Fig. 1. The effect of deoxygenated hemoglobin on the error in measurement of blood partial pressure of nitrous oxide (  closed squares ), desflurane (  closed circles ), and isoflurane (  closed triangles ) when using the standard vials, expressed as a percentage of the tonometer value. The errors when using the evacuated vials are shown (  open symbols ). The results for nitrous oxide are offset slightly to the left, and for isoflurane to the right, for clarity. The error bars indicate the SD. The results from more than one tonometer are pooled, but the range of the deoxygenated hemoglobin was no more than 0.7 g/dl in each case.