Probing softness of the parietal pleural surface at the micron scale

The pleural surfaces of the chest wall and lung slide against each other, lubricated by pleural fluid. During sliding motion of soft tissues, shear induced hydrodynamic pressure deforms the surfaces, promoting uniformity of the fluid layer thickness, thereby reducing friction. To assess pleural deformability at length scales comparable to pleural fluid thickness, we measured the modulus of the parietal pleura of rat chest wall using atomic force microscopy (AFM) to indent the pleural surface with spheres (radius 2.5 and 5 μm). The pleura exhibited two distinct indentation responses depending on location, reflecting either homogeneous or significantly heterogeneous tissue properties. We found an elastic modulus of 0.38-0.95 kPa, lower than the values measured using flat-ended cylinders >100 μm radii (Gouldstone et al., 2003, Journal of Applied Physiology 95, 2345-2349). Interestingly, the pleura exhibited a three-fold higher modulus when probed using 2.5 vs. 5 μm spherical tips at the same normalized depth, confirming depth dependent inhomogeneous elastic properties. The observed softness of the pleura supports the hypothesis that unevenness of the pleural surface on this scale is smoothed by local hydrodynamic pressure.     

Lubrication regimes in mesothelial sliding

To function normally, the lungs, heart, and other organs must undergo changes in shape and size, sliding against surrounding body walls. It is not known whether the delicate mesothelial surfaces covering these organs and body wall are in contact during sliding, or if hydrodynamic pressure in the lubricating liquid increases separation between their surfaces. To address this question, we measured the coefficient of friction (mu) of the mesothelial surface of nine rat-abdominal walls sliding in saline on a smooth glass surface. Sliding at physiological velocities of 0.0123-6.14 cm/s with normal stresses of 50-200 Pa, mu varied with velocity (P<0.001). On average, mu was relatively high at low speeds (0.078 at 0.041 cm/s), decreased to a minimum at intermediate speeds (0.034 at 1.23 cm/s), and increased slightly again at higher speeds (0.045 at 6.14 cm/s), consistent with a mixed lubrication regime in which there is at least partial hydrodynamic separation of surfaces. We conclude that mesothelial surfaces, sliding under physiological conditions, are protected from excessive shear by hydrodynamic pressures that increase separation of surfaces.     

Pleural space thickness in situ by light microscopy in five mammalian species

The thickness of the pleural space was measured by a focusing method using a light microscope (X157, 2.5-micron depth of focus). In anesthetized animals, thin transparent parietal pleural windows were made by dissection of intercostal muscle. Multiple postmortem measurements were made of the combined thickness of the pleural space and the window by focusing in sequence on the lung surface and on 1- to 2-micron tantulum particles sprayed on the window. The window thickness was measured after creating a pneumothorax and retracting the lungs. In supine rabbits the pleural space measured at various heights on the costal surface was of uniform thickness (16 micron) except for a thicker region (62 micron) located within 3 mm of the most dependent part of the lung. The thicker region reverted to the uniform thickness after it was placed in a nondependent position by inverting the animal from the supine to prone position, indicating fluid drainage by gravity. In the prone position near midchest, pleural space thickness (t) averaged 6.9 micron in the mouse, 10.2 in the rat, 17.2 in the rabbit, 18.3 in the cat, and 23.6 in the dog. Animals of larger body mass (M, kg) had a wider pleural space: t = 13.1 X M0.20. There was no contact between the two pleurae, indicating that fluid lubrication facilitates sliding between the lung and chest wall. Based on the t vs. M relationship and estimates of the viscous flow of pleural liquid, pleural fluid exchange rate would be proportional to body mass and the work of sliding as a fraction of the work of breathing would be smaller in larger animals.     

Stiffness of the pleural surface of the chest wall is similar to that of the lung.

To address the role of the parietal pleura in reduction of mesothelial shear stresses during breathing, we measured the stiffness of the parietal pleural surface of mammalian chest walls using microindentation. The pleural surface was indented over ribs and intercostal spaces with rigid flat punches (tip radii of 0.01, 0.02, and 0.1 cm) to probe stiffness at length scales comparable with those of surface asperities. We found a tissue shear modulus of 6700 dyn/cm2 and pleural membrane tension of 4900 dyn/cm, with a geometric standard deviation of 0.42. These values are similar to those measured for the lung by Hajji et al., using indentation (Hajji MA, Wilson TA, and Lai-Fook SJ. J Appl Physiol Respirat Environ Exerc Physiol 47: 175-181, 1979). Surprisingly, the pleural surface over ribs and intercostal spaces exhibited similar stiffness. In addition, caudal regions exhibited lower stiffness than cranial regions. In the context of elastohydrodynamic lubrication, these results suggest that shear-induced pressures during breathing deform the chest wall and lung surfaces to a similar extent, promoting spatial uniformity of pleural fluid thickness and reducing shear stresses.     

