Currently, fractional flow reserve (FFR) is regarded as a gold-standard invasive method to define lesion-specific ischemia and FFR-guided PCI has been proven to reduce unnecessary revascularization and to enhance patient's clinical outcomes. Therefore, current guidelines recommend FFR measurement for intermediate coronary stenosis when there is no definite evidence of lesion-specific ischemia. However, previous evidences which well demonstrated the benefit of FFR-guided strategy were mostly generated from patients with stable coronary artery disease.4 FFR may be overestimated and the hemodynamic relevance of a coronary stenosis underestimated in patients with acute coronary syndrome (ACS).Its role in ACS patients still needs to be defined although several studies have recently published addressing the value of FFR-guided PCI in ACS. In fact, recent evidence suggests that culprit lesions of patients presenting with a non-ST-segment elevation myocardial infarction that were deferred based on a "negative" FFR have a relatively high event rate, calling into question the use of FFR in that patient population.

STUDY OBJECTIVE

1. To evaluate the impact of FFR on decision for PCI in ACS patients
2. To assess the long term prognosis of deferring PCI based on FFR value in non-culprit lesion; defying the cut-off value of FFR for PCI in the non-culprit lesion of ACS patients
3. To identify the relation between OCT findings and FFR value in culprit and non-culprit lesions of ACS patients
4. To compare the long term prognosis of PCI or deferring PCI based on FFR value in non-culprit lesion of ACS patients
5. To identify OCT findings to predict the lesion progression in deferred lesions.
6. To assess the long term prognosis of post-PCI FFR value in the culprit lesion of NSTE-ACS patients
7. To assess the efficacy of routine use of FFR to guide PCI in ACS patients; angiographically guidance versus FFR guidance

Inclusion Criteria:

• Subject age 19-85 years old

  • Diagnosed as ACS (unstable angina/ Non ST elevation myocardial infarction, ST elevation myocardial infarction)

    • At least one stenosis of >50% in a non-culprit vessel ≥ 2.0 mm by visual estimation with TIMI 3 - multivessel disease after PCI for culprit lesion or single vessel disease with ambiguity for PCI ④ FFR within hospitalization for index PCI for ACS

Exclusion Criteria:

• Severe stenosis with TIMI flow ≤ II of the non-IRA artery

  • Cardiogenic shock (Killip class IV) already at presentation or the completion of culprit PCI

    • Intolerance to Aspirin, Clopidogrel, Prasugrel, Ticagrelor, Heparin, Bivaluridin, or Everolimus

      • Known true anaphylaxis to contrast medium (not allergic reaction but anaphylactic shock)

        ⑤ Pregnancy or breast feeding

        ⑥ Non-cardiac co-morbid conditions are present with life expectancy <1 year or that may result in protocol noncompliance (per site investigator's medical judgment).

        ⑦ Other primary valvular disease with severe degree: severe mitral regurgitation or mitral stenosis, severe aortic regurgitation or aortic stenosis

        ⑧ Patients with a history of Coronary Artery Bypass Graft(CABG)

        ⑨ Unwillingness or inability to comply with the procedures described in this protocol.

Patients with acute coronary syndrome and intermediate stenosis