Heart attacks occur when blood flow through the arteries to the heart (coronary arteries) is blocked for a long enough time to damage or kill a portion of heart muscle. Within days to months after the heart attack, doctors sometimes inject dye through a catheter into the coronary arteries (heart catheterization) to see where they are narrowed or blocked. Doctors do the catheterization to find out whether patients need major procedures (coronary artery bypass graft surgery or angioplasty) to help prevent future heart attacks and chest pain (angina). Some patients with heart attacks do not need catheterizations. They may have very low risks for future problems, or they may have other illnesses, such as liver failure, which preclude heart surgery. Some, but not all, research suggests that women in the United States have catheterizations after heart attacks less often than men. Even if women have these procedures less often than men, we do not know whether the procedures are inappropriately underused in women compared with men.