Effect of surface forces on oscillatory behavior of lungs

Changes in respiratory elastance after deep inspirations reflect surface film functionality in mice with acute lung injury

Pulmonary surfactant reduces surface tension in the lung and prevents alveolar collapse. Following a deep inspiration (DI), respiratory elastance first drops then gradually increases due to surface film and tissue viscoelasticity. In acute lung injury (ALI), this increase is faster and governed by alveolar collapse due to increased surface tension. We hypothesized that the rate of increase in elastance reflects the deficiency of surfactant in the lung. To test this, mice were ventilated before (baseline) and after saline lavage obtained by injecting 0.8 ml and withdrawing 0.7 ml fluid (severe ALI) or injecting 0.1 ml (mild ALI). After two DIs, elastance was tracked for 10 min followed by a full lavage to assess surfactant proteins B (SP-B) and C (SP-C) content. Following 2 DIs, the increases in elastance during 10 min ventilation (ΔH) were 3.60 ± 0.61, 5.35 ± 1.04, and 8.33 ± 0.84 cmH2O/ml in baseline mice and mice with mild and severe ALI, respectively (P < 0.0001). SP-B and SP-C in the lavage fluid dropped by 32.4% and 24.9% in the mild and 50.4% and 39.6% in the severe ALI, respectively. Furthermore, ΔH showed a strong negative correlation with both SP-B (r(2) = 0.801) and SP-C (r(2) = 0.810) content. The ΔH was, however, much smaller when the lavage fluid also contained exogeneous SP-B and SP-C. Thus ΔH can be interpreted as an organ level measure of surface film functionality in lavage-induced ALI in mice. This method could prove useful in clinical situations such as diagnosing surfactant problems, monitoring recovery from lung injury or the effectiveness of surfactant therapy.

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.

Lung parenchymal mechanics

The lung parenchyma comprises a large number of thin-walled alveoli, forming an enormous surface area, which serves to maintain proper gas exchange. The alveoli are held open by the transpulmonary pressure, or prestress, which is balanced by tissues forces and alveolar surface film forces. Gas exchange efficiency is thus inextricably linked to three fundamental features of the lung: parenchymal architecture, prestress, and the mechanical properties of the parenchyma. The prestress is a key determinant of lung deformability that influences many phenomena including local ventilation, regional blood flow, tissue stiffness, smooth muscle contractility, and alveolar stability. The main pathway for stress transmission is through the extracellular matrix. Thus, the mechanical properties of the matrix play a key role both in lung function and biology. These mechanical properties in turn are determined by the constituents of the tissue, including elastin, collagen, and proteoglycans. In addition, the macroscopic mechanical properties are also influenced by the surface tension and, to some extent, the contractile state of the adherent cells. This chapter focuses on the biomechanical properties of the main constituents of the parenchyma in the presence of prestress and how these properties define normal function or change in disease. An integrated view of lung mechanics is presented and the utility of parenchymal mechanics at the bedside as well as its possible future role in lung physiology and medicine are discussed.

Oral tolerance attenuates airway inflammation and remodeling in a model of chronic pulmonary allergic inflammation

We investigated the effects of oral tolerance (OT) in controlling inflammatory response, hyperresponsiveness and airway remodeling in guinea pigs (GP) with chronic allergic inflammation. Animals received seven inhalations of ovalbumin (1-5mg/mL-OVA group) or normal saline (NS group). OT was induced by offering ad libitum ovalbumin 2% in sterile drinking water starting with the 1st ovalbumin inhalation (OT1 group) or after the 4th (OT2 group). The induction of OT in sensitized animals decreased the elastance of respiratory system (Ers) response after both antigen and methacholine challenges, peribronchial edema formation, eosinophilic airway infiltration, eosinophilopoiesis, and airways collagen and elastic fiber content compared to OVA group (P<0.05). The number of mononuclear cells and resistance of respiratory system (Rrs) responses after antigen and methacholine challenges were decreased only in OT2 group compared to OVA group (P<0.05). Concluding, our results show that inducing OT attenuates airway remodeling as well as eosinophilic inflammation and respiratory system mechanics.

