This study will evaluate change in heart muscle function from baseline to three months and twelve months in participants who present with a heart attack and a completely occluded coronary artery. These subjects will be randomized to receive standard Percutaneous transluminal coronary angioplasty (PTCA)/Stenting to open the artery or routine PTCA/Stenting plus post conditioning. Post conditioning commences immediately upon reperfusion using four cycles of thirty second inflations with a standard angioplasty balloon followed by a thirty seconds of reperfusion. The investigators hypothesize that Postconditioning reduces the size of the heart attack when utilized with successful primary Angioplasty/stent.

Single center study involving 140 patients randomized to Post Conditioning + Percutaneous coronary intervention (PCI) versus routine PCI during their ST-Elevation Myocardial Infarction (STEMI, Heart Attack) presentation. The Post Conditioning protocol consists of performing four, 30-second PTCA (Angioplasty) balloon occlusions followed by 30-seconds of reperfusion over a total of 4 minutes. The first balloon inflation occurs immediately after an angioplasty guidewire is placed through the obstruction in the artery. Following this protocol the vessel is stented as part of the usual practice for treatment of STEMI. No other treatment differences will occur between the two groups and all patients will receive the usual post-STEMI care. Patients in both groups will receive a cardiac MRI 3-5 days following their STEMI for measurement of heart attack size and heart muscle function, among other measures. Patients will undergo collection of blood for creatine kinase (CK)-MB and troponin I every 8 hours for 24 hours following PCI. All patients will be followed by phone follow-up visits to review history and major adverse cardiac event (MACE) (death, recurrent STEMI, repeat revascularization, arrhythmias, implantable cardioverter-defibrillator (ICD) placement and hospitalization for congestive heart failure (CHF)). All patients will be required to take P2Y12 inhibitors and aspirin for the duration of the trial. All patients will be treated with angiotensin-converting enzyme inhibitor (ACE-I), beta blockers and statins.

Inclusion Criteria:

• Age > 18 years old, < 80 years old
• Able to give informed consent
• Able to undergo cMRl (cardiac magnetic resonance imaging
• ST-segment elevation infarction with 100% occlusion of a major epicardial vessel (> 2.5 mm)
• No angiographic evidence of collateral flow distal to occluded artery
• Ischemic duration between 1.0 and 6 hours
• Thrombolysis in myocardial infarction (TIMI) 3 Flow following PCI

Exclusion Criteria:

• Visible collateral blood flow to the distal vasculature of the occluded vessel
• Previous Coronary Artery Bypass Graft surgery
• Previous q-wave myocardial infarction in the same territory
• Inability to give informed consent
• Inability to undergo cMRl
• Life expectancy less than one year
• History of Non-compliance or alcohol or drug addiction
• Patients with cardiogenic shock, cardiac arrest, hypothermia on presentation
• Chronic dialysis or significant renal insufficiency (Creatinine Clearance < 35 mI/mm/i .73 m2)
• TIMI Flow > 0 on presentation
• Ischemic Time > 6 hours or < 1.0 hours
• Presence of significant valvular heart disease (>mod Aortic Stenosis, >2+ Mitral Regurgitation)
• Known Left Ventricular systolic dysfunction (Left Ventricular Ejection Fraction < 50% prior to STEMI)