In thoracentesis, fluid that has collected abnormally in the pleural space (termed a pleural effusion Pleural Effusion 
) is removed. The two principal reasons to do thoracentesis are to
Obtain a fluid sample for diagnostic testing
Relieve shortness of breath caused by fluid compressing lung tissue
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 2 ribs and into the chest cavity, but not into the lung, and withdraws some fluid into a syringe. Often the doctor uses ultrasonography Ultrasonography of the Chest Chest imaging studies include X-rays Computed tomography (CT) CT angiography Magnetic resonance imaging (MRI) read more 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 Chest Tube Insertion Chest tube insertion (also called tube thoracostomy) is a procedure in which a tube is inserted into the space between the lung and chest wall (called the pleural space). The procedure is done... read more 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 Pneumothorax A pneumothorax is partial or complete collapse of the lung due to the presence of air between the two layers of pleura (thin, transparent, two-layered membrane that covers the lungs and also... read more
)Bleeding into the pleural space or chest wall
Fainting
Infection
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fluid within the lung itself (pulmonary edema), if a large amount of fluid that has been present for weeks to months is withdrawn rapidly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 Chest Imaging Chest imaging studies include X-rays Computed tomography (CT) CT angiography Magnetic resonance imaging (MRI) read more may be done after the procedure to determine how much fluid may remain and whether complications have occurred.
(See also Medical History and Physical Examination for Lung Disorders Medical History and Physical Examination for Lung Disorders A doctor first asks the person about symptoms. Chest tightness or pain, shortness of breath ( dyspnea) either at rest or during exertion, cough, coughing up of sputum or blood (hemoptysis),... read more .)
