5.1 Ischemic Heart Disease & Myocardial Infarction

Ischemic Heart Disease & Myocardial Infarction

Step 3 Clinical Focus: The management of ischemic heart disease on the USMLE Step 3 revolves around rapid risk stratification in the acute setting, appropriate selection of reperfusion strategies, precise execution of guideline-directed medical therapy, and the diagnostic recognition of mechanical and electrical complications post-myocardial infarction (MI).

Acute Coronary Syndromes: Presentation and Evaluation

Acute Coronary Syndrome (ACS) encompasses unstable angina (UA), non-ST-elevation myocardial infarction (NSTEMI), and ST-elevation myocardial infarction (STEMI). Patients typically present with retrosternal chest pain or pressure that may radiate to the left arm, neck, or jaw, often accompanied by diaphoresis, dyspnea, and nausea. Atypical presentations (e.g., epigastric pain, unexplained dyspnea, or silent ischemia) are common in elderly patients, women, and individuals with diabetes mellitus. Initial evaluation requires a 12-lead electrocardiogram (ECG) performed and interpreted within 10 minutes of arrival. Cardiac troponins (specifically high-sensitivity troponin T or I) should be drawn at presentation and serial measurements obtained (e.g., at 1–3 hours depending on the assay and protocol) to document rise and/or fall. Unstable angina is defined by ischemic chest pain occurring at rest, of new onset, or with an accelerating pattern, without elevation of cardiac biomarkers, with or without ischemic ECG changes (such as ST-segment depression or T-wave inversion). NSTEMI requires positive biomarkers of necrosis (troponin elevation) but lacks persistent ST-segment elevation. STEMI is characterized by ST-segment elevation of ≥ 1 mm (0.1 mV) in two or more contiguous leads (except V2-V3, where age- and sex-specific criteria apply: ≥ 1.5 mm in women, ≥ 2.0 mm in men ≥ 40, and ≥ 2.5 mm in men < 40), or a new or presumably new left bundle branch block (LBBB) in the setting of clinical ischemia. Risk stratification in non-ST-segment elevation ACS (NSTE-ACS) utilizing tools like the TIMI (Thrombolysis in Myocardial Infarction) or GRACE scores guides the choice between an early invasive strategy (coronary angiography with intent to revascularize within 24 hours) versus an ischemia-guided/conservative strategy. High-risk features indicating early angiography include refractory angina, hemodynamic or electrical instability, dynamic ST-segment changes, or elevated troponins.

STEMI vs. NSTEMI Management

The management of STEMI is time-critical. The primary goal is emergency reperfusion to salvage ischemic myocardium. Percutaneous coronary intervention (PCI) is the preferred reperfusion strategy and must be performed within 90 minutes of first medical contact (FMC) at a PCI-capable facility, or within 120 minutes if the patient must be transferred from a non-PCI-capable facility. If the expected delay to PCI exceeds 120 minutes, intravenous fibrinolytic therapy (e.g., tenecteplase, alteplase, or reteplase) should be administered within 30 minutes of arrival (door-to-needle time), provided there are no absolute contraindications (e.g., active internal bleeding, history of hemorrhagic stroke, ischemic stroke within 3 months, intracranial neoplasm, or suspected aortic dissection). Following fibrinolysis, transfer to a PCI-capable center for rescue PCI (if reperfusion fails) or routine angiographic evaluation is indicated. For NSTEMI, immediate fibrinolysis is contraindicated, as it increases mortality without benefit. Treatment consists of anti-ischemic therapy, double antiplatelet therapy, and anticoagulation, followed by risk-stratified coronary angiography. Immediate invasive management (within 2 hours) is indicated for patients with refractory ischemia, severe heart failure, cardiogenic shock, or life-threatening arrhythmias. Early invasive management (within 24 hours) is indicated for stable patients with elevated troponins, dynamic ST depressions, or a TIMI score ≥ 3.

