Alveolar attachments in emphysema of human lungs

Three-Dimensional Whole-Organ Characterization of the Regional Alveolar Morphology in Normal Murine Lungs

Alveolar architecture plays a fundamental role in the processes of ventilation and perfusion in the lung. Alterations in the alveolar surface area and alveolar cavity volume constitute the pathophysiological basis of chronic respiratory diseases such as pulmonary emphysema. Previous studies based on micro-computed tomography (micro-CT) of lung samples have allowed the geometrical study of acinar units. However, our current knowledge is based on the study of a few tissue samples in random locations of the lung that do not give an account of the spatial distributions of the alveolar architecture in the whole lung. In this work, we combine micro-CT imaging and computational geometry algorithms to study the regional distribution of key morphological parameters throughout the whole lung. To this end, 3D whole-lung images of Sprague–Dawley rats are acquired using high-resolution micro-CT imaging and analyzed to estimate porosity, alveolar surface density, and surface-to-volume ratio. We assess the effect of current gold-standard dehydration methods in the preparation of lung samples and propose a fixation protocol that includes the application of a methanol-PBS solution before dehydration. Our results show that regional porosity, alveolar surface density, and surface-to-volume ratio have a uniform distribution in normal lungs, which do not seem to be affected by gravitational effects. We further show that sample fixation based on ethanol baths for dehydration introduces shrinking and affects the acinar architecture in the subpleural regions. In contrast, preparations based on the proposed dehydration protocol effectively preserve the alveolar morphology.

Neutrophilic inflammation during lung development disrupts elastin assembly and predisposes adult mice to COPD

Emerging evidence indicates that early life events can increase the risk for developing chronic obstructive pulmonary disease (COPD). Using an inducible transgenic mouse model for NF-κB activation in the airway epithelium, we found that a brief period of inflammation during the saccular stage (P3-P5) but not alveolar stage (P10-P12) of lung development disrupted elastic fiber assembly, resulting in permanent reduction in lung function and development of a COPD-like lung phenotype that progressed through 24 months of age. Neutrophil depletion prevented disruption of elastic fiber assembly and restored normal lung development. Mechanistic studies uncovered a role for neutrophil elastase (NE) in downregulating expression of critical elastic fiber assembly components, particularly fibulin-5 and elastin. Further, purified human NE and NE-containing exosomes from tracheal aspirates of premature infants with lung inflammation downregulated elastin and fibulin-5 expression by saccular-stage mouse lung fibroblasts. Together, our studies define a critical developmental window for assembling the elastin scaffold in the distal lung, which is required to support lung structure and function throughout the lifespan. Although neutrophils play a well-recognized role in COPD development in adults, neutrophilic inflammation may also contribute to early-life predisposition to COPD.

Small airway determinants of airflow limitation in chronic obstructive pulmonary disease

Background

Although a variety of pathological changes have been described in small airways of patients with COPD, the critical anatomic features determining airflow limitation remain incompletely characterised.

Methods

We examined lung tissue specimens from 18 non-smokers without chronic lung disease and 55 former smokers with COPD for pathological features of small airways that could contribute to airflow limitation. Morphometric evaluation was performed for epithelial and subepithelial tissue thickness, collagen and elastin content, luminal mucus and radial alveolar attachments. Immune/inflammatory cells were enumerated in airway walls. Quantitative emphysema scoring was performed on chest CT scans.

Results

Small airways from patients with COPD showed thickening of epithelial and subepithelial tissue, mucus plugging and reduced collagen density in the airway wall (in severe COPD). In patients with COPD, we also observed a striking loss of alveolar attachments, which are connective tissue septa that insert radially into the small airway adventitia. While each of these parameters correlated with reduced airflow (FEV₁), multivariable regression analysis indicated that loss of alveolar attachments was the major determinant of airflow limitation related to small airways. Neutrophilic infiltration of airway walls and collagen degradation in airway adventitia correlated with loss of alveolar attachments. In addition, quantitative analysis of CT scans identified an association between the extent of emphysema and loss of alveolar attachments.

Conclusion

In COPD, loss of radial alveolar attachments in small airways is the pathological feature most closely related to airflow limitation. Destruction of alveolar attachments may be mediated by neutrophilic inflammation.

