Fluid accumulates in the pleural space when the local hydrostatic forces that are pushing fluid out of the vascular space exceed the oncotic forces that are drawing fluid back into thevascular space. Pleural effusions can be transudates (intactmembrane but abnormal hydrostatic or oncotic forces)or exudates (decreased integrity of the membrane due toinflammatory processes or impaired lymphatic drainage).There are relatively few causes of transudates, the primaryones being congestive heart failure and hypoproteinemiastates, whereas the causes of exudates are legion. Almost anypulmonary inflammatory process can result in pleural fluidaccumulation. Among the most common causes of exudatesare infection (tuberculosis, bacterial pneumonia), collagen vascular diseases (systemic lupus erythematosus), andmalignancy. Chylous pleural effusions (elevated triglyceridelevels) are seen with thoracic duct injury and abnormalitiesof lymphatic drainage (lymphangiomyomatosis, lymphangiectasia). Bacterial pneumonia can lead to an accumulation ofpleural fluid (parapneumonic effusion). When this fluid is purulent or infected, then it is called an empyema, although often the terms parapneumonic effusion and empyema areused interchangeably. Parapneumonic effusion/empyemais the most common effusion in children. Most parapneumonic effusions are due to pneumonia caused by Streptococcus pneumoniae, group A streptococci, or Staphylococcus aureus.
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Small pleural effusions may be asymptomatic, but if they are large enough to compress lung tissue, then they can cause dyspnea, tachypnea, and occasionally chest pain. Effusions due to infection are usually associated with fever, malaise, poor appetite, pleuritic chest pain, and splinting. Physical findings include tachypnea, decreased breath sounds, dullness to percussion, and decreased tactile fremitus. Large effusions may occupy more than half the hemithorax and cause a mediastinal shift away from the affected side, compromising respiratory status, particularly in young children.
The presence of pleural fluid can often be confirmed by chest radiograph. In addition to anteroposterior and lateral projections, a decubitus view should be done to assess for layering of fluid. Chest ultrasonography is useful for confirming the presence of the effusion and quantifying its size. CT scans of the chest can help differentiate pleural fluid from parenchymal lesions and pleural masses. Both ultrasonography and CT scans can determine whether parapneumonic effusions contain loculations (fibrous strands that compartmentalize the effusion).
The analysis of pleural fluid is useful in differentiating a transudate from an exudate. Routine tests on pleural fluid such as cell count, pH, protein, lactose dehydrogenase, and glucose characterize the effusions. Triglyceride levels, cytology, hematocrit, gram and acid fast bacilli stain, culture, and adenosine deaminase levels can be useful in diagnosing
Pleural fluid protein/serum protein ratio greater than 0.5 OR Pleural fluid LDH/serum LDH ratio greater than 0.6 OR Pleural fluid LDH greater than two-thirds the upper limits of
Positive acid fast bacilli stain, culture Pleural fluid protein greater than 4 g/dL Pleural fluid adenosine deaminase level greater than 35-50 U/L Pleural fluid glucose 30-50 mg/dL
Pleural fluid hematocrit/serum hematocrit greater than 0.5
LDH, Lactate dehydrogenase.