A stereoradiographic technique and its application to the evaluation of lung casts

A quasi-realistic computational model development and flow field study of the human upper and central airways

The impact of drug delivery and particulate matter exposure on the human respiratory tract is influenced by various anatomical and physiological factors, particularly the structure of the respiratory tract and its fluid dynamics. This study employs computational fluid dynamics (CFD) to investigate airflow in two 3D models of the human air conducting zone. The first model uses a combination of CT-scan images and geometrical data from human cadaver to extract the upper and central airways down to the ninth generation, while the second model develops the lung airways from the first Carina to the end of the ninth generation using Kitaoka's deterministic algorithm. The study examines the differences in geometrical characteristics, airflow rates, velocity, Reynolds number, and pressure drops of both models in the inhalation and exhalation phases for different lobes and generations of the airways. From trachea to the ninth generation, the average air flowrates and Reynolds numbers exponentially decay in both models during inhalation and exhalation. The steady drop is the case for the average air velocity in Kitaoka's model, while that experiences a maximum in the 3rd or 4th generation in the quasi-realistic model. Besides, it is shown that the flow field remains laminar in the upper and central airways up to the total flow rate of 15 l/min. The results of this work can contribute to the understanding of flow behavior in upper respiratory tract.

Stochastic morphometric model of the BALB/c mouse lung

The laboratory mouse is often used as a human surrogate in aerosol inhalation studies. Morphometric data on the tracheobronchial geometry of three in situ lung casts of the Balb/c mouse lung produced by the Air Pollution Health Effects Laboratory were analyzed in terms of probability density functions and correlations among the different airway parameters. The results of this statistical analysis reveal significant differences in diameters and branching angles between major and minor progeny branching off from the same parent airway at a given airway bifurcation. Number of bronchial airways generations along a given path, expressed by the termination probability, branching angles, and daughter-to-parent diameter ratios indicate that the location of an airway with defined linear airway dimensions within the lung is more appropriately identified by its diameter (or its parent diameter) than by an assigned generation number. We, therefore, recommend classifying the mouse lung airways by their diameters and not by generation numbers, consistent with our previous analysis of the rather monopodial structure of the rat lung (Koblinger et al., J Aerosol Med 1995;8:7–19; Koblinger and Hofmann, J Aerosol Med 1995;8:21–32). Because of lack of corresponding information on respiratory airways, a partly stochastic symmetric acinar airway model was attached to the tracheobronchial model, in which the number of acinar airways along a given path was randomly selected from a measured acinar volume distribution. The computed distributions of the geometric airway parameters and their correlations will be used for random pathway selection of inhaled particles in subsequent Monte Carlo deposition calculations.

Predicted tracheobronchial and pulmonary deposition in a murine asthma model

Particulate matter dosimetry provides the critical link between exposures and initial doses reaching various sites in the respiratory tract. To extrapolate findings from animal models to humans, quantitative respiratory-tract anatomical data dosimetry in these animal models is required. The goal of this study was to provide anatomical information for the tracheobronchial and pulmonary region so predictions of particle deposition could be performed for a widely used model of asthma; the sensitized Balb/c mouse. Tracheobronchial airway morphometry of sensitized male Balb/c mice was generated from three in situ prepared lung casts. Distribution of the number of generations to terminal bronchiole for each lung lobe was determined by assigning a unique binary number to each airway. This strategy enabled the median path length to terminal bronchiole to be determined. A total of 25 median length paths to terminal bronchiole were measured (airway length, diameter, and branch angle) in each lung cast. These 25 paths were proportionately distributed among the six lobes based upon the number of median length pathways in each cast. Airway length, diameter, and branch angle were measured for each airway in the 25 median length pathways. Measurements of airway length, diameter, and branch angle for each generation were averaged to create a typical path tracheobronchial anatomy model. A pulmonary airway model was also developed so that particle deposition predictions could be performed for particle diameters of 0.2-10 micrometers. Particle deposition efficiency predictions were consistent with in vivo measured deposition.

Dosimetry counts: molecular hypersensitivity may not drive pulmonary hyperresponsiveness

Airway hyperresponsiveness is one measure of allergic asthma. One such test, the methacholine challenge, uses an inhaled aerosol to induce changes in resistance to breathing. The test is also used to test hyperresponsiveness in rodent models of asthma. For two varieties of mice, the B6C3F1 and the Balb/c, exposure to aerosolized methacholine demonstrates that the Balb/c is 12x more responsive based on the concentration of methacholine in the solution used to produce the inhaled aerosol (the normally accepted dose-metric). Here we show that the 12x difference in exposure disappears when measurements of airway dimensions of generations 1-6 are used first to calculate deposited mass of methacholine; and second to account for the physiology of airway constriction and pressure drop. These observations in mice provide one explanation of how some hyperresponsive subjects can have no underlying molecular sensitivity; and how constriction in the upper airways can have greater impact on breathing than constriction of airway generations 6-16.

