The pericardium is a thin sac that surrounds the heart and the large blood vessels closely associated with the heart. The pericardium has an inner and an outer layer with a small amount of lubricating fluid between them. Pericarditis is a condition in which the pericardium becomes inflamed (increased blood flow and a protective cell response due to injury). With pericarditis, the amount of fluid increases within the pericardial layers, squeezes the heart, and can restrict the motion of the heart. This is called a pericardial effusion. If the onset is rapid or if the volume of the pericardial effusion is large, cardiac tamponade can occur. Cardiac tamponade diminishes the force of heart contractions because of compression by the fluid trapped within the pericardial space. Among individuals with a pericardial effusion, 7% to 10% are at risk of developing a cardiac tamponade. The April 25, 2007, issue of JAMA includes an article that discusses the accuracy of the medical history, physical examination, and basic diagnostic tests for the diagnosis of cardiac tamponade.