PLEURAL EFFUSIONS | Pleural Fluid, Transudate and Exudate

Abstract

Pleural effusion is a common clinical problem. In the normal state, the pleural cavity is bathed in a small volume of physiologic pleural fluid containing mainly macrophages and lymphocytes. The volume of the pleural fluid can increase dramatically with most pathologic conditions affecting the pleura. The pleural effusion will alter the respiratory mechanics, commonly resulting in dyspnea. It is useful to differentiate the pleural effusion into transudates and exudates. Traditionally, such differentiation is made using Light's criteria, based on the protein and lactate dehydrogenase levels in pleural fluid and serum. Transudates occur as a result of altered hydrostatic and/or oncotic pressures and are usually secondary to congestive cardiac failure or hepatic cirrhosis. Exudates develop as a result of plasma extravasation, which is at least in part due to pleural or pulmonary inflammation. Evidence suggests that cytokines, such as vascular endothelial growth factor, play a role in exudative effusion formation. Parapneumonic effusion, malignant effusion, and tuberculous pleuritis are the most common causes of exudative effusions worldwide.