Post-MI Guideline-Directed Pharmacotherapy

Post-MI management requires a structured pharmacological regimen to prevent recurrent events, limit remodeling, and decrease mortality:

1. Dual Antiplatelet Therapy (DAPT): Consists of aspirin (81 mg daily indefinitely) and a P2Y12 receptor inhibitor (clopidogrel, ticagrelor, or prasugrel) for at least 12 months. Prasugrel should not be used in patients with a history of stroke or transient ischemic attack (TIA).
2. Beta-blockers: Initiate oral beta-blockers (e.g., metoprolol succinate or carvedilol) within 24 hours in patients without signs of acute heart failure, low-output states, bradycardia, or high-degree AV block. Continue for at least 3 years to reduce myocardial oxygen demand and ventricular arrhythmias.
3. Angiotensin-Converting Enzyme (ACE) Inhibitors or Angiotensin Receptor Blockers (ARBs): Indicated within 24 hours for all patients with anterior MI, ejection fraction (LVEF) ≥ 40%, heart failure, or stable chronic kidney disease/diabetes.
4. Mineralocorticoid Receptor Antagonists (MRA): Eplerenone or spironolactone is indicated within 3 to 14 days post-MI in patients already receiving an ACE inhibitor and beta-blocker who have LVEF ≥ 40% and either symptomatic heart failure or diabetes mellitus, provided serum potassium is < 5.0 mEq/L and creatinine clearance is > 30 mL/min.
5. High-Intensity Statins: Atorvastatin (80 mg) or rosuvastatin (20–40 mg) should be started immediately, targeting an LDL-C reduction of ≥ 50% and an absolute target of < 55 mg/dL.

Complications of Myocardial Infarction

Step 3 examinations frequently test the recognition of post-MI complications, categorized by timing:

• Acute (0–24 hours): Ventricular fibrillation or ventricular tachycardia (most common cause of sudden cardiac death in this window) and cardiogenic shock.
• Early (1–3 days): Postinfarction fibrinous pericarditis, presenting with pleuritic chest pain, a friction rub, and diffuse ST elevations on ECG. Treatment is high-dose aspirin (avoid NSAIDs as they impair myocardial healing).
• Subacute (3–14 days): Mechanical complications due to tissue necrosis:
  • Papillary Muscle Rupture: Typically occurs 2–7 days post-MI (often posteromedial papillary muscle supplied by the PDA). Presents with sudden-onset pulmonary edema, cardiogenic shock, and a new, harsh apical holosystolic murmur radiating to the axilla. Diagnosed via echocardiography; requires emergent surgical repair.
  • Interventricular Septum Rupture: Typically occurs 3–5 days post-MI. Presents with chest pain, dyspnea, a new holosystolic murmur at the left lower sternal border with a palpable thrill, and a step-up in oxygen saturation from the right atrium to the right ventricle. Requires urgent surgical closure.
  • Free Wall Rupture: Typically occurs 5–14 days post-MI. Presents with sudden profound shock, electromechanical dissociation (PEA), jugular venous distention, and tamponade. Frequently fatal; requires emergent pericardiocentesis and surgery.
• Late (weeks to months):
  • Left Ventricular Aneurysm: Occurs weeks to months post-MI. Presents with persistent ST-segment elevation on ECG and progressive heart failure or ventricular arrhythmias. Echocardiography shows dyskinetic thinning of the LV wall.
  • Dressler Syndrome: An autoimmune pericarditis occurring 2–10 weeks post-MI, presenting with fever, pleuritic chest pain, pericardial friction rub, and leukocytosis. Treated with NSAIDs (e.g., aspirin) plus colchicine.

Chronic Stable Angina Management

Chronic stable angina is characterized by predictable exertional chest pain relieved by rest or sublingual nitroglycerin. Initial management involves lifestyle modification and medical therapy to optimize myocardial oxygen supply and demand. First-line anti-anginal therapy begins with oral beta-blockers (titrated to a resting heart rate of 55–60 bpm). If beta-blockers are contraindicated or insufficient, long-acting calcium channel blockers (such as amlodipine, or non-dihydropyridines like diltiazem if LVEF is preserved) or long-acting nitrates are added. Ranolazine is a useful adjunctive agent for refractory angina that does not affect heart rate or blood pressure. Coronary revascularization is indicated for refractory symptoms despite optimal medical therapy, or high-risk anatomy where it improves survival. Coronary Artery Bypass Grafting (CABG) is superior to PCI for patients with left main coronary artery stenosis, triple-vessel disease, or two-vessel disease involving the proximal left anterior descending (LAD) artery with depressed ejection fraction, especially in the presence of diabetes mellitus.