Clearance of mucus by simulated cough.

We examined the relationship between mucus rheology, depth of mucus layer, and clearance by simulated cough. A model trachea was constructed of rigid Plexiglas of rectangular cross section (1 X 2 X 35 cm). The bottom of the trachea was lined with mucus simulants, gels prepared from locust bean gum cross-linked with sodium borate. Cough was simulated by opening a solenoid valve connecting the model trachea to a pressurized tank. An upstream flow-constrictive element was used to shape the flow profile of the simulated cough to approximate the pattern seen in a normal adult. Clearance of mucus was quantitated by observing the movement of contrasting marker particles floating in the mucus layer. The median particle displacement per cough maneuver was defined as the clearance index (CI). We found that CI for any initial depth of mucus increased with the driving pressure in the tank. For a given driving pressure, CI increased linearly with increasing mucus depth. For a given driving pressure and depth, CI decreased with increasing mucus cross-link density. For mucus samples with comparable levels of dynamic viscosity, samples with higher elasticity cleared less well. Mucus clearance was associated with transient wave formation in the lining layer.