Lung structure and function in elastase-treated rats: A follow-up study.

Structural and functional longitudinal alterations of the lungs were followed in an emphysema model. Rats were treated with porcine pancreatic elastase (PPE, n=21) or saline (controls, C, n=19). Before the treatment and 3, 10, 21 and 105 days thereafter, absolute lung volumes (FRC, TLC and RV) and tissue mechanical parameters (elastance: H; damping: G) were determined. At 3, 21 and 105 days the lungs were fixed in subgroups of rats. From histological samples the equivalent diameter of airspaces (Dalv), elastin (Mec) and collagen densities were assessed. In the PPE group, FRC and RV were higher from 3 days after treatment compared to controls (p<0.001), while TLC exhibited a delayed increase. H and G decreased in the PPE group throughout the study (p<0.001). Higher Mec (p<0.001) and late-phase inflammation were observed at 105 days. We conclude that during the progression of emphysema, septal failures increase Dalv which decreases H; this reveals a strong structure-function relationship.

Variability of respiratory mechanics during sleep in overweight and obese subjects with and without asthma.

Variability of respiration may provide information regarding disease states. We sought to characterize variability of ventilation and resistance in healthy and asthma, to determine how respiratory control may be altered in sleep and with bi-level positive airway pressure (BPAP). Overweight and obese subjects with and without asthma were studied during sleep at baseline and with BPAP, while measuring respiratory system resistance (Rrs) continuously. Stable periods (>20min) of wake, NREM, and REM sleep were identified and correlation metrics of respiratory parameters were calculated, including coefficient of variation (CV). Variability of Rrs was also characterized over short time scales (20 breaths) during sleep and defined as either "leading to arousal" or "not leading to arousal". Data from 10 control and 10 subjects with asthma were analyzed. CV of Rrs was decreased in asthma at baseline (p<0.001) and decreased on BPAP as compared to baseline (p<0.001). Long time scale correlations were found in respiratory parameters, but the degree of correlations was decreased from wake to sleep (p<0.05). The variance and CV of Rrs was increased preceding an arousal from sleep at baseline; however, during BPAP, the CV was decreased and was not increased preceding arousals. At baseline, resistance was greater in those with asthma, but variability was smaller. BPAP reduced both resistance and overall variability. We conclude that the BPAP-induced decrease in variability may indicate that those with asthma are more likely to remain in a low resistance state, and that low resistance variability may reduce arousals from sleep.

The effect of lung stretch during sleep on airway mechanics in overweight and obese asthma.

Both obesity and sleep reduce lung volume and limit deep breaths, possibly contributing to asthma. We hypothesize that increasing lung volume dynamically during sleep would reduce airway resistance in asthma. Asthma (n=10) and control (n=10) subjects were studied during sleep at baseline and with increased lung volume via bi-level positive airway pressure (BPAP). Using forced oscillations, respiratory system resistance (R(rs)) and reactance (X(rs)) were measured during sleep and R(rs) was partitioned to upper and lower airway resistance (R(up), R(low)) using an epiglottic pressure catheter. R(rs) and R(up) increased with sleep (p<0.01) and X(rs) was decreased in REM (p=0.02) as compared to wake. R(rs), R(up), and R(low), were larger (p<0.01) and X(rs) was decreased (p<0.02) in asthma. On BPAP, R(rs) and R(up) were decreased (p<0.001) and X(rs) increased (p<0.01), but R(low) was unchanged. High R(up) was observed in asthma, which reduced with BPAP. We conclude that the upper airway is a major component of R(rs) and larger lung volume changes may be required to alter R(low).

Acute mechanical forces cause deterioration in lung structure and function in elastase-induced emphysema.

