Nonuniform expansion of constricted dog lungs

Oscillation mechanics of the respiratory system

The mechanical impedance of the respiratory system defines the pressure profile required to drive a unit of oscillatory flow into the lungs. Impedance is a function of oscillation frequency, and is measured using the forced oscillation technique. Digital signal processing methods, most notably the Fourier transform, are used to calculate impedance from measured oscillatory pressures and flows. Impedance is a complex function of frequency, having both real and imaginary parts that vary with frequency in ways that can be used empirically to distinguish normal lung function from a variety of different pathologies. The most useful diagnostic information is gained when anatomically based mathematical models are fit to measurements of impedance. The simplest such model consists of a single flow-resistive conduit connecting to a single elastic compartment. Models of greater complexity may have two or more compartments, and provide more accurate fits to impedance measurements over a variety of different frequency ranges. The model that currently enjoys the widest application in studies of animal models of lung disease consists of a single airway serving an alveolar compartment comprising tissue with a constant-phase impedance. This model has been shown to fit very accurately to a wide range of impedance data, yet contains only four free parameters, and as such is highly parsimonious. The measurement of impedance in human patients is also now rapidly gaining acceptance, and promises to provide a more comprehensible assessment of lung function than parameters derived from conventional spirometry.

Locally measured shear moduli of pulmonary tissue and global lung mechanics in mechanically ventilated rats

This study was aimed at measuring shear moduli in vivo in mechanically ventilated rats and comparing them to global lung mechanics. Wistar rats (n = 28) were anesthetized, tracheally intubated, and mechanically ventilated in supine position. The animals were randomly assigned to the healthy control or the lung injury group where lung injury was induced by bronchoalveolar lavage. The respiratory system elastance E(rs) was analyzed based on the single compartment resistance/elastance lung model using multiple linear regression analysis. The shear modulus (G) of alveolar parenchyma was studied using a newly developed endoscopic system with adjustable pressure at the tip that was designed to induce local mechanostimulation. The data analysis was then carried out with an inverse finite element method. G was determined at continuous positive airway pressure (CPAP) levels of 15, 17, 20, and 30 mbar. The resulting shear moduli of lungs in healthy animals increased from 3.3 ± 1.4 kPa at 15 mbar CPAP to 5.8 ± 2.4 kPa at 30 mbar CPAP (P = 0.012), whereas G was ~2.5 kPa at all CPAP levels for the lung-injured animals. Regression analysis showed a negative correlation between G and relative E(rs) in the control group (r = -0.73, P = 0.008 at CPAP = 20 mbar) and no significant correlation in the lung injury group. These results suggest that the locally measured G were inversely associated with the elastance of the respiratory system. Rejecting the study hypothesis the researchers concluded that low global respiratory system elastance is related to high local resistance against tissue deformation.

Effect of airway smooth muscle tone on airway distensibility measured by the forced oscillation technique in adults with asthma

Airway distensibility appears to be unaffected by airway smooth muscle (ASM) tone, despite the influence of ASM tone on the airway diameter-pressure relationship. This discrepancy may be because the greatest effect of ASM tone on airway diameter-pressure behavior occurs at low transpulmonary pressures, i.e., low lung volumes, which has not been investigated. Our study aimed to determine the contribution of ASM tone to airway distensibility, as assessed via the forced oscillation technique (FOT), across all lung volumes with a specific focus on low lung volumes. We also investigated the accompanying influence of ASM tone on peripheral airway closure and heterogeneity inferred from the reactance versus lung volume relationship. Respiratory system conductance and reactance were measured using FOT across the entire lung volume range in 22 asthma subjects and 19 healthy controls before and after bronchodilator. Airway distensibility (slope of conductance vs. lung volume) was calculated at residual volume (RV), functional residual capacity (FRC), and total lung capacity. At baseline, airway distensibility was significantly lower in subjects with asthma at all lung volumes. After bronchodilator, distensibility significantly increased at RV (64.8%, P < 0.001) and at FRC (61.8%, P < 0.01) in subjects with asthma but not in control subjects. The increased distensibility at RV and FRC in asthma were not associated with the accompanying changes in the reactance versus lung volume relationship. Our findings demonstrate that, at low lung volumes, ASM tone reduces airway distensibility in adults with asthma, independent of changes in airway closure and heterogeneity.

