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Myocardial Infarction: Types, Diagnosis, and Management

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1. Definition

• Goldman-Cecil Medicine, Ch. 58
• Robbins & Cotran Pathologic Basis of Disease, p. 511

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2. Pathogenesis

1. An atheromatous plaque is disrupted by endothelial injury, intraplaque hemorrhage, or mechanical forces - exposing subendothelial collagen and necrotic plaque contents to blood
2. Platelets adhere, aggregate, and activate - releasing thromboxane A2, ADP, and serotonin, causing further aggregation and vasospasm
3. Activation of coagulation by tissue factor adds to the growing thrombus
4. Within minutes the thrombus can completely occlude the coronary artery lumen

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3. Classification by ECG Pattern

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4. Universal Classification by Type (4th Universal Definition 2018)

• Goldman-Cecil Medicine, Ch. 57-58

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5. Pathology: Morphologic Stages of MI

• Subendocardial infarct: <50% of wall thickness (typical of NSTEMI / short occlusion)
• Transmural infarct: Full-thickness necrosis (typical of prolonged STEMI)

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6. Diagnosis

Clinical Presentation

• Chest pain: Central, crushing, pressure-like, radiating to left arm, jaw, or back; lasting >20 minutes
• Associated symptoms: Diaphoresis, nausea, dyspnea, palpitations
• Atypical or absent pain in diabetics, elderly, women

ECG Findings

• ST-elevation ≥1 mm in 2 or more contiguous leads (or ≥2 mm in V1-V3)
• Hyperacute T waves (earliest change)
• Q waves develop (evolve over hours to days)
• ST-segment elevation then resolves; T-wave inversions persist

• ST depression, T-wave inversions
• Normal ECG does not exclude diagnosis (5% of ACS patients have a normal ECG)
• Left circumflex territory may be missed on standard 12-lead - check posterior leads V7-V9

• Proximal RCA occlusion; complicates ~30% of inferior MIs
• ST elevation ≥1 mm in right precordial leads V4R-V6R (>90% sensitive and specific)
• Management differs: requires volume loading, avoid nitrates/diuretics

Biomarkers

Imaging

• Echocardiography: New regional wall motion abnormalities; evaluate LV function; detect mechanical complications
• Coronary angiography: Defines anatomy; guides reperfusion strategy
• CT coronary angiography: >98% negative predictive value to exclude CAD in low-probability patients
• Chest X-ray: Rules out pulmonary edema, aortic dissection (widened mediastinum)

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7. Management

A. Immediate General Measures (All MI patients)

• O2 if SaO2 <90%
• IV access and continuous cardiac monitoring
• 12-lead ECG within 10 minutes of arrival
• Aspirin 325 mg chewed immediately (unless contraindicated)
• Nitroglycerin sublingual or IV for ongoing ischemia (avoid in RV infarction and hypotension)
• Morphine for pain (use cautiously - may delay P2Y12 absorption)
• Beta-blocker (oral, unless acute heart failure, cardiogenic shock, or bradycardia/AV block)

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B. STEMI Management - Time is Myocardium

Primary PCI (Preferred)

• Door-to-balloon time goal: ≤90 minutes (≤120 min if transfer required)
• Indicated for all STEMI within 12 hours of symptom onset
• Superior to fibrinolysis in mortality reduction, reinfarction, and stroke risk
• Preferred when available within the time window

Fibrinolysis (Thrombolytics)

• Indicated when PCI is not available within 120 minutes
• Must be given within 12 hours of symptom onset
• Agents: Alteplase (tPA), tenecteplase, reteplase
• Contraindications: prior intracranial hemorrhage, recent stroke, active bleeding, severe uncontrolled hypertension, aortic dissection
• Rescue PCI: if ST-elevation fails to resolve >50-70% within 1-2 hours after fibrinolysis, urgent angiography is indicated

CABG for STEMI

• When PCI has failed, ischemia is ongoing, and large areas of myocardium remain jeopardized
• Also for mechanical complications (papillary muscle rupture, VSD, free wall rupture)

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C. NSTEMI/Unstable Angina Management

Antiplatelet Therapy

Anticoagulation

• Unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) or fondaparinux
• Not required for long-term maintenance

Invasive vs. Conservative Strategy

• High-risk features (recurrent ischemia, elevated troponin, dynamic ECG changes, cardiogenic shock, heart failure, EF <40%, high TIMI/GRACE score): early invasive strategy (coronary angiography + revascularization)
• Low-risk (normal troponin, no ECG changes): conservative strategy with noninvasive testing

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D. Secondary Prevention (Post-MI - All Patients)

Recent evidence note (2025): A large individual patient data meta-analysis published in The Lancet (PMID 40897190) found that beta-blockers after MI with mildly reduced ejection fraction (41-49%) provide benefit - supporting their use even when EF is not severely reduced.

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E. Complications of MI

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8. Summary Algorithm

Chest pain presentation
        |
    12-lead ECG within 10 min
        |
  ┌─────┴─────┐
STEMI        No STEMI
  |               |
Primary PCI    Troponin + Risk stratification
(≤90 min)      (TIMI/GRACE score)
or             |            |
Fibrinolysis   High-risk   Low-risk
(if PCI       Early        Conservative
unavailable)  invasive     + noninvasive
              strategy     testing
        |
Secondary Prevention:
ASA + P2Y12 + statin + ACEi + BB

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• Goldman-Cecil Medicine, 26th Edition, Chapters 57 and 58 (NSTEMI/UA and STEMI)
• Robbins & Cotran Pathologic Basis of Disease, 10th Edition, Chapter 12 (MI pathology)
• Universal Definition of MI (Thygesen et al., Circulation 2018;138:e618-e651)
• Beta-blockers post-MI meta-analysis: Rossello X et al., Lancet 2025 (PMID 40897190)
• Mechanical circulatory support in cardiogenic shock: Thiele H et al., Lancet 2024 (PMID 39236726)