The clinical outcome of acute myocardial infarction is related to the circadian rhythm of myocardial infarction onset.

In order to study whether the circadian rhythm of acute myocardial infarction (AMI) onset has any impact on the clinical outcome, in terms of enzymatically estimated infarct size, circulatory arrest from ventricular tachyarrhythmias, and in-hospital mortality, the authors studied a representative population of 10,791 AMIs treated in the same center between 1973 and 1987. In 6,763 cases (63%) the time of symptom onset was known. In these cases a major peak in AMI incidence was observed between 7:01 AM and 10:00 AM, and minor peaks at 12:01 noon, 3:01 PM-4:00 PM, and 8:01 PM-9:00 PM. In a set of multivariate analyses including several clinical characteristics, symptom onset between 6:00 AM and 12:00 noon significantly predicted a greater infarct size, and symptom onset between 12:00 AM and 6:00 AM was associated with a significantly lower risk of circulatory arrests from ventricular tachyarrhythmias. The time of symptom onset was not significantly associated with in-hospital mortality after adjustment for other clinical characteristics, including infarct size. The authors suggest that the time of day has an impact, not only on the incidence, but also on the severity of AMI and that the ability of beta blockers to blunt the morning increase in AMI incidence may possibly contribute to the beneficial secondary preventive effects of such drugs after AMI.