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[Circadian variation in the incidence of myocardial infarction. New perceptions about the mechanisms of acute coronary disease].

The time of acute myocardial infarction (MI) was determined from the onset of symptoms and an analysis made of the time-activity course of the plasma CK-MB isoenzyme in 1741 patients of the prospective multicentre ISAM study (much greater than Intravenous Streptokinase in Acute Myocardial Infarction much less than). The occurrence of MI had a circadian variation with a marked peak between 6 a.m. and 12 noon compared with other periods during the day (P less than 0.001). Between 8 and 9 a.m., the period of highest incidence, the number of MIs was about four times higher than between midnight and 1 am (the period of lowest incidence). There was a good correlation between the clinical and enzymatic criteria for onset of MI (r = 0.95; P less than 0.001). A similar circadian pattern was obtained independent of site of MI, age and sex of patients, as well as severity and extent of the coronary heart disease. The temporal relationship between increased incidence of acute coronary heart disease (MI, sudden cardiac death, symptomatic or silent myocardial ischaemia) in the morning and a rise in platelet aggregation in the morning may point to a causal relationship.