Test of wave-speed theory of flow limitation in elastic tubes

Abstract

In elastic tubes it was hypothesized that fluid flow limits at the lowest rate which just causes the fluid velocity to equal the speed of propagation of pressure pulse waves at some point, the choke point, within the tube. Experiments were performed on excised dog tracheas and rubber membrane artificial tracheas to test this hypothesis. The internal pressure was measured along the length of the tube with a movable side-tap catheter while controlling upstream and downstream pressures. As downstream pressure was lowered below that necessary to achieve maximum flow, the upstream pressure being fixed, the pressure-distance curves remained relatively constant in an upstream section of the tube but continued to change in the downstream section, a result predicted by wave-speed theory. At the dividing point between these 2 sections, corresponding to the choke point, X-ray and Pitot tube measurements allowed calculations of tube area and stiffness under maximal flow conditions and, hence, the wave-speed limited flow. Within experimental uncertainty, the wave-speed hypothesis was supported. The effect of longitudinal tension on the pressure-distance profile and the choke point properties was also examined and shown to explain observed negative effort dependence.