The relationship between flow transients and bronchial lability in cystic fibrosis

Patients with cystic fibrosis (CF), in contrast to asthmatic subjects, often show a larger increase than decrease (bronchial lability) in peak flows following exercise. Children with CF also often have large supramaximal expiratory flow transients, produced by rapid expulsion of air from conducting airways that are being dynamically compressed. We studied the relationship between bronchial lability and flow transients to explain the peculiar form of bronchial lability found in certain CF patients. At baseline, six of seven CF patients had flow transients, suggestive of decreased resting tone of the conducting airways. Following exercise, there was a strong positive correlation (r = 0.76, P less than .01) between changes in peak flow and changes in the volume of the flow transients. Four subjects increased both, two did not change either, and in one subject both peak flow and the volume of the flow transient decreased. This study suggests that the unique, previously unexplained form of postexercise bronchial lability observed in some CF patients is probably due mainly to flow transients that contribute to peak flows. These transients are probably related to increased compliance of the conducting airways in this disease.