Contribution of inhomogeneity of lung units to the maximal expiratory flow-volume curve in children with asthma and cystic fibrosis.

Normal children as well as those with asthma and cystic fibrosis were studied to assess the contribution of lung zones emptying at different rates to the curvilinearity of the maximal expiratory flow-volume curve. Lung volumes, maximal expiratory flow-volume curves breathing air and then breathing a helium-oxygen mixture, and single-breath nitrogen washouts were measured. Forced expiratory maneuvers from lung volumes near functional residual capacity were performed to produce transients of flow exceeding maximal flow defined by the full flow-volume curve. Normal children and those with asthma and mild cystic fibrosis had small or no transients. Those with severe cystic fibrosis had large transients as well as increased phase I on the nitrogen washout curves. These large transients were associated with increased curvilinearity of the maximal expiratory flow-volume curve and smaller than normal flow response breathing helium. In severe cystic fibrosis, the large transients suggest sequential emptying of fast and slow spaces, which influences the shape of the maximal expiratory flow-volume curve. The fast space could be due to compression of an enlarged anatomic dead space. Any time constant inequality between parenchymal units present in asthma and mild cystic fibrosis does not appear to contribute significantly tof the shape of the maximal expiratory flow-volume curve.