Circadian variation and possible external triggers of onset of myocardial infarction. SPRINT Study Group.

PURPOSE: To determine whether a circadian pattern in onset of symptoms existed and possible external triggers were implicated in the precipitation of acute myocardial infarction (AMI). PATIENTS AND METHODS: One thousand eight hundred eighteen consecutive patients with AMI hospitalized in 14 of the 21 existing coronary care units in Israel during the study period were assessed. RESULTS: The frequency of onset of symptoms by 6-hour intervals showed a predominant morning peak (6 AM to noon) (32%, p < 0.01) in comparison with the other three 6-hour intervals of the day. The preponderance of the morning peak persisted for subgroup analysis by gender (males 32%, females 31%); age (less than or equal to 65 years--32%; greater than 65 years--33%); diabetes mellitus (present or absent, 32%). However, patients with peripheral vascular disease and those with stroke in the past had a predominant evening peak. Possible external triggers of onset of AMI were present in 10% of patients. Exceptional heavy physical work, violent quarrel at work or at home, and unusual mental stress were the three most frequent possible external triggers reported immediately before or within the 24 hours preceding pain onset. Patients with possible external triggers were more likely to be males (85%) and were somewhat but not significantly younger (63.1 years) in comparison with patients without external triggers (73% and 64.3 years respectively). CONCLUSIONS: In a large group of consecutive patients with AMI, a predominant cyclic morning peak of pain onset was found in comparison with the other hours of the day. Possible external triggers precipitating AMI were involved in a minority of cases, suggesting that endogenous changes occurring in the morning hours are generally responsible for the increased rate of myocardial infarction occurring after awakening.