ventilation inhomogeneity (VI) as assumptions underlying the calculation are invalid; an alternate index that has been suggested is Scond. 1 Aim To compare these two methods of CDI assessment in CF children Methods Children with cystic fibrosis (CF; 67) and healthy controls (61) performed multiple breath washout with sulphur hexaflouride measured using mass spectrometry. Scond was calculated from 1.5 to 6 turnovers and Scond* from breath 2 to 3 turnovers. Results All measures of VI were

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... antly higher for CF vs control, mean difference: LCI 4.0, Scond 0.054, Scond* 0.081. In CF, LCI correlated better with Scond* than Scond (See figure: correlation coefficient LCI vs. Scond* 0.75; LCI vs. Scond 0.42). If children with moderate-severe VI (LCI > 11) were excluded there was an improved correlation for both relationships (correlation coefficient LCI vs. Scond 0.83; LCI vs. Scond* 0.86). An asymptote for the Scond vs LCI relationship was at Scond 0.07 and Scond* 0.13. Conclusion Scond* quantifies the mechanism of VI in moderate to severe lung disease, but it may reach asymptote in very severe VI. Abstract P184 Figure 1 CF triangles, Control circles. A. Scond vs LCI b, Scond* vs LCI c. Scond* vs Scond with line of equivalence Poster sessions