What is a heart attack : A heart attack can happen when a coronary artery becomes blocked by a build-up of plaque. The plaque can rupture, forming a blood clot, which means blood and oxygen cannot reach the heart. This damages the heart muscle, leading to a heart attack. Health professionals sometimes use different names for a heart attack, such as ‘myocardial infarction’, or ‘MI’ for short, ‘STEMI’ (more severe) or ‘non-STEMI’ (less severe).