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Updated: Dec 10 2021

Myocardial Infarction

4.9

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  • Snapshot
    • A 55-year-old man presents to the emergency department due to substernal chest pain. His symptoms began a few hours ago. He describes the pain as "crushing" and it radiates down the left arm. Medical history is significant for type 2 diabetes and hypertension. On physical exam the patient is diaphoretic. An electrocardiogram demonstrates ST-segment elevations and cardiac troponins are significantly elevated.
  • Introduction
    • Clinical definition
      • death of myocardial tissue secondary to prolonged and severe ischemia
        • also known as a "heart attack"
    • Types
      • ST-segment elevation myocardial infarction (STEMI)
        • an acute coronary syndrome (ACS) with ST-segment elevations found on electrocardiogram (ECG)
        • biomarkers of myocardial necrosis are present
      • Non-STEMI (NSTEMI)
        • an ACS without ST-segment elevations found on ECG
        • biomarkers of myocardial necrosis are present
      • unstable angina
        • an ACS
          • without ST-segment elevations found on ECG
          • and no elevation biomarkers of myocardial necrosis
    • Epidemiology
      • incidence
        • increases with age
      • risk factors
        • hypertension
        • cigarette smoking
        • hyperlipidemia
        • hypercholesterolemia
        • male
        • postmenopause
        • genetic and behavioral predispositions to arteriosclerosis
          • e.g., high-fat diet
    • Etiology
      • occlusion of a coronary artery can be caused by
        • atheromatous plaque rupture with subsequent thrombi expansion
        • vasospasm
        • emboli, which can be secondary to
          • atrial fibrillation, sending an embolus from the left atrium to the coronary arteries
          • vegetations from infective endocarditis
          • material from an intracardiac prosthetic
          • paradoxical emboli
    • Pathophysiology
      • occlusion of a coronary artery disrupts the blood supply to a region in the myocardium
        • ischemia ensues, the myocytes become rapidly dysfunctional
          • when ischemia persists, this can result in myocyte death
          • after 30 minutes of severe ischemia, the damage becomes irreversible
      • infarction patterns
        • subendocardial
          • myocyte necrosis involving the inner cardiac wall
          • this is normally the least perfused portion of the myocardium
          • may be referred to as an NSTEMI
        • transmural
          • myocyte necrosis involving the full thickness of the cardiac wall
          • may be referred to as a STEMI
  • ECG Changes and STEMI
      • ECG Changes and STEMI
      • Infarction Location
      • Involved ECG Leads
      • Involved Coronary Artery
      • Inferior wall
      • II, III, and aVF
      • RCA
      • Antero-apical
      • V3 and V4
      • LAD (distal)
      • Antero-septal
      • V1 and V2
      • LAD
      • Antero-lateral
      • V5 and V6
      • LAD or LCX
      • Lateral
      • I and aVL
      • LCX
      • Posterior
      • ST depression and tall R waves in V1-3
      • V7-V9
      • Posterior descending artery
  • Evolution of MI
      • Morphological Myocardial Changes in an MI
      • Time
      • Gross Features
      • Light Microscopy
      • Complications
      • 0-24 hours
      • Initially no gross findings; however, over the course of the first 24 hours, dark mottling ensues
      • Early coagulation necrosis
      • Wavy fibers
      • Elongated myocytes
      • Neutrophil infiltration
      • Arrhythmia
      • Heart failure
      • 1-3 days
      • Mottling with a yellowish infarct center
      • Extensive coagulation necrosis
      • Brisk neutrophil infiltration
      • Fibrinous pericarditis
      • 3-14 days
      • 3-7 days
        • hyperemic with central yellowing
      • 7-10 days
        • yellow-tan with reddish tan margins
      • 10-14 days
        • reddish gray infarct borders
      • Macrophage infiltration and tissue granulation
      • Myocardial wall rupture
        • may lead to cardiac tamponade
      • Papillary muscle rupture
        • mitral regurgitation
        • posterolateral muscle rupture is more likely than anteromedial muscle rupture
