Blood flow distribution within the airway wall.

Altered perfusion of the bronchial mucosal plexus relative to the adventitial plexus may contribute to geometric changes in the airway wall and lumen. We studied bronchial perfusion distribution in sheep by using fluorescent microspheres at baseline and during intrabronchial artery challenge with methacholine chloride (MCh; n = 7). Additionally, we measured airway resistance (Raw) during MCh with control or increased perfusion (n = 9). Raw with MCh was significantly greater for high than control flow. Microspheres in histological sections lodged predominantly in the mucosa (60%), and this was not altered by MCh. However, more microspheres lodged in airways >1-mm in diameter during MCh and increased perfusion than MCh and control flow. In airways < or =1 mm in diameter, fewer microspheres lodged during control than increased flow. If the number of microspheres represents regional agonist access to airway smooth muscle, then the differences observed in Raw can be explained by the distribution of agonist. During challenge, there was greater MCh delivery to larger airways during increased flow and less delivery to smaller airways during control flow. The results demonstrate the effects of axial perfusion distribution on Raw.     

Relationship of airway narrowing, compliance, and cartilage in isolated bronchial segments.

Structural components of the airway wall may act to load airway smooth muscle and restrict airway narrowing. In this study, the effect of load on airway narrowing was investigated in pig isolated bronchial segments. In some bronchi, pieces of cartilage were removed by careful dissection. Airway narrowing was produced by maximum electrical field stimulation. An endoscope was used to record lumen narrowing. The compliance of the bronchial segments was determined from the cross-sectional area of the lumen and the transmural pressure. Airway narrowing and the velocity of airway narrowing were increased in cartilage-removed airways compared with intact control bronchi. Morphometric assessment of smooth muscle length showed greater muscle shortening to acetylcholine in cartilage-removed airways than in controls. Airway narrowing was positively correlated with airway compliance. Compliance and area of cartilage were negatively correlated. These results show that airway narrowing is increased in compliant airways and that cartilage significantly loads airway smooth muscle in whole bronchi.     

A technique for quantitating airway size from bronchial casts

To develop a technique for quantitating the size of airways at various positions in the bronchial tree, we analyzed casts of formalin-fixed excised lungs of five mature male ferrets. The left lower lobe of each cast was dissected, the diameter and position of each terminal bronchiole were entered into a computer programmed to reconstruct the airway system, and the cross-sectional areas of 120 conducting airways were measured. The fraction of the lobe served by each measured airway was estimated by dividing the sum of the squared diameters of the terminal bronchioles subtended by that airway by the summed squared diameters of all terminal bronchioles in the lobe. In each cast the relationship between an airway's cross-sectional area (Y) and the fraction of the lobe it was estimated to subtend (X) was described (0.91 less than R2 less than 0.95) by the expression ln(Y) = A + B ln(X) + C [ln(X)]2. Linear regression of ln(Y) on ln(X) for 30-50 airways estimated to serve fractions of the lobe around each of three arbitrarily selected levels (airways serving 0.7, 2.2, and 9.5% of the lobe) was adequate to characterize the area of airways at each level in each of the five animals with 95% confidence intervals narrower than 8% of the estimated area. Variability of airway size at each level among the five casts was modest, suggesting that this technique identified analagous airways in the various animals. Interindividual variability did not increase when the data were reanalyzed with terminal units defined on the basis of airway diameters rather than on the morphological identification of terminal bronchioles.     

Diameters, generations, and orders of branches in the bronchial tree

Studies of bronchial tree data by West et al. (J. Appl. Physiol. 60: 1089-1097, 1986) have shown that plots of mean diameter against generation, using log-log scales, can be represented by a power function with harmonic modulations. Other studies have shown that the mean diameter of the airways is exponentially related to order of branching. This paper demonstrates that both observations are compatible with a fractal model of branching, and because airway branching is fractal, this may explain why both are also true of the bronchial tree. Furthermore, the exponential relationship of mean diameter with generation in the larger airways, demonstrated by Weibel, is shown to result from the exponential relation of diameter with order in the fractal model.     