Inflated, dried whole lung specimens

A method is described for preparing fully-inflated whole lung specimens that are suitable for instruction or research purposes. Undamaged lungs are removed from the body and then tracheally cannulated and lavaged with tap water more than 250 times. The treatment also includes rinsing blood from vessels with water. A final filling of the lung with alcohol is optional. The multiply rinsed lung is drained and inflated to 30 cm of H2O pressure with dehumidified air and held at that pressure until the tissue is completely dry. The resulting specimens are light in color and appear to be permanent if stored properly.

Modeling of lung morphogenesis using fractal geometries

The fractal dimension (D(F)) is one measure of the space-filling features of a self-similar structure. Additionally, since D(F) varies with branching level, there may be potential critical locations that are functionality important. The authors introduce an algorithm that models lung airway structures and uses computer simulations of growth based on fractal concepts. Under these conditions, limits imposed by simple boundary constraints generate structures that are in good agreement with actual morphometric data.

Pulmonary acinus: geometry and morphometry of the peripheral airway system in rat and rabbit

The geometry and morphometry of intraacinar airways in rat and rabbit lungs were studied from silicone rubber casts. Acini, defined as the complex of alveolated airways distal to the "terminal" bronchiole, were trimmed off the bronchial tree. In both species, the acinar volume followed a log-normal distribution over a range in size of one order of magnitude. At an inflation level of 60% total lung capacity, their mean volume was 1.86 mm3 in the rat and 3.46 mm3 in the rabbit. On a representative sample of acini of different volumes, the branching pattern was characterized as irregular dichotomy, and the segment length and inner and outer diameters were measured. The average acinus had a mean of six generations in the rat and seven in the rabbit. Both showed a decrease in segment length and inner diameter with each generation. The mean longitudinal pathway length--that is, the distance from the initial acinar segment to the terminal sacs--was found to depend on the cube root of the acinar volume in both species. It was calculated at 1.46 and 1.95 mm for rat and rabbit, respectively.

Postnatal enlargement of human tracheobronchial airways and implications for particle deposition

In support of predictions for inhaled particle deposition, morphometric measurements were taken on 20 replica airway casts of people aged 11 days to 21 years. Measurements of right upper lobe airway lengths, diameters, and branching angles were made such that a growth model suitable as input to predictive equations for particle deposition efficiency was obtained. The tracheobronchial airways growth was describable by linear regressions on body length. The length-to-diameter ratio of growing airways did not change in any simple way as a function of airway generation. Airflow rates for a given state of physical activity for various ages were found from previously published data to be describable by linear regressions on body mass. Three states of physical exertion-low activity, light exertion, and heavy exertion-were used for modeling purposes. The computed particle deposition efficiencies indicate that under most circumstances smaller (younger) people will have greater tracheobronchial deposition efficiencies than larger (older) people. For example, tracheobronchial dose on a per kilogram body mass basis for 5-micron-diameter particles may be more than 6 times higher in the resting newborn than in the resting adult assuming equivalent deposition efficiencies above the larynx.

Structure of the human respiratory acinus

Silicone rubber corrosion casts of the human lung in a state of end inspiration were used to study several specimens of the human pulmonary acinus. Four of the acini were measured in detail with respect to duct length and diameter, the number of alveoli per duct, and the branching pattern of the ducts. The acini were found to have irregular branching patterns, including dichotomous, trichotomous, and side branches. There were, on the average, eight to 12 duct generations and about 7.1 X 10(3) alveoli per acinus. The polygonal alveoli had an average diameter of 250 micrometers. The lengths and diameters of the ducts varied considerably; however, the dimensions tended to decrease in the more proximal portions of the acini. The number of alveoli per duct also varied, with an average of ten alveoli per duct. On the basis of measurements, two models, a "surrogate path" model and an model being more useful for calculations such as particle deposition in the airways, and the average path model being most illustrative of the anatomical structures.

Anatomic models of the tracheobronchial and pulmonary regions of the rat

Models of the lung airways of a rat were developed from complete measurements of the tracheobronchial airways. A silicone rubber cast of the tracheobronchial airways of a rat lung was prepared and all individual airway segments down to and including the terminal bronchioles were measured to obtain the segment diameters, lengths, branching angles and angles of inclination to gravity. Models of the rat tracheobronchial airways were constructed based on the original measurements and the subsequent analysis. Some mathematical assumptions about acinar anatomy distal to terminal bronchioles were made to extend the models to include pulmonary regions. Emphasis was placed on the "Typical Path Lung Model" which used one typical pathway to represent either a whole lung or a lobe of the lung. The models are simple and can be applied in calculation of physiologic variables or particle deposition during inhalation in various lobes of the lung.