The relation between the progression of chronic obstructive pulmonary disease (COPD) and exacerbations is unclear. Currently, no animal model of acute exacerbation of COPD (AECOPD) exists. The objectives of this study were to evaluate the effects of mechanical forces induced by deep inspirations (DIs) on short-term deterioration of lung structure and function to mimic AECOPD. At 2, 7, or 21 days after treatment with elastase, mice were ventilated with or without DIs (35 cmH(2)O airway pressure for 3 s, 2 times/min) for 1 h. Functional residual capacity (FRC) was measured with body plethysmography, and respiratory compliance, resistance, and hysteresivity were obtained via forced oscillations. From hematoxylin and eosin-stained sections, equivalent airspace diameters (D), alveolar wall thickness (W(t)), number of septal ruptures (N(sr)), and attachment density (A(d)) around airways were determined. FRC, compliance, and hysteresivity statistically significantly increased with time, and both increased due to DIs. Interestingly, DIs also had an effect on FRC, compliance, resistance, and hysteresivity in control mice. The development of emphysema statistically significantly increased D and W(t) in time, and the DIs caused subtle differences in D. At 21 days, the application of DIs changed the distribution of D, increased W(t) and N(sr), and decreased A(d). These results suggest that once a critical remodeling of the parenchyma has been reached, acute mechanical forces lead to irreversible changes in structure and function, mimicking COPD exacerbations. Thus, the acute application of DIs in mice with emphysema may serve as a useful model of AECOPD.

Functional and morphological assessment of early impairment of airway function in a rat model of emphysema.

The aim of this study was to evaluate airway structure-function relations in elastase-induced emphysema in rats. Sprague-Dawley rats were treated intratracheally with 50 IU porcine pancreatic elastase (PPE, n = 8) or saline (controls, n = 6). Six weeks later, lung volumes [functional residual capacity (FRC), residual volume (RV), and total lung capacity (TLC)] and low-frequency impedance parameters (Newtonian resistance, R(N); tissue damping; tissue elastance, H) were measured, and tracheal sounds were recorded during slow inflation to TLC following in vivo degassing. The lungs were fixed and stained for standard morphometry, elastin, and collagen. In the PPE group, FRC and RV were higher [4.53 ± 0.7 (SD) vs. 3.28 ± 0.45 ml; P = 0.003 and 1.06 ± 0.35 vs. 0.69 ± 0.18 ml; P = 0.036, respectively], and H was smaller in the PPE-treated rats than in the controls (1,344 ± 216 vs. 2,178 ± 305 cmH(2)O/l; P < 0.001), whereas there was no difference in R(N). The average number of crackles per inflation was similar in the two groups; however, the crackle size distributions were different and the lower knee of the pressure-volume curves was higher in the PPE group. Microscopic images revealed different alveolar size distributions but similar bronchial diameters in the two groups. The treatment caused a slight but significant decrease in the numbers of alveolar attachments, no difference in elastin and slightly increased mean level and heterogeneity of collagen in the bronchial walls. These results suggest that tissue destruction did not affect the conventionally assessed airway resistance in this emphysema model, whereas the alterations in the recruitment dynamics can be an early manifestation of impaired airway function.

Measuring upper and lower airway resistance during sleep with the forced oscillation technique.

The forced oscillation technique (FOT) is a non-invasive technique to monitor airway obstruction in those with asthma. The aim of this study was to design and validate a system to use FOT during sleep, both with and without bi-level positive airway pressure (BPAP), and to separate upper airway resistance from lower. 8 Hz pressure oscillations were supplied, over which the subject breathed, pressure and flow measurements were then used to calculate impedance. A phase-shift induced by the pressure transducer tubing was characterized, and FOT resistance was compared to steady flow resistance both with and without BPAP. A Millar catheter was used to measure pressure at the epiglottis, allowing the separation of upper from lower airway resistance. A phase shift of -0.010 s was calculated for the pressure transducer tubing, and the average error between FOT and steady flow resistance was -0.2 ± 0.2 cmH2O/L/s without BPAP and 0.4 ± 0.2 cmH2O/L/s with BPAP. The system was tested on three subjects, one healthy, one with obstructive sleep apnea, and one with asthma. The FOT was well tolerated and resistance was separated into upper and lower airway components. This setup is suitable for monitoring both upper and lower airway obstruction during sleep in those with and without asthma.

Fluctuation analysis of lung function as a predictor of long-term response to beta2-agonists.

