Definition and Causes of Pleural Effusion

An increase in the amount of fluid between the pleura. In health there is a very small amount of fluid, only 10 to 20 milliliters, within the pleural cavity. A pleural effusion can contain upward of 2 liters of fluid, though much smaller quantities (less than 400 milliliters) are more common. Pleural effusion compresses the lungs, preventing them from fully expanding.

Pleural effusion is:

  • exudative when it results from inflammation of the pleura (pleurisy)
  • transudative when pressure changes in the body’s fluid balance (osmotic) mechanisms allow more fluid to cross the pleural membrane such as with heart failure.

A hemothorax exists when the excess fluid is blood, and a chylothorax occurs when the excess fluid is lymph.


Many people who have pleural effusion have no symptoms. When present, symptoms include

Diagnostic and Treatment

The diagnostic path typically includes chest xray, computed tomography (ct) scan or ultrasound, and thoracentesis (withdrawing a sample of the fluid using a syringe with a large needle). Treatment aims to reduce the volume of fluid as well as identify the underlying cause (such as infection).

Thoracentesis may also be therapeutic, allowing the pulmonologist to drain away the excess fluid. Doctors generally drain no more than 1.5 liters of fluid at a time because more substantial withdrawal can result in rapid fluid shifts, causing cardiovascular instability and the development of pulmonary edema (fluid accumulation in the lung tissue). Recovery depends on the condition causing the pleural effusion.


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