The role of mucus viscoelasticity in cough clearance

Abstract

The relationships between mucus rheology, depth of mucus layer and clearance by simulated cough were examined in a study employing a model plexiglass trachea lined with gels formed from locust bean gum crosslinked with sodium tetraborate. The viscoelastic properties of the mucus simulants were determined by magnetic rheometry at 100 rad/s and expressed as mechanical impedance (dynamic stress/strain ratio) and loss tangent. Cough was simulated by opening a solenoid valve connecting the model trachea to a pressurized tank, using an upstream flow-constrictive element to shape the flow profile to approximate the pattern seen in a normal adult. Mucus clearance was quantitated by observing the movement of contrasting marker particles placed in the mucus layer. The median particle displacement was defined as the clearance index, Cl. For any initial depth of mucus, Cl increased with driving pressure in the tank, and for a given driving pressure, Cl increased linearly with increasing mucus depth. For a given driving pressure and depth, Cl decreased with increasing mechanical impedance of the mucus. At constant mechanical impedance, Cl increased with increasing loss tangent, in other words, cough clearance was impeded more by elasticity than viscosity. Mucus clearance was associated with transient wave formation in the lining layer. Thus dependence on viscoelasticity is consistent with observations that airflow-mucus interaction and wave formation are impeded by elasticity. The clearance vs. loss tangent relationship for cough is opposite to that found for ciliary clearance (Biorheology 1980, 17, 249), suggesting a natural balance in viscosity and elasticity for mucus to be cleared by both mechanisms. The differences in clearance-viscoelasticity relationship also suggest that rheological adaptation of mucus should occur when the mode of clearance shifts from ciliary action to cough. © 1987.