MI refers to a dynamic process by which one or more regions of the heart experience a severe and prolonged decrease in oxygen supply because of insufficient coronary blood flow; subsequently, necrosis or death to the myocardial tissue occurs. The onset of the MI process may be sudden or gradual, and the progression of the event to completion takes approximately 3 to 6 hours.
- Acute coronary thrombosis (partial or total)associated with 90% of MIs.
- Severe CAD (greater than 70% narrowing of the artery) precipitates thrombus formation.
- The first step in thrombus formation involves plaque rupture. Platelets adhere to the damaged area.
- Activation of the exposed platelets causes expression of glycoprotein IIb/IIIa receptors that bind fibrinogen.
- Further platelet aggregation and adhesion occurs, enlarging the thrombus and occluding the artery.
- Other etiologic factors include coronary artery spasm, coronary artery embolism, infectious diseases causing arterial inflammation, hypoxia, anemia, and severe exertion or stress on the heart in the presence of significant CAD (ie, surgical procedures or shoveling snow).
- Different degrees of damage occur to the heart muscle (see Figure 13-1):
- Zone of necrosis -death to the heart muscle caused by extensive and complete oxygen deprivation; irreversible damage
- Zone of injury- region of the heart muscle surrounding the area of necrosis; inflamed and injured, but still viable if adequate oxygenation can be restored
- Zone of ischemia- region of the heart muscle surrounding the area of injury, which is ischemic and viable; not endangered unless extension of the infarction occurs
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- According to the layers of the heart muscle involved, MIs can be classified as:
- Transmural (Q wave) infarction- area of necrosis occurs throughout the entire thickness of the heart muscle.
- Subendocardial (nontransmural/non Q-wave) infarction - area of necrosis is confined to the innermost layer of the heart lining the chambers.
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- Location of MI is identified as the location of the damaged heart muscle within the left ventricle: inferior, anterior, lateral, and posterior or right ventricle.
- Left ventricle is a common and dangerous location for an MI, because it is the main pumping chamber of the heart.
- Right ventricular infarctions commonly occur in conjunction with damage to the inferior and/or posterior wall of the left ventricle.
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Region of the heart muscle that becomes damaged -determined by the coronary artery that becomes obstructed (see Figure 13-2).
- The amount of heart muscle damage and the location of the MI determine prognosis.