Abstract
Computational models of gas transport and aerosol deposition frequently utilize idealized models of bronchial tree structure, where airways are considered a network of bifurcating cylinders. However, changes in the shape of the lung during respiration affect the geometry of the airways, especially in disease conditions. In this study, the internal airway geometry was examined, concentrating on comparisons between mean lung volume (MLV) and total lung capacity (TLC). A set of High Resolution CT images were acquired during breath hold on a group of moderate persistent asthmatics at MLV and TLC after challenge with a broncho-constrictor (methacholine) and the airway trees were segmented and measured. The airway hydraulic diameter (Dh) was calculated through the use of average lumen area (Ai) and average internal perimeter (Pi) at both lung volumes and was found to be systematically higher at TLC by 13.5±9% on average, with the lower lobes displaying higher percent change in comparison to the lower lobes. The average internal diameter (Din) was evaluated to be 12.4±6.8% (MLV) and 10.8±6.3% (TLC) lower than the Dh, for all the examined bronchi, a result displaying statistical significance. Finally, the airway distensibility per bronchial segment and per generation was calculated to have an average value of 0.45±0.28, exhibiting high variability both between and within lung regions and generations. Mixed constriction/dilation patterns were recorded between the lung volumes, where a number of airways either failed to dilate or even constricted when observed at TLC. We conclude that the Dh is higher than Din, a fact that may have considerable effects on bronchial resistance or airway loss at proximal regions. Differences in caliber changes between lung regions are indicative of asthma-expression variability in the lung. However, airway distensibility at generation 3 seems to predict distensibility more distally.
Highlights
Modeling of the lungs has become an important tool for both diagnostic [1,2,3,4] and research [5,6,7,8,9,10,11] purposes
Even though several studies in the past have investigated the morphometry of the human bronchi during deep inspiration, only one study was found containing data on the Dh, calculated for airways at total lung capacity (TLC) and normalized to predicted trachea diameter [38]
Differences between lung regions have been reported in the literature for both healthy subjects [53] and COPD patients [54]
Summary
Modeling of the lungs has become an important tool for both diagnostic [1,2,3,4] and research [5,6,7,8,9,10,11] purposes. Permanent airway remodeling in asthma causes the non-uniform swelling of the airway which, combined with increased mucous secretion, can form a complex lumen shape [45]. These effects need to be taken into account during functional modeling as they will clearly affect the flow
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