A cellular model of lung elasticity

The mechanics of the lung parenchyma is studied using models comprised of line members interconnected to form 3-D cellular structures. The mechanical properties are represented as elastic constants of a continuum. These are determined by perturbing each individual cell from a reference state by an increment in stress which is superimposed upon the uniform stretching forces initially present in the members due to the transpulmonary pressure. A force balance on the distorted structure, together with a force-deformation law for the members, leads to a calculation of the strain increments of the members. Predictions based on the analysis of the 3-D isotropic dodecahedron are in good agreement with experimental values for the Young's, shear, and bulk moduli reported in the literature. The model provides an explanation for the dependence of the elastic moduli on transpulmonary pressure, the geometrical details of the structure, and the stress-strain law of the tissue.     

Effect of long-term anemia and retransfusion on central circulation during exercise

The bulk modulus and the shear modulus describe the capacity of material to resist a change in volume and a change of shape, respectively. The values of these elastic coefficients for air-filled lung parenchyma suggest that there is a qualitative difference between the mechanisms by which the parenchyma resists expansion and shear deformation; the bulk modulus changes roughly exponentially with the transpulmonary pressure, whereas the shear modulus is nearly a constant fraction of the transpulmonary pressure for a wide range of volumes. The bulk modulus is approximately 6.5 times as large as the shear modulus. In recent microstructural modeling of lung parenchyma, these mechanisms have been pictured as being similar to the mechanisms by which an open cell liquid foam resists deformations. In this paper, we report values for the bulk moduli and the shear moduli of normal air-filled rabbit lungs and of air-filled lungs in which alveolar surface tension is maintained constant at 16 dyn/cm. Elevating surface tension above normal physiological values causes the bulk modulus to decrease and the shear modulus to increase. Furthermore, the bulk modulus is found to be sensitive to a dependence of surface tension on surface area, but the shear modulus is not. These results agree qualitatively with the predictions of the model, but there are quantitative differences between the data and the model.     

Effect of vascular volume and edema on wave propagation in canine lungs

The velocities of longitudinal and transverse stress waves transmitted through inflated lung parenchyma depend on the lung stiffness, as defined by the bulk and shear moduli, and the lung density. We examined the relationship between stress wave velocities and lung density. A saline-filled reservoir was connected to the vessels of caudal dog lobes held inflated at 5 cmH2O transpulmonary pressure, and vascular volume and extravascular lung water were increased in steps by increasing vascular pressure. At each step, we measured the transmitted signals at locations 2 and 7 cm from an impulse surface distortion by means of microphones embedded in the lung surface. Longitudinal and transverse wave velocities were computed by using cross-correlation analysis of microphone signal pairs. Both wave velocities decreased as lung density increased: as a first approximation, wave velocities were inversely proportional to the square root of lung density. This behavior is consistent with the propagation of small-amplitude stress waves through an elastic continuum. Estimated bulk and shear moduli were 26 and 3.5 cmH2O, respectively, and were consistent with results from quasi-static deformation tests.     

Analysis of elastic and surface tension effects in the lung alveolus using finite element methods

Displacement method finite element theory is used to examine the structural and elastic properties of the constituent network of elastin and collagen of the alveoli that form the mammalian lung. The role of the surface tension of pulmonary surfactant of the lung is also examined using an area-dependent relationship inferred from experimental studies. The pressure-volume (PV) curves of the resulting model are found to compare favourably with measured pressure-volume curves for whole lungs filled with air (surface tension included) and saline (no surface tension effects).     

