Heart Attack Research Program: Platelet Sub-Study (HARP)

Inclusion Criteria: * Acute ischemic symptoms compatible with diagnosis of MI, such as chest pain or anginal equivalent symptoms at rest or new onset exertional anginal equivalent symptoms * Objective evidence of MI (either or both of the following): * Elevation of troponin to above the laboratory upper limit of normal (ULN) * ST segment elevation of ≥1mm on 2 contiguous ECG leads * Willing to provide informed consent and comply with all aspects of the protocol * Administration of aspirin at least 1 hour before cardiac catheterization * Administration of thienopyridine (e.g., clopidogrel, ticagrelor) at least 1 hour before cardiac catheterization * Women and men with ≥50% of any major epicardial vessel on invasive angiography may participate Exclusion Criteria: * Recent use of vasospastic agents, such as cocaine, triptans, or ergot alkaloids (≤1 month) * Alternate explanation for troponin elevation, such as hypertensive urgency, acute exacerbation of heart failure, chronic elevation due to kidney disease, pulmonary embolism, cardiac trauma * Pregnancy * Thrombolytic therapy for STEMI (qualifying event) * Use of any of the following medications: * Platelet antagonists (except aspirin and thienopyridines) within 7 days * NSAIDs (e.g., ibuprofen, naproxen) within 3 days. * Thrombocytopenia (platelet count \<100,000) * Thrombocytosis (platelet count \>500,000) * Anemia (hemoglobin \<9 mg/dl) * Hemorrhagic diathesis