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Chapter 37: Pathology of Myocardial Infarction and Sudden Death

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Which of the following statements about myocardial energy metabolism is correct?

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A. Under normal aerobic conditions, most cardiac energy is derived from glycolysis

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B. Decreased adenosine triphosphate (ATP) production due to ischemia results in decreased cytosolic Na+ and cell swelling

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C. Myocardial ischemia primarily affects mitochondrial metabolism and oxidative phosphorylation

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D. Reversibly injured myocytes are characterized by cell membrane breaks and amorphous densities in the mitochondria

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E. Only one-third of the ATP used by the heart goes to contractile shortening

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The answer is C. (Hurst’s The Heart, 14th Edition, Chap. 37) The normal function of the heart muscle is supported by high rates of myocardial blood flow, oxygen consumption, and combustion of fat and carbohydrates (glucose and lactate). Under normal aerobic conditions, cardiac energy is derived from fatty acids, supplying 60% to 90% of the energy for ATP synthesis (option A). The rest (10% to 40%) comes from the oxidation of pyruvate formed from glycolysis and lactate oxidation. Almost all of the ATP formed comes from oxidative phosphorylation in the mitochondria; only a small amount of ATP (< 2%) is produced by glycolysis. Approximately two-thirds of the ATP used by the heart goes to contractile shortening (option E), and the remaining third is used by sarcoplasmic reticulum Ca2+ ATPase and other ion pumps. Myocardial ischemia primarily affects mitochondrial metabo­lism, resulting in a decrease in ATP formation by shutting off oxidative phosphorylation (option C). Decreased ATP inhibits Na+/K+-ATPase, increasing intracellular Na+ and Cl, leading to cell swelling (option B). Reversibly injured myocytes are edematous and swollen from the osmotic overload. Irreversibly injured myocytes contain shrunken nuclei with marked chromatin margination. The two hallmarks of irreversible injury are cell membrane breaks and the mitochondrial presence of small osmiophilic amorphous densities (option D).

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A 63-year-old man presents to the emergency department with onset of chest pain while washing his car. He has a history of angina and MI due to culprit LAD disease. Which of the following could be a contributor to smaller infarct size?

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A. The absence of coronary collateral vessels

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B. The extended duration of coronary occlusion

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C. The earlier occlusion of the same coronary artery

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D. Ventricular remodeling

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E. None of the above

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The answer is C. (Hurst’s The Heart, 14th Edition, Chap. 37) Myocardial ischemia occurs when oxygen and nutrient supply do not meet myocardial demand, and necrosis or infarction occurs when ischemia is severe and prolonged. The extent of coronary collateral flow is one of the principal determinants of infarct size. Indeed, at ...

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