Alveolar surface forces and lung architecture

The entire alveolar surface is lined by a thin fluid continuum. As a consequence, surface forces at the air-liquid interface are operative, which in part are transmitted to the delicate lung tissue. Morphologic and morphometric analyses of lungs show that the alveolar surface forces exert a moulding effect on alveolar tissue elements. In particular, in lungs at low degrees of inflation, equivalent to the volume range of normal breathing, there is a derecruitment of alveolar surface area with increasing surface tensions which reflects equilibrium configurations of peripheral air spaces where the sum of tissue energy and surface energy is minimum. Thus, changes in surface tension alter the recoil pressure of the lung directly and indirectly by deforming lung tissue and hence changing tissue tensions. However, the interplay between tissue and surface forces is rather complex, and there is a marked volume dependence of the shaping influence of surface forces. With increasing lung volumes the tissue forces transmitted by the fiber scaffold of the lung become the predominant factor of alveolar micromechanics: at lung volumes of 80% total lung capacity or more, the alveolar surface area-volume relation is largely independent of surface tension. Most important, within the range of normal breathing, the surface tension, its variations and the associated variations in surface area are small. The moulding power of surface forces also affects the configuration of capillaries, and hence the microcirculation, of free cellular elements such as the alveolar macrophages beneath the surface lining layer, and of the surfaces of the peripheral airways. Still enigmatic is the coupling mechanism between the fluid continua of alveoli and airways which might also be of importance for alveolar clearance. As to the surface active lining layer of peripheral air spaces, which determines alveolar surface tension, its structure and structure-function relationship are still ill-defined owing to persisting problems of film preservation and fixation. Electron micrographs of alveolar tissue, of lining layers of captive bubbles, and scanning force micrographs of surfactant films transferred on mica plates reveal a complex structural pattern which precludes so far the formulation of an unequivocal hypothesis.     

A model of non-uniform lung parenchyma distortion

A finite element model of mammalian lung parenchyma is used to study the effect of large non-uniform distortions on lung elastic behaviour. The non-uniform distortion is a uni-axial stretch from an initial state of uniform pressure expansion. For small distortions, the parenchymal properties are linearly isotropic and described by two elastic moduli. However, for large distortions, the parenchyma has anisotropic non-linear elastic properties described by five independent elastic moduli dependent on the degree of distortion; they are computed for a range of distortions and initial pressures. Ez, the Young's modulus in the direction of stretch, increases significantly with distortion (epsilon(z)) while Ex, the Young's modulus in the plane perpendicular to the stretch, is approximately constant. The greater the initial pressure, the bigger the difference between the two moduli at larger distortion strains. The shear modulus G(xz) is approximately independent of degree of distortion except at the highest initial pressure. The Poisson's ratio, nu(xz) is approximately constant with distortion strain for lower initial pressures, but increases significantly with epsilon(z) at higher pressures. Model predictions of the relation between G(xz) and initial uniform inflation pressure show a good correlation with reported experimental data for small distortion strains in a range of species. The model also exhibits similar behaviour to the experimentally measured uni-axial large deformations of a tri-axially pre-loaded block of parenchyma (Hoppin et al., 1975, Journal of Applied Physiology 39, 742-751).     

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.     

Elastic moduli of lungs during postnatal development in the piglet

Several manifestations of lung disease during infancy suggest that mechanical interdependence can be relatively high in newborn lungs. To test this possibility, we measured elastic moduli and pleural membrane tension in lungs excised from piglets ranging in age from less than 12 h to 85 days. Near maximum inflation, newborn lungs (less than 12 h, n = 6) had no detectable pleural membrane tension, although 3- to 5-day-old lungs (n = 6) had tension greater than 5,000 dyn/cm. In contrast, parenchymal recoil was greater in the newborn lungs [19.3 +/- 3.0 (SD) vs. 14.3 +/- 2.4 cmH2O at 90% of maximum inflation volume, P less than 0.01]. Shear moduli were higher (13.5 +/- 4.6 vs. 9.2 +/- 1.5 cmH2O at 15 cmH2O transpulmonary pressure, P less than 0.05) and Poisson ratios were lower in the newborn lungs as compared with the 3- to 5-day-old lungs. Postnatal lung growth between 3 and 85 days was characterized by 1) a constant shear modulus (0.6 times transpulmonary pressure); 2) decrease in the bulk modulus (from 6.8 to 5.1 times transpulmonary pressure, P less than 0.005); and 3) evidence of gas trapping at progressively higher transpulmonary pressures. Therefore, growth of parenchyma in the piglet lung is associated with reduced stiffness to volume change but with no effect on overall stiffness to shape change. Nevertheless, a relatively great stiffness to shape change occurs transiently in newborn piglet lungs.     

