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 to the shape 
of the maximal expiratory flow-volume curve. 