What is the immediate treatment for myocardial infarction (MI)?

Immediate Treatment of Myocardial Infarction

Upon arrival to the emergency department, immediately administer aspirin 160-325 mg (chewed and swallowed), oxygen via nasal prongs (only if oxygen saturation <90%), sublingual nitroglycerin (unless systolic BP <90 mmHg or heart rate <50 or >100 bpm), and morphine for pain control, while obtaining a 12-lead ECG within 10 minutes. 1, 2

Initial Assessment and Stabilization (First 10-20 Minutes)

Time is muscle—every minute counts. The initial evaluation must be completed within 10 minutes of ED arrival, with no more than 20 minutes elapsing before assessment is complete. 1

Immediate Medications (Given Simultaneously)

• Aspirin 160-325 mg orally: Chew and swallow immediately for faster absorption unless contraindicated. 1, 2 This reduces 35-day mortality by 21% when combined with reperfusion therapy. 1

• Oxygen: Administer via nasal prongs only if oxygen saturation <90%—avoid routine oxygen in non-hypoxemic patients as hyperoxia may increase myocardial injury. 2

• Sublingual nitroglycerin: Give unless systolic BP <90 mmHg or heart rate <50 or >100 bpm. 1 Avoid completely in right ventricular infarction due to risk of profound hypotension. 3

• Morphine sulfate or meperidine: Provide adequate analgesia for chest pain. 1

Critical ECG Interpretation

• ST-segment elevation ≥1 mm in contiguous leads: Proceed immediately to reperfusion strategy (see below). 1

• New left bundle branch block: Manage identically to ST-elevation MI. 1

• No ST-elevation: Do NOT give thrombolytic therapy; primary PCI benefit remains uncertain in this group. 1

Reperfusion Strategy Decision (Within 120 Minutes)

Primary PCI is the preferred reperfusion strategy when it can be performed within 120 minutes of STEMI diagnosis by experienced operators with prompt access to emergency CABG surgery. 3, 2, 4

If Primary PCI Available Within 120 Minutes:

• Initiate dual antiplatelet therapy: Add a potent P2Y12 inhibitor (ticagrelor or prasugrel preferred over clopidogrel) before or during PCI. 3

• Anticoagulation: Administer high-dose unfractionated heparin for primary PCI. 3

• Transfer immediately to catheterization laboratory. 4

If Primary PCI Cannot Be Performed Within 120 Minutes:

• Initiate fibrinolytic therapy immediately if presenting within 12 hours of symptom onset and no contraindications exist. 3, 4

• Fibrin-specific agents preferred: Use tenecteplase, alteplase, or reteplase. 3, 4 For patients ≥75 years, reduce tenecteplase dose by 50% to minimize stroke risk. 4

• Adjunctive therapy with fibrinolysis:

  • Continue aspirin 150-325 mg 4
  • Add clopidogrel 4
  • Anticoagulation with enoxaparin IV followed by subcutaneous (preferred over unfractionated heparin) 3, 4

• Time-dependent mortality benefit: 35 lives saved per 1000 patients when given within first hour versus only 16 lives saved per 1000 when given 7-12 hours after symptom onset. 1

Post-Fibrinolysis Management Algorithm:

• Transfer all patients to PCI-capable center immediately after fibrinolysis. 4

• At 60-90 minutes: Assess ST-segment resolution in the lead with greatest initial elevation. 4

  • <50% resolution = failed fibrinolysis: Perform rescue PCI immediately. 4
  • ≥50% resolution = successful: Perform routine angiography and PCI between 2-24 hours. 4

Early Beta-Blocker Therapy

Initiate beta-blockers early in hemodynamically stable patients to reduce morbidity and mortality. 2, 5

Intravenous Metoprolol Protocol (First 48 Hours):

• Three 5 mg IV boluses at 2-minute intervals, monitoring BP, heart rate, and ECG continuously. 5

• 15 minutes after last IV dose: Start metoprolol 50 mg orally every 6 hours for 48 hours (if full IV dose tolerated). 5

• If IV dose not tolerated: Start 25-50 mg orally every 6 hours depending on degree of intolerance. 5

• After 48 hours: Transition to 100 mg orally twice daily for maintenance. 5

• Contraindications: Severe intolerance, hypotension, bradycardia, heart failure signs during acute phase. 5

Additional Pharmacotherapy (First 24 Hours)

• Intravenous nitroglycerin: Use for 24-48 hours in patients with heart failure, large anterior MI, persistent ischemia, or hypertension. 2 Critical pitfall: Absolutely avoid in right ventricular infarction. 3

• ACE inhibitors: Initiate within first 24 hours in patients with heart failure, LVEF <40%, diabetes, or anterior infarction. 3

• High-intensity statin therapy: Start as soon as possible and maintain long-term. 3

Special Consideration: Right Ventricular Infarction

Up to 50% of inferior MIs involve right ventricular infarction—this requires specific management modifications. 3

• Obtain V4R lead early: ST-elevation may resolve within 10 hours, so don't delay this assessment. 3

• Maintain RV preload: Use IV saline boluses for hypotension rather than vasodilators. 3

• Avoid nitroglycerin completely: Risk of profound hypotension. 3

Common Pitfalls to Avoid

• Do NOT routinely combine fibrinolysis with planned immediate PCI—this increases mortality and adverse outcomes. 4

• Do NOT give thrombolytics to non-ST-elevation MI—no benefit and increased bleeding risk. 1

• Do NOT delay aspirin—it should be given immediately, not after ECG confirmation. 1

• Do NOT use routine oxygen—only if SaO₂ <90%. 2

• Intracranial hemorrhage risk with thrombolytics: Higher in patients >65 years, <70 kg body weight, hypertension, or receiving tPA. 1