Pericarditis is inflammation of the pericardium. The pericardium is the thin sac (membrane) that surrounds the heart and the roots of the great blood vessels. The pericardium has an inner and outer layer with a small amount of lubricating fluid between them. When the pericardium becomes inflamed, the amount of fluid between the two layers increases. This squeezes the heart and restricts its action. Pain felt from pericarditis is usually a sharp, piercing pain over the center or left side of the chest. Often this pain gets worse if the person takes a deep breath.