Coronary Artery Disease: Plaque Rupture, Thrombosis, and Acute Coronary Syndromes

Core Idea

Atherosclerotic plaques with thin fibrous caps overlying lipid-rich cores rupture under hemodynamic stress, exposing thrombogenic material and triggering intra-plaque thrombosis. Depending on extent and location, this causes unstable angina, NSTEMI, or STEMI, with myocardial necrosis proportional to ischemic duration.

How It's Best Learned

Compare stable vs. unstable plaques histologically and relate to clinical presentation. Study time-based myocardial damage patterns from early necrosis to chronic remodeling.

Common Misconceptions

Not all coronary lesions causing acute MI are high-grade stenoses; many rupture from non-obstructive plaques with high lipid burden.

Explainer

From your study of atherosclerosis, you know that lipid-laden plaques accumulate over decades in coronary artery walls, progressively narrowing the lumen. But the most dangerous plaques are not always the biggest. The transition from chronic, stable coronary artery disease to an acute life-threatening event is driven not by plaque size but by plaque vulnerability — a structural property that determines whether a plaque will quietly persist or catastrophically rupture.

A vulnerable plaque has three features that distinguish it from a stable one: a large lipid-rich necrotic core, a thin fibrous cap (less than 65 micrometers), and active inflammation with macrophages concentrated at the cap's shoulder regions. These macrophages secrete matrix metalloproteinases that digest the collagen holding the cap together, while producing cytokines that inhibit smooth muscle cells from synthesizing new collagen. The result is a progressively thinning cap stretched over a growing lipid pool — mechanically, like a balloon with a worn spot. Hemodynamic stress from turbulent flow or a sudden surge in blood pressure (a common trigger for morning events) can rupture that cap. Importantly, this process occurs in plaques that may obstruct only 30–50% of the vessel lumen — insufficient to cause stable angina, but structurally primed to rupture.

Plaque rupture exposes the thrombogenic contents of the lipid core — tissue factor, collagen, and von Willebrand factor — directly to circulating blood. The coagulation cascade and platelet activation proceed simultaneously: platelets adhere, aggregate, and activate, while the extrinsic pathway drives thrombin generation and fibrin mesh formation. Within minutes, an intracoronary thrombus forms at the rupture site. The clinical outcome depends on whether this thrombus is partial or complete. Partial occlusion with maintained distal flow produces unstable angina (no biomarker rise) or NSTEMI (biomarker rise indicating microinfarction); complete occlusion cuts off all antegrade flow, producing the ST elevation on ECG that defines STEMI.

Time is myocardium. From the moment of complete occlusion, cardiomyocytes in the territory supplied by that artery begin dying. The wavefront of necrosis progresses from endocardium to epicardium over the first six hours. The subendocardium dies first because it is furthest from epicardial vessels and has highest oxygen demand. If reperfusion (by PCI or thrombolytics) is achieved within 90 minutes, most of the myocardium can be salvaged; by six hours, a full transmural infarct is likely. This is why the "door-to-balloon time" metric in emergency medicine is so critically watched — every minute of delay converts stunned, salvageable myocardium into scar.

The paradox that non-obstructive plaques cause more acute MIs than obstructive ones has profound clinical implications. An angiogram showing 40% stenosis looks reassuring — the lumen is wide open, flow is normal, and the patient has no symptoms. But if that plaque has a thin cap and a large lipid core, it is the more dangerous lesion. This is why aggressive medical therapy (high-intensity statins, antiplatelets) targets plaque stabilization — thickening the fibrous cap, reducing lipid core size, and suppressing macrophage inflammation — rather than merely reducing stenosis.