Effects of amiodarone on lung tissue mechanics and parenchyma remodeling

We studied the results of chronic oral administration of amiodarone on in vitro lung tissue mechanics, light and electron microscopy. Fifteen Wistar male rats were divided into three groups. In control (CTRL) group animals received saline (0.5 mL/day). In amiodarone (AMIO) groups, amiodarone was administered by gavage at a dose of 175 mg/kg 5 days per week for 6 (6AMIO) or 12 weeks (12AMIO). Lung tissue strips were analyzed 24h after the last drug administration. Tissue resistance and elastance were higher in 6AMIO and 12AMIO than in CTRL, while hysteresivity was similar in all groups. Total amount of collagen fibers in lung parenchyma increased progressively with the time course of the lesion. However, at 6 weeks there was an increase in the amount of type III collagen fibers, while in 12AMIO mainly type I collagen fibers were found. In our study amiodarone increased lung tissue impedance that was accompanied by matrix remodeling and lesion of type II pneumocytes.

Effects of chronic L-NAME treatment lung tissue mechanics, eosinophilic and extracellular matrix responses induced by chronic pulmonary inflammation

The importance of lung tissue in asthma pathophysiology has been recently recognized. Although nitric oxide mediates smooth muscle tonus control in airways, its effects on lung tissue responsiveness have not been investigated previously. We hypothesized that chronic nitric oxide synthase (NOS) inhibition by N(omega)-nitro-L-arginine methyl ester (L-NAME) may modulate lung tissue mechanics and eosinophil and extracellular matrix remodeling in guinea pigs with chronic pulmonary inflammation. Animals were submitted to seven saline or ovalbumin exposures with increasing doses (1 approximately 5 mg/ml for 4 wk) and treated or not with L-NAME in drinking water. After the seventh inhalation (72 h), animals were anesthetized and exsanguinated, and oscillatory mechanics of lung tissue strips were performed in baseline condition and after ovalbumin challenge (0.1%). Using morphometry, we assessed the density of eosinophils, neuronal NOS (nNOS)- and inducible NOS (iNOS)-positive distal lung cells, smooth muscle cells, as well as collagen and elastic fibers in lung tissue. Ovalbumin-exposed animals had an increase in baseline and maximal tissue resistance and elastance, eosinophil density, nNOS- and iNOS-positive cells, the amount of collagen and elastic fibers, and isoprostane-8-PGF(2alpha) expression in the alveolar septa compared with controls (P<0.05). L-NAME treatment in ovalbumin-exposed animals attenuated lung tissue mechanical responses (P<0.01), nNOS- and iNOS-positive cells, elastic fiber content (P<0.001), and isoprostane-8-PGF(2alpha) in the alveolar septa (P<0.001). However, this treatment did not affect the total number of eosinophils and collagen deposition. These data suggest that NO contributes to distal lung parenchyma constriction and to elastic fiber deposition in this model. One possibility may be related to the effects of NO activating the oxidative stress pathway.

On the preparation of lung strip for tissue mechanics measurement

It is widely believed that it is fundamental to degas and/or rinse the lung prior to the measurement of the tissue mechanics, so that the undesirable effects of surfactant and localized gas trapping are eliminated. However, one could hypothesize that these mechanisms are bound to disappear in the in vitro preparation since the small tissue sample remains suspended oscillating in an organ bath. To investigate the real necessity to follow these procedures, dynamic mechanical properties were studied in strips of lungs previously rinsed with saline, degassed by ventilation with 100% O(2), or without any of these prior procedures. Resistance, elastance, hysteresivity, and the amounts of airway, blood vessel, and alveolar wall were computed. There was no difference in either tissue mechanics or morphology among the groups. In conclusion, the time-consuming degassing and rinsing steps are not necessary to adequately prepare lung tissue for in vitro mechanical analysis, and eliminating these steps potentially helps preserving the intact microstructure of the tissue.

