Relationship between Tracheobronchial Particle Clearance Rates and Sites of Initial Deposition in Man

Abstract

Mucociliary clearance was compared in three groups of normal subjects; each group inhaled a different type of aerosol (sebecate, iron oxide, and aqueous) labelled with ⁹⁹Tc, to determine the relationship between deposition pattern and the subsequent total lung clearance. Standardization for deposition was accomplished by measuring a central to peripheral ratio (C/P) of deposited radioactivity in the thorax. Lung particle retention at 30, 60, 90, 120 min, and 24 h was measured, and compared at each time period to the C/P ratio. A linear relationship between the retention of aerosol at each time period and the C/P ratio was seen among all aerosols, e.g., aerosol with high C/P ratios had less retention of aerosol at a given time period than aerosols with more diffuse deposition patterns and lower C/P ratios. Furthermore, the relationship was similar to that of the more traditional measurement of regional deposition, the 24-h percentage retention. Lung particle retention was compared among aerosols, using analysis of covariance to standardize for C/P ratio. The intersubject variability was similar among all three aerosol groups. Thus, mucociliary clearance can be standardized in healthy subjects by using an initial deposition index, the C/P ratio, with similar accuracy to the 24-h percentage retention, as an index of aerosol deposition. The use of the C/P ratio, however, has distinct advantages over the 24-h retention, in that studies can be conveniently shortened, used with short lived isotopes, and be adapted to techniques investigating lung permeability instead of tracheobronchial clearance. An unexpected finding was the more rapid clearance of sebecate particles, compared with iron oxide or aqueous particles. It is uncertain whether this is due to differences in physico-chemical properties of the particles, although other explanations such as variations in study population, inhalation velocities, or particle size were not obviously evident.