Myocardial Infarction

Myocardial Infarction – High-Yield Study Guide for Medical Students

Definition

Myocardial infarction (MI) is defined as myocardial cell death due to prolonged ischemia, usually from acute thrombotic occlusion of a coronary artery on a background of atherosclerotic coronary artery disease. It is diagnosed by a rise and/or fall of cardiac troponin with at least one value above the 99th percentile upper reference limit plus evidence of myocardial ischemia (symptoms, ECG changes, imaging, or identification of coronary thrombus).

Epidemiology

MI is a leading cause of morbidity and mortality worldwide and a major component of the spectrum of acute coronary syndromes (ACS). Incidence rises sharply with age and is more common in males, though the risk in females increases after menopause. Common modifiable risk factors include hypertension, hyperlipidemia, diabetes mellitus, smoking, obesity, sedentary lifestyle, and poor diet. Non-modifiable risk factors include age, male sex, family history of premature coronary artery disease, and certain genetic dyslipidemias.

Pathophysiology

The pathogenesis of MI is closely linked to atherosclerosis of the epicardial coronary arteries:

• Atherosclerotic plaque formation: Endothelial injury and dysfunction lead to lipid deposition, macrophage infiltration, and formation of a fibrous-capped atheromatous plaque.
• Plaque rupture or erosion: Mechanical stress or inflammation causes disruption of the fibrous cap or erosion of the endothelium, exposing thrombogenic material (lipid core, collagen).
• Thrombus formation: Platelets adhere, activate, and aggregate at the site of disruption; the coagulation cascade is triggered, forming a fibrin-rich thrombus that partially or completely occludes the coronary lumen.
• Ischemia and necrosis: Coronary occlusion reduces oxygen and nutrient delivery. Subendocardium is most vulnerable. Irreversible myocyte necrosis usually begins within 20–30 minutes of severe ischemia and progresses as a wavefront from subendocardium to subepicardium over several hours.

MI is often classified as:

• ST-elevation MI (STEMI): Usually due to complete and sustained occlusion of a coronary artery with transmural infarction.
• Non–ST-elevation MI (NSTEMI): Typically due to subtotal occlusion or distal embolization, causing subendocardial infarction.

Universal definition types:

• Type 1: Spontaneous MI related to primary coronary event (plaque rupture/erosion, dissection).
• Type 2: MI secondary to ischemic imbalance (e.g., tachyarrhythmia, severe anemia, hypotension, vasospasm) without acute plaque rupture.
• Type 3: Sudden cardiac death with suggestive symptoms/ECG but biomarkers unavailable.
• Type 4 and 5: MI related to percutaneous coronary intervention (PCI) or coronary artery bypass grafting (CABG), respectively.

Clinical Presentation

The classic presentation is acute chest pain plus objective evidence of myocardial ischemia, but MI can be clinically heterogeneous.

Typical Symptoms

• Chest pain: Classically described as pressure, heaviness, squeezing, or tightness, located retrosternally or across the anterior chest.
• Radiation: Pain may radiate to the left arm, both arms, neck, jaw, back, or epigastrium.
• Duration: Usually >20 minutes and not fully relieved by rest or sublingual nitroglycerin.
• Associated symptoms: Dyspnea, diaphoresis, nausea, vomiting, palpitations, sense of impending doom, weakness, or lightheadedness.

Atypical or Silent Presentations

• Populations: Elderly, women, and patients with diabetes or chronic kidney disease have a higher rate of atypical or silent presentations.
• Symptoms: Isolated dyspnea, epigastric discomfort, indigestion-like pain, unexplained fatigue, syncope, or confusion.

Physical Examination

Physical findings can be normal, especially early, but may include:

• General: Anxiety, distress, diaphoresis, pallor.
• Vital signs: Tachycardia or bradycardia; hypo- or hypertension; tachypnea; low-grade fever after several hours.
• Cardiovascular: S4 gallop due to stiff ischemic ventricle; S3 in heart failure; new holosystolic murmur (papillary muscle dysfunction or rupture, ventricular septal defect); jugular venous distention in right ventricular MI or acute heart failure.
• Pulmonary: Crackles/rales in pulmonary edema; wheezes or normal lung exam in early or small infarcts.
• Signs of shock: Cool clammy extremities, altered mental status, oliguria in cardiogenic shock.

