Air-liquid interfacial movement in models simulating airway reopening

In this model study, we simulated the initial airway reopening event in a r igid tube model. The air-liquid interface during airway reopening was assum ed to be a simple axisymmetric meniscus similar to that of a two-phase flow in a rigid tube (radius R), where the applied pressures and the meniscus v elocities were measured experimentally for fluids of different viscosities and surface tensions (gamma). Bulk flow contribution was deducted from the applied pressure to obtain the pressure accounting for interfacial movement (P*(int)) A semi-empirical formula for the interface was generated by dime nsional analysis. The dimensionless interfacial pressure (P-int P-int*R/gam ma) was found to approach 2 for sufficiently small velocities, consistent w ith Bretherton's theoretical prediction. This formula also resembles that p reviously obtained in collapsible tubes simulating airways. The result sugg ests that the critical pressures required to reopen a collapsible airway an d a non-collapsible one with the same radius are similar in magnitude (simi lar to 2 - 3 gamma/R). However, in a collapsible airway, no significant bul k how of lining fluids would develop while the interface proceeds, leading to a much smaller overall pressure for further reopening. Airway wall colla psibility thus could play a crucial role in maintaining proper ventilation through rapid reopening of the airway. (C) 1998 IPEM. published by Elsevier Science Ltd. All rights reserved.