The response to beta(2)-agonists differs between asthmatics and has been linked to subsequent adverse events, even death. Possible determinants include beta(2)-adrenoceptor genotype at position 16, lung function and airway hyperresponsiveness. Fluctuation analysis provides a simple parameter alpha measuring the complex correlation properties of day-to-day peak expiratory flow. The present study investigated whether alpha predicts clinical response to beta(2)-agonist treatment, taking into account other conventional predictors. Analysis was performed on previously published twice-daily peak expiratory flow measurements in 66 asthmatic adults over three 6-month randomised order treatment periods: placebo, salbutamol and salmeterol. Multiple linear regression was used to determine the association between alpha during the placebo period and response to treatment (change in the number of days with symptoms), taking into account other predictors namely beta(2)-adrenoceptor genotype, lung function and its variability, and airway hyperresponsiveness. The current authors found that alpha measured during the placebo period considerably improved the prediction of response to salmeterol treatment, taking into account genotype, lung function or its variability, or airway hyperresponsiveness. The present study provides further evidence that response to beta(2)-agonists is related to the time correlation properties of lung function in asthma. The current authors conclude that fluctuation analysis of lung function offers a novel predictor to identify patients who may respond well or poorly to treatment.

Lung volumes and respiratory mechanics in elastase-induced emphysema in mice.

Absolute lung volumes such as functional residual capacity, residual volume (RV), and total lung capacity (TLC) are used to characterize emphysema in patients, whereas in animal models of emphysema, the mechanical parameters are invariably obtained as a function of transrespiratory pressure (Prs). The aim of the present study was to establish a link between the mechanical parameters including tissue elastance (H) and airway resistance (Raw), and thoracic gas volume (TGV) in addition to Prs in a mouse model of emphysema. Using low-frequency forced oscillations during slow deep inflation, we tracked H and Raw as functions of TGV and Prs in normal mice and mice treated with porcine pancreatic elastase. The presence of emphysema was confirmed by morphometric analysis of histological slices. The treatment resulted in an increase in TGV by 51 and 44% and a decrease in H by 57 and 27%, respectively, at 0 and 20 cmH(2)O of Prs. The Raw did not differ between the groups at any value of Prs, but it was significantly higher in the treated mice at comparable TGV values. In further groups of mice, tracheal sounds were recorded during inflations from RV to TLC. All lung volumes but RV were significantly elevated in the treated mice, whereas the numbers and size distributions of inspiratory crackles were not different, suggesting that the airways were not affected by the elastase treatment. These findings emphasize the importance of absolute lung volumes and indicate that tissue destruction was not associated with airway dysfunction in this mouse model of emphysema.

Alveolar macrophage activation and an emphysema-like phenotype in adiponectin-deficient mice.

Adiponectin is an adipocyte-derived collectin that acts on a wide range of tissues including liver, brain, heart, and vascular endothelium. To date, little is known about the actions of adiponectin in the lung. Herein, we demonstrate that adiponectin is present in lung lining fluid and that adiponectin deficiency leads to increases in proinflammatory mediators and an emphysema-like phenotype in the mouse lung. Alveolar macrophages from adiponectin-deficient mice spontaneously display increased production of tumor necrosis factor-alpha (TNF-alpha) and matrix metalloproteinase (MMP-12) activity. Consistent with these observations, we found that pretreatment of alveolar macrophages with adiponectin leads to TNF-alpha and MMP-12 suppression. Together, our findings show that adiponectin leads to macrophage suppression in the lung and suggest that adiponectin-deficient states may contribute to the pathogenesis of inflammatory lung conditions such as emphysema.

Impact of positive end-expiratory pressure during heterogeneous lung injury: insights from computed tomographic image functional modeling.

