Thoracocentesis, also known as thoracentesis or pleural tap, is an invasive procedure to remove fluid or air from the pleural space for diagnostic or therapeutic purposes. A cannula, or hollow needle, is carefully introduced into the thorax, generally after administration of local anesthesia.
The recommended location varies, depending upon the source: midaxillary line, in the eighth, ninth, or 10th intercostal space. Whenever possible, the procedure should be performed under ultrasound guidance, which has shown to reduce complications.

This procedure is indicated when unexplained fluid accumulates in the chest cavity outside the lung. In more than 90% of cases, analysis of pleural fluid yields clinically useful information. If a large amount of fluid is present, this procedure can also be used therapeutically to remove fluid and improve patient comfort and lung function.
The most common causes of pleural effusions are cancer, congestive heart failure, pneumonia, and recent surgery. In countries where tuberculosis is common, this is also a common cause of pleural effusions.
When cardiopulmonary status is compromised due to air (significant pneumothorax), fluid (pleural fluid), or blood (hemothorax) outside the lung, this procedure is usually replaced with tube thoracostomy, the placement of a large tube in the pleural space.

Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on October 5, 2017.