Deposition in the TB and P regions

Particles in the micrometre size range can be deposited by impaction in the TB region, particularly at the carina where the two main bronchi diverge. Using hollow casts of the trachea and main bronchi, Schlesinger et al. (1977) and Chan Lippmann (1980) found that the efficiency of deposition correlated with the impaction parameter Ppdpq. 

In the pulmonary region, air velocities are too low to impact particles small enough to reach that region, and the mechanisms of deposition are sedimentation and Brownian diffusion. The efficiency of both processes depends on the length of the respiratory cycle, which determines the stay time in the lung. If the cycle is 15 breaths/min, the stay time is of the order of a second. Table 7.1 shows the distance fallen in one second and the root mean square distance travelled by Brownian diffusion in one second by unit density particles (Fuchs, 1964). Sedimentation velocity is proportional to particle density, but Brownian motion is independent of density. Table 7.1 shows that sedimentation of unit density particles is more effective in causing deposition than Brownian diffusion when dp exceeds 1 pm, whereas the reverse is true if dp is less than 0.5 pm. For this reason, it is appropriate to use the aerodynamic diameter dA equal to pj dp when this exceeds 1 pm, but the actual diameter for submicrometre particles. 

The fraction deposited in the TB and P regions varies between subjects, but in a single subject, P/(TB + P) varies systematically with particle size (Lippmann etal., 1980 Stahlhofen etal., 1980). 

The first measurements of deposition and clearance were those of Albert Arnett (1955), who used heterogeneous particles of iron oxide labelled with 59Fe. In later work, to obtain monodisperse particles, 

In the UK, Booker et al. (1967) used a spinning disk to make monodisperse polystyrene particles. Polystyrene was dissolved in xylene, at a concentration of 0.2%, and chromium acetyl acetonate, labelled with 51Cr, was added. The spinning disk was operated at 3 x 104 rpm to produce 40-/, m droplets of xylene which evaporated to give 5- m polystyrene spheres. Few etal. (1970) adapted this method to produce particles of polystyrene labelled with mTc. This isotope has only very slight beta emission, so the dose to the lung is low, and though the radioactive half-life is only 6 h, this is adequate for estimation of the ratio P/(TB + P) and for analysis of the kinetics of the mucociliary clearance. 

---