18 patients with idiopathic pulmonary fibrosis were studied to determine if they had morphologic evidence of small airways disease and if physiologic testing could predict morphologic findings. In the presence of normal airway function by standard physiologic studies (forced expiratory volume in 1 s/forced vital capacity and airway resistance by plethysmography), dynamic compliance, maximum expiratory flow-volume curves, and maximum flow-static recoil curves were measured to detect physiologic alterations consistent with small airways abnormalities. These physiologic data were then compared with estimates of small airways diameter made in lung biopsy specimens. 94% (17 of 18) of the patients had peribronchiolar fibrosis or peribronchiolar inflammation or bronchiolitis. 67% (12 of 18) had an overall estimate of small airways diameter of 'narrowed', whereas 33% (6 of 18) had airways that overall were 'not narrowed'. 59% (10 of 17) had frequency-dependent dynamic compliance, 50% (9 of 18) had abnormal maximum expiratory flow-volume curves, and 39% (7 of 18) had abnormal maximum flow-static recoil curves. Comparisons between morphologic and physiologic data revealed a significant correlation between the results of dynamic compliance and the overall estimate of small airways diameter (P = 0.001), and the results of maximum flow-volume curves and the overall estimate of small airways diameter (P = 0.009); there was no significant correlation between the results of maximum flow-static recoil curves and the overall estimate of small airways diameter (P = 0.1). The results of this study suggest that: idiopathic pulmonary fibrosis is a disease of small airways as well as alveoli; dynamic compliance and the maximum expiratory flow-volume curve can predict the overall status of small airways diameter in idiopathic pulmonary fibrosis; and whereas the maximum flow-static recoil curve predicts the overall estimate of small airways diameter in most patients with this disease, it is the least sensitive of these 3 monitors of small airways.
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