Management of acute coronary syndrome is targeted against the effects of reduced blood flow to the affected area of the heart muscle, usually because of a blood clot in one of the coronary arteries, the vessels that supply oxygenated blood to the myocardium. This is achieved with urgent hospitalization and medical therapy, including drugs that relieve chest pain and reduce the size of the infarct, and drugs that inhibit clot formation; for a subset of patients invasive measures are also employed (coronary angiography and percutaneous coronary intervention). Basic principles of management are the same for all types of acute coronary syndrome. However, some important aspects of treatment depend on the presence or absence of elevation of the ST segment on the electrocardiogram, which classifies cases upon presentation to either ST segment elevation myocardial infarction (STEMI) or non-ST elevation acute coronary syndrome (NST-ACS); the latter includes unstable angina and non-ST elevation myocardial infarction (NSTEMI). Treatment is generally more aggressive for STEMI patients, and reperfusion therapy is more often reserved for them. Long-term therapy is necessary for prevention of recurrent events and complications. General principles Acute coronary syndromes are caused by sudden and critical reduction of blood flow in one of the coronary arteries, the vessels that supply oxygenated blood to the myocardium (heart muscle), typically by a blood clot. The principal symptom is typically chest pain, known as angina pectoris; people who present with angina must prompt evaluation for possible acute coronary syndrome. Acute coronary syndromes are classified to two major categories, according to the patient's electrocardiogram, and specifically the presence or absence of persistent (>20 min) ST segment elevation (or left bundle branch block). Patients with acute coronary syndrome and ST elevation are said to have ST-elevation myocardial infarction (STEMI) and they tend to have one of their coronary arteries totally blocked. Damage is reversible for approximately 20-30 minutes after complete obstruction of blood flow; thereafter myocardial cell death ensues and progresses as time passes. Therefore, complete and sustained restoration of blood flow must be as prompt as possible to ensure maximum salvage of functional myocardium, a principle expressed in the maxim "time is muscle". This is achieved with reperfusion therapy, which is based on invasive reopening of the affected coronary artery with primary percutaneous coronary intervention, or non-invasive breaking up of the responsible blood clot with a thrombolytic drug. Patients without ST segment elevation are said to have non-ST-elevation