Particle deposition, alveolar

Fig. 7-2. Particle deposition as a function of particle diameter in various regions of the lung. The nasopharyngeal region consists of the nose and throat the tracheobronchial region consists of the windpipe and large airways and the pulmonary region consists of the small bronchi and the alveolar sacs. Source Task Group on Lung Dynamics, Health Phys. 12, 173 (1966).

The ICRP deposition model estimates the fraction of inhaled material initially retained in each compartment (see Figure 3-2). The model was developed with five compartments (1) the anterior nasal passages (ET,) (2) all other extrathoracic airways (ET2) (posterior nasal passages, the naso- and oropharynx, and the larynx) (3) the bronchi (BB) (4) the bronchioles (bb) and (5) the alveolar interstitium (AI). Particles deposited in each of the regions may be removed and redistributed either upward into the respiratory tree or to the lymphatic system and blood by different particle removal mechanisms. 

Particle deposition in the respiratory tract can initiate inflammatory responses. With repeated deposition, inflammation becomes chronic, and the site or sites of deposition beeome laden, not only with the particulates, but with several types of cells—fibroblasts, macrophages, leukocytes, and lymphocytes. These cells are normal constituents of the lung, an organ composed predominantly of connective tissue. Lung connective tissue forms the thin membrane that defines the functional alveolar-capillary unit. Inside this air sac and on the membrane are specialized eells required for gas exchange, maintenance, and repair (Fig. 3.6). 

Particles deposited on or in the lung parenchyma are cleared primarily by alveolar macrophages. These phagocytized particles migrate to the ciliated epithelium or to the... 

Fig. 7.4. Deposition of particles in alveolar region (open symbols) and whole respiratory tract (closed symbols) (14 or 15 breaths/min by mouth, tidal volume 1.0 to 1.51). Experimental results of Chan Lippmann, 1980 (C>), Stahlhofen etal., 1980 ( ), Foord etal., 1978 (O), Pritchard etal., 1980 (A). Error bars are Is.e. Lines are theoretical calculations of Yu Diu, 1982.

This is particle deposition resulting from settling under gravity. It becomes increasingly important for particles that reach airways where the airstream velocity is relatively low, e g the bronchioles and alveolar region. The fraction of particles depositing by this mechanism will be dependent upon the time the particles spend in these regions. 

With mass respirable sampling, an attempt is made to separate the aerosol into two fractions representing the mass that would be deposited in the alveolar region and the mass that would not be deposited in this region. To do this, it is necessary to define the size distribution of particles deposited in the alveolar region. This material is defined as respirable dust. 

Asgharian B, Wood R, Schlesinger RB. 1995. Empirical modeling of particle deposition in the alveolar region of the lungs A basis for interspecies extrapolation. Fundam Appl Toxicol 27 232-238. 

Stober W, Morrow PE, Hoover MD. 1989. Compartmental modeling of the long-term retention of insoluble particles deposited in the alveolar region of the lung. Fundam Appl Toxicol 13 823-842. 

To achieve benefit from aerosolized antibiotic therapy, an adequate amount of medication must reach the site of infection [3,4,8]. Optimal particle size for deposition in the lower respiratory tract and alveoli lies between 1 and 5 micrometers (pm). For alveolar deposition, particle sizes of 1 -2 pm are optimal. Particle diameters of 3 and 4 pm reach the lower airway, while 5-pm particles deposit in the central airways. Particles below this range are likely exhaled, and... 

In the alveolar region, more particles are deposited during slow than dnring fast breathing (Fig. 11) and particle deposition is due to diffusion and/or sedimentation. 

Heyder et al. found that particles <3 pm do penetrate beyond the nose and deposit primarily in the alveolar region. However, the number of particles that bypass the nose appears to be lower than what is predicted from the model (i.e., -20%). In addition, only about 3% of 1- to 5-pm particles deposit in the bronchial airways during nose breathing. Therefore, if one is interested in targeting those airways, it will be necessary to administer large amounts of aerosol to compensate for the losses in the nose. 

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