Lung imaging in asthmatic patients: the picture is clearer

Imaging of the lungs in patients with asthma has evolved dramatically over the last decade with sophisticated techniques, such as computed tomography, magnetic resonance imaging, positron emission tomography, and single photon emission computed tomography. New insights into current and future modalities for imaging in asthmatic patients and their application are discussed to potentially shed a clearer picture of the underlying pathophysiology of asthma, especially severe asthma, and the proposed clinical utility of imaging in patients with this common disease.

Stress and strain in the contractile and cytoskeletal filaments of airway smooth muscle

Stress and strain are omnipresent in the lung due to constant lung volume fluctuation associated with respiration, and they modulate the phenotype and function of all cells residing in the airways including the airway smooth muscle (ASM) cell. There is ample evidence that the ASM cell is very sensitive to its physical environment, and can alter its structure and/or function accordingly, resulting in either desired or undesired consequences. The forces that are either conferred to the ASM cell due to external stretching or generated inside the cell must be borne and transmitted inside the cytoskeleton (CSK). Thus, maintaining appropriate levels of stress and strain within the CSK is essential for maintaining normal function. Despite the importance, the mechanisms regulating/dysregulating ASM cytoskeletal filaments in response to stress and strain remained poorly understood until only recently. For example, it is now understood that ASM length and force are dynamically regulated, and both can adapt over a wide range of length, rendering ASM one of the most malleable living tissues. The malleability reflects the CSK's dynamic mechanical properties and plasticity, both of which strongly interact with the loading on the CSK, and all together ultimately determines airway narrowing in pathology. Here we review the latest advances in our understanding of stress and strain in ASM cells, including the organization of contractile and cytoskeletal filaments, range and adaptation of functional length, structural and functional changes of the cell in response to mechanical perturbation, ASM tone as a mediator of strain-induced responses, and the novel glassy dynamic behaviors of the CSK in relation to asthma pathophysiology.

Functional imaging: CT and MRI

Numerous imaging techniques permit evaluation of regional pulmonary function. Contrast-enhanced CT methods now allow assessment of vasculature and lung perfusion. Techniques using spirometric controlled multi-detector row CT allow for quantification of presence and distribution of parenchymal and airway pathology; xenon gas can be employed to assess regional ventilation of the lungs, and rapid bolus injections of iodinated contrast agent can provide a quantitative measure of regional parenchymal perfusion. Advances in MRI of the lung include gadolinium-enhanced perfusion imaging and hyperpolarized gas imaging, which allow functional assessment, including ventilation/perfusion, microscopic air space measurements, and gas flow and transport dynamics.

Magnetic resonance elastography of the lung: technical feasibility

Magnetic resonance elastography (MRE) is a phase-contrast technique that can spatially map shear stiffness within tissue-like materials. To date, however, MRE of the lung has been too technically challenging-primarily because of signal-to-noise ratio (SNR) limitations and phase instability. We describe an approach in which shear wave propagation is not encoded into the phase of the MR signal of a material, but rather from the signal arising from a polarized noble gas encapsulated within. To determine the feasibility of the approach, three experiments were performed. First, to establish whether shear wave propagation within lung parenchyma can be visualized with phase-contrast MR techniques, MRE was performed on excised porcine lungs inflated with room air. Second, a phantom consisting of open-cell foam filled with thermally polarized (3)He gas was imaged with MRE to determine whether shear wave propagation can be encoded by the gas. Third, preliminary evidence of the feasibility of MRE in vivo was obtained by using a longitudinal driver on the chest of a normal volunteer to generate shear waves in the lung. The results suggest that MRE in combination with hyperpolarized noble gases is potentially useful for noninvasively assessing the regional elastic properties of lung parenchyma, and merits further investigation.