Celebration of the 50-Year Anniversary of the National Heart, Lung, and Blood Institute Division of Lung Diseases: A Half-Century of Landmark Clinical Trials

The National Institutes of Health (NIH)-National Heart, Lung, and Blood Institute's (NHLBI) Division of Lung Diseases is celebrating its 50th anniversary. On this occasion, we are reviewing the major landmark clinical trials that were initiated by the NHLBI's Division of Lung Disease and that have had substantial impact on our understanding of chronic obstructive pulmonary disease (COPD) and how it is best treated. Although some of these trials did not show hypothesized treatment benefits for COPD, they have enabled clinicians to provide care for individuals with COPD relying on the most rigorous evidence. The 5 trials that are reviewed here are: the Intermittent Positive Pressure Breathing Trial, the Nocturnal Oxygen Treatment Trial, the Lung Health Study, the National Emphysema Treatment Trial, and the Long-term Oxygen Treatment Trial. These clinical trials have not only set the standards for COPD care but have served as models for the state-of-the-art conduct of clinical research in COPD.

Micro-Computed Tomography Comparison of Preterminal Bronchioles in Centrilobular and Panlobular Emphysema

RATIONALE

Very little is known about airways that are too small to be visible on thoracic multidetector computed tomography but larger than the terminal bronchioles.

OBJECTIVES

To examine the structure of preterminal bronchioles located one generation proximal to terminal bronchioles in centrilobular and panlobular emphysema.

METHODS

Preterminal bronchioles were identified by backtracking from the terminal bronchioles, and their centerlines were established along the entire length of their lumens. Multiple cross-sectional images perpendicular to the centerline were reconstructed to evaluate the bronchiolar wall and lumen, and the alveolar attachments to the outer airway walls in relation to emphysematous destruction in 28 lung samples from six patients with centrilobular emphysema, 20 lung samples from seven patients with panlobular emphysema associated with alpha-1 antitrypsin deficiency, and 47 samples from seven control (donor) lungs.

MEASUREMENTS AND MAIN RESULTS

The preterminal bronchiolar length, wall volume, total volume (wall + lumen), lumen circularity, and number of alveolar attachments were reduced in both centrilobular and panlobular emphysema compared with control lungs. In contrast, thickening of the wall and narrowing of the lumen were more severe and heterogeneous in centrilobular than in panlobular emphysema. The bronchiolar lumen was narrower in the middle than at both ends, and the decreased number of alveolar attachments was associated with increased wall thickness in centrilobular emphysema.

CONCLUSIONS

These results provide new information about small airways pathology in centrilobular and panlobular emphysema and show that these changes affect airways that are not visible with thoracic multidetector computed tomography scans but located proximal to the terminal bronchioles in chronic obstructive pulmonary disease.

The effects of emphysema on airway disease: correlations between multi-detector CT and pulmonary function tests in smokers

BACKGROUND

Chronic obstructive pulmonary disease (COPD) is characterized by airflow limitation caused by emphysema and small airway narrowing. Quantitative evaluation of airway dimensions by multi-detector computed tomography (MDCT) has revealed a correlation between airway dimension and airflow limitation. However, the effect of emphysema on this correlation is unclear.

OBJECTIVE

The goal of this study was to determine whether emphysematous changes alter the relationships between airflow limitation and airway dimensions as measured by inspiratory and expiratory MDCT.

METHODS

Ninety-one subjects underwent inspiratory and expiratory MDCT. Images were evaluated for mean airway luminal area (Ai), wall area percentage (WA%) from the third to the fifth generation of three bronchi (B1, B5, B8) in the right lung, and low attenuation volume percent (LAV%). Correlations between each airway index and airflow limitation were determined for each patient and compared between patients with and without evidence of emphysema.

RESULTS

In patients without emphysema, Ai and WA% from both the inspiratory and expiratory scans were significantly correlated with FEV1. No correlation was detected in patients with emphysema. In addition, emphysematous COPD patients with GOLD stage 1 or 2 disease had significantly lower changes in B8 Ai than non-emphysematous patients.

CONCLUSIONS

A significant correlation exists between airway parameters and FEV1 in patients without emphysema. Emphysema may influence airway dimensions even in patients with mild to moderate COPD.

ANALYSIS OF STRESS AND PRESSURE IN THE HUMAN ALVEOLAR WALL BEFORE BURSTING

A strain energy function for the human alveolar wall in vivo is developed based on its length-tension properties. Using large deformation theory, this function is employed to determine the relationship between circumferential stress and stretch ratio as well as between alveolar pressure and stretch ratio. We find that both the circumferential stress and the pressure in the alveolar wall rise very rapidly if the wall is stretched even slightly more than that which occurs at the pressures attained during mechanical in-exsufflation (MI-E). MI-E involves producing a high inspiratory air pressure and then quickly switching to a large negative air pressure in order to remove mucus from the airways.