Diagnosis

Diagnosis of MI requires integration of history, ECG, biomarkers, and imaging. The universal definition requires a rise and/or fall in cardiac troponin with at least one value above the 99th percentile upper reference limit plus at least one of the following:

• Symptoms of ischemia.
• New ischemic ECG changes (ST elevation, ST depression, or new left bundle branch block) or new pathological Q waves.
• Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality.
• Identification of a coronary thrombus by angiography or autopsy.

Electrocardiogram (ECG) Findings

The ECG is the first-line diagnostic test and guides urgent management.

• STEMI:
  • New ST-segment elevation at the J point in two contiguous leads with cut-offs: ≥1 mm in all leads other than V2–V3; in leads V2–V3, ≥2 mm in men ≥40 years, ≥2.5 mm in men <40 years, or ≥1.5 mm in women.
  • Reciprocal ST depression may appear in opposite leads.
  • Evolution: hyperacute T waves, ST elevation, then T wave inversion and development of Q waves.
• NSTEMI/Unstable angina:
  • ST-segment depression (horizontal or downsloping) and/or T wave inversions in contiguous leads.
  • No persistent ST elevation; biomarkers differentiate NSTEMI (positive troponins) from unstable angina (normal troponins).
• Localization:
  • Inferior MI: ST elevation in II, III, aVF (usually right coronary artery).
  • Anterior MI: ST elevation in V1–V4 (usually left anterior descending artery).
  • Lateral MI: ST elevation in I, aVL, V5–V6 (circumflex or diagonal branches).
  • Posterior MI: ST depression and tall R waves in V1–V3; ST elevation in posterior leads V7–V9 if recorded.
  • Right ventricular MI: Inferior MI pattern plus ST elevation in right-sided leads (e.g., V4R).

Cardiac Biomarkers

• Cardiac troponins (I and T):
  • Most sensitive and specific biomarkers of myocardial necrosis.
  • Rise within 3–4 hours, peak at 12–24 hours, remain elevated for 7–10 days (sometimes longer).
  • High-sensitivity troponin assays detect even small necrosis and allow earlier diagnosis and rule-out protocols.
• CK-MB:
  • Less specific than troponin; can be useful to detect reinfarction because levels normalize sooner (return to baseline in 2–3 days).
• Other markers: Myoglobin rises earliest but is nonspecific; largely supplanted by troponin.

Imaging

• Echocardiography:
  • Assesses regional wall motion abnormalities, global left ventricular function, and complications (papillary muscle rupture, ventricular septal defect, free wall rupture, mural thrombus).
  • Helpful when ECG is non-diagnostic or in presence of baseline abnormalities (LBBB, paced rhythm).
• Coronary angiography:
  • Gold-standard for defining coronary anatomy, identifying culprit lesion, and enabling PCI.
  • Indicated emergently in STEMI and in many high-risk NSTEMI cases.
• Other imaging (CT, MRI):
  • Cardiac MRI can characterize infarct size, viability, and complications but is not typically used in the acute setting.

Initial Emergency Management

Management of MI begins as soon as it is suspected, even before confirmation. Priorities are stabilization, analgesia, antithrombotic therapy, and rapid reperfusion when indicated.

