Acute Coronary Syndrome or ACS is a term that encompasses several conditions including ST and non-ST myocardial infarctions (more commonly known as heart attacks), unstable angina and Prinzmetal's angina.  These conditions are characterized by the rapidly developing imbalance in the myocardium's need for blood, and the amount of blood it actually receives, resulting in myocardial ischemia.  Whereas typical or stable angina is characterized by symptoms that can be reliably reproduced by exertion or emotional stress, ACS is characterized by anginal symptoms that developed within the preceding 4 weeks; occurs at rest or with minimal exertion; or that are prolonged, more frequent or severe. 
Most patients with ACS have existing narrowing or obstruction of the coronary arteries.  The rapid progression of their symptoms is often caused by erosion of a cholesterol laden plaque over which a thrombus or clot forms, effectively obstructing the flow of blood.  Alternatively, the flow of blood can also be restricted by the rapid growth of an arterial plaque or spasm of the artery.  These symptoms can also be precipitated by conditions that increase the myocardium's requirements for blood such as hyperthyroidism (i.e., an overactive thyroid) or anemia. 
Acute coronary syndrome is a serious condition that requires urgent medical attention.
  Abraham Salacata, MD, FACC