A geometric model analysis of conductive airways in expiratory flow limitation during artificial ventilation

Expiratory flow limitation (EFL) was simulated in mechanical ventilation through a recently proposed tracheobronchial tree model which reproduces breathing mechanics in the time-domain using Weibel's symmetrical description of lung anatomy considered as 17 upper generations conducting air and seven deeper generations where gas exchange takes place. The model allows for flow turbulence, inertance and tissue viscoelasticity to be represented nonlinearly and distributed along the tracheobronchial tree in a lumped parameter description. An electric analogue of the simulator, consisting of a RLC ladder network, was numerically implemented by Matlab-Simulink software. Normal conditions and chronic obstructive pulmonary disease (COPD) were simulated by setting model parameters. An extra negative expiratory pressure (NEP) was introduced to detect EFL. The transmural pressures of conductive airways, their viscoelastic characteristics and their related distributions of transversal geometrical changes over the breathing cycle were evaluated. The results showed that during mechanical ventilation, large modifications in the elastic characteristics of the conductive airways explain EFL in simulated COPD. In particular, a marked expiratory reduction in medium bronchi diameter, up to 20-30% of their maximum value, and a progressive decrease in small bronchi diameter during expiration were observed. A remarkably lower reduction in medium bronchi diameter and almost constant behaviour of small bronchi diameter during expiration were found in the simulated healthy subject. Application of NEP confirmed the absence of EFL for normal cases whereas COPD was flow limited over most of expiration.