Functional effect of longitudinal heterogeneity in constricted airways before and after lung expansion

Aerosol deposition predictions in computed tomography-derived skeletons from severe asthmatics: A feasibility study

BACKGROUND

The authors numerically investigated the correlation between airway skeletons of severe asthmatic human subjects and predicted aerosol deposition to shed light on the effect of environmental factors on asthma risk. We hypothesized that there are asthmatic subjects whose airway skeletal structure can expose the subject to a risk of higher local aerosol deposition compared to subjects with a more common/normal branching pattern.

METHODS

From a population of severe asthmatics studied at total lung capacity via computed tomography we randomly selected 8 subjects whose Forced Expiratory Volume in 1s, percent predicted fell below 45% predicted. To simulate aerosol motion in the human lungs, we employed in-house three-dimensional eddy-resolving computational fluid dynamics and particle tracking models utilizing 3 of the 8 severe asthmatic subjects. One of the 3 subjects was found to have a distinct, localized airway narrowing chosen for further investigation. In the simulation, we controlled flow rate and luminal area, i.e., Reynolds and Stokes numbers, in each branch of the computed tomography-derived airway skeletons.

FINDINGS

We found a distinct enhancement of aerosol deposition associated with the narrowed branches of one subject even when the luminal area was numerically adjusted from its narrowed state to that of a non-asthmatic subject. The branching angle, freed of luminal narrowing persisted in demonstrating a marginally significant increase in local particle deposition compared with the subjects without the initial constriction.

INTERPRETATION

These results demonstrate the possibility that inherent airway structure may influence localized constriction found in severe asthmatics.

Elevation in lung volume and preventing catastrophic airway closure in asthmatics during bronchoconstriction

Background

Asthma exacerbations cause lung hyperinflation, elevation in load to inspiratory muscles, and decreased breathing capacity that, in severe cases, may lead to inspiratory muscle fatigue and respiratory failure. Hyperinflation has been attributed to a passive mechanical origin; a respiratory system time-constant too long for full exhalation. However, because the increase in volume is also concurrent with activation of inspiratory muscles during exhalation it is unclear whether hyperinflation in broncho-constriction is a passive phenomenon or is actively controlled to avoid airway closure.

Methods

Using CT scanning, we measured the distensibility of individual segmental airways relative to that of their surrounding parenchyma in seven subjects with asthma and nine healthy controls. With this data we tested whether the elevation of lung volume measured after methacholine (MCh) provocation was associated with airway narrowing, or to the volume required to preventing airway closure. We also tested whether the reduction in FVC post-MCh could be attributed to gas trapped behind closed segmental airways.

Findings

The changes in lung volume by MCh in subjects with and without asthma were inversely associated with their reduction in average airway lumen. This finding would be inconsistent with hyperinflation by passive elevation of airway resistance. In contrast, the change in volume of each subject was associated with the lung volume estimated to cause the closure of the least stable segmental airway of his/her lungs. In addition, the measured drop in FVC post MCh was associated with the estimated volume of gas trapped behind closed segmental airways at RV.

Conclusions

Our data supports the concept that hyperinflation caused by MCh-induced bronchoconstriction is the result of an actively controlled process where parenchymal distending forces on airways are increased to counteract their closure. To our knowledge, this is the first imaging-based study that associates inter-subject differences in whole lung behavior with the interdependence between individual airways and their surrounding parenchyma.

Contributions of Kinetic Energy and Viscous Dissipation to Airway Resistance in Pulmonary Inspiratory and Expiratory Airflows in Successive Symmetric Airway Models With Various Bifurcation Angles

The aim of this study was to investigate and quantify contributions of kinetic energy and viscous dissipation to airway resistance during inspiration and expiration at various flow rates in airway models of different bifurcation angles. We employed symmetric airway models up to the 20th generation with the following five different bifurcation angles at a tracheal flow rate of 20 L/min: 15 deg, 25 deg, 35 deg, 45 deg, and 55 deg. Thus, a total of ten computational fluid dynamics (CFD) simulations for both inspiration and expiration were conducted. Furthermore, we performed additional four simulations with tracheal flow rate values of 10 and 40 L/min for a bifurcation angle of 35 deg to study the effect of flow rate on inspiration and expiration. Using an energy balance equation, we quantified contributions of the pressure drop associated with kinetic energy and viscous dissipation. Kinetic energy was found to be a key variable that explained the differences in airway resistance on inspiration and expiration. The total pressure drop and airway resistance were larger during expiration than inspiration, whereas wall shear stress and viscous dissipation were larger during inspiration than expiration. The dimensional analysis demonstrated that the coefficients of kinetic energy and viscous dissipation were strongly correlated with generation number. In addition, the viscous dissipation coefficient was significantly correlated with bifurcation angle and tracheal flow rate. We performed multiple linear regressions to determine the coefficients of kinetic energy and viscous dissipation, which could be utilized to better estimate the pressure drop in broader ranges of successive bifurcation structures.

