Thoracentesis is a procedure where fluid or air is removed from the space between the lung and the thoracic wall either for therapeutic purposes or diagnostic purposes. The space or sac is lined with a membrane called the pleural, and this space can fill with fluid for various reasons or sometimes air. If this happens the lung will not expand with inspiration and the fluid will also compress the lung tissue as it accumulates. The causes for this fluid include: cancer, tuberculosis, congestive heart failure, ammonia, trauma, and emphysema. Thoracentesis is accomplished by inserting a large bore (needle) into the pleural space, usually under local anesthesia and aspirating the fluid, blood or air. This fluid is sent off for further evaluation to rule out cancer (infection, cultures, stains, cell counts).