Measurement of the tracheobronchial clearance of particles after aerosol bolus inhalation

The technique of using an aerosol bolus of defined volume injected at a selected moment during inhalation has been applied to investigate the clearance of radioactively labelled particles from the tracheobronchial (TB) tree. In previous experiments subjects inhaled 1-6 μm aerodynamic diameter (d_ae) particles at a low flow-rate (250 cm³ s^-1) and particles were deposited primarily by sedimentation during a breath-hold at the end of inhalation. The fraction (A) of the lung deposit that was not cleared in the rapid phase of mucociliary clearance was close to 1.0 when the bolus was injected at the start of a breath. When the bolus was injected at the end of a breath A was consistently about 0.5 for 1-3 μm d_ae particles, but reduced to 0.25 at d_ae = 6 μm. Three subjects inhaled 3.8 μm d_ae particles at a high flow-rate (660-750 cm³ s^-1) to promote deposition by impaction in the bronchi. The value of A was again about 0.5 and thus the results do not support the hypothesis that the observed particle retention is related to specific sites of deposition within the TB tre.