best heart clinics in yelahanka Pericardial effusion is a serious medical condition where excess fluid accumulates in the pericardial sac—the double-layered sac that surrounds the heart. This buildup of fluid puts pressure on the heart, impairing its ability to pump blood effectively and potentially leading to cardiac tamponade, a life-threatening complication. Causes range from infections and autoimmune diseases to heart conditions, trauma, and cancer.