Postpneumonectomy airway growth in the ferret

To investigate the participation of the conducting airways in compensatory growth following partial lung resection, bronchial casts of six ferrets having undergone right-sided pneumonectomy at 8 wk of age were compared with those of five sham-operated control animals. At maturity, the left lungs of the postpneumonectomy animals were 65% larger than those of the controls. Central airway cross-sectional areas at 10 specific locations in each cast were 12% larger in the postpneumonectomy animals compared with controls. To characterize the size of more peripheral airways, the size and number of the terminal bronchioles subtended by each airway in each left lower lobe cast were identified so that the fraction of the lobe served by that airway could be estimated. The characteristic cross-sectional areas of airway serving 0.7, 2.2, and 9.5% of the left lower lobe in postpneumonectomy animals were 18, 13, and 13% larger than those of controls, but this difference was statistically significant only at the two more peripheral levels. Although airway areas were larger in postpneumonectomy animals, the ratio of airway cross-sectional area to the 0.67 power of lung volume was 20-26% smaller in operated than in control animals at each of the four levels. Following pneumonectomy in the weanling ferret, central and peripheral conducting airways increase in cross-sectional area to similar degrees, but this airway growth is less than the compensatory increase in lung volume.     

Understanding the contribution of native tracheobronchial structure to lung function: CT assessment of airway morphology in never smokers

Background

Computed tomographic (CT) airway lumen narrowing is associated with lower lung function. Although volumetric CT measures of airways (wall volume [WV] and lumen volume [LV]) compared to cross sectional measures can more accurately reflect bronchial morphology, data of their use in never smokers is scarce. We hypothesize that native tracheobronchial tree morphology as assessed by volumetric CT metrics play a significant role in determining lung function in normal subjects. We aimed to assess the relationships between airway size, the projected branching generation number (BGN) to reach airways of <2mm lumen diameter –the site for airflow obstruction in smokers- and measures of lung function including forced expiratory volume in 1 second (FEV₁) and forced expiratory flow between 25% and 75% of vital capacity (FEF 25–75).

Methods

We assessed WV and LV of segmental and subsegmental airways from six bronchial paths as well as lung volume on CT scans from 106 never smokers. We calculated the lumen area ratio of the subsegmental to segmental airways and estimated the projected BGN to reach a <2mm-lumen-diameter airway assuming a dichotomized tracheobronchial tree model. Regression analysis was used to assess the relationships between airway size, BGN, FEF 25–75, and FEV₁.

Results

We found that in models adjusted for demographics, LV and WV of segmental and subsegmental airways were directly related to FEV₁ (P <0.05 for all the models). In adjusted models for age, sex, race, LV and lung volume or height, the projected BGN was directly associated with FEF 25–75 and FEV₁ (P = 0.001) where subjects with lower FEV₁ had fewer calculated branch generations between the subsegmental bronchus and small airways. There was no association between airway lumen area ratio and lung volume.

Conclusion

We conclude that in never smokers, those with smaller central airways had lower airflow and those with lower airflow had less parallel airway pathways independent of lung size. These findings suggest that variability in the structure of the tracheobronchial tree may influence the risk of developing clinically relevant smoking related airway obstruction.

Electronic supplementary material

The online version of this article (doi:10.1186/s12931-015-0181-y) contains supplementary material, which is available to authorized users.     

