Evaluation of Respiratory Impedance in Asthma and COPD by an Impulse Oscillation System

Objective The purpose of this study was to clarify the differences in physiological properties of the airways between asthma and COPD using an impulse oscillation system (IOS). Patients and Methods Subjects comprised 95 stable COPD patients, 52 never-smoker asthma patients and 29 healthy never-smokers > 60 years old, all matched for age, in whom respiratory impedance was examined by IOS. Results In both asthma and COPD patients, a significant increase in respiratory resistance (Rrs5) and more negative value of respiratory reactance (Xrs5) at 5 Hz of oscillatory frequency with an increase in resonant frequency (fres) were observed when compared with healthy never-smokers. In asthma, a significant increase in respiratory resistance at 20 Hz (Rrs20) was also observed when compared with healthy never-smokers and COPD. The increases in Rrs5 and relative changes of Xrs5 to more negative were remarkable with increasing severity of COPD. On the other hand, among patients with asthma, these changes in Rrs5 and Xrs5 were also observed in asthmatics with normal FEV1/FVC. Interestingly, Xrs5 showed further changes to more negative in expiration of tidal breath in severe COPD, whereas no significant changes in Xrs5 to more negative in expiration was observed in healthy never-smokers and asthmatics with and without normal FEV1/FVC. Conclusion IOS may be useful for detecting pathophysiological changes of respiratory system in accordance with severity of COPD and even in asthmatics with normal FEV1/FVC. The larger within-breath changes of Xrs5 to more negative in severe COPD may represent easy collapsibility of small airways in expiration of tidal breath. These properties may help to analyze airway mechanics and to identify abnormalities of the airways that cannot be found by spirometry alone.