Immediate Measures (Pre-hospital and Emergency Department)

• Rapid assessment: Focused history, physical exam, 12-lead ECG within 10 minutes, and continuous cardiac monitoring.
• Oxygen: Administer if hypoxemic (e.g., SpO₂ <90%) or in respiratory distress; routine use in normoxic patients is not recommended.
• Aspirin: Chewable non-enteric-coated aspirin (typically 150–325 mg loading dose), unless true anaphylactic allergy.
• Nitroglycerin: Sublingual nitroglycerin (e.g., 0.3–0.4 mg) every 5 minutes up to 3 doses for ongoing chest pain, provided there is no hypotension, severe aortic stenosis, or suspected right ventricular infarct.
• Analgesia: Intravenous opioids (e.g., morphine) may be used for severe pain unrelieved by nitrates, with caution due to potential hypotension and association with poorer outcomes.
• Beta-blocker: Oral or IV beta-blocker (e.g., metoprolol) in the absence of contraindications (hypotension, bradycardia, heart block, acute heart failure, severe reactive airway disease).
• Statin: High-intensity statin therapy (e.g., atorvastatin 40–80 mg) initiated early, unless contraindicated.

Reperfusion Therapy in STEMI

Early reperfusion salvages myocardium and reduces mortality.

• Primary percutaneous coronary intervention (PCI):
  • Preferred reperfusion strategy when it can be performed by an experienced team within guideline-recommended time (e.g., door-to-balloon time typically ≤90 minutes).
  • Involves coronary angiography with balloon angioplasty and stent placement in the culprit lesion.
  • Requires dual antiplatelet therapy and anticoagulation.
• Fibrinolytic (thrombolytic) therapy:
  • Indicated when timely PCI is not available and there are no contraindications.
  • Agents include alteplase, tenecteplase, reteplase.
  • Most effective when given as early as possible, ideally within 12 hours of symptom onset; benefit decreases with time.
  • Should be followed by angiography (pharmaco-invasive strategy) in many patients.

Antiplatelet and Anticoagulant Therapy

• Aspirin: Given as loading dose, then continued indefinitely at a maintenance dose (e.g., 75–100 mg daily).
• P2Y12 inhibitors (Dual antiplatelet therapy, DAPT):
  • Agents: clopidogrel, prasugrel, ticagrelor.
  • Given as a loading dose followed by maintenance therapy, usually for at least 12 months after stenting, depending on bleeding risk and stent type.
• Anticoagulation:
  • Unfractionated heparin, low-molecular-weight heparin (e.g., enoxaparin), or bivalirudin may be used during acute MI, particularly when undergoing PCI or receiving fibrinolysis.
  • Dosing depends on renal function, bleeding risk, and chosen strategy.

Management of NSTEMI and Unstable Angina

NSTEMI/UA management focuses on risk stratification, anti-ischemic therapy, antiplatelet/anticoagulant therapy, and early invasive evaluation in higher-risk patients.

• Initial therapy: Similar to STEMI (aspirin, nitrates as tolerated, beta-blockers, statin, analgesia prn).
• Dual antiplatelet therapy: Aspirin plus P2Y12 inhibitor in nearly all patients, unless contraindicated.
• Anticoagulation: Typically unfractionated heparin, enoxaparin, or fondaparinux.
• Risk stratification: Use clinical scores such as TIMI or GRACE to determine early invasive strategy versus conservative approach.
• Invasive strategy: Early coronary angiography with PCI or CABG for high-risk features (recurrent ischemia, hemodynamic instability, significant troponin elevation, dynamic ECG changes, high risk scores).

Adjunctive and Long-Term Medical Therapy

• Beta-blockers: Reduce myocardial oxygen demand, arrhythmias, and mortality; aim for early initiation and long-term continuation unless contraindicated.
• ACE inhibitors/ARBs: Started within 24 hours in patients with LV dysfunction, anterior MI, diabetes, or hypertension; improve remodeling and survival.
• Aldosterone antagonists: Eplerenone or spironolactone in selected patients with LV systolic dysfunction and heart failure or diabetes, already on ACEI/ARB and beta-blocker.
• High-intensity statins: Indefinite therapy to achieve LDL-C lowering and plaque stabilization.
• Nitrates: For symptom control of angina; can be used orally, topically, or intravenously.
• Calcium channel blockers: Consider in patients with contraindication to beta-blockers or persistent symptoms, particularly in vasospastic angina.
• Glycemic control: Optimize in patients with diabetes; avoid overly aggressive glucose lowering in the acute phase.

