1.Plaque rupture or erosion with superimposed nonocclusive thrombus,is believed to be the most common cause
2. Dynamic obstruction [e.g., coronary spasm, as in Prinzmetal's variant angina]
3. Progressive mechanical obstruction [e.g., rapidly advancing coronary atherosclerosis or restenosis following percutaneous coronary intervention (PCI)]
4.Secondary UA related to increased myocardial oxygen demand and/or decreased supply (e.g., anemia).
More than one of these processes may be involved in many patients.
