Introduction and Hypothesis In cystic fibrosis (CF), but not in PCD (Am J Respir Crit Care Med. 2013;188:545–549), lung clearance index (LCI) and spirometry are correlated. The difference may be related to differences in small airway disease. To explore this further, the novel MBW analyses Curv, Scond* and Sacin* were calculated in PCD and CF (Eur Respir J. 2013;42(suppl 2):380–388). Curv assesses specific ventilation inhomogeneity calculated as the ratio of the slope of the first half to the second half of the washout, and unlike LCI is not sensitive to deadspace effects. Scond and Sacin are not useful in severe obstructive lung disease; Scond* and Sacin* are recalculations corresponding respectively to VI in the conducting airways and the acinar region. (Scond* is measured from the slopes of the increase in phase III modified to include the 0–3 lung turnovers and Sacin** is phase III over the first breath of the washout, minus the contribution of Scond*). We hypothesised that these novel indices will differ in PCD compared to CF due to differences in small airways disease.
Methods 38 PCD (14 male, group mean (range) age 21.8 (7.2–59.1) years, FEV1 Z score -3.18 ((-6–0.17)) and CF (14 male, group mean (range) age 10.9 (6.8–19.1) years, FEV1 Z score -2.72 ((-5.4–0.9)) patients matched for P. aeruginosa status and 24 healthy controls recorded spirometry and MBW. LCI, Curv, Scond* and Sacin* were calculated.
Results There was no difference in LCI, FEV1 and Curv between the patient groups. LCI was correlated with Scond* (CF p = 0.0006, r = 0.5, PCD, p = 0.03 r = 0.3), Sacin* (CF p < 0.0001, r = 0.7, PCD p < 0.0001, r = 0.6) and Sacin (CF p < 0.0001, r = 0.7, PCD p = 0.0003, r = 0.5), whereas Scond was not. There was no difference in Sacin** between the groups, but Scond* was significantly lower in PCD, approaching that of healthy controls.
Conclusions Curv is similarly impaired in PCD and CF. Scond* is nearly normal in PCD but not CF, supporting the hypothesis that there are differences in distal airway disease between these conditions. Finally, the results suggest that the new indices may be better discriminators between diseases in severe obstructive lung disease.