Phasic flow mechanisms of mucus clearance.

Excessive bronchial secretions within the airways can be transported by two-phase gas-liquid flow with airflow rates encountered during normal tidal breathing. The transport speed of secretions with this mechanism is as effective as the mucus clearance rate in normal subjects. The thickness of bronchial secretions required to effect two-phase gas-liquid flow is about 10% of the airways diameter which is not an unusual situation for patients who have bronchial hypersecretion. Periodic airflow as encountered in normal breathing is more effective than continuous flow in moving secretions in one direction or another. However, to propel the secretions toward the larynx, expiratory airflow velocity should be higher than inspiratory airflow velocity. This pattern can be achieved by imposing a controlled pattern of mechanical ventilation and is also probably the basis for the "huffing manoeuvre" taught by chest physiotherapists to clear airway secretions. Increased elasticity and decreased viscosity of secretions promote higher transport rates by two-phase gas-liquid flow. Conditions for two-phase gas-liquid flow during tidal breathing can be met at the 8th to 9th generation of the airways.