Rôle of mucus viscoelasticity in clearance by cough

We examined the relationship between mucus rheology, depth of mucus layer, and clearance by simulated cough. A model trachea constructed of rigid plexiglass was lined with mucus simulants--borate-crosslinked locust bean gum gels. Their viscoelastic properties were determined by magnetic rheometry and expressed as mechanical impedance (dynamic stress/strain ratio or vectorial sum of elasticity and viscosity) and loss tangent (viscosity/elasticity ratio). 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 per cough manoeuvre was defined as the clearance index, C.I. We found that C.I., for any initial depth of mucus, increased with the driving pressure in the tank. For a given driving pressure, C.I. increased linearly with increasing mucus depth. For a given driving pressure and depth, C.I. decreased with increasing mechanical impedance of mucus. At constant mechanical impedance, C.I. increased with increasing loss tangent. Mucus clearance was associated with transient wave formation in the lining layer. Thus the dependence on viscoelasticity is consistent with the 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, suggesting a natural balance in viscosity and elasticity for mucus to be cleared by both mechanisms.