Increased volume of conducting airways in idiopathic pulmonary fibrosis is independent of disease severity: a volumetric capnography study

Bronchiectasis, bronchiolectasis, and bronchiolisation of alveolar regions are salient features of idiopathic pulmonary fibrosis (IPF). We asked whether IPF was associated with physiological changes consistent with increases in the volume of conducting airways, and whether airway volume was related to the severity of lung fibrosis. Patients with IPF (N  =  57, vital capacity-VC: 73  ±  20%), patients with non-IPF interstitial lung disease (non-IPF ILD, N  =  24, VC  =  78  ±  18%) and controls without lung disease (N  =  51, VC  =  112  ±  21%) underwent volumetric capnography for the determination of conducting airway volume using Fletcher's equal area method, reported to predicted total lung capacity to control for the effect of lung size (VDaw/TLCp, mL/L). VDaw/TLCp was higher in patients with IPF (45.3  ±  12.8 ml L(-1)) in comparison with controls (34.2  ±  11.0 ml L(-1), p  <  0.0001) and patients with non-IPF ILD (39.5  ±  9.2 ml L(-1), p  =  0.0496). The same differences were observed when analysis was restricted to subjects with moderate IPF (VC  ⩾  80% predicted). Among IPF patients, VDaw/TLCp was correlated with neither the mMRC dyspnea scale, nor VC, nor carbon monoxide transfer factor, nor computed tomography fibrosis scores. Volumetric capnography showed higher conducting airway volume in IPF patients in comparison with controls and non-IPF ILDs, independent of disease severity. This result is consistent with either anatomical predisposition or dilation/longitudinal growth of conducting airways in IPF.