Tests claimed to detect small airway disease were evaluated in children. Fifty-two subjects who were normal, 30 with cystic fibrosis (CF), and 35 with asthma were selected on the basis of normal spirometry, informed consent being obtained from them and their parents. Tests undertaken were measurement of maximum expiratory flow volume curves breathing air and a helium-oxygen gas mixture, single breath nitrogen washout curves, lung volumes, and, except in the normal subjects, measurement of arterial oxygen tension. Closing volumes were normal in most children with CF and asthma, whereas the slope of the alveolar plateau was abnormal in 83%. Maximum expiratory flow rate at 50% of total lung capacity was reduced in 65% and maximum mid-expiratory flow rate in 57%; residual volume increased in 51%, and results of other tests were abnormal in less than 50% of children.There was evidence suggesting that lung disease was more patchy in children with mild CF than in those with interval phase asthma. These tests do not detect specific pathological processes in small airways but reflect the consequences of small airway abnormalities, which may vary in the same disease as well as in different diseases. No one test alone appears most useful. Together they contribute to understanding the functional abnormalities present in each individual.
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