Ultrasonic diagnosis of pleurisy

Ultrasound investigation of changes in the pleural cavity was conducted in 146 patients with infectious and infectious-allergic pleuritides, beginning with the first clinical manifestations to the outcome of the disease. The following stages were noted: thickening of the pleural layer in a limited area, sometimes of the endothoracic fascia and connective tissue layer between it and the parietal pleura; the appearance of fibrinous effusion; fluid accumulation; encapsulation and multilocular pleurisy; exudate resorption, fibrinolysis; fibrin involvement by connective tissue and formation of adhesions. Besides, an ultrasound picture of the normal chest wall and pleural cavity is presented.     

Intrapleural fluid movements described by a porous flow model.

We injected technetium-labeled albumin (at a concentration similar to that of the pleural fluid) in the costal region of anesthetized dogs (n = 13) either breathing spontaneously or apneic. The decay rate of labeled activity at the injection site was studied with a gamma camera placed either in the anteroposterior (AP) or laterolateral (LL) projection. In breathing animals (respiratory frequency approximately 10 cycles/min), 10 min after the injection the activity decreased by approximately 50% on AP and approximately 20% on LL imaging; in apneic animals the corresponding decrease in activity was reduced to approximately 15 and approximately 3%, respectively. We considered label translocation from AP and LL imaging as a result of bulk flows of liquid along the costomediastinal and gravity-dependent direction, respectively. We related intrapleural flows to the hydraulic pressure gradients existing along these two directions and to the geometry of the pleural space. The pleural space was considered as a porous medium partially occupied by the mesh of microvilli protruding from mesothelial cells. Solution of the Kozeny-Carman equation for the observed flow velocities and pressure gradients yielded a mean hydraulic radius of the pathways followed by the liquid ranging from 2 to 4 microns. The hydraulic resistivity of the pleural space was estimated at approximately 8.5 x 10(5) dyn.s.cm-4, five orders of magnitude lower than that of interstitial tissue.     

No evidence for mesothelial cell contact across the costal pleural space of sheep

Pleural space width was measured by four morphological approaches using either frozen hydrated or freeze-substituted blocks of chest wall and lung. Anesthetized sheep were held in the lateral (n = 2), sternal recumbent (n = 2), or vertical (head-up; n = 2) position for 30 min. The ribs and intercostal muscles were excised along a 20-cm vertical distance of the chest wall region, which was sprayed with liquid Freon 22, cooled with liquid nitrogen, to facilitate the fastest possible freezing of the visceral and parietal pleura. We measured pleural space width in frozen hydrated blocks by reflected-light and low-temperature scanning electron microscopy and in freeze-substituted, fixed, and embedded tissue blocks by light and transmission electron microscopy. We combined the data from the two groups of sheep held sternally recumbent and vertical because the results were comparable. The average arithmetic mean data for pleural space width determined by reflected-light analysis for samples near the top (18.5 microns) and bottom (20.3 microns) of the chest, separated by 15 cm of lung height, varied inversely with lung height (n = 4; P less than 0.009). The average harmonic mean data demonstrated a similar gravity-dependent gradient (17.3 and 18.8 microns, respectively; P less than 0.02). Therefore a slight vertical gradient of approximately -0.10 micron/cm of lung height was found for costal pleural space width. Pleural space width in the most dependent recesses, such as the costodiaphragmatic recess, reached 1-2 mm. We never found any contacts between the visceral and parietal pleura with either of the frozen hydrated preparations. No points of mesothelial cell contact were revealed in the light- and transmission electron microscopic views of the freeze-substituted tissue, despite an apparent narrower pleural space associated with the tissue-processing steps. We conclude that the pleural space has a slightly nonuniform width, contacts if they occur must be very infrequent, and pleural liquid clearance is probably facilitated by liquid accumulation in dependent regions where lymphatic pathways exist.     

