Alveolar duct expansion greatly enhances aerosol deposition: a three-dimensional computational fluid dynamics study

Abstract

Obtaining in vivo data of particle transport in the human lung is often difficult, if not impossible. Computational fluid dynamics (CFD) can provide detailed information on aerosol transport in realistic airway geometries. This paper provides a review of the key CFD studies of aerosol transport in the acinar region of the human lung. It also describes the first ever three-dimensional model of a single fully alveolated duct with moving boundaries allowing for the cyclic expansion and contraction that occurs during breathing. Studies of intra-acinar aerosol transport performed in models with stationary walls (SWs) showed that flow patterns were influenced by the geometric characteristics of the alveolar aperture, the presence of the alveolar septa contributed to the penetration of the particles into the lung periphery and there were large inhomogeneities in deposition patterns within the acinar structure. Recent studies have now used acinar models with moving walls. In these cases, particles penetrate the alveolar cavities not only as a result of sedimentation and diffusion but also as a result of convective transport, resulting in a much higher deposition prediction than that in SW models. Thus, models that fail to incorporate alveolar wall motions probably underestimate aerosol deposition in the acinar region of the lung.

Figure 1

Duct model geometry. Axial planes (A) and radial planes (B) delineate the individual alveoli (C). Model dimensions at the beginning of inspiration are 267 μm for the lumen diameter, 606 μm for the outer diameter and 600 μm for the duct length. Length, average width and depth of the alveoli are 150×137×169.5 μm. Model is shown in horizontal position with gravity force (g) acting downwards.

Figure 2

Flow pattern in a longitudinal section of the three-dimensional alveolated duct model with expanding walls 1 s after the beginning of a 2 s inspiration, i.e. at time T/4 where T is the breathing period. (a,b) Streamlines coloured by velocity magnitude (in 10⁴ cm s⁻¹) in an alveolated duct representative of generation 18 of the Weibel symmetric lung model (Weibel 1963; Haefeli-Bleuer & Weibel 1988) in the stationary and MW cases, respectively, are shown. A parabolic velocity profile was imposed at the inlet of the duct and corresponded to a flow rate at the mouth of approximately 500 ml s⁻¹. (c) Streamlines in the MW model representative of generation 23 of the Weibel symmetric model are shown. Flow was induced as a result of the motions of the alveolar walls. In all panels, flow was from right to left. Note the presence of significant radial flow in the expanding structures compared with the SW structure.

Figure 3

Deposition as a function of particle size in the three-dimensional alveolated duct representative of generation 18 of the Weibel model (Z=18). (a) Deposition for both the stationary wall (SW; circles) and the moving wall (MW; triangles) case is shown. (b) The percentage increase in deposition predicted in the MW structure compared with the SW structure is shown.

Figure 4

Deposition as a function of particle size in the three-dimensional alveolated duct representative of generation 23 of the Weibel model (Z=23). Data are shown in the same format as in figure 3. (a) Deposition for both the stationary wall (SW; circles) and the moving wall (MW; triangles) case. (b) The percentage increase in deposition predicted in the MW structure compared with the SW structure.

Figure 5

Symmetric six-generation structure of fully alveolated ducts of the adult human lung developed by Darquenne showing deposition pattern for 2 μm diameter particles after one breath cycle. The grey triangles and black circles represent the particles that were deposited during inspiration and expiration, respectively. The white and dark grey circles (shown outside the structure) represent the particles that did not deposit but remained in suspension or escaped the structure at the end of expiration, respectively. Adapted with permission from Darquenne (2001).

Figure 6

(a–c) Flow pattern in a model of a single alveolus with expanding walls developed by Tsuda et al. for three different ratios between alveolar and ductal flow with the ratio increasing from (a) the entrance of the acinus to (c) the periphery of the lung. Note that the smaller the ratio, the larger the recirculation zone. Adapted with permission from Tsuda et al. (1995).