A pericardial effusion describes the effect when fluid or solid tissue surrounds the cavity around the heart. The heart is surrounded by an empty space to allow it to contract freely. When fluid or other tissue enters this space (perhaps from bleeding or a malfunctioning lymphatic system), it compresses the heart and prevents it from beating properly. There are several underlying causes of the effusion, including infection, inflammation, cancer and kidney failure.
See also Pleural effusion, a similar condition where fluid surrounds the lungs.