Chest wall distortion during resistive inspiratory loading

We studied six (1 naive and 5 experienced) subjects breathing with added inspiratory resistive loads while we recorded chest wall motion (anteroposterior rib cage, anteroposterior abdomen, and lateral rib cage) and tidal volumes. In the five experienced subjects, transdiaphragmatic and pleural pressures, and electromyographs of the sternocleidomastoid and abdominal muscles were also measured. Subjects inspired against the resistor spontaneously and then with specific instructions to reach a target pleural or transdiaphragmatic pressure or to maximize selected electromyographic activities. Depending on the instructions, a wide variety of patterns of inspiratory motion resulted. Although the forces leading to a more elliptical or circular configuration of the chest wall can be identified, it is difficult to analyze or predict the configurational results based on insertional and pressure-related contributions of a few individual respiratory muscles. Although overall chest wall respiratory motion cannot be readily inferred from the electromyographic and pressure data we recorded, it is clear that responses to loading can vary substantially within and between individuals. Undoubtedly, the underlying mechanism for the distortional changes with loading are complex and perhaps many are behavioral rather than automatic and/or compensatory.     

Regional ventilation during spontaneous breathing and mechanical ventilation in dogs

We evaluated the effects of the different patterns of chest wall deformation that occur with different body positions and modes of breathing on regional lung deformation and ventilation. Using the parenchymal marker technique, we determined regional lung behavior during mechanical ventilation and spontaneous breathing in five anesthetized recumbent dogs. Regional lung behavior was related to the patterns of diaphragm motion estimated from X-ray projection images obtained at functional residual capacity (FRC) and end inspiration. Our results indicate that 1) in the prone and supine positions, FRC was larger during mechanical ventilation than during spontaneous breathing; 2) there were significant differences in the patterns of diaphragm motion and regional ventilation between mechanical ventilation and spontaneous breathing in both body positions; 3) in the supine position only, there was a vertical gradient in lung volume at FRC; 4) in both positions and for both modes of breathing, regional ventilation was nonlinearly related to changes in lobar and overall lung volumes; and 5) different patterns of diaphragm motion caused different sliding motions and differential rotations of upper and lower lobes. Our results are inconsistent with the classic model of regional ventilation, and we conclude that the distribution of ventilation is determined by a complex interaction of lung and chest wall shapes and by the motion of the lobes relative to each other, all of which help to minimize distortion of the lung parenchyma.     

Pleural pressure distribution and its relationship to lung volume and interstitial pressure

The mechanics of the pleural space has long been controversial. We summarize recent research pertaining to pleural mechanics within the following conceptual framework, which is still not universally accepted. Pleural pressure, the force acting to inflate the lung within the thorax, is generated by the opposing elastic recoils of the lung and chest wall and the forces generated by respiratory muscles. The spatial variation of pleural pressure is a result of complex force interactions among the lung and other structures that make up the thorax. Gravity contributes one of the forces that act on these structures, and regional lung expansion and pleural pressure distribution change with changes in body orientation. Forces are transmitted directly between the chest wall and the lung through a very thin but continuous pleural liquid space. The pressure in pleural liquid equals the pressure acting to expand the lung. Pleural liquid is not in hydrostatic equilibrium, and viscous flow of pleural liquid is driven by the combined effect of the gravitational force acting on the liquid and the pressure distribution imposed by the surrounding structures. The dynamics of pleural liquid are considered an integral part of a continual microvascular filtration into the pleural space. Similar concepts apply to the pulmonary interstitium. Regional differences in lung volume expansion also result in regional differences in interstitial pressure within the lung parenchyma and thus affect regional lung fluid filtration.     

Electronmicroscopic observations on the visceral and parietal rat's pleura after contralateral pneumonectomy

The mechanism of compensatory growth and healing of the pleura remains unresolved. Contralateral visceral and parietal (diaphragmatic and costal) pleura were investigated by transmission electron microscopy, following an experimental pneumonectomy (EP). Fifteen young-adult Wistar rats were divided into three groups and with survival times of 1, 5 and 8 days respectively after EP. Three sham-operated (thoracic cavity opened and closed) and three unoperated rats served as controls. One day following EP the superficial mesothelial cells have more microvilli and microvesicles, but a lower number of specialized contacts. Multiplication of extravasal cells leads to an increase of the thickness of the layer over the basal lamina and of the submesothelial layer. Five days after EP the superficial cells show a stratified arrangement in longer sectors of both pleural sheets. Along with typical mesothelial cells there are three new populations of cells: (1) with an abundant granular endoplasmic reticulum and secretory granules, (2) with fibroblast-like characteristics and (3) with a more extensive lysosomal system. The submesothelial layer is thickened due to newly formed blood vessels and collagen bundles. Eight days after EP the mesothelial cells build multi-row arrangement sectors and surround intercellular dilatations covered with microvilli. 'Activated' high mesothelial cells characterize the monolayer sectors. The submesothelial layer remains thicker due to larger collagen bundles and elastic fibers. The changes in the mesothelium and in the connective tissue layer suggest the existence of two periods. The first one is characterized by different mesothelial cell populations, new vasculogenesis and starting of fibrillogenesis. In the second period there are 'activated' mesothelial cells, pleural villi, groups of lymphatic lacunae and significant fibrillogenesis.     

