Expiratory flow limitation in mechanically ventilated patients: a simulation study

A non-linear lumped-parameter model of the tracheobronchial tree has been recently proposed to simulate expiratory flow limitation (EFL) in mechanical ventilation. The model is based on Weibel\´s symmetrical description of lung anatomy subdivided into 24 generations, where the first 17 generations correspond to conductive airways and the last 7 generations belong to the respiratory zone. Thanks to an accurate description of airway viscoelastic properties, the model enables both normal and severe chronic obstructive pulmonary disease (COPD) conditions to be reproduced. In particular, in the present work, normal and COPD cases have been studied and compared during tidal breathing. Presence of EFL in COPD conditions has been detected by means of the "external resistance method," which on the contrary revealed no EFL in normal case. Changes in diameter and resistance of conductive airways have been analyzed giving an increased airways resistance in COPD condition with respect to normal case. Simulation results show as, in patient suffering from COPD, EFL can be ascribed to large modifications in geometry and in elastic characteristics of the conductive airways, combined with a high resistance of lower airways.