Flow limitation, cough, and patterns of aerosol deposition

We studied deposition of radioactive monodisperse 1.5-μm aerosol in humans following inhalation during quiet breathing. Two groups were studied: normal, defined by tidal loops below the maximum expiratory flow-volume (MEFV) envelope [forced expiratory volume at 1 s at percent of forced vital capacity (FEV₁%) 62-78]; and flow-limited, with tidal loops superimposed on MEFV relationship (FEV₁% 21-57) and flow-limiting segments (FLS) known to exist in central airways. During simultaneous imaging with a gamma camera, fraction of inhaled aerosol deposited in the lung (DF) was determined by right-angle light scattering. With regions of interest defined by an equilibrium image of ¹³³Xe, regional deposition was normalized for area and lung thickness and expressed as a central-to-peripheral (C/P) ratio. Deposition was uniform throughout the lung in normal subjects [C/P 1.02 ± 0.07 (SD), n = 6]. In flow-limited group, central deposition predominated (C/P 1.98 ± 0.64, n = 6, P < 0.05). Tidal volume and inspiratory flow, forces thought to influence deposition during inspiration, were not different between groups. Spontaneous cough occurred in five flow-limited subjects during aerosol inhalation, with further increase in central deposition when compared with quiet breathing (C/P 1.85 ± 0.60 to 2.69 ± 0.60, P < 0.01). During cough, tidal volume (ml) was reduced significantly (576 ± 151 to 364 ± 117, P < 0.01) with no change in inspiratory flow (l/s) (1.37 ± 0.23 to 1.38 ± 0.40, P = NS). DF, however, was unaffected by cough (0.34 ± 0.13 to 0.61 ± 0.12, P = NS). Preservation of DF during cough, in spite of reduction in tidal volume, and marked increase in central deposition demonstrate an enhanced deposition efficiency of central airways during expiratory dynamic compression. In conclusion, FLS have a significant influence on particle deposition in central airways of humans.