Airway bifurcations

Whipple, Chen, and Wang S showed that the distribution of an inhaled aerosol bolus depends on the orientation of the successive airway bifurcations and the volume of the bolus. On the basis of skewed velocity profiles, they made theoretical calculations of the distribution of aerosol boli in branching airways that were in fair agreement with the experimental data. Their results suggested that slow and shallow breaths should show greater differences in dispersion of irritant gases in the airways. 

Particles with diameters between 1 and 5 pm are deposited in the tracheobronchial region as a result of either inertial impaction at airway bifurcations or gravitational sedimentation onto other airway surfaces. Undissolved particles may then be removed by the action of the mucociliary defense system working as an escalator particles trapped in the mucus are propelled toward the pharynx by the action of thin cilia located on the surface membrane of specialized cells. Once in the pharynx, the particles may be swallowed. The efficiency of the escalator defense system may be greatly impaired by various environmental contaminants, like sulfur dioxide, ozone, and cigarette smoke that are known to paralyze the activity of the ciliated cells and consequently the upward movement of the mucus. 

Balashazy I, Hofman W, Heistracher T. Computation of local enhancement factors for the quantification of particle deposition patterns in airway bifurcations. J Aerosol... 

Influence of pneumoconstriction on dust deposition. Bronchoconstriction in man can be caused by exposure to cigarette smoke as shown by Loomis (1956), Nadel and Comroe (1961), and Guyatt et al. (1970), or by exposure to inert dusts as shown by Dautrebande et al. (1948), and Dubois and Dautrebande (1958). The mechanisms involved have been discussed by Nadel et al. (1965), Nadel (1968) and Dubois (1969). Bronchoconstriction, by reducing the cross section for flow in the conductive airways, results in increased air velocities and turbulence. Increased velocity can result in greatly increased deposition by impaction at the airway bifurcations, while increased turbulence can account for an increase in deposition by eddy diffusion (Lehmann 1938, Worth and Schiller 1951). 

Effects of airflow and turbulence on particle deposition in the lung were examined by Chan et al. using the airflow measurements in hollow casts and airway bifurcation models. Heyder et al. measured total and regional aerosol depositions through the mouth and the nose. Experimental studies for inspiratory particle depositions in single and double bifurcation airways were reported by Johnston and Schroter, Kim and Iglesias, Kim et al., and Kim and Fisher. ... 

Balashazy, I., Hofmann, W., and Hdstracher, T. (1999). Computation of Local Enhancement Factors for the Quantification of Particle Deposition Patterns in Airway Bifurcations. J. Aerosol Sci., Vol. 30, pp. 185-203. 

Mazaheri, A.R. and Ahmadi, G. (2002). Inspiratory Particle Deposition in the Upper Three Airway bifurcation. 21st Annual Conference of the American Association for Aerosol Research, AAAR 2002, Charlotte, NC. 

Figure 3 Mechanisms of particle deposition in the respiratory system. Illustrated are the three main deposition mechanisms, impaction, sedimentation, and diffusion, in an airway bifurcation during inhalation.

Balashazy 1, Hofmann W. Particle deposition in airway bifurcations—I. Inspiratory flow. J Aerosol Sci 1993 24 745-772. 

Balashazy I, Hofmann W. Deposition of aerosols in asymmetric airway bifurcations. J Aerosol Sci. 1995 26 273-292. 

The estimated number of tubes in each airway generation depends on the bifurcation model used in describing the tracheobronchial tree. Though bronchial bifurcations are asymmetric, symmetric models, exemplified by Weibel, or asymmetric models, such as one suggested by Horsfield, can... 

Bifurcation In the airway, a relatively large bronchi divides into two... 

The deposition of ultrafine particles has been measured in replicate hollow casts of the human tracheobronchial tree. The deposition pattern and efficiency are critical determinants of the radiation dose from the short lived decay products of Rn-222. The experimental deposition efficiency for the six airway generations just beyond the trachea was about twice the value calculated if uniform deposition from laminar flow is assumed. The measured deposition was greater at bifurcations than along the airway lengths for 0.2 and 0.15 ym diameter particles ... 

After exposure, the outside surface of the cast was cleansed until the activity of the washes was less than 10X the background of a gamma well scintillation counter. The cast was cut into separate bifurcations and airway sections and each section was counted to determine the amount of aerosol deposited. Sane samples contained both airway and bifurcation sections because of the complex configuration of the cast. For combination samples, the total activity deposited was equally apportioned between each of the airways and bifurcations. End airways were included for determination or total deposition but not in any of the analyses because flow disturbances at open ends may have affected deposition. The surface area of each sample was measured separately. The surface density for each cast segment was calculated by dividing the activity measured in the sample by the interior surface area of that sample. 

The mean measured activity per unit surface area are shown for airways and bifurcations separately in Table II. These data are for those segments which contained only airway lengths bifurcations. The results are given as the number of particles which deposit per cm2 for 10 particles which enter the trachea. This assumes that the particle and activity distributions are equivalent. For the 0.2 and 0.15 ym particles the surface density at the bifurcations is greater than that along the airway lengths at p <. 01 when the paired data are compared by a one tailed t-test. 

The air stream velocity profile downstream of a bifurcation is asymmetrical. The peak velocity occurS near the inner wall of the daughter branches in the plane of the bifurcation (Olson, et al., 1973). We observed this skewed distribution and unsteady flow when the velocity was measured near the open end of recently bifurcated airways for this model cast (Sussman, et al., 1985). 

Clearance in the upper, or ciliated, region is governed by the rate of mucus transport along the airways. These rates have been measured in the human nose and in dogs, rats, and other species. Asmundsson and Kilbum, Hilding, and Iravani established that mucociliary clearance rates increase from the distal bronchi toward the trachea. Because bronchial openings retard mucus flow, bifurcations receive an accumulation of mucus and associated particles. The rate of mucus production and mucus thickness and velocity vary from one person to another. Thickness increases and velocity decreases greatly when some toxic elements are present in the airway. 

To calculate more precisely the average uptake or the local variation in uptake in each airway, the local variations in velocity and concentration profiles must be taken into account. For example, thin momentum and concentration boundary layers occur at bifurcations and gradually increase in thickness with distance downstream. Bell and Friedlander showed that particle and gas transfer to the airway wall is greatest where the boundary layers are thinnest, e.g., at the carina or apex of bifurcations. 

The prediction of such hot spots of gas transfer at bifurcations is supported by experimental data on ozone-exposed rabbits. Longitudinal slices of airways from these rabbits showed at low magnification that desquamation of the ciliated epithelial cells was focal and sometimes more intense at a bifurcation. 

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