Computed tomography studies of lung mechanics

The study of lung mechanics has progressed from global descriptions of lung pressure and volume relationships to the high-resolution, three-dimensional, quantitative measurement of dynamic regional mechanical properties and displacements. X-ray computed tomography (CT) imaging is ideally suited to the study of regional lung mechanics in intact subjects because of its high spatial and temporal resolution, correlation of functional data with anatomic detail, increasing volumetric data acquisition, and the unique relationship between CT density and lung air content. This review presents an overview of CT measurement principles and limitations for the study of regional mechanics, reviews some of the early work that set the stage for modern imaging approaches and impacted the understanding and management of patients with acute lung injury, and presents evolving novel approaches for the analysis and application of dynamic volumetric lung image data.

Bronchospasm and its biophysical basis in airway smooth muscle

Airways hyperresponsiveness is a cardinal feature of asthma but remains unexplained. In asthma, the airway smooth muscle cell is the key end-effector of bronchospasm and acute airway narrowing, but in just the past five years our understanding of the relationship of responsiveness to muscle biophysics has dramatically changed. It has become well established, for example, that muscle length is equilibrated dynamically rather than statically, and that non-classical features of muscle biophysics come to the forefront, including unanticipated interactions between the muscle and its time-varying load, as well as the ability of the muscle cell to adapt rapidly to changes in its dynamic microenvironment. These newly discovered phenomena have been described empirically, but a mechanistic basis to explain them is only beginning to emerge.

Frequency dependence of respiratory system mechanics during induced constriction in a murine model of asthma

Airway dysfunction in asthma is characterized by hyperresponsiveness, heterogeneously narrowed airways, and closure of airways. To test the hypothesis that airway constriction in ovalbumin (OVA)-sensitized OVA-intranasally challenged (OVA/OVA) mice produces mechanical responses that are similar to those reported in asthmatic subjects, respiratory system resistance (Rrs) and elastance (Edyn,rs) spectra were obtained in OVA/OVA and control mice during intravenous methacholine (MCh) infusions. In control mice, MCh at 1,700 microg x kg(-1) x min(-1) produced 1) a 495 and 928% increase of Rrs at 0.5 Hz and 19.75 Hz, respectively, 2) a 33% rise in Edyn,rs at 0.5 Hz, and 3) a mild frequency (f)-dependent increase of Edyn,rs. The same MCh dose in OVA/OVA mice produced 1) elevations of Rrs at 0.5 Hz and 19.75 Hz of 1,792 and 774%, respectively, 2) a 390% rise in Edyn,rs at 0.5 Hz, and 3) marked f-dependent increases of Edyn,rs. During constriction, the f dependence of mechanics in control mice was consistent with homogeneous airway narrowing; however, in OVA/OVA mice, f dependence was characteristic of heterogeneously narrowed airways, closure of airways, and airway shunting. These mechanisms amplify the pulmonary mechanical responses to constrictor stimuli at physiological breathing rates and have important roles in the pathophysiology of human asthma.

Airway stability and heterogeneity in the constricted lung

The effect of bronchoconstriction on airway resistance is known to be spatially heterogeneous and dependent on tidal volume. We present a model of a single terminal airway that explains these features. The model describes a feedback between flow and airway resistance mediated by parenchymal interdependence and the mechanics of activated smooth muscle. The pressure-tidal volume relationship for a constricted terminal airway is computed and shown to be sigmoidal. Constricted terminal airways are predicted to have two stable states: one effectively open and one nearly closed. We argue that the heterogeneity of whole lung constriction is a consequence of this behavior. Airways are partitioned between the two states to accommodate total flow, and changes in tidal volume and end-expiratory pressure affect the number of airways in each state. Quantitative predictions for whole lung resistance and elastance agree with data from previously published studies on lung impedance.

