Thoracentesis Health Article

Definition

Thoracentesis is a procedure in which pleural fluid is removed from the space between the lung and the chest wall. The space in which this fluid collects is called the pleural space. It is formed in between the serous membrane covering each lung, called the visceral pleura, and the serous membrane covering the chest wall, called the parietal pleura. Normally very little fluid is present in the pleural space, and it serves to lubricate the two pleural surfaces, so they can easily slip across each other during respiration.

Purpose

Abnormal quantities of pleural fluid may accumulate in various conditions. Removal of pleural fluid for analysis is commonly performed in order to determine the cause of fluid accumulation. Sometimes the effusion is so large that it interferes with normal lung function. In such cases, thoracentesis may be performed to relieve the respiratory distress caused by lung compression.

An excess of pleural fluid is called an effusion. Laboratory analysis is directed at distinguishing between two types of effusion, transudates and exudates. Transudates are caused by hemodyndamic changes out-side the lungs that increase the movement of fluid from the capillaries in the parietal pleura into the pleural space. These include increased hydrostatic pressure (i.e., high blood pressure); decreased oncotic pressure (i.e., low plasma protein due to liver or renal disease); increased pleural capillary permeability; and lymphatic obstruction. Exudates are caused by injury, infection, inflammation, or malignancy. Exudates usually involve the lungs, but in some cases such as esophageal rupture or pancreatitis, they do not.

Precautions

Practitioners should be aware that many pleural fluids display some characteristics of both transudates and exudates. These conditions have many causes which may be present concurrently, making the distinction complicated. The physician performing thoracentesis must take great care to avoid puncturing the lung, which can cause air to enter the pleural space (pneumothorax) and result in lung collapse. A blood sample should be collected at the time of thoracentesis to provide a basis for comparison to certain pleural fluid results. When collecting pleural fluid or blood, the physician and other members of the health care team should observe universal precautions for the prevention of transmission of bloodborne pathogens. If pH is to be measured, the syringe containing the fluid must be capped, placed in an ice bath, and sent immediately to the laboratory.

Preparation

Written consent should be obtained before the procedure is begun. X ray of the chest is performed prior to the procedure. A special view of a pleural effusion, called a lateral decubitus film, may be ordered. In this view, the patient lies down on the side on which the effusion is known to exist. If the effusion is "free-flowing," gravity will cause it to spread up the lateral chest wall. If an effusion is not free-flowing, it may be more difficult to access for thoracentesis, and ultrasound or CT guidance may be helpful. A thorough history is performed to determine if any conditions such as a bleeding disorder are present that may complicate the procedure. The history should also document the medications that the patient is currently taking, and allergies to drugs or anesthetics. Prior to the procedure, a blood sample should be collected and a platelet count and prothrombin time should be performed. These tests determine whether there is an abnormally high risk of uncontrolled bleeding from the site that may contraindicate the procedure.

Description

Generally the effusion has been identified already on chest x ray, and may be noticeable by percussion of the chest wall. If there is any question about the location of the excess fluid, ultrasound or computed tomography (CT) may be used as a guide for the procedure. The patient should be seated upright, generally on the edge of a bed or chair, with arms propped up on a stable surface. The lateral chest wall is scrubbed with an antiseptic preparation, local anesthesia is administered, and a needle inserted between two ribs known to overlie the effusion. Generally the needle enters the chest below the armpit. Using a syringe, the appropriate amount of fluid is removed. The fluid should be collected in a heparinized syringe or transferred to a tube containing heparin or EDTA, and delivered to the lab for analysis. If the effusion is large, recurrent, or particularly concerning (e.g. very low pH and signs of infection), a chest tube may be placed and attached to a one-way system to promote continued drainage and prevent air from entering the pleural space. A pulse oximeter can be used to monitor the patient's oxygenation, and oxygen can be administered via a nasal cannula if needed. Generally oxygen therapy is not required, but if a pneumothorax occurs as a complication, or a large volume of pleural fluid is removed in a short period of time, lung function can be compromised.

Transudates form from diseases that occur outside the lungs. They are most frequently caused by congestive heart failure which accounts for up to 90% of all pleural effusions, pulmonary embolism (which sometimes causes exudates), cirrhosis of the liver, myxedema (hypothyroidism) or kidney disease. Exudates are generally due to infection, malignancy, trauma, pulmonary infarction, ruptured esophagus, pancreatitis systemic lupus erythmatosus, and rheumatoid arthritis.

Sometimes bloody fluid is found in the pleural space. This may be due to major trauma that has severed blood vessels in the chest. This is termed a hemothorax, and will produce a hematocrit that approximates that of blood. Malignancies involving the pleural fluid cause an increased red blood cell count but usually do not cause massive bleeding into the pleural space. Occasionally a thoracentesis sample may appear milky (chylothorax). This can be caused by a perforated or torn thoracic duct which carries lymph from the intestines to the heart. Chylothorax can also be caused by an aggressive cancer which blocks the flow of lymph. A similar appearance to the fluid can result from necrosis which causes formation of a pseudochylous effusion. Such fluids are characterized by foul odor, cholesterol, and high cellularity. Chylous effusions are odorless and have high triglycerides.

Malignancy is a common cause of pleural effusions and exudative fluids should always be examined for malignant cells. Approximately 35% of lung cancers, 25% of breast cancers, and 10% of lymphatic cancers shed cells into the pleural fluid.