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Why can a plaque that narrows an artery by only 50 percent still cause a heart attack?

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The role of coronary artery disease in heart attack — cardiovascular disease This micrograph shows a cross section of a coronary artery narrowed by an atherosclerotic plaque (purplish matter inside the artery). Most heart attacks are caused by the rupture of an atherosclerotic plaque, which triggers the formation of a clot that abruptly blocks blood flow to part of the heart muscle.

A plaque that narrows an artery by only 50 percent can still cause a heart attack because of the risk of acute thrombosis, rather than just the physical size of the blockage itself.1

While chronic, stable atherosclerosis typically reduces blood flow only when lumen occlusion exceeds 70 percent, an atherosclerotic plaque is also susceptible to disruptive influences. If the surface of a plaque is disrupted, it can trigger the formation of a blood clot, or thrombus, directly on the plaque.1

This process causes a rapid, acute occlusion of the coronary artery. Because this blockage happens suddenly, the body lacks the time to compensate through mechanisms like collateral circulation. The resulting sudden loss of blood flow, or ischemia, leads to irreversible tissue damage and a myocardial infarction. Therefore, the danger lies not only in the degree of narrowing but in the plaque's potential to cause a sudden, total blockage.1