What causes the formation of a pleural effusion (fluid on the lungs)?

Updated: Dec 28, 2018
  • Author: Kamran Boka, MD, MS; Chief Editor: Guy W Soo Hoo, MD, MPH  more...
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Answer

The normal pleural space contains approximately 10 mL of fluid, representing the balance between (1) hydrostatic and oncotic forces in the visceral and parietal pleural capillaries and (2) persistent sulcal lymphatic drainage. Pleural effusions may result from disruption of this natural balance.

Presence of a pleural effusion heralds an underlying disease process that may be pulmonary or nonpulmonary in origin and, furthermore, that may be acute or chronic. [4, 5] Although the etiologic spectrum of pleural effusion can be extensive, most pleural effusions are caused by congestive heart failure, pneumonia, malignancy, or pulmonary embolism.

The following mechanisms may play a role in the formation of pleural effusion:

  • Altered permeability of the pleural membranes (eg, inflammation, malignancy, pulmonary embolism)
  • Reduction in intravascular oncotic pressure (eg, hypoalbuminemia due to nephrotic syndrome or cirrhosis)
  • Increased capillary permeability or vascular disruption (eg, trauma, malignancy, inflammation, infection, pulmonary infarction, drug hypersensitivity, uremia, pancreatitis)
  • Increased capillary hydrostatic pressure in the systemic and/or pulmonary circulation (eg, congestive heart failure, superior vena cava syndrome)
  • Reduction of pressure in the pleural space (ie, due to an inability of the lung to fully expand during inspiration); this is known as "trapped lung" (eg, extensive atelectasis due to an obstructed bronchus or contraction from fibrosis leading to restrictive pulmonary physiology)
  • Decreased lymphatic drainage or complete lymphatic vessel blockage, including thoracic duct obstruction or rupture (eg, malignancy, trauma)
  • Increased peritoneal fluid with microperforated extravasation across the diaphragm via lymphatics or microstructural diaphragmatic defects (eg, hepatic hydrothorax, cirrhosis, peritoneal dialysis)
  • Movement of fluid from pulmonary edema across the visceral pleura
  • Persistent increase in pleural fluid oncotic pressure from an existing pleural effusion, causing further fluid accumulation

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