Heart attacks occur when the blood flow through the arteries to the heart (coronary arteries) is blocked for a time long enough to damage or kill a portion of heart muscle. Most, but not all, heart attacks cause symptoms, such as severe, crushing chest pain; nausea; shortness of breath; and a feeling of impending doom. Some people with heart attacks die immediately, before they receive medical attention. Others are admitted to hospitals and treated for their symptoms. Several things, such as a eating a healthy diet, avoiding tobacco, exercising, and taking aspirin and cholesterol-lowering medicines, may help prevent heart attacks. In addition, many treatments given after a heart attack prevent death and complications. People's lifestyles and the way doctors try to prevent and treat heart attacks have changed over time. We do not know how these changes have affected the occurrence (incidence) and outcomes of heart attacks.