The FEF25–75/FVC Ratio Is Associated with Methacholine Airway Responsiveness

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Abstract
Airway responsiveness is known to be partly explained by geometric and anatomic factors. This cross-sectional investigation was undertaken to determine whether FEF25–75/FVC as a surrogate measure of airway size relative to lung size is associated with airway responsiveness to methacholine. Posteroanterior chest radiographs and spirometry were performed on a group of 929 middle aged and older men from an ongoing longitudinal study, the Normative Aging Study, who returned for their regularly scheduled examination between 1984 and 1989. Subjects had a mean age of 60.5 ± 7.7 yr. FEV1, FEF25–75, and FVC were taken from spirometric results and FEF25–75/FVC ratios were obtained. Main bronchus (MB) and tracheal (TR) diameters and lung area (LA) were obtained from chest radiographs, and ratios of MB/LA and TR/LA were calculated for each subject and compared with FEF25–75/ FVC as measures of airway size relative to lung size. In a multiple linear regression model adjusting for age, height, initial FEV1, smoking, eosinophil count, and IgE level, FEF25–75/FVC was significantly related to the degree of methacholine airway responsiveness as measured by Log10 dose response slope ( β = − 0.37, p < 0.001). Controlling for the same variables, both MB/LA ( β = − 149.07, p < 0.001) and TR/LA ( β = − 125.87, p < 0.001) were significant predictors of the degree of bronchial responsiveness in separate regression models; however, their effects were greatly attenuated when FEF25–75/ FVC was present in the same model. Similar results were obtained after excluding subjects with FEV1/ FVC ⩽ 0.70 and subjects who had any smoking history. We conclude that FEF25–75/FVC as a surrogate measure of airway size relative to lung size is significantly associated with airway responsiveness.