Simulation of airway closure during forced vital capacity.

Airway closure, which disconnects peripheral respiratory units from the trachea, has been observed during expiration to residual volume. It is attributed to dynamic compression that may cause unstable collapse and closure of small airways. During forced vital maneuvers, airway closure is expected to be more significant owing to the maximum expiratory effort. In the present study we have added a simulation of airway closure to the model developed by Elad and associates which simulated flow limitation during forced expiration. Progressive closure is simulated by variation in the number of branches and their cross-sectional areas rather than by change in tube law. The results demonstrate that peripheral airway closure may explain the reduction in maximal flow rate at small lung volumes. It can reproduce either the abrupt fall in maximal expiratory flow-volume curves as observed in dog lungs or the gradual decrease that has been observed in humans.