Interrelationship of Pleural and Pulmonary Interstitial Liquid

When the lung interstitium fills with liquid, lung interstitial liquid may flow across the visceral pleura into the pleural space. This route of interstitial liquid flow does not apparently depend on the type of edema because it is used equally in both hydrostatic and increased permeability edema. The route of flow depends mostly on the quantity and location of extravascular lung water. In experimental studies of hydrostatic and increased permeability edema, pleural effusion develops when extravascular lung water has reached a certain level for a certain amount of time. The necessary level of edema appears to be greater than 5.0 g/g dry lung, whether in hydrostatic edema [5.2-6.2 g/g dry lung (14); 5.0-6.5 g/g dry lung (4)] or in increased permeability edema due either to ANTU [5.1-9.3 g/g dry lung (45)], oleic-acid [6.5-8.0 g/g dry lung (58)], or alveolar P. aeruginosa [5.4-6.7 g/g dry lung (J. Wiener-Kronish, unpublished observations)]. The amount of lung edema necessary for pleural effusion formation is similar to the amount of edema Bhattacharya and co-workers found to be associated with the steep rise in subpleural pressure (7). The time before appearance of pleural effusion appears to be approximately 2 hr after the development of lung edema, whether the effusion results from hydrostatic edema (14) or increased permeability edema due to ANTU (48) or oleic acid (58). This delay may be necessary for the interstitial liquid to accumulate and the interstitial pressure to increase in the subpleural tissues to the point that liquid flows across the visceral pleura at a rate greater than the rate at which the parietal lymphatics can clear it. Consequences of a pleural route of edema clearance are several. Pleural effusion that develops in the course of lung edema represents lung interstitial edema and can be sampled in lieu of sampling lung lymph. Indeed, pleural liquid may be a better sample of lung interstitial liquid than is alveolar edema liquid. The flow of lung interstitial liquid to the pleural space can protect against the development of alveolar edema by transferring excess liquid from the lung interstitium to the pleural space, where the effects on lung function are relatively minor. Obliteration of the pleural space may either lower the threshold for development of alveolar edema in the adjacent lungs, or slow the clearance of established edema; with chronic edema, other routes of clearance would likely adapt to compensate for the loss of the pleural clearance route.