Image Functional Modeling (IFM) synthesizes three dimensional airway networks with imaging and mechanics data to relate structure to function. The goal of this study was to advance IFM to establish a method of exploring how heterogeneous alveolar flooding and collapse during lung injury would impact regional respiratory mechanics and flow distributions within the lung at distinct positive end-expiratory pressure (PEEP) levels. We estimated regional respiratory system elastance from computed tomography (CT) scans taken in 5 saline-lavaged sheep at PEEP levels from 7.5 to 20 cmH(2)O. These data were anatomically mapped into a computational sheep lung model, which was used to predict the corresponding impact of PEEP on dynamic flow distribution. Under pre-injury conditions and during lung injury, respiratory system elastance was determined to be spatially heterogeneous and the values were distributed with a hyperbolic distribution in the range of measured values. Increases in PEEP appear to modulate the heterogeneity of the flow distribution throughout the injured lung. Moderate increases in PEEP decreased the heterogeneity of elastance and predicted flow distribution, although heterogeneity began to increase for PEEP levels above 12.5-15 cmH(2)O. By combining regional respiratory system elastance estimated from CT with our computational lung model, we can potentially predict the dynamic distribution of the tidal volume during mechanical ventilation and thus identify specific areas of the lung at risk of being overdistended.

Heterogeneous airway versus tissue mechanics and their relation to gas exchange function during mechanical ventilation.

We have advanced a commercially available ventilator (NPB840, Puritan Bennett/Tyco Healthcare, Pleasanton, CA) to deliver an Enhanced Ventilation Waveform (EVW). This EVW delivers a broadband waveform that contains discrete frequencies blended to provide a tidal breath, followed by passive exhalation. The EVW allows breath-by-breath estimates of frequency dependence of lung and total respiratory resistance (R) and elastance (E) from 0.2 to 8 Hz. We hypothesized that the EVW approach could provide continuous ventilation simultaneously with an advanced evaluation of mechanical heterogeneities under heterogeneous airway and tissue disease conditions. We applied the EVW in five sheep before and after a bronchial challenge and an oleic acid (OA) acute lung injury model. In all sheep, the EVW maintained gas exchange during and after bronchoconstriction, as well as during OA injury. Data revealed a range of disease conditions from mild to severe with heterogeneities and airway closures. Correlations were found between the arterial partial pressure of oxygen (PaO2) and the levels and frequency-dependent features of R and E that are indicative of mechanical heterogeneity and tissue disease. Lumped parameter models provided additional insight on heterogeneous airway and tissue disease. In summary, information obtained from EVW analysis can provide enhanced guidance on the efficiency of ventilator settings and on patient status during mechanical ventilation.

Effect of sighs on breathing memory and dynamics in healthy infants.

Deep inspirations (sighs) play a significant role in altering lung mechanical and airway wall function; however, their role in respiratory control remains unclear. We examined whether sighs act via a resetting mechanism to improve control of the respiratory regulatory system. Effects of sighs on system variability, short- and long-range memory, and stability were assessed in 25 healthy full-term infants at 1 mo of age [mean 36 (range 28-57) days] during quiet sleep. Variability was examined using moving-window coefficient of variation, short-range memory using autocorrelation function, and long-range memory using detrended fluctuation analysis. Stability was examined by studying the behavior of the attractor with use of phase-space plots. Variability of tidal volume (VT) and minute ventilation (VE) increased during the initial 15 breaths after a sigh. Short-range memory of VT decreased during the 50 breaths preceding a sigh, becoming uncorrelated (random) during the 10-breath presigh window. Short-range memory increased after a sigh for the entire 50 breaths compared with the randomized data set and for 20 breaths compared with the presigh window. Similar, but shorter duration, changes were noted in VE. No change in long-range memory was seen after a sigh. Coefficient of variation and range of points located within a defined attractor segment increased after a sigh. Thus control of breathing in healthy infants shows long-range stability and improvement in short-range memory and variability after a sigh. These results add new evidence that the role of sighs is not purely mechanical.

Acoustic evidence of airway opening during recruitment in excised dog lungs.