Emphysema: the challenge of the remodelled lung

Emphysema is recognized as the component of chronic obstructive airways disease that is responsible for airways obstruction. Different patterns of emphysema are, however, recognized, suggesting possible different pathogenetic processes within the lung. This, coupled with the associated idea of susceptibility factors to the development of emphysema, has led to studies of genes that may be involved in the defence of the lung from proteolytic and oxidative damage. These studies have been driven by the goal of finding a treatment for emphysema, but appear to have lost sight of the fundamental remodelling of the lung that has occurred in patients with emphysema and the fact that it is not a single morphological entity.

Animal models of chronic bronchitis and their relevance to studies of particle-induced disease

Chronic bronchitis is a significant cause of morbidity and mortality. Chronic irritation of the conducting airways by inhaled substances, most importantly cigarette smoke, air pollution, and occupational exposures, is thought to be a key factor in the pathogenesis of chronic bronchitis. Microbial infections have been implicated in acute exacerbations of bronchitis and in its progression. Several animal models of chronic bronchitis have been developed. This review examines similarities and dissimilarities among commonly used animal models of bronchitis and the human disease. The most commonly used animal models of chronic bronchitis are those employing SO2, tobacco smoke, lipopolysaccharide (endotoxin), proteases, and secretagogues. Bronchiolitis induced by nickel and nitric acid have also been reported. Rats, hamsters, and dogs are the species most frequently used; sheep and monkeys have been used less frequently. These models vary in the extent or location of mucous-cell hyperplasia and metaplasia, airway inflammation, chronicity, ease of induction, and reproducibility. Frequently, the deficiencies in these models are attributable to anatomic differences between human and animal airways, differences in the severity or chronicity of inflammation or fibrosis, or lack of complete characterization of the responses and their time course in the animal model. These animal models may be useful for investigating how, and under what exposure conditions, ambient pollutants might exacerbate airway inflammation, mucus hypersecretion, and airflow limitation.

Airway alveolar attachment points and exposure to cigarette smoke in utero

The harmful effects of in utero cigarette smoke exposure include increased asthma symptoms and reduced lung function during the neonatal period, increased airway responsiveness to inhaled stimuli, and an increased risk of sudden infant death syndrome. Altered lung function may result from altered airway/lung structure. Airway dimensions, alveolar attachment points, and parenchymal elastin content were measured in 32 infants who died from sudden infant death syndrome and were grouped according to their perinatal cigarette smoke exposure. Compared with those without any exposure to cigarette smoke, the distance between alveolar attachments on airways was greater (p < 0.001) in infants exposed to cigarette smoke only in utero or both in utero and during the postnatal period but not different in those with only postnatal exposure. The percentage of elastin within the alveolar walls was similar in all the exposure groups. These findings suggest that in utero cigarette smoke exposure may result in abnormal airway function due to a reduction of the forces opposing airway narrowing.

Relationship between lung function, ventilation-perfusion inequality and extent of emphysema as assessed by high-resolution computed tomography

The development of the high-resolution computed tomography (HRCT) has improved the ability to detect and quantify emphysema in various groups of patients with chronic airflow obstruction (COPD). Significant correlations have previously been found between indices of air flow obstruction, hyperinflation, reduced diffusing capacity for carbon monoxide (DLCO), and the extent of emphysema (emph.%) assessed by HRCT. However, the relationship between emph.% and ventilation-perfusion (V(A)/Q) inequality in COPD is unknown. Twenty COPD patients with a mean forced expiratory volume in 1 s (FEV1) of 38.2 (+/- 15.5)% in percent of predicted value (%P), a mean PaO2 value of 9.6 (+/- 1.3) kPa, and a mean diffusing capacity of 43.6 (+/- 23.0)%P, were subjected to measurements by the multiple elimination inert gas technique (MIGET). The extent of emphysema was determined by HRCT at both full inspiration, emph.I(%) and at full expiration, emph.E(%), with a cut-off limit of -910 Hounsfield Units (HU) using the "Density Mask" method. The ventilation directed towards high V(A)/Q areas was 73 (+/- 10.2)% and the mean ventilation (V-mean) was elevated about three times compared to normal. The mean emph.(I)% and emph.(E) was 45.6 (+/- 16.9) and 32.7 (+/- 190)%, respectively. Significant correlations were shown between the emphysema extent and several lung function parameters, but no correlation was found between the emphysema extent and the V(A)/Q relationships or the blood gas values. Reduced DLCO%P correlated with less high V(A)/Q ventilation (r=0.73, P < 0.05) for the subgroup of COPD patients with DLCO(%P) less than 50% (n=12).

