Tracheobronchial deposition and clearance in small airways in asthmatic subjects

Asthma tends to impair mucociliary clearance, as assessed from measurements in large airways. However, very little is known about clearance in the smallest airways of the tracheobronchial region. Deposition and clearance was estimated in 11 subjects with stable asymptomatic asthma and 10 healthy subjects after inhalation of 6 microns (aerodynamic diameter) monodisperse Teflon particles labelled with 111In. The particles were inhaled at an extremely slow flow, 0.05 L.s-1. Theoretical calculations and experimental data in healthy subjects using this slow flow support an enhanced deposition in the tracheobronchial region, in particular in the small ciliated airways (bronchioles). Lung retention was measured at 0, 24, 48 and 72 h. Clearance was significant every 24 h both for asthmatic and healthy subjects, with similar fractions of retained particles at all time-points. The fractions of tracheobronchially-deposited particles were on average 41 and 47% for asthmatic and healthy subjects, respectively, as compared to a maximal deposition of 30% using a normal inhalation flow (0.5 L.s-1). No significant correlation was found between lung retention and lung function, either in asthmatics or in healthy subjects. Our results indicate that particles clear equally well from small ciliated airways in asthmatic and healthy subjects, maybe as a consequence of an optimal asthma therapy. Furthermore, our results show that it is possible to enhance tracheobronchial deposition both in healthy and asthmatic subjects, i.e. practically independent of airway dimensions, by inhaling rather large aerosol particles extremely slowly. This may be a useful therapeutic approach.