ST-segment elevation myocardial infarction (STEMI) is caused by acute coronary occlusion due to rupture of vulnerable plaque or superficial erosion of a proteoglycan-rich plaque.Although, primary percutaneous coronary intervention (PCI) is a standard therapy to treat STEMI by opening the infarct-related artery (IRA), microvascular obstruction without myocardial perfusion is sometimes occurred just after balloon angioplasty or coronary stenting, and associated with an increased infarct size, hibernating myocardium, and mortality.The microvascular obstruction is related to distal thrombus embolization from atherosclerotic plaque or thrombotic materials in the IRA.