Ventilation distribution efficiency or lung clearance index: Does it matter?

<bold>Background</bold> The ratio of 1/lung clearance index (LCI) has been proposed as an improved index of ventilation inhomogeneity as it may be less influenced by dead space ventilation than LCI (Sandvik R.M. et al. J Appl Physiol 2023). We aimed to investigate 1/LCI for longitudinal tracking to assess the influence of physiological dead space changes, and to capture differences between interfaces. <bold>Methods</bold> This is a secondary analysis of an observational study (Stanojevic, S. et al. Eur Respir J 2021) of 64 cystic fibrosis (CF) children and 48 healthy controls followed for 12 and 24 months during preschool and school-age, respectively. Nitrogen multiple breath washout (MBW) was performed quarterly with the Exhalyzer D device and reported from Spiroware 3.3.2. To assess the impact of equipment dead space, 30 children performed MBW tests using mask and mouthpiece on the same day, in random order. <bold>Results</bold> During preschool, both 1/LCI and LCI were stable in healthy children, while a small, yearly decrease in 1/LCI(%) (Δ-0.13, 95% CI -0.23, -0.02; p=0.02) and a corresponding increase in LCI (Δ0.04, 95% CI 0.00, 0.08; p=0.03) were found during school-age. Individual slopes were highly correlated in both healthy children and CF (r=0.96; p<0.001). Both 1/LCI (r= -0.41; p=0.01) and LCI (r= 0.41; p=0.01) correlated with dead space ventilation (Vd/Vt) in healthy preschool and school age children. Comparing mask vs. mouthpiece, we found decreased 1/LCI(%) (Δ-1.44 (95% CI -1.68, -1.20); p<0.001) and increased LCI (Δ0.62 (95% CI 0.51, 0.73; p<0.001) in children using masks. <bold>Conclusion</bold> In this longitudinal study, 1/LCI mimicked the changes in LCI and did not prove to be a more useful outcome measure for longitudinal tracking.