The effect of disease and respiration on airway shape in patients with moderate persistent asthma

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.

[Mechanisms of non-specific airway hyperresponsiveness: Methacholine-induced alterations in airway architecture]

Multiple mechanisms drive non-specific airway hyperresponsiveness in asthma. At the organ level, methacholine inhalation induces a complex bronchomotor response involving both bronchoconstriction and, to some extent, paradoxical bronchodilatation. This response is heterogeneous both serially, along a single bronchial axis, and in parallel, among lung regions. The bronchomotor response to methacholine induces contraction of distal airways as well as focal airway closure in select lung territories, leading to anatomically defined ventilation defects and decreased vital capacity. In addition, loss of the bronchoprotector and bronchodilator effects of deep inspirations is a key contributor to airway hyperresponsiveness in asthma.

Development and Analysis of Patient-Based Complete Conducting Airways Models

The analysis of high-resolution computed tomography (CT) images of the lung is dependent on inter-subject differences in airway geometry. The application of computational models in understanding the significance of these differences has previously been shown to be a useful tool in biomedical research. Studies using image-based geometries alone are limited to the analysis of the central airways, down to generation 6-10, as other airways are not visible on high-resolution CT. However, airways distal to this, often termed the small airways, are known to play a crucial role in common airway diseases such as asthma and chronic obstructive pulmonary disease (COPD). Other studies have incorporated an algorithmic approach to extrapolate CT segmented airways in order to obtain a complete conducting airway tree down to the level of the acinus. These models have typically been used for mechanistic studies, but also have the potential to be used in a patient-specific setting. In the current study, an image analysis and modelling pipeline was developed and applied to a number of healthy (n = 11) and asthmatic (n = 24) CT patient scans to produce complete patient-based airway models to the acinar level (mean terminal generation 15.8 ± 0.47). The resulting models are analysed in terms of morphometric properties and seen to be consistent with previous work. A number of global clinical lung function measures are compared to resistance predictions in the models to assess their suitability for use in a patient-specific setting. We show a significant difference (p < 0.01) in airways resistance at all tested flow rates in complete airway trees built using CT data from severe asthmatics (GINA 3-5) versus healthy subjects. Further, model predictions of airways resistance at all flow rates are shown to correlate with patient forced expiratory volume in one second (FEV1) (Spearman ρ = -0.65, p < 0.001) and, at low flow rates (0.00017 L/s), FEV1 over forced vital capacity (FEV1/FVC) (ρ = -0.58, p < 0.001). We conclude that the pipeline and anatomical models can be used directly in mechanistic modelling studies and can form the basis for future patient-based modelling studies.

Methacholine-Induced Variations in Airway Volume and the Slope of the Alveolar Capnogram Are Distinctly Associated with Airflow Limitation and Airway Closure

Mechanisms driving alteration of lung function in response to inhalation of a methacholine aerosol are incompletely understood. To explore to what extent large and small airways contribute to airflow limitation and airway closure in this context, volumetric capnography was performed before (n = 93) and after (n = 78) methacholine provocation in subjects with an intermediate clinical probability of asthma. Anatomical dead space (VDaw), reflecting large airway volume, and the slope of the alveolar capnogram (slope3), an index of ventilation heterogeneity linked to small airway dysfunction, were determined. At baseline, VDaw was positively correlated with lung volumes, FEV₁ and peak expiratory flow, while slope3 was not correlated with any lung function index. Variations in VDaw and slope3 following methacholine stimulation were correlated to a small degree (R² = -0.20). Multivariate regression analysis identified independent associations between variation in FEV₁ and variations in both VDaw (Standardized Coefficient-SC = 0.66) and Slope3 (SC = 0.35). By contrast, variation in FVC was strongly associated with variations in VDaw (SC = 0.8) but not Slope3. Thus, alterations in the geometry and/or function of large and small airways were weakly correlated and contributed distinctly to airflow limitation. While both large and small airways contributed to airflow limitation as assessed by FEV1, airway closure as assessed by FVC reduction mostly involved the large airways.