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Snapshot
  • A 55-year-old man presents to the emergency department due to substernal chest pain. His symptoms began approximately 20 minutes 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
  • 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, 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 day, dark mottling ensues
  • Early coagulation necrosis
  • Wavy fibers
  • Neutrophil infiltration
  • Arrhythmia
  • Heart failure
1-3 days
  • Mottling with a yellow-ish 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 red-tan margins
  • 10-14 days
    • red-gray infarct borders
  • Macrophage infiltration and tissue granulation  
  • Myocardial wall rupture
    • may lead to cardiac tamponade
  • Papillary muscle rupture
    • mitral regurgitation 
  • 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
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
  • Biomarkers
    • Troponin
      • preferred marker as it has a high sensitivity and specificity for myocardial necrosis
      • troponin I increases after 4 hours and peaks at 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
Treatment
  • Conservative
    • lifestyle modification
      • e.g., smoking cessation
  • Medical
    • initial medical treatment includes
      • aspirin
      • oxygen
      • nitroglycerin 
      • morphine
        • only give if there is an unacceptable pain 
          • appears to be associated with a mortality increase
    • P2Y12 (ADP) receptors blockers
      • indication
        • given in addition to aspirin in patients with MI
    • β-blockers
      • indication
        • given to all patients
    • statin
      • indication
        • given to all patients
    • angiotensin-converting enzyme (ACE) inhibitor
      • indication
        • given to patients with a myocardial infarction
        • recommended when
          • anterior infarction
          • heart failure
          • left ventricular ejection fraction < 40%
        • reduces mortality
      • contraindication
        • shock
        • bilateral renal artery stenosis
        • allergy
  • Reperfusion therapy
    • percutaneous coronary intervention (PCI)
      • indication
        • perform PCI within 90-120 minutes
        • if fibrinolytic therapy are contraindicated
    • fibrinolytic therapy
      • indication
        • for patients who cannot receive PCI within 120 minutes
Complications
  • Heart failure
  • Sudden cardiac death
  • Arrhythmia
 

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