane is 33.2kPa. Assuming an atmospheric pressure of 101.325kPa it can be seen that in the vaporising chamber, a saturated mixture of about one third isoflurane in carrier gas will be produced (recall Dalton’s law of partial pressures). Amagat’s law of partial volumes states that at a fixed temperature and pressure, the sum of the volumes of components of a gas mixture equals the total volume of that mixture (admittedly for ideal gases). Some texts incorrectly state that as 5% of the fresh gas flow (FGF) enters the vaporising chamber, which contains 33% isoflurane, then when this mixture rejoins the 95% that travelled through the bypass channel the resulting concentration is 0.05 x 0.33 = 0.0165 or 1.65%.

This fails to acknowledge that the volume leaving the vaporising chamber is greater than the volume that entered the chamber as there has been volatile added to the gas with no increase in pressure. Consider the volumes in one minute. FGF 1000mL. Splitting ratio 5%. 50mL enters the chamber. This 50mL of carrier gas must also exit the chamber but a volume of pure isoflurane vapour has been added to it such that the exit gas proportion of isoflurane is one third (SVP/Patm as previously established). It can be seen that 25mL of isoflurane has been added to the carrier gas (25/(50 + 25) is one third). At the exit of the chamber the carrier gas (still 50mL) with added isoflurane (25mL) rejoins the 950mL of bypass carrier gas that now leaves at the vaporiser outlet.

25mL of isoflurane in 1025mL total volume is 2.4%.