Historical perspective on airway smooth muscle: the saga of a frustrated cell

Despite the lack of a clearly defined physiological function, airway smooth muscle receives substantial attention because of its involvement in the pathogenesis of asthma. Recent investigations have turned to the ways in which the muscle is influenced by its dynamic microenvironment. Ordinarily, airway smooth muscle presents little problem, even when maximally activated, because unending mechanical perturbations provided by spontaneous tidal breathing put airway smooth muscle in a perpetual state of "limbo," keeping its contractile machinery off balance and unable to achieve its force-generating potential. The dynamic microenvironment affects airway smooth muscle in at least two ways: by acute changes associated with disruption of myosin binding and by chronic changes associated with plastic restructuring of contractile and cytoskeletal filament organization. Plastic restructuring can occur when dynamic length changes occur between sequential contractile events or within a single contractile event. Impairment of these normal responses of airway smooth muscle to its dynamic environment may be implicated in airway hyperresponsiveness in asthma.

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.

Determination of regional pulmonary parenchymal strain during normal respiration using spin inversion tagged magnetization MRI

In clinical practice, the assessment of lung mechanics is limited to a global physiological evaluation, which measures, in the aggregate, the contributions of the pulmonary parenchyma, pleura, and chest wall. In this study, we used an MR imaging methodology which applies two-dimensional bands of inverted magnetization directly onto the pulmonary parenchyma, thus allowing for the quantification of local pulmonary tissue deformation, or strain, throughout inhalation. Our results showed that the magnitude of strain was maximal at the base and apex of the lung, but was curtailed at the hilum, the anatomical site of the poorly mobile bronchial and vascular insertions. In-plane shear strain mapping showed mostly positive shear strain, predominant at the apex throughout inhalation, and increasing with expanding lung volume. Anisotropy mapping showed that superior-inferior axial strain was greater than medial-lateral axial strain at the apex and base, while the opposite was true for the middle lung field. This study demonstrates that localized pulmonary deformation can be measured in vivo with tagging MRI, and quantified by applying finite strain definitions from continuum mechanics.

Distribution of pulmonary ventilation using Xe-enhanced computed tomography in prone and supine dogs

Xe-enhanced computed tomography (CT; Xe-CT) is a method for the noninvasive measurement of regional pulmonary ventilation in intact subjects, determined from the washin and washout rates of the radiodense, nonradioactive gas Xe, as measured in serial CT scans. We used the Xe-CT ventilation method, along with other quantitative CT measurements, to investigate the distribution of regional lung ventilation and air content in healthy, anesthetized, mechanically ventilated dogs in the prone and supine postures. Vertical gradients in regional ventilation and air content were measured in five mongrel dogs in both prone and supine postures at four axial lung locations. In the supine position, ventilation increased with dependent location, with a mean slope of 7.3%/cm lung height, whereas no ventilation gradients were found at any location in the prone position. These results agree quantitatively with other published studies. In addition, six different animals were studied (3 supine, 3 prone) to examine the longitudinal distribution of ventilation and air content. The prone lungs were more uniformly inflated compared with the supine, which were less well expanded at the base than apex. Ventilation index, a measure of regional ventilation relative to whole lung ventilation, increased steeply from apex to base in the supine animals, whereas it was again more uniform in the prone condition. We conclude that the Xe-CT method provides a reasonable, quantitative measurement of regional ventilation and promises to be a valuable tool for the noninvasive determination of regional lung function.

Relationship between heterogeneous changes in airway morphometry and lung resistance and elastance

We present a dog lung model to predict the relation between inhomogeneous changes in airway morphometry and lung resistance (RL) and elastance (EL) for frequencies surrounding typical breathing rates. The RL and EL were sensitive in distinct ways to two forms of peripheral constriction. First, when there is a large and homogeneous constriction, the RL increases uniformly over the frequency range. The EL is rather unaffected below 1 Hz but then increases with frequencies up to 5 Hz. This increase is caused by central airway wall shunting. Second, the RL and EL are extremely sensitive to mild inhomogeneous constriction in which a few highly constricted or nearly closed airways occur randomly throughout the periphery. This results in extreme increases in the levels and frequency dependence of RL and EL but predominantly at typical breathing rates (<1 Hz). Conversely, the RL and EL are insensitive to highly inhomogeneous airway constriction that does not produce any nearly closed airways. Similarly, alterations in the RL and EL due to central airway wall shunting are not likely until the preponderance of the periphery constricts substantially. The RL and EL spectra are far more sensitive to these two forms of peripheral constriction than to constriction conditions known to occur in the central airways. On the basis of these simulations, we derived a set of qualitative criteria to infer airway constriction conditions from RL and EL spectra.

