Analysis of flow limiting mechanisms during forced expiration

Abstract

Flow limitation in the airways plays a fundamental role in constituting the maximal expiratory flow-volume (MEFV) curve, and thereby in lung function diagnosis based on spirometry. There are three main mechanisms responsible for flow limitation during the forced expiration, namely wave-speed of pressure disturbance propagation along the airway walls, turbulent dissipation, and viscous dissipation of pressure in the flexible airways. The location of flow limitation is referred to as a choke point. One of the primary questions concerns the role of the above mechanisms in determining flow during successive phases of the forced expiration. In this work the expressions enabling calculation of critical flows in case of wave-speed, turbulent or viscous limitation were derived. Then we used a computational model for the forced expiration to analyze the contribution of these mechanisms to flow limitation. The degree and position of flow limitation caused by each of the mechanisms were tracked by calculation of the ratios between actual and critical flows in the airways. We conclude that the most probable locations of the choke points are the regions of airway junctions. The wave-speed mechanism is responsible for flow limitation for most of vital capacity (VC) (up to 94% of VC in our simulations) and viscous dissipation of pressure for the last part of the test. Turbulent dissipation, however, may play a significant role as a supporting factor in transition between wave-speed and viscous flow limitation.