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Myocardial infarction results from acute coronary occlusion causing transmural or subendocardial necrosis. Ischemia initiates a cascade of metabolic derangement, calcium overload, and reactive oxygen species production; reperfusion paradoxically accelerates cell death through inflammation and apoptosis.
Study the temporal progression of necrosis (12–24 hours for full transmural involvement) and correlate with biomarker rise (troponin, CK-MB). Understand reperfusion injury as a distinct mechanism from ischemic injury.
Troponin elevation begins 2–4 hours post-infarction, not immediately—early angiography shows no enzyme change. Reperfusion is not uniformly beneficial; it can paradoxically increase mortality in certain settings.
You've already studied how atherosclerotic plaque forms in coronary arteries — a process that narrows the lumen and stiffens the vessel wall over decades. Myocardial infarction is what happens when that slow process suddenly becomes acute. The precipitating event is almost always plaque rupture or erosion: the fibrous cap overlying a lipid-rich, necrotic atherosclerotic core tears, exposing the highly thrombogenic subendothelial contents to flowing blood. Within seconds, platelets adhere and activate at the rupture site; within minutes, the coagulation cascade generates fibrin; and within an hour, a fully occlusive thrombus can cut off perfusion to the downstream myocardium.
The ischemic cascade begins immediately after occlusion. Cardiomyocytes are obligate aerobic metabolizers with almost no glycogen reserve — they exhaust ATP within seconds to minutes of ischemia. Anaerobic glycolysis acidifies the cell, Na⁺/K⁺-ATPase fails as ATP is depleted, sodium accumulates intracellularly, and osmotic water influx causes cell swelling. The critical step in irreversible injury is calcium overload: as Na⁺/K⁺-ATPase fails, the Na⁺/Ca²⁺ exchanger reverses and floods the cell with calcium. Mitochondrial calcium overload activates destructive enzymes — phospholipases, proteases, endonucleases — and triggers mitochondrial permeability transition. Beyond approximately 20–40 minutes of complete ischemia, cardiomyocyte death by coagulative necrosis becomes irreversible. The wave of necrosis progresses from the subendocardium outward; complete transmural infarction takes 12–24 hours to develop fully.
Ischemia-reperfusion injury is the paradox at the heart of myocardial infarction treatment. When coronary blood flow is restored — by thrombolysis or percutaneous coronary intervention — there is unambiguous net benefit: salvaging living but stunned myocardium. But reperfusion also causes harm. Re-oxygenation of ischemic mitochondria generates a burst of reactive oxygen species. Calcium that accumulated during ischemia now enters mitochondria at high concentrations, triggering the mitochondrial permeability transition pore (mPTP) to open permanently, collapsing the proton gradient and releasing cytochrome c to initiate apoptosis. Neutrophil influx with reperfusion adds further inflammatory injury. The result is that some cells — viable at the moment of reperfusion — die in subsequent hours because of the reperfusion process itself, not the original ischemia.
The temporal sequence of biomarker release reflects the cellular destruction sequence directly. Troponin I and T are structural proteins bound to the cardiac contractile apparatus; they are released into the bloodstream as the cardiomyocyte membrane is destroyed. Because they must diffuse from dead cells into lymphatics and then blood, they don't rise until 2–4 hours post-infarction and peak at 24 hours. CK-MB (the cardiac isoform of creatine kinase) rises faster and clears faster, making it useful for detecting reinfarction. A patient presenting 30 minutes after chest pain onset may have a normal troponin despite active, ongoing infarction — the evolving troponin trend rather than any single value tells the story of necrosis progressing in real time.