Determinants of friction in soft elastohydrodynamic lubrication

Elastohydrodynamic lubrication (EHL) protects soft tissues from damage and wear in many biological systems (e.g. synovial joints, cornea of the eye, and pleural surfaces of the lung and chest wall). Among studies of lubrication of deformable solids, few have examined the effects of external loads, geometry, and material properties on EHL of soft tissues. To examine these effects, we studied the tribology of soft tissues in a two-dimensional finite element simulation of a thin layer of fluid separating a sliding rigid surface from a soft asperity or bump with an initial sinusoidal shape. We computed the frictional force, deformation of the solid, and change in fluid thickness as functions of independent variables: sliding velocity, normal load, material properties, and bump amplitude and length. Double-logarithmic regression was used to determine the exponents of the scaling relationships of friction coefficient and minimum fluid thickness to the independent variables. The analysis showed that frictional shear force is strongly dependent on velocity, viscosity, and load, moderately dependent on bump length and elasticity, and only weakly dependent on the bump amplitude. The minimum fluid thickness is strongly dependent on velocity and viscosity, and changes moderately with load, elasticity, amplitude, and length. The shape of the bump has little effect. The results confirm that the shear-induced deformation of an initially symmetrical shape, including generalizations to other symmetrical geometries such as quadratic or piecewise linear bumps, leads to load-supporting behavior.     

Measurement of alveolar pressure in closed-chest dogs during flow interruption

We have developed a technique for installing alveolar capsules in dogs with intact chest wall, by exposing a region of parietal pleura between a pair of ribs and gluing the parietal and visceral pleura together around a small region of lung. This allows the direct measurement of alveolar pressure during spontaneous breathing. We measured alveolar pressure in normal dogs using this technique while suddenly interrupting flow at the trachea during passive expiration. Tracheal pressure exhibited a very rapid rise immediately on interruption that we showed to be composed of two distinct and roughly equal parts: one was the resistive pressure drop across the airways, and the other was a resistive pressure drop across tissues. By simultaneously measuring pleural pressure we showed that the tissues responsible were only in the chest wall and not in the lungs.     

Hydraulic conductivity of canine parietal pleura in vivo

The hydraulic conductivity (Lp) of the parietal pleura was measured in vivo in spontaneously breathing anesthetized dogs in either the supine (n = 8) or the prone (n = 7) position and in an excised portion of the chest wall in which the pleura and its adjacent tissue were intact (n = 3). A capsule was glued to the exposed parietal pleura after the intercostal muscles were removed. The capsule was filled with either autologous plasma or isotonic saline. Transpleural fluid flow (V) was measured at several transpleural hydrostatic pressures (delta P) from the rate of meniscus movement within a graduated pipette connected to the capsule. Delta P was defined as the measured difference between capsule and pleural liquid pressures. The Lp of the parietal pleura was calculated from the slope of the line relating V to delta P by use of linear regression analysis. Lp in vivo averaged 1.36 X 10(-3) +/- 0.45 X 10(-3) (SD) ml.h-1.cmH2O-1.cm-2, regardless of whether the capsule was filled with plasma or saline and irrespective of body position. This value was not significantly different from that measured in the excised chest wall preparation (1.43 X 10(-3) +/- 1.1 X 10(-3) ml.h-1.cmH2O-1.cm-2). The parietal pleura offers little resistance to transpleural protein movement, because there was no observed difference between plasma and saline. We conclude that because the Lp for intact parietal pleura and extrapleural interstitium is approximately 100 times smaller than that previously measured in isolated stripped pleural preparations, removal of parietal pleural results in a damaged preparation.     

A model of the respiratory pump

The interaction of forces that produce chest wall motion and lung volume change is complex and incompletely understood. To aid understanding we have developed a simple model that allows prediction of the effect on chest wall motion of changes in applied forces. The model is a lever system on which the forces generated actively by the respiratory muscles and passively by impedances of rib cage, lungs, abdomen, and diaphragm act at fixed sites. A change in forces results in translational and/or rotational motion of the lever; motion represents volume change. The distribution and magnitude of passive relative to active forces determine the locus and degree of rotation and therefore the effect of an applied force on motion of the chest wall, allowing the interaction of diaphragm, rib cage, and abdomen to be modeled. Analysis of moments allow equations to be derived that express the effect on chest wall motion of the active component in terms of the passive components. These equations may be used to test the model by comparing predicted with empirical behavior. The model is simple, appears valid for a variety of respiratory maneuvers, is useful in interpreting relative motion of rib cage and abdomen and may be useful in quantifying the effective forces acting on the rib cage.     