CONCLUSIONS

In COPD patients, suffering from moderate to severe emphysema without severe blood gas impairment, no correlation was shown between the extent of emphysema, as assessed by HRCT, and the severity of ventilation-perfusion inequality. A substantial collateral ventilation in severe emphysema may be a mechanism that prevents a deterioration in V(A)/Q relationships and in blood gas levels.

Effects on diffusing capacity and ventilation--perfusion relationships of budesonide inhalations for 2 months in chronic obstructive pulmonary disease (COPD)

Forced expiratory volumes are reduced in chronic obstructive pulmonary disease (COPD), mainly as a result of inflammatory and morphological changes in the small airways (with a diameter < 2 mm) and in the alveoli. However, it is difficult to detect minor changes in small airways by spirometry measurements. To study the effects on small airways of inhaled corticosteroids (ICS), 19 stable COPD patients were investigated; 15 were evaluated by ventilation-perfusion (V(A)/Q) relationships, assessed by the multiple inert gas elimination technique, and by diffusing capacity for carbon monoxide (DL(CO)), assessed by the single breath technique. Measurements were repeated after 2 months of budesonide inhalations (800 microg) twice daily. Before ICS treatment: mean forced expiratory volume in 1 sec (FEV1) as a percentage of predicted (% P) was 40.1 (+/- 16.0)%, DL(CO)% P was 45.7 (+/- 25.0)% and 6.0 (+/- 6.4)% of the ventilation was directed at high V(A)/Q areas. The mean of the V(A)/Q ratio for ventilation (V-mean) was 2.7 times higher than normal. After 2 months of ICS: the mean of DL(CO)% P increased by 8.6 (+/- 19.4)%, and FEV1/vital capacity decreased by 6.9 (+/- 11.3)%. Basal morning and ACTH-stimulated S-cortisol levels were significantly reduced. All the V(A)/Q relationships remained unchanged. In conclusion, a significant increase in diffusion capacity for carbon monoxide levels after treatment with corticosteroid inhalations for 2 months was shown, but no significant improvements were found in forced expiratory airflow, lung volumes, or V(A)/Q relationships.

The importance of spirometry in COPD and asthma: effect on approach to management

COPD is characterized by airflow limitation. The diagnosis is suggested by history and physical examination and is confirmed by spirometry (ie, a low FEV(1) level that is unresponsive to bronchodilators). Once diagnosed, there is no widely accepted staging or severity scoring system. COPD presently is graded using a single measurement such as FEV(1), which, unlike the case with asthma, has a limited role in disease management. A more comprehensive staging system is required incorporating, for example, age, arterial blood gases, dyspnea, body mass index, and distance walked, in addition to FEV(1). These criteria should allow for more evidence-based recommendations for management of this condition. Asthma is an inflammatory disease also characterized by airflow limitation. But in contrast with COPD, the airflow limitation is highly reversible either spontaneously or with therapy. Repeated lung function measurements using portable peak flowmeters have resulted in improved outcomes. Therefore, frequent flow determination is recommended in the routine management of asthma. Treatment with anti-inflammatory agents and close monitoring of lung function should help decrease the morbidity and mortality associated with asthma.

Analysis of stress distribution in the alveolar septa of normal and simulated emphysematic lungs

The alveolar septum consists of a skeleton of fine collagen and elastin fibers, which are interlaced with a capillary network. Its mechanical characteristics play an important role in the overall performance of the lung. An alveolar sac model was developed for numerical analysis of the internal stress distribution and septal displacements within the alveoli of both normal and emphysematic saline-filled lungs. A scanning electron micrograph of the parenchyma was digitized to yield a geometric replica of a typical two-dimensional alveolar sac. The stress-strain relationship of the alveolar tissue was adopted from experimental data. The model was solved by using commercial finite-element software for quasi-static loading of alveolar pressure. Investigation of the state of stresses and displacements in a healthy lung simulation yielded values that compared well with experimentally reported data. Alteration of the mechanical characteristics of the alveolar septa to simulate elastin destruction in the emphysematic model induced significant stress concentrations (e.g., at a lung volume of 60% total capacity, tensions at certain parts in an emphysematic lung were up to 6 times higher than those in a normal lung). The combination of highly elevated stress sites together with the cyclic loading of breathing may explain the observed progressive damage to elastin fibers in emphysematic patients.