Effect of stochastic heterogeneity on lung impedance during acute bronchoconstriction: a model analysis

In a previous study (J. H. T. Bates, A. M. Lauzon, G. S. Dechman, G. N. Makaym, and T. F. Schuessler. J. Appl. Physiol. 76: 616-626, 1994), we investigated the acute changes in isovolume lung mechanics immediately after a bolus injection of histamine. We found that dynamic resistance and elastance increased progressively in the 80-s period after injection, whereas the estimated tissue hysteresivity reached a stable plateau after approximately 25 s. In the present study, we developed a computer model of the lung to investigate the mechanisms responsible for these observations. The model conforms to Horsfield's morphometry, with the addition of compliant airways and structural damping tissue units. Using this model, we simulated the time course of acute bronchoconstriction after intravenous administration of a bolus of bronchial agonist. Heterogeneity was induced by randomly varying the values of the maximal airway smooth muscle contraction and the tissue response to the agonist. Our results demonstrate that much of the increase in lung impedance observed in our previous study can be produced purely by the effects of airway heterogeneity. However, we were only able to reproduce the plateauing of hysteresivity by assigning a minimum radius to each airway, beyond which it would immediately snap completely shut. We propose that airway closure played an important role in our experimental observations.

Partitioning of lung tissue response and inhomogeneous airway constriction at the airway opening

During a bronchial challenge, much of the observed response of lung tissues is an artifactual consequence of inhomogeneous airway constriction. Inhomogeneities, in the sense of time constant inequalities, are an inherently linear phenomenon. Conversely, if lung tissues respond to a bronchoagonist, they become more nonlinear. On the basis of these distinct responses, we present an approach to separate real tissue changes from airway inhomogeneities. We developed a lung model that includes airway inhomogeneities in the form of a continuous distribution of airway resistances and nonlinear viscoelastic tissues. Because time domain data are dominated by nonlinearities, whereas frequency domain data are most sensitive to inhomogeneities, we apply a combined time-frequency domain identification scheme. This model was tested with simulated data from a morphometrically based airway model mimicking gross peripheral airway inhomogeneities and shown capable of recovering all tissue parameters to within 15% error. Application to our previously measured data suggests that in dogs during histamine infusion 1) the distribution of airway resistances increases widely and 2) lung tissues do respond but less so than previously reported. This approach, then, is unique in its ability to differentiate between airway and tissue responses to an agonist from a single broadband measurement made at the airway opening.

Regional ventilation in statically and dynamically hyperinflated dogs

Using the parenchymal marker technique in normal anesthetized dogs, we compared the dynamics of regional lung expansion between two ventilation strategies designed to increase mean thoracic volume. Dynamic hyperinflation (DH was produced by ventilating the lungs at a rate of 50 breaths/min and with a duty cycle of 0.5. Static hyperinflation (SH) was produced through the application of extrinsic positive end-expiratory pressure while the lungs were ventilated at a rate of 15 breaths/min and with a duty cycle of 0.15. Regional tidal volume (VT,r), regional functional residual volume, and the time delay between regional expansion and the flow signal at the common airway were computed for up to 100 regions/lobe in 5 animals. Ventilation strategy had no effect on the overall variance of VT,r within lobes. Although the VT,r measured during SH correlated with VT,r measured during DH, the average correlation coefficient was only 0.69. Ventilation rate-related differences in VT,r and regional functional residual capacity varied with the regional time delay in ways qualitatively consistent with parallel inhomogeneity of unit time constants. However, a large component of frequency-dependent behavior remains unexplained by established mechanisms. We conclude that DH and SH should not be considered equivalent lung unit recruitment strategies.