The pleural fluid has a low protein concentration ( <2 grams per deciliter ) with a pH and glicose similar to that of blood. Pleural fluid is formed primarily from the parietal pleura, and part of its turnover depends on the same Starling forces that govern vascular and interstitial fluid exchange. The parietal pleura has a hydrostatic pressure similar to that of the systemic circulation (30 cm H2O ), whereas that of the visceral pleura depends on the pulmonary circulation (10 cm H2O ). Oncotic pressure is similar in both ( 25 cm H2O), but the pressure within the pleural cavity is affected by the gravity gradient. The stomas present over the parietal surface of the low mediastinum, low chest wall, and diaphragm, seem to empty into lymphatics. The subpleural lymphatics represent the major pathway for liquid and solute drainage. The lymphatics have the capacity to absorb 20 times more fluid than is normally formed. Pleural effusion may develop when there is excess pleural fluid formation or when there is decreased fluid removal.
History and Physical Examination
Although suggestive, a patient’s history of pain, dyspnea, or cough is neither sensitive nor specific. These symptoms may be absent in some large effusions and in critically ill patients. When present, pain is usually unilateral and sharp and worsens with inspiration and cough.It may irradiate to shoulder, neck, or abdomen. Dyspnea may result from compression of lung tissue, or mechanical alterations in the respiratory muscles as the fluid changes their length-tension relationship. The physical examination shows decreased breath sounds and excursions in the affected hemithorax. Percussion shows dullness with absent tactile fremitus over the area. Frequently there are E to A changes (egobronchophony) at the upper fluid border.
Radiologic Examination
An effusion is suspected when there is blunting and medial displacement of the sharp costophrenic angle. Fluid accumulation between the lung and the diaphragm (subpulmonic effusion ) is suspected when there is apparent elevation of the hemidiaphragm or widening of the shadow between the gas-containing stomach and the lower left lung margin. The costovertebral angle is usually the first place where minimal pleural effusions are detected. Up to 300 ml of fluid may fail to be seen in a posteroanterior chest roentgenogram, whereas as little as 150 ml may be seen in a lateral decubitus view. A supine film (frequent in patients in intensive care units) may obscure the diagnosis as the fluid displaces posteriorly.
Thoracentesis and Pleural Fluid Analysis
A pleural effusion can occur as a complication of many different diseases (Tables 1 and 2 ). An exudative pleural effusion results from disease of the pleural surface itself, while a transudative pleural effusion results from alterations in the systemic factors that influence the movement of fluid in and out of the pleural space.
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Thoracentesis may be performed for diagnosis or therapy. Although there is no absolute contraindications to a diagnostic thoracentesis, relative contraindications include a bleeding diathesis, anticoagulation, a small volume, mechanical ventilation, and low benefit-to-risk ratio.The criteria established by Light et al for segregating transudates from exudates have been widely accepted and rely on serum protein ratio greater than 0.5 , a pleural fluid to serum LDH ratio greater than 0.6 ,and a pleural LDH concentration more than two thirds normal upper limit for serum. If any one of these critical values is exceeded, the effusion is an exudate.
Table 3 illustrates the usefulness of different analytes in the diagnosis of exudative effusions. In a study performed in Spain, the following was taken as cutoff points for diagnosing exudate values commonly used in the literature: 10,000 red blood cells (RBC) per cubic millimeter ; 1,000 leucocytes (WBC ) per cubic millimeter ; pleural protein concentration (PRO) of 3 g/dL ;pleural LDH concentration (LDH ) of 300 IU/L ( two thirds of the upper normal limit for serum in their laboratory ) ; pleural to serum protein ratio of 0.5 (PRO r ) ; and pleural to serum LDH ratio ( LDH r ) of 0.6. As shown, the classic criteria of Light et al showed great accuracy ; the LDH r was the best single parameter ; PRO, LDH, and PRO r were similar ; and RBC and WBC counts showed very little diagnostic value.
| Parameters | Sensitivity(%) | Specificity(%) | Accuracy | |
|---|---|---|---|---|
| RBC > 10.000 | 32,2 | 89,7 | 43,8 | |
| WBC > 1.000 | 61,9 | 82,1 | 66,0 | |
| PRO > 3,0 | 84,0 | 82,1 | 83,6 | |
| LDH > 300 | 87,4 | 87,2 | 87,4 | |
| PROr >0,5 | 85,5 | 86,5 | 85,7 | |
| LDHr > 0,6 | 91,5 | 83,3 | 89,9 | |
| LDH or PROr | 97,4 | 81,1 | 94,2 | |
| PROr or LDHr | 97,3 | 80,0 | 94,0 | |
| LDH or LDHr | 95,3 | 83,3 | 93,0 | |
| Light et al criteria | 98,7 | 77,8 | 94,7 |
The diagnosis that can be established by thoracentesis include malignancy, empiema , tuberculosis , fungal infection, lupus pleuritis, chylothorax ,urinothorax , and esophageal rupture. Others findings in thoracentesis can only suggest a causative disease (Table 4 ) .The complications of thoracentesis include pain, bleeding ( local, pleural, or abdominal ), pneumothorax, infection, and spleen or liver puncture.
Percutaneous Pleural Biopsy
Biopsy is indicated to evaluate patients with undiagnosed exudative effusion (particulary those with lymphocitic predominance ) because the most frequently diagnosed disease is malignancy or tuberculosis.The contraindications are a small or loculated pleural effusion, an uncooperative patient, and anticoagulation or bleeding diathesis including azotemia with abnormal bleeding time.
Exploration of the Pleura
Biopsy can also be obtained through thoracoscopy (introducing a rigid scope with a cold light source ). Thoracoscopy may be performed under local anesthesia, but in some cases it is necessary to perform an open pleural biopsy under general anesthesia. The main advantage is the possibility of obtaining larges specimens and concomitant lung tissue.