Effect of flow-limiting segments and cough on particle deposition and mucociliary clearance in the lung

Functional narrowing of airways at flow-limiting segments may explain the patterns of aerosol deposition and impaired mucociliary clearance observed in patients with lung disease. Using intact dogs, we studied the effects of flow limitation (cough) on the regional deposition of inhaled particles as they left the lung during forced expiration. When compared to passive expiration, cough produced a significant increase in the deposition of 1-μm particles in the region immediately down-stream from the flow-limiting segment (p<0.05). Total deposition at the flow-limiting segment, although increased with cough, was unaffected by gas density or driving pressure. Deposition over the whole lung excluding the flow-limiting segment was unchanged during flow limitation. In a separate group of animals, the effect of cough on tracheal mucociliary clearance was assessed. Using radioactive saline droplets, measurements of tracheal mucociliary clearance were made before and after 50 to 100 coughs. In individual dogs (n=6) the radioactive droplets moved uniformly and reproducibly as single units during control periods of 2 to 4 h. Control tracheal mucociliary clearance was 0.88 ± 0.54 cm/min (n=19). Immediately after the coughs, tracheal mucociliary clearance decreased to zero at the flow-limiting segment in all studies (n=8). Upstream from the flow-limiting segment (toward alveoli) slow, erratic movements with breakup of radio-active droplets was observed (tracheal mucociliary clearance = 0.13 ± 0.16 cm/min for n=8; p<0.01). Clearance downstream was relatively unaffected (0.79 ± 0.54 cm/min for n=6; p value, NS). We conclude that the change in geometry at the flow-limiting segment during the flow limitation associated with cough significantly increases particle deposition at the flow-limiting segment and locally impairs mucociliary clearance.