Parenchymal mechanics and asthma

The essence of asthma is impairment of expiratory flow. Expiratory flow requires lung recoil forces to supply the driving pressure and to tether the airways open. Lung recoil forces arise within the parenchymal structures, particularly the air-liquid interface and elastin. Lung recoil is mainly elastic, but shows dissipative properties and contractility, and may be changed by volume history, time dependency, and plasticity. Lung recoil tethers the airway with peribronchial forces approximating pleural pressure but the peribronchial pressure departs systematically from this when the airway constricts, ie, "interdependence." In asthma, lung recoil may decrease due to changes in surfactant, stretching of connective tissues, and growth. Parenchymal-airway coupling may be changed by local changes in parenchymal properties and particularly by swelling of the adventitia due to edema, inflammation, or matrix remodeling. Such changes in lung recoil and airway coupling may explain some of the reductions of expiratory flow seen in asthma.