A Pleural Effusion is excess fluid AROUND the lung (within the pleural cavity). The pleural space is a cavity outside of the lungs. A tiny amount of lubricating fluid fills this space. The pleura’s main job is to lubricate the chest wall and the lungs.  After all, the chest is always in motion for breathing! Importantly, there’s only one exit from the pleural cavity, and that’s through the lymphatic system, but we’ll get to that more in a second. Imagine we have a pleural effusion. When fluid builds up, it restricts the ability of the lung to expand, causing dyspnea. That pool of fluid is sitting right underneath the rib cage, and we can use that fact to our advantage during our exam. Percussion of the effusion will produce a dull sound, unlike the drum-like resonance of healthy lungs. The pool of fluid will also obstruct the passage of sound waves, which makes it harder for us to hear the underlying lung sounds. So why would a pleural effusion occur? They don’t typically happen in healthy young adults, but rather in someone with another disease process in the chest (e.g., pneumonia, CHF, cancer) 

Exudative fluid is caused by inflammation. Inflammation increases nearby vascular permeability so as to recruit some white blood cells, but a lot of protein spills out too. The pleural fluid has more protein, more cells, more acid (↓pH)  and more LDH than transudative effusions. Levels of proteins, LDH and pH will approach the levels found in the blood. Exudative effusions are pretty sinister - they’re due to cancer, pneumonia, TB, trauma or burns. Transudative fluid is usually dark or bloody in color. They need to be drained ASAP. 

Transudative fluid is caused by high pressure inside the blood vessels. Most often because of CHF or cirrhosis. The increased pressure will push water out of the vessels and into the cavity, but other plasma contents like protein and LDH remain in the vessels. The transudative fluid is watery and clear (no cells). Light criteria is negative. 

Lymphatic fluid is an overflow from the lymphatic system. Lymphatic channels are sometimes damaged after surgery or radiation therapy - but the most common cause is cancer. The pleural fluid is milky and fatty (from Chylomicrons).  

The diagnosis is initially made with some imaging, usually a chest x-ray or perhaps an ultrasound or CT. A CXR will show blunting of the cardiophrenic and costophrenic angles. Look for a meniscus (cup-shaped curvature), because this indicates fluid. . But your job isn’t done after imaging -- now you have to identify the cause! Do that with a thoracentesis (needle into the chest wall) to draw out some fluid, and send that fluid to the lab. Thoracocentesis is both diagnostic and therapeutic! The lab will examine it for Lactate Dehydrogenase (LDH) and “Protein.” Using Light’s Criteria, you can determine what caused the effusion (although it's only 75% specific).  

Using Beer to Visualize Pleural Effusions

Most texts use the term “serosanguinous” to describe fluid that’s somewhat pinkish and somewhat yellowish. But due to the traumatic nature of thoracentesis (you are stabbing their chest with a big needle after all), there’s going to be a bit of blood in every tap. The term is applied broadly and lazily, so I find it difficult to understand a lot of the time. Credit to Taylor Warmouth for coming up with this scrumdiddly model for understanding the appearance of pleural effusion.