Deposition and Clearance of Fine Particles in the Human Respiratory Tract

Abstract

Total deposition of respirable particles in the human respiratory tract is well determined by experimental data and model calculations whereas regional deposition is still under discussion, particularly in the size range from 0.1-1.0 urn particle diameter. Experimental data on regional deposition of radiolabelled aerosol particles can be derived from gamma camera images on the basis of the particle clearance kinetics during the first few days after inhalation, since the limited resolution of the planar image allows no separation between the peripheral and the bronchial airways. When particles are deposited in the human respiratory tract, two distinct phases of clearance from the thorax are usually observed: a fast cleared fraction during the first hours after inhalation followed by much slower clearance. It is generally assumed that the fast phase, completed within about 24 h, represents predominantly mucociliary clearance of particles deposited in the tracheobronchial tree, whereas the slow phase represents predominantly clearance from the alveolar region (Lippmann, 1977; Stahlhofen etal., 1980). Nevertheless, during recent years the assumption that there also exists a slow cleared component from the tracheobronchial region has been confirmed by a large number of experiments (Scheuch, 1991; Stahlhofen et al, 1994). Recently, a new radio-aerosol Technegas (TcG; Tetley Manufacturing Ltd, Lucas Heights, Australia) was developed for lung ventilation scans in nuclear medicine. Technegas (TcG) is a condensation type aerosol consisting of hydrophobic inert graphite particles. For the production of TcG 20-100 \i of Na m TcO4 in saline is put into the cavity of a graphite crucible which is dried at 50-70°C. Thereafter the cavity of the crucible is heated to 2500°C in an atmosphere of pure argon for 5 s such that carbon as well as the salt and 99m Tc evaporates, followed by condensation during cooling. This procedure is similar to the production of the C-60 molecules, named buckminster-fullerenes (Kroto et al., 1985). Therefore, it was assumed that TcG consists of ultrafine spherical particles which are formed via homogenous condensation from fullerene vapor at temperatures > 500°C. More recent investigations have shown that these ultrafine particles agglomerate rapidly to fine particles (Lemb et al., 1993). The latter is in agreement with the fact that the distribution of deposited TcG applied to the lungs of normal subjects was found in the entire ventilated lungs, that is, it is transported in the lungs like a gas.