Myocardial Infarction (MI) Quiz

Myocardial Infarction (MI)

• Definition: Irreversible myocardial damage due to sudden coronary artery occlusion
• Risk factors: Hypertension, Diabetes Mellitus, Smoking, Hyperlipidemia, Old age, Male gender, Family history, Stressful Life

Types of MI

• Trans-mural infarction:
  • Affects the full thickness of ventricles
  • Due to atherosclerosis superimposed by thrombus formation
  • Usually affects the left ventricle, but the right ventricle can be affected rarely
• Sub-endocardial infarction:
  • Limited to the inner 1/3 of the ventricular wall
  • Sites of coronary occlusion include:
    • Anterior descending branch of the left coronary artery, leading to infarction at the anterior surface of the left ventricle
    • Lateral circumflex branch, leading to infarction at the lateral surface of the left ventricle
    • Right coronary artery occlusion, leading to infarction at the posterior surface of the left ventricle and posterior portion of the interventricular septum

Causes of Acute Coronary Artery Occlusion

• Thrombosis on top of atherosclerosis
• Polyarteritis nodosa (PAN)
• Emboli

Pathogenesis of MI

• The major cause of MI is coronary atherosclerosis (atheroma)
• Hemorrhage, ballooning, and fissuring of atheroma lead to vascular occlusion and MI
• Non-complicated atheroma occludes 75% of the cross-diameter of the affected vessel
• If occlusion persists for more than 20 minutes, irreversible myocardial damage occurs

Pathology of MI

• The trans-mural infarct undergoes a characteristic sequence of gross and microscopic changes

Diagnosis of MI

• Clinical: Chest pain, dyspnea, and arrhythmia
• ECG: Elevated ST segment and inversion T wave
• Enzyme changes:
  • Increased CPK (Creatine Phosphokinase) - specific, peaks at 24 hours, and falls in 3 days
  • Increased LDH (Lactic Dehydrogenase) - peaks at 48-72 hours, and falls in 7-10 days
  • Increased SGOT (Serum Glutamic Oxaloacetic Transaminase) - not specific
  • Leucocytosis

Complications of MI

• Arrhythmia (90%)
• Cardiogenic shock
• Ventricular aneurysms
• Mural thrombosis
• Rupture causing cardiac tamponade
• Congestive Heart Failure (CHF)
• Sudden death

Atherosclerosis

• Definition: Chronic degenerative disease of arteries characterized by raised yellowish-white sub-intimal plaques formed of lipid deposition, proliferated smooth muscle cells, and extracellular matrix
• Risk factors:
  • Major: Hypertension, Smoking, Diabetes Mellitus, Hyperlipidemia
  • Minor: Obesity, Old age, Male gender, Sedentary and stressful life, Oral contraceptive pills, Positive family history
• Pathogenesis:
  • Endothelial injury theory: Physical and chemical trauma leading to endothelial cell injury, increase in endothelial permeability to lipids, platelet and monocyte activation, and macrophage engulfing lipids
  • Smooth muscle proliferation theory: Migration of smooth muscles from media to intima, followed by proliferation and synthesis of extracellular matrix
• Pathological features:
  • Sites: Large arteries (aorta and its main branches) and small arteries (coronary, cerebral, retinal, and renal)
  • Gross picture: Raised yellowish-white atheromatous plaques (1-1.5 cm) in large and small arteries protruding into the vascular lumen
  • Complications: Fissuring, calcification, or ulceration leading to thrombosis and vascular occlusion
  • Microscopically: Atheroma is composed of superficial fibrous cap, cellular zone, central necrotic zone, and internal elastic lamina