Patients presenting with large myocardial infarction and signs of persistent ischemia after successful percutaneous coronary intervention, have a poor prognosis with respect to outcome and development of heart failure in the future.

The hypothesis of this study is that in patients in whom persistent ischemia is present, use of intra-aortic balloon pump will be beneficial and improve outcome.

In some patients presenting with large myocardial infarction and poor hemodynamic condition, intra-aortic balloon counterpulsation is effective in alleviating cardiogenic shock. In others, the use of intra-aortic balloon pump has no effect at all. The investigators believe this is dependent on the presence of persisting ischemia after successful epicardial reperfusion, known as no-reflow.

In the presence of persisting ischemia, the investigators believe us of the intra-aortic balloon pump will relieve ischemia and improve outcome.

Inclusion Criteria:

• Acute ST-segment elevation myocardial infarction with summed ST-segment deviation ≥15 mm.
• Insufficient ST-segment resolution (<50%) on the ECG made 10-30 minutes after the primary PCI in the catheterization laboratory.

Exclusion Criteria:

• Initial summed ST-segment deviation less than 15 mm
• ST-segment resolution > 50% on the ECG performed in the catheterization laboratory
• Chest pain onset less than 2 or more than 8 hours before arrival
• Severe aortic valve stenosis/regurgitation
• Aortic abnormalities prohibitive for use of intra aortic balloon pump
• Full blown cardiogenic shock with immediate requirement of left ventricular assist device as judged necessary by the operator
• Inability to provide informed consent
• Pregnancy
• Inability to perform coronary angiography by the femoral approach