In thoracentesis, fluid that has collected abnormally in the pleural space (termed a pleural effusion) is removed. The two principal reasons to do thoracentesis are to
During the procedure, the person sits comfortably and leans forward, resting the arms on supports. A small area of skin on the back is cleaned and numbed with a local anesthetic. Then a doctor inserts a needle between two ribs and into the chest cavity, but not into the lung, and withdraws some fluid into a syringe. Often the doctor uses ultrasonography for guidance (to determine where to insert the needle). The collected fluid is analyzed to assess its chemical makeup and to determine whether bacteria or cancerous cells are present.
If a large volume of fluid has accumulated, it may need to be removed through a plastic catheter and it may be necessary to use a fluid container that is larger than a syringe. The fluid may need to be drained over several days, in which case a larger tube (chest tube or drainage catheter) is left in the chest and suctioned continuously.
The risk of complications during and after thoracentesis is low. A person may feel some pain as the lung fills with air and expands against the chest wall or may feel the need to cough. Also, a person may briefly feel light-headed and short of breath. Other possible complications (listed roughly in order of frequency) include
Puncture of the lung with leakage of air into the pleural space (pneumothorax)
Bleeding into the pleural space or chest wall
Puncture of the spleen or liver
Accumulation of fluid within the lung itself (pulmonary edema), if a large amount of fluid that has been present for weeks to months is withdrawn rapidly
A chest x-ray may be done after the procedure to determine how much fluid may remain and whether complications have occurred.