Lung clearance index in adult cystic fibrosis patients: the role of convection-dependent lung units

We investigated the extent of convective ventilation heterogeneity contributing to the observed lung clearance index values in adult cystic fibrosis patients, as the result of two simulated scenarios, either 1) a fixed part of the lungs becoming increasingly less compliant, or 2) an increasingly greater part of the lung being less compliant. In 25 cystic fibrosis patients and 25 matched controls, we computed the lung clearance index and also quantified curvilinearity of the washout concentration curve, where curvilinearity is equal to 0 (linear in semilog plot) reflects homogeneous ventilation and curvilinearity equal to 1 corresponds to the presence of an infinitesimally slowly emptying lung unit. In the cystic fibrosis group (forced expiratory volume in 1 s 27-100% predicted), lung clearance index and curvilinearity average±sd values were 10.3±2.3 and 0.57±0.13, respectively, and were significantly different from control values (6.2±0.4 and 0.18±0.07; both p<0.001); lung clearance index and curvilinearity were also correlated (R = 0.67; p<0.001). The average curvilinearity value in the cystic fibrosis group was found to be compatible with a cumulative volume of underventilated lung of 40-50%, depending on the simulation scenario considered. The degree of washout curvilinearity observed here indicates that a major determinant of the abnormal lung clearance index values observed in adult cystic fibrosis patients is ventilation heterogeneity generated between convection-dependent lung units, while the remainder of lung clearance index abnormality with respect to normal controls potentially represents the small airways within these lung zones.