Circadian variations in the onset of acute myocardial infarction

BACKGROUND: Several studies have observed a circadian pattern in the onset of acute myocardial infarction (AMI), with a peak incidence in the morning hours. It has been suggested that different circadian rhythms may exist in various subgroups of patients. METHODS: This study sought to determine whether the circadian incidence of AMI varied by sex, age, cardiovascular risk factors, previous history of ischemic accidents, the site of AMI, and the short-term outcome. These possibilities were examined in a population of 597 consecutive patients with AMI, admitted to the coronary care unit. 548 patients have been included in the study, 442 men (80.6%) and 106 women (19.4 %); mean age 64.5 years. RESULTS: A peak incidence of AMI was found between 06.01 a.m. and 12.00 a.m. (32.4%; p<0.0002). This peak was present in patients 65 years old (33.2%; p<0.005), in men (32.5%; p<0.0002) but not in women, in smokers (32.1%; p<0.0005) and in those that did not smoke (33.0%; p<0.04), in patients with hypercholesterolemia (34.9%; p<0.006 ) and without hypercholesterolemia (31.1%; p<0.03). A circadian rhythm was absent in diabetics, hypertensives and in patients with a history of previous cardiovascular events. Regarding the site of AMI, inferior AMI showed an increased incidence between 06.01 a.m. and 12.00 a.m. (36.2%; p<0.002), while the circadian distribution of anterior AMI, as well as non-Q wave AMI, did not show this incidence. Finally, higher mortality was reported in patients with an AMI onset at night (22.3%). CONCLUSIONS: These results give further clues in understanding the external and inner factors acting in the morning hours as triggers for AMI.