functional residual capacity (FRC) and LCI measurements at varying degrees of DS and ventilation heterogeneity. An estimate was also made of the effects in children, based upon extrapolation adult clinical data. Results Excreted nitrogen had a small effect on accuracy of FRC (1.8%) in the healthy adult model. The error in LCI calculated with true FRC was greater (6.3%), and excreted nitrogen contributed 21% of the total nitrogen concentration at the end of the washout. Increasing DS caused

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... er increase in measurement error, whilst increasing ventilation heterogeneity caused a bell shaped curve with initially increasing error falling again as ventilation heterogeneity approached a maximum. LCI was increased by 6-13% in a CF child model, and excreted nitrogen increased the end of washout nitrogen concentration by 24-49%. Conclusions Excreted nitrogen appears to have complex but clinically significant effects on washout progress, particularly in the presence of abnormal gas mixing. This may explain much of the previously described differences in washout outcomes between SF 6 and nitrogen. Caution should be applied in applying conclusions derived from using one tracer gas to another.