Pleural Effusion

Pleural effusion is excess fluid that accumulates in the pleural cavity, the fluid-filled space that surrounds the lungs. This excess can impair breathing by limiting the expansion of the lungs. Various kinds of pleural effusion, depending on the nature of the fluid and what caused its entry into the pleural space, are hydrothorax (serous fluid), hemothorax (blood), urinothorax (urine), chylothorax (chyle), or pyothorax (pus). Pneumothorax is the accumulation of air in the pleural space.

A pleural effusion will show up as an area of whiteness on a standard posteroanterior X-ray. Normally the space between the two layers of the lung, the visceral pleura and the parietal pleura, cannot be seen. A pleural effusion infiltrates the space between these layers. Because the pleural effusion has a density similar to body fluid or water, it can be seen on radiographs. Since the effusion has greater density than the rest of the lung, it will gravitate towards the lower portions of the pleural cavity. The pleural effusion behaves according to basic fluid dynamics, conforming to the shape of the lung and chest cavity. If the pleural cavity contains both air and fluid, then the fluid will have a "fluid level" that is horizontal instead of conforming to the lung space. Chest radiographs acquired in the lateral decubitus position (with the patient lying on his side) are more sensitive and can pick up as little as 50 ml of fluid. At least 300 ml of fluid must be present before upright chest films can pick up signs of pleural effusion.

A common cause of pleural effusions are bacterial pneumonia, cancer (with lung cancer, breast cancer, and lymphoma causing approximately 75% of all malignant pleural effusions), viral infection, and pulmonary embolism.

Pleural effusion is often experienced by pleural mesothelioma and pericardial mesothelioma patients as the disease progresses.