thoracentesis

Thoracentesis is the procedure of drawing fluid out of the pleural space — the thin gap between the lung and the chest wall — with a needle. Normally this space holds only a film of lubricating fluid, but in many diseases fluid builds up there (a pleural effusion), pressing on the lung and causing breathlessness. Thoracentesis lets a doctor tap that fluid the way you might drain water that has collected between a balloon and its surrounding bag.

It serves two purposes. Diagnostically, even a small sample is analysed to find out why the fluid is there — measuring protein and other markers to classify it (for example as a transudate from pressure imbalance or an exudate from inflammation, infection or cancer), and examining it for cells, microbes and blood. Therapeutically, draining a larger volume relieves the shortness of breath caused by a compressed lung.

The needle is placed between the ribs over the fluid, after numbing the skin and, increasingly, with ultrasound to map the safest spot and avoid the lung, diaphragm and vessels. It is generally safe; the main risks are pneumothorax, bleeding and, if a very large volume is removed too quickly, a rare lung re-expansion problem. Ultrasound guidance has substantially reduced complications.