Fig. 2. The effect of excluding nitrous oxide from the correction factor used in the calculation of blood partial pressure of desflurane (  circles ) and isoflurane (  triangles ) when using the standard vials. The errors introduced to the results from the three fully oxygenated tonometers are expressed as a percentage of the value calculated with nitrous oxide in the correction factor. The results for desflurane are offset slightly to the left, and for isoflurane to the right, for clarity. The SDs of the errors are less than the size of the symbols.