[Mechanisms of myocardial ischemia and circadian fluctuations of ischemic episodes].

The circadian variation of angina pectoris, myocardial infarction and sudden death, with a first peak in the early morning and a second one in the late afternoon, suggests that physical activity plays an important role in triggering ischemic events by an increase of cardiac oxygen demand. But there is also evidence of decreased oxygen supply related to a paradoxical catecholamine-induced vasoconstriction which reflects the endothelial dysfunction of atherosclerotic coronary arteries. Elevated blood pressure during daytime and other mechanical factors can facilitate a plaque-rupture which triggers formation of coronary thrombosis. Hypercoagulability in the early morning hours by increased platelet aggregability and by elevated levels of plasminogen-activator inhibitors may contribute to this fatal mechanism.