Mechanical and Electrical Complications

Complications can occur early or late and significantly influence prognosis.

• Arrhythmias:
  • Ventricular fibrillation and sustained ventricular tachycardia (major causes of early sudden death).
  • Accelerated idioventricular rhythm, atrial fibrillation, bradyarrhythmias, and AV block (especially in inferior MI).
• Heart failure and cardiogenic shock:
  • Due to extensive LV dysfunction; presents with pulmonary edema, hypotension, and signs of hypoperfusion.
• Mechanical complications:
  • Papillary muscle rupture (acute severe mitral regurgitation).
  • Ventricular septal rupture (acute VSD with left-to-right shunt and heart failure).
  • Free wall rupture (hemopericardium and tamponade, usually catastrophic).
  • Left ventricular aneurysm (persistent ST elevation, heart failure, mural thrombus, arrhythmias).
• Pericardial complications:
  • Acute pericarditis early after MI.
  • Dressler syndrome: immune-mediated pericarditis weeks to months after MI.
• Thromboembolic events: LV mural thrombus formation with systemic embolization risk (e.g., stroke).

Secondary Prevention and Rehabilitation

• Lifestyle modification: Smoking cessation, weight management, healthy diet (e.g., Mediterranean-style), regular aerobic exercise.
• Blood pressure and lipid control: Treat according to current guidelines, often requiring multiple agents.
• Glycemic control: Target individualized HbA1c in diabetics; consider agents with cardiovascular benefit (e.g., SGLT2 inhibitors, GLP-1 agonists) based on evolving evidence.
• Cardiac rehabilitation: Structured programs improve functional capacity, adherence to therapy, risk factor control, and quality of life.
• Adherence to medications: Emphasize continuation of antiplatelets, beta-blocker, ACEI/ARB, and statin as indicated.

Key Clinical Pearls for Exams and Practice

• Time is myocardium: Early recognition and reperfusion are critical; always prioritize rapid ECG acquisition in suspected MI.
• STEMI vs NSTEMI: STEMI requires emergent reperfusion; NSTEMI requires urgent but not always immediate invasive evaluation based on risk stratification.
• Troponin interpretation: Remember that non-ischemic conditions (e.g., myocarditis, sepsis, renal failure, tachyarrhythmias) can also elevate troponin; correlate with clinical picture and ECG.
• Right ventricular infarction: Suspect in inferior STEMI with hypotension, clear lungs, and elevated JVP; avoid nitrates and excessive preload reduction, and consider right-sided ECG leads.
• Beware of contraindications to fibrinolysis: Prior intracranial hemorrhage, known structural cerebral lesions, ischemic stroke within 3 months (except acute ischemic stroke within 4.5 hours), active bleeding, and significant head trauma are major contraindications.
• New systolic murmur after MI: Think mechanical complication (papillary muscle rupture or VSD) and evaluate urgently with echocardiography.
• Persistent ST elevation weeks after MI: Suggestive of a left ventricular aneurysm rather than ongoing acute ischemia.
• Post-MI pericarditis vs Dressler syndrome: Early fibrinous pericarditis occurs in days; Dressler syndrome occurs weeks later and is autoimmune in nature.
• Long-term therapy: For exam memory: "ABCDS" – Aspirin (plus P2Y12), Beta-blocker, Cholesterol (high-intensity statin), Dilators/afterload reducers (ACEI/ARB), Smoking cessation and secondary prevention.

Summary

Myocardial infarction is a common, life-threatening manifestation of atherosclerotic coronary artery disease. For medical students, mastery involves understanding the pathophysiology of plaque rupture and thrombosis, recognizing typical and atypical clinical presentations, interpreting ECG patterns and cardiac biomarkers, and knowing the principles of urgent reperfusion, antithrombotic therapy, and secondary prevention. A systematic, guideline-based approach improves outcomes and forms a foundation for both exam success and clinical practice.