This study is being conducted in patients with a major heart attack caused by a blocked artery undergoing Percutaneous Coronary Intervention (PCI) to open up the blockage. Up to 50% of people with an heart attack are found to have one or more additional narrowings that did not cause the heart attack. At present the best way to manage these additional blockages is not known. Many cardiologist recommend opening these blockages at at a later time after the heart attack. The present study is examining if PCI of all blockages at the same time as the PCI for the artery that caused the major heart attack (SS-PCI) will reduce the amount of heart damage compared to performing PCI of additional blockages 2-45 days later (IRA-PCI). Clinical follow up will be completed at 3, 12 and 24 months to determine heart function and monitor adverse events.

Inclusion Criteria:

• ST elevation myocardial infarction evidenced by new ST elevation at the J point in at least 2 contiguous leads of ≥ 2 mm (0.2mV) in men or ≥ 1.5mm (0.15mV) in women in leads V2-V3 and/or ≥ 1mm (0.1mV) in other contiguous chest leads or limb leads.
• MVD as evidenced by > one significant (>70% by visual assessment or FFR < 0.80 for 50-70% stenosis) stenosis in the non-IRA >2mm in diameter.
• Successful IRA-PCI with <30% residual angiographic stenosis and TIMI III flow
• Written informed consent

Exclusion Criteria:

• Age ≤ 18 years
• Prior coronary artery bypass graft (CABG) surgery
• Administration of thrombolytic therapy.
• Hemodynamic instability as evidenced by SBP<90 mmHg, Killip class ≥3, and/or need for inotropes/vasopressors.
• Known renal insufficiency (estimated GFR < 30 ml/min/1.73m2)
• Non-IRA stenosis is a chronic total occlusion (CTO)
• Plan by treating physician to treat non-IRA stenosis with coronary artery bypass surgery