Myocardial Infarction - Pathophysiology

MI can be considered the end-point of CHD. Unlike the temporary ischemia that occurs with angina, prolonged unrelieved ischemia causes irreversible damage to the myocardium. Cardiac cells can withstand ischemia for about 15 minutes before they die. Because the myocardium is metabolically active, manifestations of ischemia can be seen within 8 to 10 seconds of decreased blood flow. When the heart does not receive blood and oxygen, It converts to anaerobic metabolism, creating less adenosine triphosphate (ATP) and more lactic and as a by-product. Myocardial cells are very sensitive to changes in PH and become less functional. Acidosis causes the myocardium to become more vulnerable to the effects of the lysosomal enzymes within the cell. Acidosis leads to conduction system disorders, and dysrhythmias develop. Contractility is also reduced, decreasing the hearts ability to pump. As the myocardial cells necrose, intracellular enzymes are introduced into the bloodstream, where they can be detected by laboratory tests.

Zones of infarction and necrosis

Cellular necrosis occurs in one layer of myocardial tissue in subendocardial, intramural and subepicardial infarctions. In a transmural infarction, cellular necrosis is present in all three layers of myocardial tissue. The infarct site is called the zone of infarction and necrosis. Around it is a zone of hypoxic injury also called a penumbra. This zone can return to normal but may also become necrotic if blood flow is not restored. The outermost zone is called the zone of ischemia.

Transmural infarctions cause changes in the architecture of the left ventricle, called remodeling, which can result in acute or chronic heart failure. Within few hours of MI the necrotic area stretches in a process called infarct expansion.

The most common site of an MI is the anterior wall of the left ventricle near the apex, resulting from thrombosis of the descending branch of the left coronary artery. Other sites include the posterior wall of the left ventricle and the inferior surface of the heart.

Category: Medicine Notes