A Study of Low-dose Intracoronary Thrombolytic Therapy in STEMI (Heart Attack) Patients.

Inclusion Criteria: 1. Adult men and women aged over 18 who present with STEMI within 6 hours of symptom onset. Patients will be eligible if they have symptoms consistent with myocardial ischaemia (chest pain, dyspnoea) for at least 20 minutes accompanied by definite ECGs indicating STEMI as defined by Australian National Heart Foundation (NHF) guidelines 2. Willing and able to comply with all study requirements, including treatment, assessment and clinic visit attendances 3. Able to personally read and understand the Participant Information and Consent Form and provide written, signed and dated informed consent to participate in the study 4. (At time of PCI) Patient has received metallic drug-eluting stent 5. Participant consents to have a 3-7 day (discharge) and 6 month follow up cardiac MRI Exclusion Criteria: At the time of screening and/or prior to randomisation, no known; 1. Previous coronary bypass grafting 2. Other residual lesions with ≥50% diameter stenosis in the culprit vessel 3. Prior myocardial infarction in the target territory 4. Presence of contraindications to thrombolytic therapy (including history of stroke and recent brain surgery active internal bleeding; history of cerebrovascular accident; intracranial or intraspinal surgery, or trauma within 2 months; intracranial neoplasm, arteriovenous malformation, or aneurysm; known bleeding diathesis; and severe uncontrolled hypertension) 5. Presence of contraindications to adenosine infusion for IMR measurement including sinus node disease, moderate to severe bronchoconstrictive disease and second or third-degree atrioventricular (AV) block 6. Diagnosis of metastatic disease 7. Concurrent illness, including severe infection that may jeopardise the ability of the patient to undergo the procedures outlined in this protocol with reasonable safety 8. Serious medical or psychiatric conditions that might limit the ability of the patient to comply with the protocol 9. Pregnancy, lactation, or inadequate contraception. Women must be post-menopausal, infertile, or use a reliable means of contraception. Women of childbearing potential must have a negative pregnancy test done within 7 days prior to registration. Men must have been surgically sterilised or use a (double if required) barrier method of contraception. 10. Participation in any investigational study in the previous 30 days Other exclusion criteria: 11. (Cardiac MRI cohort only) Presence of contraindications to contrast enhanced MRI including severe claustrophobia, pregnancy, pacemakers, non-MRI compatible aneurysm clips, defibrillators and estimated glomerular filtration rate of \<30mL/min. (At time of PCI) 12. Patients who received GpIIb/IIIa treatment prior to IMR measurement 13. Patients who do not undergo primary PCI due to lack of severity of culprit lesion or other reasons.