Simulation of regional lung emptying during slow and forced expirations.

Regional lung emptying was simulated by means of a bialveolar lung model. The influence of bronchial asymmetry and the vertical pleural pressure gradient was evaluated. The model suggests that 1) in vivo the influence of the pleural pressure gradient prevails over that of the bronchial asymmetry; 2) in the presence of this gradient, the shape of phases III and IV of the single-breath washout curves obtained following inspiration of a tracer gas bolus at residual volume is determined by the recoil pressure-volume curve of the lung, by the vertical displacements of the alveoli, and,, at higher flow rates, by the elastic characteristics of the airways; 3) if the pleural pressure gradient is independent of lung volume and of flow rate, the factors mentioned in 2 suffice to produce single-breath washout curves (phases III and IV) and regional vs. overall lung volume relationships corresponding to those observed in vivo; 4) the configuration of the maximal expiratory flow-volume curve is relatively insensitive to pulmonary and bronchial asymmetry, at least in healthy individuals.