Structure of the dodecahedral penton particle from human adenovirus type 3

The sub-viral dodecahedral particle of human adenovirus type 3, composed of the viral penton base and fiber proteins, shares an important characteristic of the entire virus: it can attach to cells and penetrate them. Structure determination of the fiberless dodecahedron by cryo-electron microscopy to 9 Angstroms resolution reveals tightly bound pentamer subunits, with only minimal interfaces between penton bases stabilizing the fragile dodecahedron. The internal cavity of the dodecahedron is approximately 80 Angstroms in diameter, and the interior surface is accessible to solvent through perforations of approximately 20 Angstroms diameter between the pentamer towers. We observe weak density beneath pentamers that we attribute to a penton base peptide including residues 38-48. The intact amino-terminal domain appears to interfere with pentamer-pentamer interactions and its absence by mutation or proteolysis is essential for dodecamer assembly. Differences between the 9 Angstroms dodecahedron structure and the adenovirus serotype 2 (Ad2) crystallographic model correlate closely with differences in sequence. The 3D structure of the dodecahedron including fibers at 16 Angstroms resolution reveals extra density on the top of the penton base that can be attributed to the fiber N terminus. The fiber itself exhibits striations that correlate with features of the atomic structure of the partial Ad2 fiber and that represent a repeat motif present in the amino acid sequence. These new observations offer important insights into particle assembly and stability, as well as the practicality of using the dodecahedron in targeted drug delivery. The structural work provides a sound basis for manipulating the properties of this particle and thereby enhancing its value for such therapeutic use.     

Surface forces in lungs. II. Microstructural mechanics and lung stability

Recent lung microstructural models describing interactions between alveolar surface tension (gamma) and forces in structural elements of the alveolar duct predict that the component of lung recoil pressure due to gamma (P gamma) is proportional to gamma/V1/3, where V is the total lung volume. This relation is tested against experimental data obtained from pressure-volume measurements of excised rabbit lungs with different constant values of gamma. It is found that for values of gamma less than approximately 18 dyn/cm the data generally agree with the model predictions. With higher values of gamma, a mismatch between the data and predictions first occurs at low and high volumes and then spreads over the entire volume range. The mismatch at the lower volumes coincides with the appearance of nonuniformities of lung expansion. The nonuniformities are characterized by a coexistence of under- and overexpanded regions of the parenchyma referred to as a mixture of phases. These nonuniformities, as well as a pressure-volume curve with a shape similar to the shape of measured curves, are predicted from an analysis of lung stability. Results of this work indicate that if the lung expands uniformly, P gamma proportional to gamma/V1/3 is a good approximation over a wide range of volumes. The stability analysis indicates that the equilibrium configurations of the lung parenchyma when gamma is independent of interfacial area and elevated above normal values are nonuniform states of expansion, characterizable as a mixture of phases. This result confirms that a dependence of gamma on surface area is normally required to achieve stable, uniform states of lung expansion.     

A concentration-dependent mechanism by which serum albumin inactivates replacement lung surfactants.

Endogenous lung surfactant, and lung surfactant replacements used to treat respiratory distress syndrome, can be inactivated during lung edema, most likely by serum proteins. Serum albumin shows a concentration-dependent surface pressure that can exceed the respreading pressure of collapsed monolayers in vitro. Under these conditions, the collapsed surfactant monolayer can not respread to cover the interface, leading to higher minimum surface tensions and alterations in isotherms and morphology. This is an unusual example of a blocked phase transition (collapsed to monolayer form) inhibiting bioactivity. The concentration-dependent surface activity of other common surfactant inhibitors including fibrinogen and lysolipids correlates well with their effectiveness as inhibitors. These results show that respreading pressure may be as important as the minimum surface tension in the design of replacement surfactants for respiratory distress syndrome.     

