A heart attack, or myocardial infarction (MI), occurs when a portion of the heart muscle becomes oxygen-starved due to clogged blood vessels. This can damage or kill the affected portion of muscle. Depending upon the amount of muscle involved, heart attacks range from mild to life-threatening. The best predictor of whether a heart attack sufferer will survive an acute MI is their ability to identify that a heart attack is taking place and get medical help.