The aim of this study was to test the hypothesis that the mechanism of recruitment and the lower knee of the pressure-volume curve in the normal lung are primarily determined by airway reopenings via avalanches rather than simple alveolar recruitments. In isolated dog lung lobes, the pressure-volume loops were measured, and crackle sounds were recorded intrabronchially during both the first inflation from the collapsed state to total lobe capacity and a second inflation without prior degassing. The inflation flow contained transients that were accompanied by a series of crackles. Discrete volume increments were estimated from the flow transients, and the energy levels of the corresponding crackles were calculated from the sound recordings. Crackles were concentrated in the early phase of inflation, with the cumulative energy exceeding 90% of its final value by the lower knee of the pressure-volume curve. The values of volume increments were correlated with crackle energy during the flow transient for both the first and the second inflations (r(2) = 0.29-0.73 and 0.68-0.82, respectively). Because the distribution of volume increments followed a power law, the correlation between crackle energy and discrete volume increments suggests that an avalanche-like airway opening process governs the recruitment of collapsed normal lungs.

Relation between structure, function, and imaging in a three-dimensional model of the lung.

Previous studies have reported morphometric models to predict function relations in the lung. These models, however, are not anatomically explicit. We have advanced a three-dimensional airway tree model to relate dynamic lung function to alterations in structure, particularly when constriction patterns are imposed heterogeneously inspecific anatomic locations. First we predicted the sensitivity of dynamic lung resistance and elastance (RL and EL) to explicit forms of potential constriction patterns. Simulations show that severe and heterogeneous peripheral airway constriction confined to a single region in the lung (apex, mid, or base) will not produce substantial alterations in whole lung properties as measured from the airway opening. Conversely, when measured RL and EL are abnormal, it is likely that significant (but not necessarily homogeneous) constriction has occurred throughout the entire airway tree. We also introduce the concept of image-assisted modeling. Here positron emission tomographic imaging data sensitive to ventilation heterogeneity is synthesized with RL and EL data to help identify which airway constriction conditions could be consistent with both data sets. An ultimate goal would be personalized predictions.

How does airway inflammation modulate asthmatic airway constriction? An antigen challenge study.

During the late-phase (LP) response to inhaled allergen, mediators from neutrophils and eosinophils are released within the airways, resembling what occurs during an asthma attack. We compared the distribution of obstruction and degree of reversibility that follows a deep inspiration (DI) during early-phase (EP) and LP responses in nine asthmatic subjects challenged with allergen. Heterogeneity of constriction was assayed by determining frequency dependence of dynamic lung resistance and elastance, airway caliber by tracking airway resistance during a DI, and airway inflammation by measuring inflammatory cells in induced sputum postchallenge. Despite a paucity of eosinophils in the sputum at baseline (<1% of nonsquamous cells), asthmatic subjects showed a substantial EP response with highly heterogeneous constriction and reduced capacity to maximally dilate airways. The LP was associated with substantial airway inflammation in all subjects. However, five subjects showed only mild LP constriction, whereas four showed more marked LP constriction characterized by heterogeneous constriction similar to EP. Bronchoconstriction during LP was fully alleviated by administration of a bronchodilator. These findings, together with the impaired bronchodilatory response during a DI, indicate a physiological abnormality in asthma at the smooth muscle level and indicate that airway inflammation in asthma is associated with a highly nonuniform pattern of constriction. These data support the hypothesis that variability in responsiveness among asthmatic subjects derives from intrinsic differences in smooth muscle response to inflammation.

Roles of mechanical forces and collagen failure in the development of elastase-induced emphysema.

Emphysema causes a permanent destruction of alveolar walls leading to airspace enlargement, loss of elastic recoil, decrease in surface area for gas exchange, lung hyperexpansion, and increased work of breathing. The most accepted hypothesis of how emphysema develops is based on an imbalance of protease and antiprotease activity leading to the degradation of elastin within the fiber network of the extracellular matrix. Here we report novel roles for mechanical forces and collagen during the remodeling of lung tissue in a rat model of elastase-induced emphysema. We have developed a technique to measure the stress-strain properties of tissue sections while simultaneously visualizing the deformation of the immunofluorescently labeled elastin-collagen network. We found that in the elastase treated tissue significant remodeling leads to thickened elastin and collagen fibers and during stretching, the newly deposited elastin and collagen fibers undergo substantially larger distortions than in normal tissue. We also found that the threshold for mechanical failure of collagen, which provides mechanical stability to the normal lung, is reduced. Our results indicate that mechanical forces during breathing are capable of causing failure of the remodeled extracellular matrix at loci of stress concentrations and so contribute to the progression of emphysema.

