Rupture of a coronary artery plaque leads to thrombotic occlusion of the coronary artery and would present as ST segment elevation myocardial infarction. Early treatment with aspirin and early primary percutaneous coronary intervention are indicated. Anticoagulation therapy, usually with unfractionated heparin, is required during percutaneous coronary intervention.

Investigators hypothesis is that pretreatment with unfractionated heparin in addition to aspirin at first medical contact may facilitate spontaneous reperfusion of culprit artery and procedural thrombotic complication in patients with ST elevation myocardial infarction without significant risk of bleeding complications.

Randomization of patients with STEMI at first medical contact in a 1:1 ratio into an intervention group receiving 100 IU of unfractionated heparin (UFH) per kilogram of body weight IV and later additionally UFH after diagnostic coronary angiography according to the activated clotting time (ACT) and a control group receiving only UFH after diagnostic coronary angiography at the dose of 100 IU per kilogram of body weight.

The primary end point of the study is TIMI flow at coronary angiography. Secondary endpoints are: Bleeding complications (defined by BARC score), occurrence of cardiogenic shock, and 30-day mortality.

Investigators plan to randomize 600 patients.

Inclusion Criteria:

• Adult patients with STEMI referred for primary PCI
• Duration of symptoms less than 6 hours before presentation

Exclusion Criteria:

• Pregnancy
• Cardiogenic shock at presentation (hemodynamic instability)
• Cardiac arrest before randomization
• Duration of symptoms for more than 6 hours before presentation

Interventional cardiologists evaluating coronary angiography will be blinded to assigned group