Effects of Airway Smooth Muscle Activation and Unloading on Short-Term Variability of Inspiratory Impedance in Healthy and Asthmatic Subjects

Background: Short-term temporal variability of respiratory impedance is increased in asthma. Aim: To examine whether airway smooth muscle unloading alters the pattern of bronchoconstrictor response as assessed by changes in inspiratory reactance (Xrs) and resistance (Rrs). Methods: 14 mild asthmatics and 9 healthy male subjects were studied at control conditions, during methacholine (MCh)-induced bronchoconstriction, after chest wall strapping and during MCh-induced bronchoconstriction with chest wall strapped. Rrs and Xrs were measured at 5 Hz by forced oscillation technique. Rrs and Xrs variability over 5-min periods were estimated from the interquartile range of frequency distribution (RrsIQR and XrsIQR, respectively). Results: The percent increments of XrsIQR with MCh, strapping and strapping plus MCh were larger than those of RrsIQR, though statistical significance was achieved only with the combination of strapping plus MCh (p Conclusions: Activation of unloaded airway smooth muscle causes greater instability in the airways contributing to the increase in Xrs than in those contributing to Rrs in both healthy and asthmatic subjects, suggesting occurrence of clustered bronchoconstriction.