Frequency dependence of compliance in the evaluation of patients with unexplained respiratory symptoms

Frequency dependence of compliance (FDC) reflects non-homogeneous ventilato ry distribution and, in the presence of a normal measured airway resistance , suggests peripheral airways dysfunction. This study evaluated peripheral airway function and bronchial reactivity in irritant exposed or non-exposed individuals with normal routine pulmonary function tests (PFTs) who had pe rsistent unexplained lower respiratory symptoms. Twenty-two patients were identified with persistent respiratory symptoms an d with normal chest X-ray and PFTs. Twenty were non-smokers; two had stoppe d smoking more than 10 years before evaluation. Twelve patients had been ex posed to irritants in their workplaces or at home. Non-specific bronchial h yper-reactivity (nsBHR) and FDC, pre- and post-bronchodilator, were measure d in all patients. Studies were repeated in 6/12 irritant-exposed subjects after exposure removal and inhaled corticosteroid treatment. Whereas 12/22 patients had nsBHR, all 22 subjects demonstrated FDC [dynamic lung compliance/static lung compliance C-dyn,C-1 / C-st,C-1 at respiratory frequency 60 min(-1) (f60), mean 46%, range 27-67%]. After bronchodilator administration, a 15% improvement C-dyn,C-1 was observed most consistently at f60 (mean% improvement 26%, 95% CI 14-38%) and in subjects without nsBHR . However, C-dyn,C-1 at f60 did not return to normal after inhaled bronchod ilator. Irritant-exposed and unexposed individuals appeared similar in resu lts of testing for FDC and nsBHR. FDC and its response to bronchodilators provide objective physiological mea sures of an airway abnormality which may provide a basis for clinical sympt oms in patients with normal routine pulmonary function studies. The presenc e of persistently abnormal FDC after bronchodilator (BD) and on follow up s tudies may reflect chronic inflammatory and/or structural changes in the ai rways in addition to bronchoconstriction.