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

Heterogeneity in narrowing among individual airways is an important contributor to airway hyperresponsiveness. This paper investigates the contribution of longitudinal heterogeneity (the variability along the airway in cross-sectional area and shape) to airway resistance (R(aw)). We analyzed chest high-resolution computed tomography scans of 8 asthmatic (AS) and 9 nonasthmatic (NA) subjects before and after methacholine (MCh) challenge, and after lung expansion to total lung capacity. In each subject, R(aw) was calculated for 35 defined central airways with >2 mm diameter. Ignoring the area variability and noncircular shape results in an underestimation of R(aw) (%U(total)) that was substantial in some airways (～50%) but generally small (median <6%). The average contribution of the underestimation of R(aw) caused by longitudinal heterogeneity in the area (%U(area)) to %U(total) was 36%, while the rest was due to the noncircularity of the shape (%U(shape)). After MCh challenge, %U(area) increased in AS and NA (P < 0.05). A lung volume increase to TLC reduced %U(total) and %U(area) in both AS and NA (P < 0.0001, except for %U(total) in AS with P < 0.01). Only in NA, %U(shape) had a significant reduction after increasing lung volume to TLC (P < 0.005). %U(area) was highly correlated, but not identical to the mean-normalized longitudinal heterogeneity in the cross-sectional area [CV(2)(A)] and %U(shape) to the average eccentricity of the elliptical shape. This study demonstrates that R(aw) calculated assuming a cylindrical shape and derived from an average area along its length may, in some airways, substantially underestimate R(aw). The observed changes in underestimations of R(aw) with the increase in lung volume to total lung capacity may be consistent with, and contribute in part to, the differences in effects of deep inhalations in airway function between AS and NA subjects.     

Changes in airway length after unilateral pneumonectomy in weanling ferrets

We have previously shown that airway cross-sectional area increases 15-20% after pneumonectomy in weanling ferrets by measuring bronchial casts. We have now reanalyzed these same casts to quantitate changes in airway length after pneumonectomy. In each cast the distance from each of 120 airways to the terminal bronchiole along its axial pathway was measured. The relationship between the logarithm of this distance and that of the fraction of the lobe subtended by an airway could be described by a quadratic equation with a correlation coefficient greater than 0.85. Subsegmental and more distal airways of postpneumonectomy animals were 33-47% longer than those of controls. Overall the main axial pathway of airways in the left lower lobes was 12% longer in operated animals, but this increase was primarily accounted for by an increase in the length of the more peripheral airways. Central airways were little if any longer in operated animals. After pneumonectomy in weanling ferrets, subsegmental and peripheral airway lengths increase to a greater degree than lung volume and airway cross-sectional area, whereas central airway lengths increase to a lesser extent if at all. The mechanisms responsible for this difference between central and intralobar compensatory airway growth are unknown.     

Radial heat and water transport across the airway wall

The thermal profiles in the airways of healthy human volunteers and patients with asthma differ after cessation of hyperpnea. The asthmatic patients rewarm their airways more rapidly. To identify thermal properties and processes that could account for the difference between these populations, we developed a model describing the radial transport of heat and water across the trachea. A distinctive feature of the model is a variable parameter describing blood supply to the mucosal and submucosal layers. Simulations performed with the model are initiated by a breath-hold maneuver and are propagative in time. Blood perfusion rates in the airway wall, the thickness of the layer of airway surface liquid, and the mucosa-submucosa thickness, all thought to be more pronounced in asthmatic patients, were varied by changing model parameters and initial conditions. Increasing the thickness of the liquid layer by more than an order of magnitude had little effect on the temperature or water content in the airway lumen. Doubling the blood flow to the mucosa-submucosa resulted in a slight increase in airway temperature. When this effect was coupled, however, with an increase in the thickness of the mucosa-submucosa layer, the increase in temperature was more pronounced. Because the bronchial circulation is the major source of heat to the airway, these results indicate that differences in airway wall thickness coupled with differences in the magnitude or responsiveness of the bronchial microcirculation could account for the differences in intra-airway temperature between the two populations.     

Indirect assessment of mucosal surface temperatures in the airways: theory and tests