      • Pseudoaneurysm of a ventricular wall
        • may rupture
      • 2 weeks - several months
      • 2-8 weeks
        • gray-white scar
      • > 2 months
        • complete scar
      • Collagenous scar
      • Dressler syndrome
      • Heart failure
      • True ventricular aneurysm
        • a thrombus may form
  • Presentation
    • Symptoms
      • chest pain
        • features
          • squeezing
          • crushing
          • substernal
        • radiation
          • jaw
          • neck
          • left shoulder or down the arm
      • nausea and vomiting
      • dyspnea
      • asymptomatic
        • typically seen in patients with diabetic neuropathy
          • nerve fibers are damaged and impair their ability to sense pain
    • Physical exam
      • diaphoresis
      • variable findings
        • e.g., S3 or S4, signs of heart failure, bradycardia (in cases of an inferior wall MI)
  • Imaging
    • Coronary angiography
      • indication
        • diagnostic study to assess coronary anatomy and to determine where the occlusion is
  • Studies
    • 12-lead ECG
      • perform as soon as possible
      • findings
        • STEMI
          • hyperacute or peaked T-waves
            • earliest finding
          • ST elevation
          • Q waves
            • a late finding (~2 weeks post-MI)
          • new left bundle branch block (LBBB)
            • considered to be an equivalent to a STEMI
        • NSTEMI
          • ST depression
          • T wave inversion
        • Late
          • pathologic Q waves
            • Q wave in leads V2–V3 lasting longer than 0.02 seconds
            • Q wave anywhere lasting longer than 0.03 seconds with an amplitude greater than 0.1 mV
    • Biomarkers
      • Troponin
        • preferred marker as it has a high sensitivity and specificity for myocardial necrosis
        • troponin I increases after 4 hours and peaks around 24 hours
          • remains elevated for 7-10 days
      • CK-MB
        • a sensitive but not specific biomarker since skeletal muscle can also release it
        • useful for assessing reinfarction
  • Differential
    • Unstable angina
      • differentiating factor
        • no elevation in cardiac biomarkers
    • Costochondritis
      • differentiating factor
        • chest pain that is reproducible with palpation
  • Treatment
    • Conservative
      • lifestyle modification
        • e.g., smoking cessation
    • Medical
      • initial medical treatments include
        • aspirin
        • oxygen
        • nitroglycerin
          • hypotension in an inferior wall infarction secondary to reduced preload
        • morphine
          • only give if there is unacceptable pain
            • appears to be associated with a mortality increase
      • P2Y12 (ADP) receptors blockers
        • indication
          • given in addition to aspirin
      • heparin
        • indication
          • given in addition to antiplatelet therapy
      • β-blockers
        • indication
          • given to all patients if there are no contraindications
      • statin
        • indication
          • given to all patients
      • angiotensin-converting enzyme (ACE) inhibitor
        • indication
          • given to patients with a myocardial infarction
          • recommended when there is
            • anterior infarction
            • heart failure
            • left ventricular ejection fraction < 40%
          • reduces mortality
        • contraindication
          • shock
          • bilateral renal artery stenosis
          • allergy
    • Reperfusion therapy
      • percutaneous coronary intervention (PCI)
        • indications
          • if STEMI symptoms developed in < 12 hours and the procedure can be performed within 90-120 minutes
          • if fibrinolytic therapy is contraindicated
      • coronary artery bypass graft (CABG)
        • indication
          • when coronary anatomy does not allow for PCI
          • 3 vessel occlusion or 2 vessel occlusion in a patient with diabetes
          • significant stenosis of the left main coronary artery
      • fibrinolytic therapy
        • indication
          • for patients who cannot receive PCI within 120 minutes
  • Complications
    • Heart failure
    • Sudden cardiac death
    • Arrhythmia
    • Myocardial stunning
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