Physiological and pathological considerations for aerosol deposition: expiration and models of deposition

Theoretical models are often used to predict fractional and regional deposition of inhaled particles in the respiratory tract. The distribution of particle diameters in the aerosol, airway geometry, breathing pattern, and local flow profiles are major determinants of deposition in the lung. However, most models predicting deposition consider airway geometry to be fixed and concentrate on inspiratory events in their calculations. When particle losses during expiration are estimated, inspiratory and expiratory flow patterns and airspace geometry are usually considered to be similar with similar effects on deposition. The theme of this presentation will be the analysis of events during expiration that influence particle deposition. In the normal lung, during quiet breathing, experiments performed on excised lungs have suggested that convective forces may be different between inspiration and expiration that significantly affect deposition. Bennett and Smaldone, in excised dog lungs, by regulating the duty cycle of tidal breathing found that more particles deposited during inspiration than expiration and that the effects were density dependent. In human subjects with obstructive lung disease, the situation is reversed. Major differences in large airway geometry between inspiration and expiration can occur with each tidal breath. Once the FEV(1) decreases to about 60% of the FVC, flow-limiting segments (FLS) are known to form in central airways. Large pressure drops can occur over short lengths of airway indicating disturbed regions of convective streamlines that are not present during inspiration. Using radiolabeled monodisperse particles, Smaldone and Messina have determined that FLS can be a major determinant of deposition in central airways. Theoretical predictive models of particle deposition and clearance should consider inspiratory and expiratory differences in airway physiology in health and disease.