Simulation of muscle and adipose tissue deformation in the passive human pharynx

Quantifying the contribution of passive mechanical deformation in the human pharynx to upper airway collapse is fundamental to understanding the competing biomechanical processes that maintain airway patency. This study uses finite element analysis to examine deformation in the passive human pharynx using an intricate 3D anatomical model based on computed tomography scan images. Linear elastic properties are assigned to bone, cartilage, ligament, tendon, and membrane structures based on a survey of values reported in the literature. Velopharyngeal and oropharyngeal cross-sectional area versus airway pressure slopes are determined as functions of Young's moduli of muscle and adipose tissue. In vivo pharyngeal mechanics for small deformations near atmospheric pressure are matched by altering Young's moduli of muscle and adipose tissue. The results indicate that Young's moduli ranging from 0.33 to 14 kPa for muscle and adipose tissue matched the in vivo range of area versus pressure slopes. The developed anatomical model and determined Young's moduli range are expected to be useful as a starting point for more complex simulations of human upper airway collapse and obstructive sleep apnea therapy.     

离体人肺弹性增量模量的实验研究

本文就离体人肺进行实验研究和理论分析,最后求出肺在不同的膨胀程度下的弹性模量值。从压力一容积曲线的增量回滞曲线求出膨胀模量K,利用K值与凹痕实验获得的荷载-变形关系相结合,获得其它弹性模量。计算结果表明:膨胀模量K约为膨胀压力的3倍,杨氏模量E约为膨胀压力的4倍,剪切模量约为膨胀压力的1.5倍,泊松系数值约在0.3左右。最后与其它文献中所列的猫、兔、狗的各种弹性模量值进行了比较。     

Shape and tension distribution of the active canine diaphragm

Both diaphragm shape and tension contribute to transdiaphragmatic pressure, but of the three variables, tension is most difficult to measure. We measured transdiaphragmatic pressure and the global shape of the in vivo canine diaphragm and used principles of mechanics to compute the tension distribution. Our hypotheses were that 1) tension in the active diaphragm is nonuniform with greater tension in the central tendon than in the muscular regions; 2) maximum tension is essentially oriented in the muscle fiber direction, whereas minimum tension is orthogonal to the fiber direction; and 3) during submaximal activation change in the in vivo global shape is small. Metallic markers, each 2 mm in length, were implanted surgically on the peritoneal surface of the diaphragm at 1.5- to 2.0-cm intervals along the muscle bundles at the midline, ventral, middle, and dorsal regions of the left costal diaphragm and along a muscle bundle of the crural diaphragm. Postsurgery, a biplane videofluoroscopic system was used to determine the in vivo three-dimensional coordinates of the markers at end expiration and end inspiration during quiet breathing as well as at end-inspiratory efforts against an occluded airway at lung volumes of functional residual capacity and at one-third maximum inspiratory capacity increments in volume to total lung capacity. A surface was fit to the marker locations using a two-dimensional spline algorithm. Diaphragm surface was modeled as a pressurized membrane, and tension distribution in the active diaphragm was computed using the ANSYS finite element program. We showed that the peak of the diaphragm dome was closer to the ventral surface than to the dorsal surface and that there was a depression or valley in the crural region. In the supine position, during inspiratory efforts, the caudal displacement of the dorsal region of the diaphragm was greater than that of the dome, and the valley along the crural diaphragm was accentuated. In contrast, at lower lung volumes in the prone posture, the caudal displacement of the dome was greater than that of the crural region. At end of inspiration, transdiaphragmatic pressure was approximately 6.5 cmH2O, and tensions were nonuniform in the diaphragm. Maximum principal stress sigma(1) of central tendon was found to be greater than sigma(1) of the costal region, and that was greater than sigma(1) of the crural region, with values of 14-34, 14-29, and 4-14 g/cm, respectively. The corresponding data of the minimum principal stress sigma(2) were 9-18, 3-9, and 0-1.5 g/cm, respectively. Maximum principal tension was approximately parallel to the muscle fibers, whereas minimum tension was essentially orthogonal to the longitudinal direction of the muscle fibers. In the muscular region, sigma(1) was approximately 3-fold sigma(2), whereas in the central tendon, sigma(1) was only approximately 1.5-fold sigma(2.).     

