EXPIRATORY FLOW LIMITATION IN A LUNG-LIKE MODEL.

Forced expiration is a diagnostic test of pulmonary function, which is sensitive to lung disease. Experiments in excised lungs show that flow limitation occurs, with an abrupt pressure decrease just downstream of the flow limiting site. The latter moves upstream (i. e. peripherally) for lower lung volumes. The accepted explanation of flow limitation is that the fluid velocity reaches the local value of wave speed. The flow rate then has a maximum value ('choking') and becomes independent of the downstream conditions for unchanged upstream conditions. This paper presents a simple model designed to give deeper fluid mechanical insights and to address such as-yet unsolved questions as: How does a choke point move upstream for lower lung volumes? What is the flow behavior downstream of the choke point? Do elastic jumps (shock-like waves) occur?