Pathophysiology and initial management of the acute coronary syndromes.

Acute coronary syndromes are responsible for more than half a million hospital admissions each year in the United States alone. Plaque rupture is the precipitating pathophysiologic event. The degree of narrowing of plaques that rupture is not necessarily severe, in the range of 30% to 70% diameter stenosis. Plaques containing large lipid pools with only thin fibrous caps are most at risk. The site of rupture is most often at the shoulder of the plaque, where stress is highest. Clusters of macrophages are often seen at these points. Most plaque ruptures heal without causing symptoms, perhaps leaving a narrowing somewhat more severe than before. Plaque ruptures that expose larger areas of thrombogenic intramural debris to flowing blood in areas of high turbulence are most likely to provoke more extensive thrombosis. Risk factors, particularly smoking and hypercholesterolemia, cause increased thrombin deposition at the site of deep arterial injury. Thrombin deposition causes local coronary vasoconstriction that may contribute to the development of ischemia. Whether plaque rupture with thrombosis causes infarction, unstable angina, or no symptoms at all depends on the site of the lesion, its severity, and whether the jeopardized myocardium is served by collaterals. Aspirin, heparin, and, potentially, the newer agents provide benefit in each of the acute coronary syndromes.