Membranous bronchioles and connective tissue network of normal and emphysematous lungs

Three-dimensional reconstructions of the septal system of normal human lungs revealed that airways course within the interlobular septa, i.e., between the two blades formed by the peripheral boundaries of adjacent lobuli of whatever order, and enter the supplied pulmonary unit at its side. This is not in keeping with the classic view of a peripheral airway in the center of a lung unit and submitted to radial traction by attached alveolar septa. The basic design of the lung fibrous scaffold appears to be in conformity with the laws of fractal geometry. Similar reconstructions in centrilobular emphysema disclosed tortuosities of both intra-acinar and interlobular septa, with consequent distortions of the corresponding intraseptal bronchioles and collapse of lung units of different sizes. It is suggested that in centrilobular emphysema competition for space, besides intrinsic airways narrowing because of inflammation and loss of elastic recoil, is a cause of flow limitation.

Current thoughts regarding treatment of chronic obstructive pulmonary disease

The available evidence indicates that pulmonary rehabilitation benefits patients with symptomatic COPD. The effect of pulmonary rehabilitation programs on health care use is promising but requires further investigation. In contrast, aerobic lower extremity training is of benefit in several areas of importance to patients with COPD. These areas include exercise endurance, perception of dyspnea, quality of life, and self-efficacy. The exact role of upper extremity exercise training programs requires further studies but should be used in patients who develop symptoms with arm activities. Psychological support improves the awareness of the patient and increases his or her understanding of the disease, but when used alone it is of limited value. Pulmonary rehabilitation when coupled with smoking cessation, optimization of blood gases, and medications offers the best treatment option for patients with symptomatic airflow obstruction.

Physiological changes due to age. Implications for respiratory drug therapy

The pulmonary system is modified in various ways over time and it is particularly vulnerable to environmental insults. Of particular interest are the implications of aging for therapy of respiratory illnesses. The changes in pulmonary structure and function due simply to aging, and changes due to diseases, should be distinguished from each other. The great reserve function of the lung permits reasonable physical capacity in healthy individuals despite aging changes. In principle, loss of function equivalent to more than one lung is necessary to impair aerobic capacity at any age. Elderly people are subject to the same respiratory diseases as younger adults but may manifest them differently. They may present in atypical ways such as in bacterial pneumonia, tuberculosis, and asthma, all modified by anatomical alterations or deterioration of immunological defence mechanisms. Accumulation of toxic substances over time such as cigarette smoke or environmental pollutants may give rise to chronic bronchitis, emphysema, bronchogenic carcinoma and interstitial lung disease. Changes in the number or function of airway receptors modulate responses to bronchodilator drugs. Chronic inflammation of the bronchial wall has blurred the distinction between traditional asthma and chronic bronchitis and emphysema, and similar drug therapy can be useful for all. Adverse reactions to respiratory drugs such as theophylline, oral corticosteroids, and isoniazid increase with age. As more data accumulate, drug therapy of respiratory diseases in older patients will become more effective and safer.

Relation between distal airspace size, bronchiolar attachments, and lung function

BACKGROUND--Smoking related fixed airway obstruction may be due to airway scarring and narrowing or decreased support due to loss of adjacent alveolar walls. In this study of resected specimens, preoperative pulmonary function was compared with results of a morphometric study of lung structure. METHODS--Morphometric measurements were made on 42 inflation fixed lung specimens as follows: airspace wall surface area per unit volume (AWUV) was measured on at least 25 l mm2 histological fields from each specimen, expressed as a mean, and the mean of the lowest five measurements for each case (LF5). Minimum diameter, maximum diameter, diameter ratio (ellipticality), lumen area, and lumen circumference were measured on at least 16 non-respiratory bronchioles from each lung. Peribronchiolar alveolar support was measured as mean interalveolar attachment distance (IAAD). Measurements of pulmonary function included forced expiratory volume in one second (FEV1) (absolute and % predicted values; n = 42), slope of phase III (single breath nitrogen test; n = 28), closing volume (expressed as a percentage of vital capacity (CV/VC%); n = 28). RESULTS--Bronchiolar size was not independently related to the tests of lung function used. Percentage predicted FEV1 was related to mean IAAD, ellipticality, and mean AWUV. CV/VC% showed significant relation with ellipticality, mean AWUV, and LF5 AWUV. Slope of phase III increased with increasing IAAD. Significant correlations were found between ellipticality and AWUV (mean and LF5), and between ellipticality and IAAD. Both IAAD and ellipticality were significantly increased in patients with abnormally low FEV1. CONCLUSION--Destruction of airspace walls, particularly those attached to the peripheral bronchioles, is more influential in determining airflow limitation than bronchiolar size.