Enhancement of particle deposition by flow-limiting segments in humans

Severe chronic obstructive pulmonary disease is associated with central deposition of inhaled aerosols. This pattern may be due to functional narrowing of the large airways during expiration at flow-limiting segments (FLS). Using a gamma camera and 2.5-micron particles, we compared the pattern of aerosol deposition following quiet breathing with that after a controlled forced expiration (cough) when FLS are known to form in central airways. Lung size measurement by 133Xe allowed construction of regions of interest over the central airways and lung periphery. Deposition in these regions was normalized for area and lung thickness and expressed as a central-to-peripheral (C/P) ratio. In addition, using right-angle light scattering, the fraction of inhaled particles deposited with each breath (DF) was determined. During control studies, airflow and tidal volume were continuously monitored to insure that tidal loops were well below the maximum expiratory flow-volume (MEFV) curve. To create dynamic compression, cough was used to generate a partial MEFV curve, while inspiratory flow, tidal volume, and functional residual capacity were maintained close to quiet breathing. With cough, C/P ratios increased markedly from 1.04 +/- 0.18 to 2.21 +/- 0.61 (P less than 0.01, n = 6). DF for the lung and airways did not significantly change (0.43 +/- 0.11 to 0.45 +/- 0.09, P = NS). The greater enhancement of regional deposition in the central airways with deposition unchanged over the whole lung demonstrates that, during cough, peripheral deposition is actually reduced when compared with quiet breathing. We conclude that dynamic compression at FLS can be an important factor in the central deposition of inhaled particles.