Dynamic simulation of expiratory flow limitation in normal airways during mechanical ventilation

A non linear dynamic morphometric model of breathing mechanics during artificial ventilation is described. Based on the Weibel symmetrical representation of the tracheobronchial tree, the model incorporates the geometrical and mechanical characteristics of the conductive zone and packs the respiratory zone into a viscoelastic Voigt body. The model also accounts for the main mechanisms limiting expiratory flow (wave speed limitation and viscous flow limitation), in order to satisfactorily reproduce the expiratory flow limitation phenomenon occurring in normal subjects when the difference between alveolar pressure and tracheal pressure (driving pressure) is high. Several expirations characterized by different levels of driving pressure are simulated and expiratory flow limitation is detected by plotting the flow-volume curves. The model is used to study the expiratory time course of lateral pressure, total cross-sectional area and the ratio of fluid velocity to wave speed, in conductive airway generations. The results highlight that in mechanically ventilated patients, the coupling between dissipative pressure losses and airway compliance is predominant in limiting flow during expiration with respect to wave speed mechanism.