Updated: 8/13/2021

Myocardial Infarction

Review Topic
  • A 60-year-old man presents to the emergency department due to substernal chest pain. He describes the pain as "crushing" and it radiates down the left arm. Medical history is significant for hypertension and hypercholesterolemia. On physical exam the patient is diaphoretic. An electrocardiogram demonstrates ST-segment elevations and cardiac troponins are significantly elevated. After appropriate acute therapy, the coronary catheterization lab is activated and cardiology is consulted.
  • 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
      • cocaine use
      • male
      • postmenopause
      • genetic and behavioral predispositions to arteriosclerosis
        • e.g., high-fat diet
      • coronary heart disease risk equivalents
        • diabetes mellitus 
        • chronic kidney disease
        • noncoronary atherosclerotic disease 
          • carotid artery disease
          • abdominal aortic aneurysm
          • peripheral artery disease
  • 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 
  • V3 and V4
  • LAD (distal)
  • V1 and V2
  • LAD
  • V5 and V6
  • LAD or LCX
  • I and aVL
  • LCX
  • ST depression and tall R waves that can be seen anywhere in V2-5 (not all leads mandatory) 
  • ST elevations in V7-V9
  • Posterior descending artery
Evolution of MI
Morphological Myocardial Changes in an MI
Gross Features Light Microscopy
0-24 hours
  • Initially no gross findings; however, over the course of the first 24 hours, dark mottling ensues
  • Early coagulation necrosis
  • Wavy fibers
  • 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
    • sudden hypotension, tachycardia, and pulseless electrical activity 
    • 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
    • evaluate with echocardiograph  
    • ECG with persistent ST elevation in original MI leads with a deep QS wave 
  • 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, and bradycardia (in cases of an inferior wall MI)
  • Coronary angiography
    • indication
      • diagnostic study to assess coronary anatomy and to determine where the occlusion is
  • 12-lead ECG
    • perform as soon as possible
    • findings
      • STEMI  
        • hyperacute or peaked T-waves 
          • earliest finding 
        • ST elevation
          • ST depression may present initially
          • followed by pseudonormalization of the ST segment 
          • followed by ST elevation
        • Q-waves
          • a late finding (~2 weeks post-MI) or indicative or previous infarction 
        • new left bundle branch block (LBBB)
          • in isolation does not denote a STEMI
          • ECG findings 
            • prolonged QRS duration
            • dominant S wave in V1
            • broad monophasic or M-shaped R wave in the lateral leads (I, aVL, and V5-V6)
            • prolonged R wave peak time in the left precordial leads (V5-6)
              • can complicate diagnosis of MI - use Sgarbossa criteria (3 points or more concerning for MI) 
                • ST elevation 1 mm or greater in a lead with an upward QRS (5 points)
                • ST depression 1 mm or greater in V1, V2, or V3 (3 points)
                • ST elevation or depression 5 mm or greater in a lead with a downward QRS complex (2 points)
        • heart block 
          • can include first-, second-, and third-degree heart block
          • more common in inferior infarctions
            • the right coronary artery most commonly serves the AV node
        • fasicular blocks 
          • left anterior and posterior fascicular blocks
          • may occur from other stressors or secondary to ischemia
      • 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 around 24 hours
        • remains elevated for 7-10 days
      • can be elevated in other conditions including states of physiologic stress such as hypotension or sepsis 
    • CK-MB
      • a sensitive but not specific biomarker since skeletal muscle can also release it
      • useful for assessing reinfarction
  • Unstable angina
    • differentiating factor
      • no elevation in cardiac biomarkers
  • Hibernating myocardium 
    • decreased contractility of the myocardium
      • secondary to ischemia without necrosis
      • reversible when stenting restores bloodflow
  • Brugada syndrome 
    • sodium channelopathy
    • coved ST elevation
    • risk for sudden cardiac death
  • Wellens syndrome 
    • deeply inverted or biphasic T waves
    • indicative of critical left anterior descending artery stenosis
  • De Winter T waves 
    • ST depression and peaked T waves in precordial leads
    • anterior STEMI equivalent
    • requires cardiac catheterization
  • Cerebral T waves 
    • inverted and wide T waves
    • prolonged QT
    • associated with head trauma and intracranial pathology (such as a subarachnoid hemorrhage) 
  • Arrhythmogenic right ventricular dysplasia 
    • common cause of syncope and sudden death
    • epsilon wave on ECG
  • Precordial catch syndrome 
    • sudden, sharp, and severe paroxsyms of chest pain
    • no ECG or troponin abnormalities
  • Spontaneous coronary artery dissection (SCAD)
    • more common in women, postpartum period, and connective tissue disorders
    • may cause unstable angina, NSTEMI, or STEMI 
  • Conservative
    • lifestyle modification
      • e.g., smoking cessation
  • Medical
    • initial medical treatments include
      • aspirin 
        • first priority even before diagnostic work-up
        • confers significant benefit to mortality when given early
      • oxygen
      • nitroglycerin 
        • Reduces chest pain by lowering preload and thus myocardial oxygen demand 
        • contraindicated in a right inferior wall infarction (give fluids to maintain blood pressure)  
          • reduces preload leading to cardiovascular collapse 
      • morphine
        • only give if there is unacceptable pain 
          • appears to be associated with a mortality increase
      • benzodiazepines in cocaine-induced myocardial ischemia
        • typically given with nitrogylcerin
    • 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
      • contraindication
        • acute decompensated heart failure 
        • bradycardia
    • 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
    • cardiac catheterization and percutaneous coronary intervention (PCI) 
      • indications 
        • if STEMI symptoms developed in < 12 hours and the procedure can be performed within 90 minutes
        • highest mortality lowering intervention  
      • complications
        • cholesterol embolism   
        • retroperitoneal hematoma 
    • 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 90 minutes
·· Three coronary vessels with > 70 percent stenosis
·· Left main coronary artery stenosis > 50–70 percent
·· 2 vessels in a diabetic
·· 2 or 3 vessels with low ejection fraction
  • Heart failure
  • Sudden cardiac death
  • Arrhythmia
  • Myocardial stunning

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