We developed and tested a method, based on conduction heat transfer analysis, to infer airway mucosal temperatures from airstream temperature-time profiles during breath-hold maneuvers. The method assumes that radial conduction of heat from the mucosal wall to inspired air dominates heat exchange during a breath-hold maneuver and uses a simplified conservation of energy analysis to extrapolate wall temperatures from air temperature vs. time profiles. Validation studies were performed by simultaneously measuring air and wall temperatures by use of a retractable basket probe in the upper airways of human volunteers and intrathoracic airways of paralyzed intubated dogs during breath holding. In both protocols, a good correlation was demonstrated between directly measured wall temperatures and those calculated from adjacent airstream temperature vs. time profiles during a breath hold. We then calculated intrathoracic bronchial wall temperatures from breath-hold airstream temperature-time profiles recorded in normal human subjects after cold air hyperpnea at 30 and 80 l/min. The calculations show airway wall temperatures in the upper intrathoracic airways that are below core body temperature during hyperpnea of frigid air and upper thoracic airways that are cooler than more peripheral airways. The data suggest that the magnitude of local intrathoracic heat/water flux is not represented by heat/water loss measurements at the airway opening. Both the magnitude and locus of heat transport during cold gas hyperventilation vary with changes in inspired gas temperature and minute ventilation; both may be important determinants of airway responses.     

Activation of smooth muscle in the airway wall, force production, and airway narrowing.

Airway narrowing depends on smooth muscle force production and muscle shortening, but the structural and geometric properties exhibited by individual generations of the bronchial tree largely determine the extent and characteristics of airway narrowing. Properties of major importance include the nature and integrity of the epithelium, the structural and mechanical properties of the airway wall, as well as airway diameter. The influence of these properties on airway narrowing measured as flow or flow resistance in large and small diameter segments of airways from pig lung is described using a novel preparation, the perfused bronchial segment.     

Modeling stochastic and spatial heterogeneity in a human airway tree to determine variation in respiratory system resistance

Asthma is a variable disease with changes in symptoms and airway function over many time scales. Airway resistance (Raw) is variable and thought to reflect changes in airway smooth muscle activity, but just how variation throughout the airway tree and the influence of gas distribution abnormalities affect Raw is unclear. We used a multibranch airway lung model to evaluate variation in airway diameter size, the role of coherent regional variation, and the role of gas distribution abnormalities on mean Raw (Raw) and variation in Raw as described by the SD (SDRaw). We modified an anatomically correct airway tree, provided by Merryn Tawhai (The University of Auckland, New Zealand), consisting of nearly 4,000 airways, to produce temporal and spatial heterogeneity. As expected, we found that increasing the diameter variation by twofold, with no change in the mean diameter, increased SDRaw more than fourfold. Perhaps surprisingly, Raw was proportional to SDRaw under several conditions-when either mean diameter was fixed, and its SD varied or when mean diameter varied, and SD was fixed. Increasing the size of a regional absence in gas distribution (ventilation defect) also led to a proportionate increase in both Raw and SDRaw. However, introducing regional dependence of connected airways strongly increased SDRaw by as much as sixfold, with little change in Raw. The model was able to predict previously reported Raw distributions and correlation of SDRaw on Raw in healthy and asthmatic subjects. The ratio of SDRaw to Raw depended most strongly on interairway coherent variation and only had a slight dependence on ventilation defect size. These findings may explain the linear correlation between variation and mean values of Raw but also suggest that regional alterations in gas distribution and local coordination in ventilation amplify any underlying variation in airway diameters throughout the airway tree.     

Radiographic visualization of airway wall movement during oscillatory flow in dogs

It has been suggested that radial movement of the central airway walls during oscillatory flow might contribute to the increased frequency dependence of compliance seen in chronic obstructive pulmonary disease (COPD) (J. Appl. Physiol. 26: 670-677, 1969). Radial airway wall motion has also been invoked to explain the frequency-dependent decreases in the efficiency of gas exchange during low-volume high-frequency ventilation (HFV) in histamine-bronchoconstricted dogs and in patients with respiratory insufficiency. To test the possibility that airway wall motion increases with bronchoconstriction, we measured central airway diameters using cinebronchoradiography in anesthetized tracheostomized dogs during oscillatory HFV [50 and 100 ml tidal volume (VT) at frequencies (f) of 2, 6, and 12 Hz], under control conditions, during electrical stimulation of the vagi, and after exposure to histamine aerosol. Cineradiobronchograms from two dogs were evaluated quantitatively for tracheal diameter and for lengths and diameters of a number of major airways. Under control conditions, the diameter of the airways fluctuated 7-9% of the mean with VT of 50 ml and 9-18% with VT of 100 ml in the range of frequencies studied. Bronchoconstriction produced by aerosolized histamine increased radial airway wall movement to 10-47% with VT of 50 ml, and during vagal stimulation diameters changed 7-20% at VT of 50 ml. After histamine, the central airways displayed large diameter changes during HFV, whereas more peripheral airways were markedly constricted and did not change in diameter.(ABSTRACT TRUNCATED AT 250 WORDS)     

Airway remodeling in asthma amplifies heterogeneities in smooth muscle shortening causing hyperresponsiveness.