Viscoelastic behavior of a lung alveolar duct model

A study is conducted into the oscillatory behavior of a finite element model of an alveolar duct. Its load-bearing components consist of a network of elastin and collagen fibers and surface tension acting over the air-liquid interfaces. The tissue is simulated using a visco-elastic model involving nonlinear quasi-static stress-strain behavior combined with a reduced relaxation function. The surface tension force is simulated with a time- and area-dependent model of surfactant behavior. The model was used to simulate lung parenchyma under three surface tension cases: air-filled, liquid-filled, and lavaged with 3-dimenthyl siloxane, which has a constant surface tension of 16 dyn/cm. The dynamic elastance (Edyn) and tissue resistance (Rti) were computed for sinusoidal tidal volume oscillations over a range of frequencies from 0.16-2.0 Hz. A comparison of the variation of Edyn and Rti with frequency between the model and published experimental data showed good qualitative agreement. Little difference was found in the model between Rti for the air-filled and lavaged models; in contrast, published data revealed a significantly higher value of Rti in the lavaged lung. The absence of a significant increase in Rti for the lavaged model can be attributed to only minor changes in the individual fiber bundle resistances with changes in their configuration. The surface tension was found to make an important contribution to both Edyn and Rti in the air-filled duct model. It was also found to amplify any existing tissue dissipative properties, despite exhibiting none itself over the small tidal volume cycles examined.

A comparative study of elastic properties of rat and guinea pig parenchymal strips

Constricted guinea pig (GP) airways are much less sensitive to changes in transpulmonary pressure (Ptp) than are those of the rat. The object of this study was to investigate whether differences in the mechanical behavior of the lung parenchyma could explain differences between the two species in the interdependence of the airway and parenchyma. Subpleural lung strips from guinea pigs and rats were excised and suspended in an organ bath. One end of each strip was attached to a force transducer and the other to a servo-controlled lever arm that effected length (L) changes in the strip. Sinusoidal oscillations at varying frequencies and amplitudes were applied at different resting tensions. Measurements of L and resting tension (T) were recorded during baseline conditions and after acetylcholine (ACh) challenge. Elastance (E) and resistance (R) were calculated by fitting changes in T and L to the equation of motion. During sinusoidal oscillations, E and R in the two species were different in both the unconstricted and constricted states. The effect of T on E was significantly different in rats and GPs; E was less dependent on T in GPs. Insofar as E is a measure of the load against which airway smooth muscle (ASM) contracts, this difference may represent a potential mechanism to explain why constricted GP airways are less sensitive to changes in Ptp.

Respiratory impedances and acinar gas transfer in a canine model for emphysema

We examined how the changes in the acini caused by emphysema affected gas transfer out of the acinus (Taci) and lung and chest wall mechanical properties. Measurements were taken from five dogs before and 3 mo after induction of severe bilateral emphysema by exposure to papain aerosol (170-350 mg/dose) for 4 consecutive wk. With the dogs anesthetized, paralyzed, and mechanically ventilated at 0.2 Hz and 20 ml/kg, we measured Taci by the rate of washout of 133Xe from an area of the lung with occluded blood flow. Measurements were repeated at positive end-expiratory pressures (PEEP) of 10, 5, 15, 0, and 20 cmH2O. We also measured dynamic elastances and resistances of the lungs (EL and RL, respectively) and chest wall at the different PEEP and during sinusoidal forcing in the normal range of breathing frequency and tidal volume. After final measurements, tissue sections from five randomly selected areas of the lung each showed indications of emphysema. Taci during emphysema was similar to that in control dogs. EL decreased by approximately 50% during emphysema (P < 0.05) but did not change its dependence on frequency or tidal volume. RL did not change (P > 0.05) at the lowest frequency studied (0.2 Hz), but in some dogs it increased compared with control at the higher frequencies. Chest wall properties were not changed by emphysema (P > 0.05). We suggest that although large changes in acinar structure and EL occur during uncomplicated bilateral emphysema, secondary complications must be present to cause several of the characteristic dysfunctions seen in patients with emphysema.