Protein transduction into human cells by adenovirus dodecahedron using WW domains as universal adaptors

BACKGROUND: Direct protein transduction is a recent technique that involves use of peptide vectors. In this study, we demonstrate that adenovirus dodecahedron (Dd), a virus-like particle devoid of DNA and able to penetrate cells with high efficiency, can be used as a vector for protein delivery. METHODS: Taking advantage of Dd interaction with structural domains called WW, we have elaborated a universal adaptor to attach a protein of interest to this vector. RESULTS: A tandem of three WW structural domains derived from the Nedd4 protein enables the formation of stable complexes with Dd, without impairing its endocytosis efficiency. Our protein of interest fused to the triple WW linker is delivered by the dodecahedron in 100% of cells in culture with on average more than ten million molecules per cell. CONCLUSION: These data demonstrate the great potential of adenovirus dodecahedron in combination with WW domains as a protein transduction vector.     

Elastic properties of air- and liquid-filled lung parenchyma

Pressure-volume measurements and the punch indentation test are used to obtain the bulk modulus (kappa) and the shear modulus (mu) of lung parenchyma of air- and liquid-filled rabbit lungs. Plots of kappa and mu vs. transpulmonary pressure obtained from these measurements indicate that there is very little difference between the elastic behavior of the air- and liquid-filled lung, suggesting that the mechanism of resisting deformation in both cases is similar. On the other hand, from plots of kappa and mu vs. lung volume, it appears that the elastic moduli are higher in the air-filled lung than in the liquid-filled lung at the same volume. These differences, referred to as kappa gamma and mu gamma, as well as the difference in transpulmonary pressures (P gamma), are presumably due to the additional elastic recoil of the air-filled lung provided by alveolar surface tension (gamma). No conclusion could be reached about the shape of the kappa gamma vs. P gamma curve. However, the mu gamma vs. P gamma relationship appears to be approximately linear, with a slope of approximately 0.5. This result agrees qualitatively with the model (T. A. Wilson and H. Bachofen, J. Appl. Physiol. 52: 1064-1070, 1982) in which the part of the parenchyma that provides P gamma is pictured as mechanically analogous to an open cell liquid foam, having mu gamma = 0.4P gamma (J. Appl. Mech. Trans. ASME 51: 229-231, 1984), but it is statistically significant only at high lung volumes.     

Constrained sessile drop as a new configuration to measure low surface tension in lung surfactant systems.

Existing methodology for surface tension measurements based on drop shapes suffers from the shortcoming that it is not capable to function at very low surface tension if the liquid dispersion is opaque, such as therapeutic lung surfactants at clinically relevant concentrations. The novel configuration proposed here removes the two big restrictions, i.e., the film leakage problem that is encountered with such methods as the pulsating bubble surfactometer as well as the pendant drop arrangement, and the problem of the opaqueness of the liquid, as in the original captive bubble arrangement. A sharp knife edge is the key design feature in the constrained sessile drop that avoids film leakage at low surface tension. The use of the constrained sessile drop configuration in conjunction with axisymmetric drop shape analysis to measure surface tension allows complete automation of the setup. Dynamic studies with lung surfactant can be performed readily by changing the volume of a sessile drop, and thus the surface area, by means of a motor-driven syringe. To illustrate the validity of using this configuration, experiments were performed using an exogenous lung surfactant preparation, bovine lipid extract surfactant (BLES) at 5.0 mg/ml. A comparison of results obtained for BLES at low concentration between the constrained sessile drop and captive bubble arrangement shows excellent agreement between the two approaches. When the surface area of the BLES film (0.5 mg/ml) was compressed by about the same amount in both systems, the minimum surface tensions attained were identical within the 95% confidence limits.     