Although airway remodeling and inflammation in asthma can amplify the constriction response of a single airway, their influence on the structural changes in the whole airway network is unknown. We present a morphometric model of the human lung that incorporates cross-sectional wall areas corresponding to the adventitia, airway smooth muscle (ASM), and mucosa for healthy and mildly and severely asthmatic airways and the influence of parenchymal tethering. A heterogeneous ASM percent shortening stimulus is imposed, causing distinct constriction patterns for healthy and asthmatic airways. We calculate lung resistance and elastance from 0.1 to 5 Hz. We show that, for a given ASM stimulus, the distribution of wall area in asthmatic subjects will amplify not only the mean but the heterogeneity of constriction in the lung periphery. Moreover, heterogeneous ASM shortening that would produce only mild changes in the healthy lung can cause hyperresponsive changes in lung resistance and elastance at typical breathing rates in the asthmatic lung, even with relatively small increases in airway resistance. This condition arises when airway closures occur randomly in the lung periphery. We suggest that heterogeneity is a crucial determinant of hyperresponsiveness in asthma and that acute asthma is more a consequence of extensive airway wall inflammation and remodeling, predisposing the lung to produce an acute pattern of heterogeneous constriction.     

Reduction in airway hyperresponsiveness to methacholine by the application of RF energy in dogs.

We delivered controlled radio frequency energy to the airways of anesthetized, ventilated dogs to examine the effect of this treatment on reducing airway narrowing caused by a known airway constrictor. The airways of 11 dogs were treated with a specially designed bronchial catheter in three of four lung regions. Treatments in each of the three treated lung regions were controlled to a different temperature (55, 65, and 75 degrees C); the untreated lung region served as a control. We measured airway responsiveness to local methacholine chloride (MCh) challenge before and after treatment and examined posttreatment histology to 3 yr. Treatments controlled to 65 degrees C as well as 75 degrees C persistently and significantly reduced airway responsiveness to local MCh challenge (P < or = 0.022). Airway responsiveness (mean percent decrease in airway diameter after MCh challenge) averaged from 6 mo to 3 yr posttreatment was 79 +/- 2.2% in control airways vs. 39 +/- 2.6% (P < or = 0.001) for airways treated at 65 degrees C, and 26 +/- 2.7% (P < or = 0.001) for airways treated at 75 degrees C. Treatment effects were confined to the airway wall and the immediate peribronchial region on histological examination. Airway responsiveness to local MCh challenge was inversely correlated to the extent of altered airway smooth muscle observed in histology (r = -0.54, P < 0.001). We conclude that the temperature-controlled application of radio frequency energy to the airways can reduce airway responsiveness to MCh for at least 3 yr in dogs by reducing airway smooth muscle contractility.     

Smooth muscle development during postnatal growth of distal bronchioles in infant rhesus monkeys.

Development of smooth muscle in conducting airways begins early in fetal life. Whereas the pattern and regulation of smooth muscle differentiation are well-defined, the impact of airway growth on the process is not. To evaluate the transformations in organization during postnatal growth, smooth muscle bundle organization (size, abundance, and orientation) was mapped in five generations of distal airways of infant rhesus monkeys (5 days and 1, 2, 3, and 6 mo old). On the basis of direct measurement of the bronchiole proximal to the terminal bronchiole, length increased by 2-fold, diameter by 1.35-fold, and surface area by 2.8-fold between 5 days and 6 mo of age. Smooth muscle bundle size was greater in proximal bronchioles than in respiratory bronchioles and did not change with age. However, relative bundle size decreased in proportion to airway size as the airways grew. Relative bundle abundance was constant regardless of airway generation or age. The distribution of smooth muscle bundle orientation changed with age in each airway generation, and there were significant changes in the terminal and respiratory bronchioles. We conclude that smooth muscle undergoes marked organizational changes as airways grow during postnatal development.     

