Myocardial Infarction: A High-Yield USMLE Review

Myocardial infarction (MI) is the death of cardiac muscle from acute coronary artery occlusion, presenting across the acute coronary syndrome spectrum as STEMI or NSTEMI. It is among the highest-yield topics on the exam because the reperfusion decision, its timing, and antiplatelet strategy directly change survival. Recognizing STEMI equivalents and correctly localizing the infarct to a coronary territory on ECG are frequently tested reasoning steps.

Presentation

• Acute chest pain, classically lasting hours from onset (e.g., a patient presenting several hours after pain begins)
• STEMI: ST-segment elevation on ECG, such as ST elevation in the anteroseptal leads (V1–V4) indicating an LAD-territory infarct; extension into V5–V6 (lateral leads) suggests a larger anterolateral infarct
• Posterior STEMI equivalent: ST depression in V1–V3 with tall R waves and tall upright T waves (mirror-image changes), with direct ST elevation seen on posterior leads V7–V9
• NSTEMI: chest pain with elevated troponin but without ST elevation
• Rising troponin in the setting of ongoing ischemic symptoms

Diagnosis

• 12-lead ECG: identifies ST elevation in STEMI and localizes the territory (e.g., V1–V4 anteroseptal/LAD); detects STEMI equivalents such as posterior MI (ST depression with tall R waves in V1–V3), which posterior leads V7–V9 confirm as direct ST elevation
• Troponin: elevated/rising level confirms myocardial injury and is used to diagnose NSTEMI and risk-stratify
• Coronary angiography: defines culprit lesion and stenosis severity (e.g., high-grade proximal LAD stenosis) and guides revascularization

Management

• STEMI reperfusion: primary PCI if achievable within 120 minutes of first medical contact; otherwise give fibrinolytic therapy (e.g., tenecteplase) immediately and transfer for angiography
• After successful fibrinolysis (pain resolution, >50% ST reduction), perform angiography within 3–24 hours; if fibrinolysis fails (persistent pain and ST elevation at 60–90 minutes), proceed to urgent rescue PCI
• Posterior STEMI and other STEMI equivalents require emergent reperfusion—treat like a STEMI, not as NSTEMI
• Antiplatelet therapy: aspirin plus a P2Y12 inhibitor (dual antiplatelet therapy); aspirin should never be stopped, and stopping clopidogrel early risks acute stent thrombosis
• NSTEMI: risk-stratify and proceed to catheterization; medical therapy includes aspirin and a P2Y12 inhibitor

High-yield

• Anteroseptal STEMI = ST elevation in V1–V4 = LAD territory; V5–V6 are lateral leads (LCx or LAD diagonal), and involvement points to a larger anterolateral infarct
• Posterior MI is a STEMI equivalent: V1–V3 show ST depression, tall R waves, and tall upright T waves as mirror-image changes; confirm with posterior leads V7–V9
• PCI within 120 minutes of first medical contact is the reperfusion goal; fibrinolysis is the fallback when PCI cannot be achieved in time
• Acute stent thrombosis presents as STEMI and carries 20–40% mortality
• Delay elective surgery until at least 6–12 months after drug-eluting stent placement; if urgent, continue aspirin and hold clopidogrel for the shortest time possible
• In stable CAD (stable angina), PCI plus optimal medical therapy does not reduce death or MI versus optimal medical therapy alone (COURAGE, ISCHEMIA)

Pitfalls

• Misreading posterior STEMI as NSTEMI—ST depression in V1–V3 with tall R waves is a STEMI equivalent needing emergent PCI, not a delayed strategy
• Labeling V1–V6 collectively as 'anterior' leads—V1–V4 are anteroseptal (LAD), while V5–V6 are lateral leads; conflating them mislocalizes the culprit artery
• Stopping antiplatelet therapy prematurely after stent placement—early clopidogrel cessation can trigger fatal acute stent thrombosis, and aspirin should never be stopped
• Assuming a patient with stable angina needs a stent to 'prevent a heart attack'—revascularization improves symptoms but does not reduce hard outcomes versus optimal medical therapy in most stable patients
• Treating an MI decision as purely medical—a patient with capacity may refuse catheterization even when it would lower mortality; disagreement with the choice does not negate capacity