Home > Encyclopedia of Nursing & Allied Health > Thoracentesis

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.

Laboratory evaluation

Pleural fluid is generally evaluated for gross appearance and volume, protein, specific gravity, glucose, lactate dehydrogenase, blood cell counts, pH, cytology, culture and Gram stain. Other tests may be requested such as lactate, amylase, flow cytometry, triglycerides, complement, other enzymes, bilirubin, and tumor markers.

Normal pleural fluid has a volume of 3-5 mL, but effusions of several hundred milliliters are not uncommon. The fluid should be clear and light yellow (strawcolored). Turbidity can be caused by a traumatic tap or by an abnormal condition. Bloody taps are associated with streaking of the fluid as it is collected, and a clear supernatant after centrifugation. Turbidity can result from infection, mucin, or fat in the fluid. It takes very little blood to turn the pleural fluid red. In addition to a traumatic tap, red tinged fluids are caused by trauma, malignancy, and pulmonary infarction. Turbid, yellow fluids are associated with infection. Turbid, green fluids are associated with rheumatoid arthritis, and milky-white fluids with lymph containing chyle. The specific gravity of the fluid should be equal to or less than plasma. Exudates are associated with a specific gravity of 1.015 or higher, but transudates sometimes overlap this cutoff.

Chemistry tests are performed on pleural fluid by the same methods used for plasma. The pleural fluid glucose should be the same as the plasma glucose. Low levels are significant. Pleural fluid glucose below 40 mg/dL are associated with infection, malignancy, and rheumatic disease (i.e., rheumatoid arthritis and systemic lupus erythematosus. LD is the single best test to differentiate transudates from exudates. Pleural fluid LD in excess of 200 U/L or a fluid to serum LD ratio of 0.6 or higher indicates an exudate. Lactate levels are increased in exudative fluids as well but cannot differentiate between the causes. Total protein in pleural fluid is increased when the fluid is exudative, but the interpretation is difficult whenever there is bleeding or a traumatic tap. A total protein of less than 3.0 g/dL is consistent with a transudate. Pleural fluid amylase is increased in both chronic and acute pancreatitis, in amylase producing

cancers that infiltrate the pleura, and in rupture of the esophagus. pH is below 7.45 in exudative fluids and is extremely low (7.0-7.3) in malignancy, bacterial infection, rupture of the esophagus, tuberculosis, and rheumatoid arthritis. A pH below 7.0 is seen only in empyema (bacterial infection with a white count greater than 10,000 per microliter), esophageal rupture, and rheumatoid arthritis. Triglycerides are increased (greater than 110 mg/dL) in chylous effusions.

The white blood cell (WBC) count of pleural fluid is performed manually. Transudates have a WBC count of less than 1,000 per microliter. Exudates have a WBC count of 10,000 per microliter or higher. WBC counts in excess of 50,000 per microliter signal infection of the pleura. A WBC differential is always performed on pleural fluid using a method to concentrate the cells. No single cell type should predominate. A predominance of lymphocytes (greater than 50%) occurs in lymphoid cancers (lymphoma), lymphocytic leukemias, and tuberculosis. Greater than 50% neutrophils occurs in acute infections, acute injuries (such as pulmonary infarction and rupture of the esophagus), malignancies, and granulocytic leukemia. Increased eosinophils are seen in pneumothorax, pulmonary infarction, congestive heart failure, parasitic infestation, and some infections. Red blood cell counts are also performed manually. Red counts in excess of 100,000 per microliter are associated with trauma, malignancy, and pulmonary infarctions.

A Gram stain and culture should be performed on the sediment of all pleural fluids. The Gram stain of sediment is positive in about 50% of persons with pleural infections. Cultures for tuberculosis are frequently requested because this disease is associated with approximately 8% of pleural fluid effusions. Cultures should be performed using blood agar plates, chocolate (heated blood) agar plates, and thioglycolate broth. Transudative fluids are usually negative for growth. The most common bacterial isolates are Staphylococcus aureus and gram negative bacilli.

Cytological analysis of pleural fluid is usually requested and should be performed on a concentrate of any fluid that is exudative. As with microbiological culture, the sensitivity of cytology is proportional to the volume of fluid concentrated. Metastatic carcinoma,

sarcomas, mesothelioma, Hodgkin's and nonHodgkin's lymphoma, and leukemias can cause cellular infiltration of the pleura and produce exudative effusions. Activated and phagocytic mesothelial cells are often seen in inflammatory pleural fluids, and are difficult to distinguish from malignant mesothelial cells. Cytology is performed on both Wright and Papanicolaou stains. Special cytochemical stains and flow cytometry are often used to differentiate reactive from malignant mesothelial cells and identify the type of other malignant cells present.

Aftercare

Vital signs are assessed every fifteen minutes until stable. A chest x ray is ordered to document changes in the appearance of the lung fields, and to look for possible pneumothorax. Examination of the chest with a stethoscope is also useful for documenting bilateral breath sounds that make pneumothorax very unlikely. The site of the needle puncture is covered with a simple dressing and monitored for bleeding or drainage.

Complications

With any procedure which breaks the skin, bleeding and infection are possibilities, although very unlikely if careful and sterile technique are followed. Pneumothorax is a very real complication, and may need to be treated with a chest tube. If very large effusions are drained quickly, pulmonary edema and low oxygen levels can occur, requiring oxygen and possibly other support measures for the patient. A chest x ray should be ordered right after the procedure. If the pH and glucose are very low (e.g. pH below 7.2), white blood cells are found to be greater than 25,000 per microliter, or there are other signs of frank infection, a chest tube may need to be placed.

Results

Representative normal values for pleural fluid are shown below:

• Volume: less than 10 mL.
• Appearance: clear, light yellow.
• Specific gravity: less than 1.015.
• Protein: less 3.0 g/dL.
• Lactate dehydrogenase: less than or equal to 200 U/L.
• Pleural fluid: serum LD ratio: less than 0.6.
• pH: 7.65 (transudates 7.4-7.5).
• Glucose: greater than 60 mg/dL (pleural fluid: serum ratio greater than 0.5).
• Triglycerides: 13-107 mg/dL.
• WBC count: less than 1000 per microliter.
• Neutrophils: less than 50%.
• Lymphocytes: less than 50%.
• Eosinophils: less than 10%.

Health care team roles

A physician performs the thoracentesis, and orders and interprets the results of the laboratory tests. Nursing staff will be very involved in documenting a patient's response to the procedure, and providing support and instruction for the patient during thoracentesis. Careful observation of respiratory status and pulse oximetry is important to aid in speedy intervention if necessary. Clinical laboratory scientists/medical technologists perform all of the laboratory tests done on the pleural fluid with the exception of cytological evaluation which is performed by a pathologist. Radiology technicians will perform x rays and other imaging studies before and after thoracentesis.

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KEY TERMS

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Pulse oximeter—A non-invasive device which uses a beam of light that passes through a digit or earlobe to assess oxygenation.

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Erika J. Norris