Reflex tracheal contraction evoked in dogs by bronchodilator prostaglandins E2 and I2

Bronchodilator prostaglandins E2 and I2 may cause airway irritation and bronchoconstriction in human subjects. These experiments were designed to test the hypothesis that this paradoxical bronchoconstriction is a vagal reflex triggered by stimulation of airway afferents. We recorded smooth muscle tension in an innervated upper tracheal segment in anesthetized dogs and injected prostaglandins into the general circulation or into a bronchial artery or administered them as aerosol to the lungs. Prostaglandins usually caused tracheal contraction, which survived vagal cooling to 5-7 degrees C but was abolished at 0 degrees C. Vagally mediated tracheal contraction was also evoked when prostacyclin was injected into the pulmonary circulation of dogs whose pulmonary and systemic circulations were independently pump perfused. Recordings of afferent vagal impulses indicated that bronchial arterial injection of prostaglandins stimulated bronchial C-fibers; aerosols of prostaglandin stimulated pulmonary and bronchial C-fibers and C-fibers in extrapulmonary airways. We postulate that in susceptible human subjects concentrations of these prostaglandins too low to have direct bronchodilator effects may cause reflex bronchoconstriction by stimulating afferent vagal C-fibers in the lower airways.     

Effect of lung inflation in vivo on airways with smooth muscle tone or edema

Brown, Robert H., Wayne Mitzner, Yonca Bulut, and ElizabethM. Wagner. Effect of lung inflation in vivo on airways with smoothmuscle tone or edema. J. Appl.Physiol. 82(2): 491-499, 1997.Fibrousattachments to the airway wall and a subpleural surrounding pressurecan create an external load against which airway smooth muscle mustcontract. A decrease in this load has been proposed as a possible causeof increased airway narrowing in asthmatic individuals. To study theinteraction between the airways and the surrounding lung parenchyma, weinvestigated the effect of lung inflation on relaxed airways, airwayscontracted with methacholine, and airways made edematous by infusion ofbradykinin into the bronchial artery. Measurements were made inanesthetized sheep by using high-resolution computed tomography tovisualize changes in individual airways. During methacholine infusion,airway area was decreased but increased minimally with increases intranspulmonary pressure. Bradykinin infusion caused a 50% increase inairway wall area and a small decrease in airway luminal area. Incontrast to airways contracted with methacholine, the luminal areaafter bradykinin increased substantially with increases intranspulmonary pressure, reaching 99% of the relaxed area at totallung capacity. Thus airway edema by itself did not prevent fulldistension of the airway at lung volumes approaching total lungcapacity. Therefore, we speculate that if a deep inspiration fails torelieve airway narrowing in vivo, this must be a manifestation ofairway smooth muscle contraction and not airway wall edema.

     

Ontogenetic changes in the fractal geometry of the bronchial tree in Rattus norvegicus

Respiration and metabolism change dramatically over the course of the development of vertebrates. In mammals these changes may be ascribed to organogenesis and differentiation of structures involved in gas exchange and transport and the increase in size. Since young as well as mature individuals must be well-designed if the species is to survive, the physiological changes during the development should be matched with geometrical or structural adjustments of the respiratory system. The aim of this study was to evaluate changes in the fractal geometry of the bronchial tree during the postnatal development of the rat. The average fractal dimension of the bronchial tree of the rats was 1.587, but that of juveniles was larger than that of the adults. We found a significant negative correlation between age and fractal dimension. This correlation could be considered be misleading because of the difficulty of separating age/body size effects. Nevertheless, because fractal dimensions of the bronchial tree of rabbits and humans are known to be similar, 1.58 and 1.57 respectively, the body size effect may be nil. To our knowledge, this is the first report of ontogenetic changes in the fractal dimension of the bronchial tree in mammals.