Stress failure in pulmonary capillaries

In the mammalian lung, alveolar gas and blood are separated by an extremely thin membrane, despite the fact that mechanical failure could be catastrophic for gas exchange. We raised the pulmonary capillary pressure in anesthetized rabbits until stress failure occurred. At capillary transmural pressures greater than or equal to 40 mmHg, disruption of the capillary endothelium and alveolar epithelium was seen in some locations. The three principal forces acting on the capillary wall were analyzed. 1) Circumferential wall tension caused by the transmural pressure. This is approximately 25 dyn/cm (25 mN/m) at failure where the radius of curvature of the capillary is 5 microns. This tension is small, being comparable with the tension in the alveolar wall associated with lung elastic recoil. 2) Surface tension of the alveolar lining layer. This contributes support to the capillaries that bulge into the alveolar spaces at these high pressures. When protein leakage into the alveolar spaces occurs because of stress failure, the increase in surface tension caused by surfactant inhibition could be a powerful force preventing further failure. 3) Tension of the tissue elements in the alveolar wall associated with lung inflation. This may be negligible at normal lung volumes but considerable at high volumes. Whereas circumferential wall tension is low, capillary wall stress at failure is very high at approximately 8 x 10(5) dyn/cm2 (8 x 10(4) N/m2) where the thickness is only 0.3 microns. This is approximately the same as the wall stress of the normal aorta, which is predominantly composed of collagen and elastin. The strength of the thin part of the capillary wall is probably attributable to the collagen IV of the basement membranes. The safety factor is apparently small when the capillary pressure is raised during heavy exercise. Stress failure causes increased permeability with protein leakage, or frank hemorrhage, and probably has a role in several types of lung disease.     

Equilibrium mechanics of monolayered epithelium

In order to fully understand the epithelial mechanics it is essential to integrate different levels of epithelial organization. In this work, we propose a theoretical approach for connecting the macroscopic mechanical properties of a monolayered epithelium to the mechanical properties at the cellular level. The analysis is based on the established mechanical models—at the macroscopic scale the epithelium is described within the mechanics of thin layers, while the cellular level is modeled in terms of the cellular surface (cortical) tension and the intercellular adhesion. The macroscopic elastic energy of the epithelium is linked to the energy of an average epithelial cell. The epithelial equilibrium state is determined by energy minimization and the macroscopic elastic moduli are calculated from deformations around the equilibrium. The results indicate that the epithelial equilibrium state is defined by the ratio between the adhesion strength and the cellular surface tension. The lower and the upper bounds for this ratio are estimated. If the ratio is small, the epithelium is cuboidal, if it is large, the epithelium becomes columnar. Importantly, it is found that the cellular cortical tension and the intercellular adhesion alone cannot produce the flattened squamous epithelium. Any difference in the surface tension between the apical and basal cellular sides bends the epithelium towards the side with the larger surface tension. Interestingly, the analysis shows that the epithelial area expansivity modulus and the shear modulus depend only on the cellular surface tension and not on the intercellular adhesion. The results are presented in a general analytical form, and are thus applicable to a variety of monolayered epithelia, without relying on the specifics of numerical finite-element methods. In addition, by using the standard theoretical tools for multi-laminar systems, the results can be applied to epithelia consisting of layers with different mechanical properties.     

Optical monitoring of bubble size and shape in a pulsating bubble surfactometer.

The pulsating bubble surfactometer (PBS) is often used for in vitro characterization of exogenous lung surfactant replacements and lung surfactant components. However, the commercially available PBS is not able to dynamically track bubble size and shape. The PBS therefore does not account for bubble growth or elliptical bubble shape that frequently occur during device use. More importantly, the oscillatory volume changes of the pulsating bubble are different than those assumed by the software of the commercial unit. This leads to errors in both surface area and surface tension measurements. We have modified a commercial PBS through the addition of an image-acquisition system, allowing real-time determination of bubble size and shape and hence the accurate tracking of surface area and surface tension. Compression-expansion loops obtained with the commercially available PBS software were compared with those provided by the image-analysis system for dipalmitoylphosphatidylcholine, Infasurf, and Tanaka lipids (dipalmitoylphosphatidylcholine-palmitoyloleoylphosphatidyl-glycerol-palmitic acid, 68:22:9) at concentrations of 0.1 and 1.0 mg/ml and at frequencies of 1 and 20 cycles/min. Whereas minimum surface tension as determined by the image-analysis system is similar to that measured by the commercially available software, the maximum surface tension and the shapes of the interfacial area-surface tension loops are quite different. Differences are attributable to bubble drift, nonsinusoidal volume changes, and variable volume excursions seen with the modified system but neglected by the original system. Image analysis reveals that the extent of loop hysteresis is greatly overestimated by the commercial device and that an apparent, rapid increase in surface tension upon film expansion seen in PBS loops is not observed with the image-analysis system. The modified PBS system reveals new dynamic characteristics of lung surfactant preparations that have not previously been reported.