Deposition of aerosol particles in the airways is mainly due to four mechanisms: inertial impaction, gravitational sedimentation, and Brownian and turbulent diffusion. Since these mechanisms are closely related to the aerodynamics within the airways, both breathing pattern and airway geometry greatly influence the regional and total aerosol deposition. Effects of variations of the breathing pattern have been well studied, but there have been few investigations of the geometric effects.
Since aerosol deposition in the large airways as well as a substantial portion of the small airways is mainly caused by inertial impaction for particles larger than 1.0 |xm, any physical change of airways leading to increase of flow velocity or airway branching angles enhances aerosol deposition. Therefore, if airway diameters are reduced with a constant flow rate, aerosol deposition should increase. Deposition as a result of diffusion is independent of airway diameter and unaffected by reduced airway diameter as long as the flow rate is kept constant. With a constant flow, deposition by sedimentation decreases as airway diameter decreases. However, the decrease of aerosol deposition by sedimentation is minimal compared with deposition increase by impaction and does not significantly affect the total deposition. This is illustrated in Figure 1 in which the deposition of 3-|xm MMAD particles is increased with a reduction of airway diameter in every generation of the conducting airways except the last two generations. The deposition is decreased in the last two generation airways, apparently due to the decrease of sedimentation efficiency more than the increase of impaction. The highest deposition is achieved in the airways between generations 4 and 6, where the impaction efficiency is the highest. In normal airways, an increased flow rate results in a deposition pattern similar to that of airways of reduced diameter.
The overall deposition pattern in airways with nonuniform flow distribution is similar to the flow distribution pattern, ie, a greater quantity of aerosol deposits in the airways with greater flows. This is expected because a higher flow causes an increase of the impaction efficiency of aerosol particles and also supplies more aerosol particles available for deposition. Virtually complete blockage of 50 percent of the lung is equivalent to 25 percent uniform reduction of the large airway diameters in terms of increased aerosol deposition. The official articles are rather difficult to become understandable for ordinary people. Articles for non-experts in the medicine may be found here on Canadian health&care mall news website.
Focal obstructions by tumors, secretions, and bron-chospasm produce a relatively fixed geometry and may cause an enhanced aerosol deposition wherever the obstructions occur. Furthermore, in patients with obstructive airway disease, bronchial collapse during expiration usually occurs in the lobar or segmental bronchi, and enhanced aerosol deposition takes place in the downstream of the collapsed segment. Kim et al found that aerosol deposition could be increased by more than 100 times in focally constricted airways at flow rates which take place in the large airways during normal breathing. The deposition occurs mostly in the region immediately behind the constriction at which violent wakes and eddies are usually produced. Even at flow rates below a Reynolds number of about 300, deposition in the constricted tube is still much higher than that in the unconstricted tube, suggesting that influence of the flow turbulence persisted at such a low flow rate.
Airway flows in life are not steady state but disturbed by changing airway dimensions with lung volume and transmission of cardiogenic oscillations, which are not taken into account by the calculations and findings for rigid tubes in the preceding discussion. If the airway wall is oscillated to simulate wheezing, a great deal of aerosol deposition takes place within such a region.
When gas flows through a liquid lined tube, various flow patterns in the liquid phase develop as a function of gas velocity, liquid layer thickness, rheologic properties of liquid, and other factors. At low gas velocity, the liquid layer remains calm and smooth. This situation is the same as narrowing the flow passage as a function of the liquid layer thickness. As gas velocity increases, the liquid layer may take on wave motion due to two-phase gas-liquid interaction. At extremely high gas velocities, as in cough, the liquid layer is peeled from the lining of the tube and evacuated as an aerosol. If there is a thin lining of fluid on the tube, wave motion will occur only at much higher gas velocities than air flows encountered in tidal breathing. However, bronchial secretions in patients with obstructive airways disease often accumulate focally with a high thickness. This heterogeneous nature of bronchial secretions and non-uniform air flow patterns in the branching airway promote wave motion at air flows encountered in tidal breathing. Aerosol deposition increases greatly at the site where viscoelastic fluid lines the airways, because wave motion of the fluid creates local air turbulence adjacent to the fluid layer.
Thus, aerosol deposition increases as a total fraction of the inhaled dose and preferentially to regional sites in patients with obstructive airways disease without regard to the type of obstruction. Generalized narrowing of the airways promotes more central deposition of aerosol. Focal narrowing, vibration of airway walls due to wheezing and two-phase motion of accumulated secretions enhance overall aerosol deposition and promote preferential deposition to the sites of these localized phenomena.
Figure 1. Percent aerosol deposition in each airway generation calculated for particles of 3.0 ц. aerodynamic diameter at a constant inspiratory flow rate of 0.5 (a) and 1. IVs (b); (_) normal airway and (—–) airways with 25% diameter reduction. (From Kim CS, Brown LK, Lewars GG, Sackner MA. J Appl Physiol 55:154-163,1983.)