PLEURAL DISEASES

In the United States, approximately one million patients each year develop a pleural effusion. Pleural effusions have classically been divided into transudative and exudative pleural effusions. A transudative pleural effusion occurs when the systemic factors influencing pleural fluid formation and reabsorption are altered so that pleural fluid accumulates; an exudative pleural effusion occurs when the local factors influencing pleural fluid formation and reabsorption are altered, allowing accumulation of pleural fluid. The leading causes of transudative pleural effusions are left ventricular failure and cirrhosis with ascites. The leading causes of exudative pleural effusions are pneumonia, malignancy, and pulmonary embolization. Transudative pleural effusions can be differentiated from exudative pleural effusions by measurement of the pleural fluid protein and lactic dehydrogenase (LDH) levels. The ratio of the pleural fluid protein to the serum protein is less than 0.5, the ratio of the pleural fluid LDH to the serum LDH is less than 0.6, and the absolute value of the pleural fluid LDH level is less than two thirds of the upper normal limit for serum with transudative pleural effusions while at least one of these criteria is not met with exudative effusions. Most patients who have pleural effusion with congestive heart failure have left ventricular failure. It is believed that the transudation of the pulmonary interstitial fluid across the visceral pleura overwhelms the capacity of the lymphatics to remove the fluid. Most patients with cirrhosis who have a pleural effusion also have ascites. It is also believed that the pleural effusions form when the fluid moves directly from the peritoneal cavity into the pleural cavity through pores in the diaphragm. Approximately 40% of patients with pneumonia will have a pleural effusion. If these patients have a significant amount of pleural fluid, a diagnostic thoracentesis should be performed. Chest tubes should be inserted if the pleural fluid is gross pus, id the Gram stain of the pleural fluid is positive, if the pleural fluid glucose level is below 40 mg/dl, or if the pleural fluid pH level is less than 7.00. If the drainage with the chest tubes is unsatisfactory, either streptokinase or urokinase should be injected intrapleurally. If the drainage is still unsatisfactory, a decortication should be considered. The three leading malignancies that have an association pleural effusion are breast carcinoma, lung carcinoma, lymphomas and leukemias. The diagnosis of pleural malignancy is made most commonly with pleural fluid cytology; in recent years immunohistochemical tests have proved invaluable in differentiating benign from malignant pleural effusions. If a patient has dyspnea from a malignant pleural effusion that is relieved with a therapeutic thoracentesis, the patient should be treated with either a chemical pleurodesis or a pleuroperitoneal shunt. There are many other causes of exudative pleural effusions besides pneumonia and malignancy. One diagnosis that should always be considered is pulmonary embolization. If the etiology of the effusion is not clear after the initial evaluation, including cytology and cultures of the pleural fluid, needle biopsy of the pleura, and perfusion scan of the lung, it is recommended that a nonaggressive approach be taken if the patient's symptoms are improving. No diagnosis is ever established in 20% of patients with exudative pleural effusions and it is believed that many of these are the result of a viral infection.