Analysis of the harmonic content of the tidal flow waveforms in infants.

The aim of this study was to examine whether the spectral characteristics of tidal flow waveform reflect the interaction between the control of breathing and lung mechanics in 10 healthy infants (H), 10 infants with a history of wheezing disorders (W), and 10 infants with chronic lung disease (CLD). From the flow waveform, we calculated a shape index, the harmonic distortion (k(d)), which quantifies the extent to which a periodic signal deviates from a sine wave. The k(d) of the entire tidal flow waveform did not significantly discriminate between diagnostic groups. However, k(d) was sensitive to maturation: it increased from 0.26 at 1 mo to 0.37 at 6 mo of age (P < 0.002). Furthermore, the frequency (f) spectra of the flow (V) amplitudes between 0.13 and 10 Hz followed a power law: V(f) approximately f(-s), where s (slope) is the exponent in the power law. The exponent of the healthy infants s(H) was 4.24 [95% confidence interval (CI) = 0.2] at 1 mo, 4.39 (CI = 0.16) at 6 mo, and 4.35 (CI = 0.19) at 12 mo and not significantly changing with age. The mean value of s(W) was marginally lower (4.09 +/- 0.28; P < 0.05) than that of s(H). The mean s(CLD) was significantly lower (3.04 +/- 0.31; P < 0.001). Lower values of s and higher values of k(d) indicate an increased complexity of the feedback mechanisms determining tidal flow waveform and may be associated with disease.

Characterization of the branching structure of the lung from "macroscopic" pressure-volume measurements.

We analyze the problem of fluid flow in a bifurcating structure containing random blockages that can be removed by fluid pressure. We introduce an asymmetric tree model and find that the predicted pressure-volume relation is connected to the distribution Pi(n) of the generation number n of the tree's terminal segments. We use this relation to explore the branching structure of the lung by analyzing experimental pressure-volume data from dog lungs. The Pi(n) extracted from the data using the model agrees well with experimental data on the branching structure. We can thus obtain information about the asymmetric structure of the lung from macroscopic, noninvasive pressure-volume measurements.

Avalanche dynamics of crackle sound in the lung.

We analyze a sequence of short transient sound waves, called "crackles," which are associated with explosive openings of airways during lung inflation. The distribution of time intervals between consecutive crackles Delta(t) shows two regimes of power law behavior. We develop an avalanche model which fits the data over five decades of Delta(t). We find that the regime for large Delta(t) is related to the dynamics of distinct avalanches in a Cayley tree, and the regime for small Delta(t) is determined by the dynamics of crackle propagation within a single avalanche. We also obtain a mean-field solution of the model which provides information about lung inflation.

Hysteresivity of the lung and tissue strip in the normal rat: effects of heterogeneities.

We measured lung impedance in rats in closed chest (CC), open chest (OC), and isolated lungs (IL) at four transpulmonary pressures with a optimal ventilator waveform. Data were analyzed with an homogeneous linear or an inhomogeneous linear model. Both models include tissue damping and elastance and airway inertance. The homogeneous linear model includes airway resistance (Raw), whereas the inhomogeneous linear model has a continuous distribution of Raw characterized by the mean Raw and the standard deviation of Raw (SDR). Lung mechanics were compared with tissue strip mechanics at frequencies and operating stresses comparable to those during lung impedance measurements. The hysteresivity (eta) was calculated as tissue damping/elastance. We found that 1) airway and tissue parameters were different in the IL than in the CC and OC conditions; 2) SDR was lowest in the IL; and 3) eta in IL at low transpulmonary pressure was similar to eta in the tissue strip. We conclude that eta is primarily determined by lung connective tissue, and its elevated estimates from impedance data in the CC and OC conditions are a consequence of compartment-like heterogeneity being greater in CC and OC conditions than in the IL.