Overview of Pleural Effusion

Fluid that accumulates in the pleural space because of trauma or disease is called pleural effusion. This condition may result from heart failure, cancer, pulmonary embolism, or inflammation.

  • If blood is in the accumulating fluid, the condition is called "hemothorax;";
  • if pus is present, it is called "empyema;"
  • if air enters the space, it is called "pneumothorax;" and
  • if there is chyle (milky fluid consisting of lymph and fat), it is called "chylothorax."

There are two types of pleural effusion, transudative and exudative. An imbalance between the venous-arterial pressure and the pressure within the pleural space (oncotic pressure) causes transudative effusions. Transudates contain few protein cells and little solid material content and have a clear, pale yellow color. Cardiac failure and, less commonly, liver and kidney disease cause transudative pleural effusion.

Exudative effusions are caused by inflammation, infection, and cancer. Exudates have large amounts of protein cells, white blood cells, and immune cells that have migrated into the pleural fluid and deposited in tissues or on tissue surfaces. Exudates also are pale yellow in color but have a cloudy appearance. If pus is present because of infection (empyema), the fluid is yellow, cloudy, and has a foul odor. Pneumonia, tuberculosis, pulmonary embolism (blocked pulmonary artery), cancer, and trauma are common causes of exudative pleural effusion.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 31 May 2000

Last Modified: 01 Oct 2015