Acute coronary syndrome (ACS) is an event that occurs when part of the heart muscle is not getting enough blood and oxygen. ACS includes unstable angina and a heart attack. In a heart attack, the heart muscle is injured or starts to die. ACS is an emergency.

ACS is most often caused by a narrowed or blocked coronary artery. The coronary arteries supply blood to the heart muscle. The narrowing or blockage is usually the result of coronary artery disease, in which fatty deposits called plaque (say "plak") build up inside the arteries. If the plaque breaks open or ruptures, a blood clot may form and limit or block blood flow to the heart muscle.