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.