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A5837 - Modelling Apparent Diffusivity Within Lung Acinus in Health and Disease
Author Block: H. Kumar, K. Clampitt, G. Thiranja Babarenda, M. H. Tawhai, A. R. Clark; Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand.
Rationale: Emphysema is typically associated with destruction of alveolar walls. Healthy aging has also been shown to alter alveolar duct and alveolar shape and size but the two are difficult to distinguish from clinical imaging. Short-range apparent diffusivity can be used to (indirectly) investigate the effect of normal aging compared to emphysematic destruction at the alveolar level. In a normal healthy acinus, the intricate alveolar structures impede helium (He) diffusion, and over time-ranges of order 2ms, the typically measured apparent diffusivity of He is reduced by about 4 times compared with diffusion in free space. Both aging and emphysema are known to increase this apparent value. The relationship between acinar geometry and apparent diffusivity is unresolved. Here the relative effects of normal aging and emphysematic destruction of alveolar wall on predicted short-range apparent diffusivity was investigated using models of the acinus.
Methods: Simple mathematical models of branched acinar airways of one and two generations were used for the normal healthy case. Each alveolus was represented in three-dimensions as a truncated octahedron, and alveoli were arranged around cylindrical ducts. The older alveolus was represented by a shallowing of alveolar depths and wider alveolar ducts. Severity of emphysema was included by removing alveolar septal walls progressively. A finite element mesh was generated in the alveolar structure. Advection-diffusion scalar equation was solved starting with 100% scalar concentration at the acinus entrance. Using the scalar concentration, apparent diffusivity was deduced.
Results: When the surface area to volume ratio (SA/V) of the alveolar duct was decreased by 28% compared to normal (representing aging), the average concentration in the first 1ms increased by 75% showing a significant effect of alveolar duct dimensions on diffusivity. Model alveolar changes typical of mild emphysema (causing about 3% reduction in SA/V) had negligible effect on the resulting concentration and hence observed diffusivity for the first 1ms. Changes in SA/V due to progressive destruction of alveolar walls (about 7% increase in mean linear intercept) had a greater impact on apparent diffusivity when the alveolar destruction occurred predominantly near the alveolar opening than when it was distributed through the alveolus (centrilobar vs panlobar emphysema).
Conclusion: Our model suggests that age-related changes in alveolar structure may present differently to early stage emphysema with respect to their effects on apparent diffusion. Our model provides a means to relate age-related changes in alveolar structure to observed diffusivity.
Acknowledgements: The Royal Society of New-Zealand Marsden Fund