Pericardial effusion (per-e-KAHR-dee-ul uh-FU-zhun) is the buildup of too much fluid in the double-layered, saclike structure around the heart (pericardium). The space between these layers typically contains a thin layer of fluid. Lung cancer is the most common cause of the malignant pericardial effusion. Trauma: Blunt, penetrating, and iatrogenic injury to the myocardium, aorta, or coronary vessels can lead to the accumulation of blood within the pericardial sac.