Residual Volume and Total Lung Capacity to Assess Reversibility in Obstructive Lung Disease

BACKGROUND: Reversibility of obstructive lung disease is traditionally defined by changes in FEV1 or FVC in response to bronchodilators. These may not fully reflect changes due to a reduction in hyperinflation or air-trapping, which have important clinical implications. To date, only a handful of studies have examined bronchodilators' effect on lung volumes. The authors sought to better characterize the response of residual volume and total lung capacity to bronchodilators. METHODS: Responsiveness of residual volume and total lung capacity to bronchodilators was assessed with a retrospective analysis of pulmonary function tests of 965 subjects with obstructive lung disease as defined by the lower limit of normal based on National Health and Nutritional Examination Survey III prediction equations. RESULTS: A statistically significant number of subjects demonstrated response to bronchodilators in their residual volume independent of response defined by FEV1 or FVC, the American Thoracic Society and European Respiratory Society criteria. Reduced residual volume weakly correlated with response to FEV1 and to FVC. No statistically significant correlation was found between total lung capacity and either FEV1 or FVC. CONCLUSIONS: A significant number of subjects classified as being nonresponsive based on spirometry have reversible residual volumes. Subjects whose residual volumes improve in response to bronchodilators represent an important subgroup of those with obstructive lung disease. The identification of this subgroup better characterizes the heterogeneity of obstructive lung disease. The clinical importance of these findings is unclear but warrants further study.