Monday, March 26, 2012

Causes, Pathogenesis, Morphology of Acute Myocardial Ischemia and Myocardial Infarction (MI)


Pathogenesis
  1. Thrombosis superimposed on a complicated stenosing atheroma is the basic cause of acute myocardial ischemia and myocardial infarction.
  2. The risk factors of atherosclerosis are hypercholesterolemia, cigarette smoking, diabetes mellitus, physical inactivity and use of oral contraceptives. Checkout risk factors.
  3.  Myocardial necrosis begins within approximately 20-30 minutes after coronary artery occlusion.
  4. Subendocardial region of myocardium is the most poorly perfused region of the ventricular wall and therefore myocardial infarct typically begins in the subendocardial region. The infarct usually reaches it full size within a period of 3-6 hours (During this period of infarction in evolution lysis of thrombus by administration of thrombolytic agents e.g. streptokinase or tissue plasminogen activator may limit the size of the infarct.
  5. Location of infarct depends on the coronary artery involved e.g.
    1. Occlusion of anterior descening artery (LAD) leads to infarction in anterior and apical areas of left ventricle and adjacent anterior 2/3 of interventricular septum.
    2. Occlusion of right coronary artery (RCA) leads to infarction in posterior wall of left ventricle, posterior 1/3 of interventricular septum.
    3. Occlusion of left circumflex artery (LCx) leads to infarction in lateral wall of left ventricle.
  6. Size of infarct depends on the following factors:
    1. Which segment of artery is blocked: Occlusion of proximal segments of the coronary arteries produces larger infarcts, involving the full thickness of the myocardium. Conversely, occlusion in more distal arterial branches tend to cause smaller infarcts.
    2. Degree of collateral circulation: In patients with long-standing coronary atherosclerosis, collateral circulation may develop over time in response to chronic ischemia. Such collateral vessels may limit the size of the infarct.

Consequences of Acute Myocardial Ischemia
The Acute myocardial ischemia has four possible consequences:
1.      It may only induce an attack of angina.
2.      More severe ischemia may result in myocardial infarction limited to inner 1/2  some portion or the entire circumference of the left ventricular wall to produce subendocardial foci of ischemic necrosis, also called subendocardial infarct.
3.      The ischemic necrosis (infarction) may traverse the entire thickness of some portion of the left ventricular wall, creating transmural infarct.
4.      Sudden cardiac death.

Morphology
1.      Transmural infarct: Greater than 2.5 cm and traverses from endocardium to subepicrdial myocardium. It is more common than subendocardial one.
2.      Subendocardial infarct: Multifocal area of necrosis confined to inner 1/3 – 1/2 of the left ventricular wall.
3.      Sequence of Morphologic Changes in Transmural Infarct

Morphologic changes in myocardial infarction depend on the time passed from onset. There is coagulation necrosis and inflammation followed by formation of granulation tissue, resorption of necrotic material and finally organization of the granulation tissue to form a fibrous scar.

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