In contrast, the σ d of the mediastinal pleura in the pig was reported to be between 0.02 and 0.05, indicating little restriction in the movement of large molecules ( 3). For example, the σ d of the canine visceral pleura combined with the endothelium has been reported to exceed 0.80 ( 3), indicating a marked restriction in the movement of large molecules such as albumin. Widely varying values for σ d have been reported. Where r is the liquid movement L p is the filtration coefficient/unit area or the hydraulic water conductivity of the membrane A is the surface area of the membrane P and π are the hydrostatic and oncotic pressures, respectively, of the capillary (cap) and pleural (pl) space and σ d is the solute reflection coefficient for protein, a measure of the membrane’s ability to restrict the passage of large molecules ( 3). The use of the micromanometer should circumvent some of the problems associated with esophageal balloons. It has been demonstrated that reliable measurements of esophageal pressures can be made with micromanometers ( 9). Moreover, the balloon must be short and must be placed in the lower part of the esophagus. The volume of air within the balloon must be small so that the balloon is not stretched and the esophageal walls are not displaced otherwise, pleural pressure estimates are falsely elevated. Estimation of pleural pressure by means of an esophageal balloon is not without difficulties ( 8). Situated between the two pleural spaces, esophageal pressure measurements provide a close approximation of the pleural pressure at the level of the balloon in the thorax ( 7, 8). Because the esophagus is a compliant structure Rather, the pleural pressure is measured indirectly by a balloon positioned in the esophagus ( 6, 7). Direct measurement of the pleural pressure is not usually made because of the danger of producing a pneumothorax or of introducing infection into the pleural space. Pleural pressure can be measured directly by inserting needles, trocars, catheters, or balloons into the pleural space. It should be noted, however, that there is still a school of researchers who believe in the presence of two different pressures ( 4, 5). It now appears that there is only one pressure, the pleural surface pressure, and that the discrepancies in the pressures arose because of the distortion from the catheters ( 3). This pressure was designated the pleural surface pressure and represented the balance between the outward pull of the thoracic cavity and the inward pull of the lung. If the pressure was measured using surface balloons or suction cups, then a gradient of 0.3 cm H 2O/cm vertical height was obtained. This pressure was designated the pleural liquid pressure and was believed to represent the pressure that influenced the absorption of fluid. If the pressure was measured using fluid-filled catheters, the vertical gradient obtained was approximately 1.0 cm H 2O/cm vertical height. The two different pressures had been proposed to explain a discrepancy obtained when the pleural pressure was measured in two different ways. There has been a controversy for many years as to whether there are two pleural pressures or one ( 2).
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