Objectives: To analyze the circadian pattern of the first signs of myocardial infarction in France.
Methods: The USIK study was conducted in November 1995 among patients hospitalized in cardiac intensive care units within 48 hours after the first signs of confirmed myocardial infarction. This prospective study recorded the day and hour of onset for each eligible patient.
Results: Three hundred seventy-three centers throughout France participated, including 2563 patients (71% men), mean age 68 years, with confirmed myocardial infarction. The day of the week in which the first signs occurred was recorded between November 3 and 30, 1995. The pattern did not show any peaks, notably on Monday as reported in the literature. The hour of onset was known in 2468 patients (96.3%). Demographic features and past history did not vary for the overall population. Hour of onset was: between midnight and 6 a.m. 20.1%; between 6 and 12 a.m. 30.6%; between noon and 6 p.m. 25.7%; between 6 p.m. and midnight 23.6% (p < 0.001). This pattern was also studied in several subgroups of patients defined by age, sex, presence or absence of different cardiovascular risk factors or pertinent cardiovascular history. Univariate analysis showed that the distribution in the 6 a.m.-noon period was significantly different by age group. Older patients had a higher morning peak than younger patients (32.6% versus 28.7%; p = 0.03). Inversely, this peak was lower in smokers compared with non-smokers (26.7% versus 32.4%; p = 0.005); likewise, in patients with recurrent infarction compared with first infarction patients (24.7% versus 31.9%; p = 0.003). Other variables did not affect the circadian pattern. After multivariate analysis, only 2 of these 3 factors had an influence on distribution in the 6-12 a.m. period: smoking habits (odds ratio = 0.75; p < 0.01) and prior myocardial infarction (odds ratio = 0.70; p < 0.01).
Conclusion: This large epidemiology study confirms that there is a morning peak in the circadian pattern of myocardial infarction in France. Smoking and past history of infarction significantly affect the circadian pattern.