Cytology of pleural effusion associated with disseminated infection caused by varicella-zoster virus in an immunocompromised patient. A case report

BACKGROUND: Pleural effusion caused by varicella-zoster virus (VZV) is rare. We report a case of a woman with acute lymphocytic leukemia (ALL) who developed a pleural effusion caused by VZV infection. CASE: A 55-year-old woman with ALL treated with consolidation therapy developed skin vesicles and a pleural effusion. Pleural fluid smears contained numerous mesothelial cells, which had ground-glass nuclei or eosinophilic nuclear inclusions. Some multinucleated giant cells were also seen. Electron microscopic examination revealed intranuclear virus particles, about 150 nm in diameter, in some mesothelial cells. Tissue samples from the skin, lungs, pleura, liver, pancreas, kidneys and gastrointestinal tract, obtained at autopsy, contained many virus-infected cells. They were positive for VZV glyco-protein 1 by immunohistochemistry. CONCLUSION: VZV infection should be considered in the differential diagnosis of an unexplained exudative pleural effusion, especially in immunocompromised hosts.     

Clearance of lung edema into the pleural space of volume-loaded anesthetized sheep

To determine whether lung edema leaks into the pleural space, we measured flow rates of visceral pleural liquid from exposed sheep lungs during volume loading and then compared the protein concentration of visceral pleural liquid and lung interstitial liquids (lymph and peribronchovascular cuff liquid). For 4 h, we volume loaded 24 anesthetized ventilated sheep with one side, both sides, or neither side of the chest open. During the experiment, we collected visceral pleural liquid from a bag surrounding the exposed lung and lung lymph; after the experiment, we collected peribronchovascular cuff liquid. We found that during volume loading visceral pleural liquid flow increased significantly by 2 h, and its protein concentration over the final hour was the same as that of lung interstitial liquids. The volume of visceral pleural liquid correlated with excess lung water and wedge pressure elevation. By our estimates, clearance of edema from the lung into the pleural space constituted 23-29% of all edema liquid collected, similar to measured lymph edema clearance. We conclude that edema liquid leaks directly from edematous sheep lungs into the pleural space and that this leakage provides an important additional route of edema clearance.     

Compliances of the liquid-filled lungs and chest wall during development in fetal sheep.

Our aim was to measure the compliance of the liquid-filled lungs (CL), and the compliance of the chest wall (CW) in fetal sheep in utero. CL and CW were measured in 6 fetuses. The compliance of the lungs and chest wall combined (respiratory system, Crs) was measured in 9 fetuses. Pressure differences across the lungs (PL), chest wall (PW) and respiratory system (Prs) were measured while the lungs were deflated and inflated with liquid from their resting lung liquid volume (V1). V1 was measured using an indicator dilution technique. Specific compliance values were obtained by normalizing the values of CL, CW and Crs with respect to values of V1. From values obtained during stepwise inflation from V1, specific compliances (ml/cm H2O/ml of lung liquid) were: lungs, 0.22 +/- 0.02; chest wall, 0.41 +/- 0.07; respiratory system, 0.13 +/- 0.01. Specific compliances of the lungs, chest wall and respiratory system did not change significantly with advancing gestational age from 120 to 143 days. Our baseline data will be valuable in assessing the in utero progress of the structural development of the lungs following manipulations known to cause altered lung growth.     

Mycobacterium kansasii infection diagnosed by pleural fluid cytology: a case report

BACKGROUND: Identification of disseminated nontuberculous Mycobacterium infection is a challenge, especially when it occurs in patients without a known cause of immunosuppression. Acid-fast organisms in the pleural fluid are rare and easily missed, especially when they occur in patients without a clinical suspicion of infection. The classical cytologic picture of tuberculous pleural fluid with lymphocytosis and paucity of mesothelial cells is not seen. CASE: A 57-year-old man presented with chronic neutrophilia of unknown etiology together with chest pain and bilateral pleural effusions. Pleural fluid cytology revealed organisms seen in the cytoplasm of numerous macrophages and neutrophils, creating a "negative image" on Diff-Quik smears. Acid-fast stains demonstrated intracellular acid-fast bacilli consistent with mycobacteria. Microbiologic studies with DNA probe technology resulted in identification of the mycobacterial organism as Mycobacterium kansasii. CONCLUSION: Nontuberculous Mycobacterium should be included in the differential diagnosis in patients with inflammatory, exudative pleural effusions.