What is the initial treatment for a patient with myocardial infarction (MI)?

Initial Treatment of Myocardial Infarction

Immediately administer aspirin 160-325 mg (chewed), provide oxygen, give sublingual nitroglycerin (if blood pressure permits), obtain a 12-lead ECG within 10 minutes, and initiate reperfusion therapy as soon as possible—preferably primary PCI within 90-120 minutes or fibrinolytic therapy if PCI is unavailable. 1, 2

Immediate Actions Upon Arrival (First 10-20 Minutes)

Core Interventions

• Aspirin 160-325 mg orally (chewed) should be given immediately unless absolutely contraindicated 3, 1, 2
• Oxygen by nasal prongs for all patients 3, 2
• Sublingual nitroglycerin unless systolic blood pressure <90 mmHg, heart rate <50 bpm, or heart rate >100 bpm 3, 1
• Morphine sulfate or meperidine for adequate analgesia to minimize pain and anxiety 3, 1
• 12-lead ECG must be obtained within 10 minutes of arrival to identify ST-segment elevation 3, 2

Monitoring Setup

• Continuous cardiac monitoring for arrhythmias and conduction disturbances 1
• Emergency equipment must be immediately available: atropine, lidocaine, transcutaneous pacing patches, transvenous pacemaker, defibrillator, and epinephrine 3, 1

Reperfusion Strategy (The Critical Decision)

Primary PCI (Preferred Method)

Primary percutaneous coronary intervention is the preferred reperfusion strategy and must be performed within 90-120 minutes of first medical contact if available at a center with 24/7 capability 2. For patients with ST-segment elevation ≥1 mV in contiguous leads or new left bundle branch block, this represents the highest priority intervention 3.

• Patients should bypass the emergency department and go directly to the catheterization laboratory 2
• The benefit is greatest when initiated within 6 hours of symptom onset, though definite benefit exists up to 12 hours 3, 2
• High-dose intravenous heparin is required during the procedure 3, 2

Fibrinolytic Therapy (When PCI Unavailable)

Administer fibrinolytic therapy if PCI cannot be performed within 120 minutes and no contraindications exist 3, 1. This produces a 21% proportional reduction in 35-day mortality, saving 35 lives per 1000 patients when given within the first hour 3.

• Greatest benefit occurs within the first hour of symptom onset (35 lives saved per 1000 treated) 3
• Still beneficial when given 7-12 hours after symptom onset (16 lives saved per 1000 treated) 3
• Do not give thrombolytic therapy to patients without ST-segment elevation 3

Antiplatelet and Anticoagulation Therapy

Dual Antiplatelet Therapy

• Aspirin 160-325 mg daily should be continued indefinitely after the initial dose 3, 2
• Add a potent P2Y12 inhibitor (prasugrel or ticagrelor preferred over clopidogrel) before or at the time of PCI 2
• Continue dual antiplatelet therapy for 12 months unless excessive bleeding risk exists 2
• For fibrinolytic therapy, add clopidogrel 75 mg daily to aspirin 1, 2

Anticoagulation

• Intravenous heparin for 48 hours is current practice after alteplase administration 3
• For streptokinase, APSAC, or urokinase, evidence for heparin benefit is limited 3
• Anticoagulation is required until revascularization or for hospital stay up to 8 days with fibrinolysis 2

Beta-Blocker Therapy

Administer early intravenous beta-blocker therapy followed by oral therapy unless contraindications exist (hypotension, acute heart failure, AV block, or severe bradycardia) 3, 1, 2.

Intravenous Administration

• Metoprolol 5 mg IV over 1-2 minutes, repeated every 5 minutes for a total of 15 mg 3, 4
• Monitor heart rate, blood pressure, and continuous ECG during administration 3, 4
• Auscultate for rales and bronchospasm 3

Oral Continuation

• Begin oral metoprolol 25-50 mg every 6 hours for 48 hours, starting 15 minutes after the last IV dose 3, 4
• Maintenance dose of 100 mg twice daily thereafter 3, 4
• Target resting heart rate of 50-60 bpm unless limiting side effects occur 3

Important Caveat

Recent high-quality evidence from the 2024 REDUCE-AMI trial showed that in patients with preserved ejection fraction ≥50%, long-term beta-blocker therapy did not reduce death or reinfarction 5. However, beta-blockers remain indicated for patients with heart failure and/or LVEF <40% 2. The FDA indication supports use in hemodynamically stable patients to reduce cardiovascular mortality 4.

Nitroglycerin Therapy

Initiate intravenous nitroglycerin for 24-48 hours if no hypotension, bradycardia, or excessive tachycardia is present 3, 1. This allows dose titration based on hemodynamic response, unlike oral preparations 3.

• Do not use nitroglycerin as a substitute for narcotic analgesics 3
• Titrate with frequent blood pressure and heart rate monitoring 3

Additional Pharmacotherapy

ACE Inhibitors

Initiate ACE inhibitors within the first 24 hours in patients with heart failure, LV systolic dysfunction, diabetes, or anterior infarct 2.

Statins

Start high-intensity statin therapy as early as possible and maintain long-term 2.

Medications to Avoid

Do not administer calcium channel blockers—they have not been shown to reduce mortality and may be harmful 3, 1.

Special Considerations for Inferior MI

Assess for Right Ventricular Involvement

Up to 50% of inferior MIs involve the right ventricle, which increases in-hospital mortality to 25-30% compared to 6% in uncomplicated inferior MI 6.

Look for the clinical triad: hypotension, clear lung fields, and elevated jugular venous pressure 1, 6.

Obtain right-sided ECG leads, particularly V4R—ST elevation ≥1mm has 88% sensitivity and 78% specificity for RV infarction 1, 6.

Management of RV Infarction

• Volume loading with IV normal saline to maintain RV preload 1, 6
• Avoid nitrates and diuretics—they can cause profound hypotension by reducing preload 1, 6
• Inotropic support with dobutamine if cardiac output fails to increase after volume loading 1, 6
• Consider intra-aortic balloon pump for persistent shock 1, 6

Conduction Disturbances

• IV atropine 0.5 mg (repeated up to 2.0 mg total) for symptomatic sinus bradycardia 1, 6
• Temporary pacing for symptomatic high-degree AV block unresponsive to atropine 1, 6
• Maintain AV synchrony through AV sequential pacing for symptomatic high-degree heart block 1, 6

Common Pitfalls to Avoid

• Never give nitrates in RV infarction—this can cause profound hypotension 1, 6
• Record lead V4R early—ST elevation can resolve within 10 hours 1, 6
• Do not administer beta-blockers or calcium channel antagonists to patients in a low-output state due to pump failure 3, 1
• Discontinue NSAIDs (except aspirin) due to increased risks of mortality and reinfarction 1
• Recognize that volume depletion may mask signs of RV involvement 6

Hemodynamic Monitoring

• Consider pulmonary artery catheter monitoring for progressive hypotension unresponsive to fluid administration 1
• Use intra-arterial pressure monitoring for cardiogenic shock 1
• Perform echocardiography to evaluate LV and RV function, detect mechanical complications, and exclude LV thrombus 1, 2

Activity Restriction

Limit physical activities for at least 12 hours 1.