Pleural effusion is a collection of fluid in the pleural space.

Many of the changes of congestive heart failure mentioned elsewhere in this module are seen in this film. The heart is enlarged, the hila are somewhat enlarged and indistinct, and there is "reversal of flow" (i.e., the vessels going to the upper lung fields are more prominent than normal -- normally this degree of prominence is evident only in vessels of the lower lobes). The ground glass opacity at the right base of the x-ray represents the pleural effusion.