• Myocardial infarct size [ Time Frame: 3-5 days after first medical contact ]
  The primary clinical endpoint will be myocardial infarct size measured by late gadolinium enhancement CMR imaging at 3-5 days from first medical contact.
• Feasibility outcomes [ Time Frame: Assessed at the end of the study ]
  Primary feasibility outcomes would include the rate of recruitment, the number of patients undergoing cardiac MRI within the stipulated time frame, and completeness of the study data collection.

• Myocardial salvage [ Time Frame: 3-5 days after infarction ]
  Myocardial salvage as measured by T2-weighted short tau inversion recovery on CMR assessed at 3-5 days after infarction
• Left ventricular ejection fraction [ Time Frame: 3-5 days after infarction ]
  Left ventricular ejection fraction on CMR at 3-5 days
• ST-segment elevation resolution [ Time Frame: 90-minutes after thrombolysis ]
  ST-segment elevation resolution at 90 minutes after thrombolysis as assessed by the worst lead on electrocardiogram (ECG core lab).
• TIMI frame count in infarct related artery [ Time Frame: During index coronary angiogram which will be performed within 24 hours of admission ]
  TIMI frame count on baseline coronary angiogram in the infarct-related artery
• Creatine kinase MB area under the curve [ Time Frame: 24 hours after admission ]
  Creatine kinase MB area under the curve through 24 hours
• Von Willebrand factor fragmentation [ Time Frame: At the time of angiography, assessed up to 7 days from admission ]
  The proportion of Von Willebrand factor multimers in the low, intermediate and high molecular weight form at the time of angiography
• Bleeding [ Time Frame: From time of randomization until the date of discharge or date of death from any cause, whichever came first, assessed up to 90 days ]
  Bleeding research consortium type II, III and V bleeding; safety outcome
• Allergic reactions [ Time Frame: From time of randomization, upto 48 hours ]
  Allergic reactions including hypotension (SBP< 90 mm Hg or a fall in BP >30 mm Hg below baseline), urticaria, flushing, wheezing and/or angioedema

• Experimental: Intravenous N-Acetylcysteine arm
  On arrival at the recruiting hospital, eligible and consenting STEMI patients randomly allocated to the experimental arm would be administered an intravenous N-Acetylcysteine bolus of 1200 mg over 0.5 hours (in 5% Dextrose) followed by 600mg/hour for the remaining 47.5 hours (in 5% dextrose). A total N-acetylcysteine dose of 29.7 grams is administered over 48 hours.
  Intervention: Drug: Intravenous N-Acetylcysteine
• No Intervention: Control arm
  Patients randomized to this arm would receive no experimental therapies and would continue to receive all standard guideline recommended medical therapies and interventions.

Inclusion Criteria:

Patients presenting with STEMI within 3 hours of symptom onset and satisfying all of the following criteria:

1. Patient age ≥ 18 years
2. Have received thrombolysis, with intend to pursue a pharmaco-invasive reperfusion strategy. Onset of chest pain to reperfusion time of < 3hrs.
3. STEMI involving anterior and/or inferior wall
4. An absence of baseline Q-waves on the initial ECG: The presence of Q waves defined at baseline using the Selvester QRS screening criteria

5. Have a high-risk STEMI ECG defined as:

   • ≥2mm ST-segment elevation in 2 anterior or lateral leads; or
   • ≥2 mm ST-segment elevation in 2 inferior leads coupled with ST-segment depression in 2 contiguous anterior leads for a total ST-segment deviation of ≥4 mm

Exclusion Criteria:

1. Previous myocardial infarction
2. Known to have moderate to severe LV systolic dysfunction (LV EF< 45%)
3. Known allergy to thrombolytic therapy or NAC
4. Presence of left bundle branch block
5. Cardiogenic shock (defined as systolic blood pressure of < 90mm Hg, for at least 30 minutes, not responsive to fluid resuscitation)
6. Permanent pacemaker or cardioverter defibrillator implanted previously
7. Patients with contra-indications to thrombolytic therapy
8. Patients with loss of consciousness or confusion
9. Patients with known chronic kidney disease (GFR < 30ml/min/m2) or on dialysis
10. Current pregnancy
11. Planned therapy with primary PCI