The purpose of this study is to assess whether the Index of Microcirculatory Resistance (IMR) can be considered a prognostic predictor for the occurrence of events at one year of follow up after primary Percutaneous Coronary Intervention (PCI) in ST-Elevation Myocardial Infarction (STEMI) patients.

Any correlation between IMR and the short and medium term outcomes, defined as cardiovascular death, re-Myocardial Infarct (MI), re-hospitalization for Heart Failure (HF), resuscitation or Implantable Cardioverter Defibrillator (ICD) appropriate shock, will be assessed in the study.

Prospective, multicentre study designed to evaluate IMR ability to predict events occurrence, defined as Cardiovascular death, re-MI, re-hospitalization for HF, resuscitation or ICD appropriate shock, during a 1 year follow-up period.

All participants will have the culprit lesion treated following clinical practice and guidelines; Fractional Flow Reserve (FFR) and IMR will be measured after the primary PCI procedure to evaluate treatment success and myocardial viability. Non-culprit lesions will be functionally evaluated through FFR index and will be treated if FFR will show functional stenosis. FFR and IMR will be measured to evaluate treatment success and myocardial viability. Patients will be followed-up at 1m, 6m and 1y periods.

Inclusion Criteria:

1. Patient of legal age in hosting country able and willing to provide informed consent form
2. Hospital admission either within 12 h of symptom onset or between 12 and 24 h after onset with evidence of continuing ischemia
3. Electrocardiographic ST-segment elevation ≥1 mm in two or more contiguous ECG leads, or with a left bundle-branch block (LBBB)
4. Multivessel diseased patients with lesions in the proximal 2/3 part of the vessels
5. Culprit Lesion EyeBall (EB) identified during evaluation of basal angiography
6. Presence of at least one non-culprit lesion >50% EB detected in the basal angiography and eligible for PCI for which the operators decision is to perform a staged pre-discharge angioplasty procedure

Exclusion Criteria:

1. Patients who cannot give informed consent
2. A life expectancy of less than 1 year
3. Patients who are pregnant or nursing
4. Contra-indication to angiography
5. Allergy/intolerance to Adenosine
6. Contra-indication/Allergy/Intolerance to contrast media or to medical therapy foreseen for PCI
7. Documented allergy to Adenosine diphosphate (ADP) inhibitors (aspirin and clopidogrel)
8. New infarct on the same area of a previous infarct
9. Critical non treatable Lesion EB>70% downstream of the culprit lesion
10. Absence of non-culprit lesion/s
11. Patient with hemodynamic instability not controllable with medical therapy and/or need intra aortic balloon pump implantation (IABP)
12. Prior Coronary Artery Bypass Graft (CABG) or indication for CABG
13. Patients with Left Main (LM) coronary artery disease requiring revascularization