This study will compare the microcirculatory resistance (IMR) of infarct-related artery (IRA) in patients who underwent immediate versus deferred stenting during percutaneous coronary intervention (PCI) for acute myocardial infarction.

Primary PCI with immediate stenting (IS) is the current standard of reperfusion strategy for STEMI. However, it is thought that IS may cause additional myocardial injury by increasing distal embolization of clot and atheromatous plaque debris. Only about 35% of patients without cardiogenic shock can achieve optimal myocardial tissue perfusion at the microvascular level, even after restoration of epicardial coronary artery patency. IS in highly pro-thrombotic and inflammatory milieu of IRA during primary PCI would increase distal embolization of clot and atheromatous plaque debris, and provoke the inflammation process, so deferred stenting after a cooling down period of IRA for several days, have a potential to mitigate or prevent microvascular obstruction (MVO). Among several methods to evaluate MVO after STEMI, IMR has been well known as an good indicator of MVO and strong predictor for short and long term clinical outcomes.

Inclusion Criteria:

1. More than 30 minutes in duration of typical chest pain
2. Thrombolysis In Myocardial Infarction (TIMI) flow 0, 1 or 2 prior to the procedure

Exclusion Criteria:

1. Cardiogenic shock,
2. Previous history of myocardial infarction, or coronary artery bypass surgery
3. Rescue PCI after fibrinolysis
4. Life expectancy of less than 1 year
5. Acute occlusion of left main coronary artery
6. STEMI due to stent thrombosis
7. Major coronary dissection (type D~F) following procedures achieving TIMI 3 flow

Patients undergoing primary precutaneous coronary intervention for ST- segment elevation or Non ST-segment elevation myocardial infarcation with ongoing myocardial ischemia