Pericardial effusion (per-e-KAHR-dee-ul uh-FU-zhun) is the accumulation of too much fluid in the double-layered, sac-like structure around the heart (pericardium).
The space between the layers normally contains a thin layer of fluid. But if the pericardium is diseased or injured, the resulting inflammation can lead to excess fluid. Fluid can also build up around the heart without inflammation, such as from bleeding after a chest trauma.
Pericardial effusion puts pressure on the heart, affecting the heart's function. If untreated, it can lead to heart failure or death.