An acute coronary syndrome is a set of signs and symptoms related to the heart. ACS is compatible with a diagnosis of acute myocardial ischemia, but it is not pathognomonic. The sub-types of acute coronary syndrome include unstable angina and two forms of myocardial infarction, in which heart muscle is damaged. These types are named according to the appearance of the electrocardiogram as non-ST segment elevation myocardial infarction and ST segment elevation myocardial infarction. There can be some variation as to which forms of MI are classified under acute coronary syndrome.