Circadian and weekly variations in pain onset of acute myocardial infarction.

A seasonal variation with more myocardial infarctions in the winter months due to cold weather has been reported. Other reports have described excess numbers of acute myocardial infarction (AMI) in the summer in Southern USA due to hot temperatures. To determine whether circadian and seasonal variations affect the incidence of AMI in the subtropical area of Taiwan, 480 consecutive patients with AMI admitted to our coronary care units were analyzed.Six-hourly intervals over 24 hours (4 periods), daily intervals in a week (7 days) and monthly intervals in a year (12 months) were respectively studied. The distributions of the numbers of AMI occurring in the six-hour intervals were tested for differences among the four periods using the chi-squared test for goodness of fit.We found that there was a circadian variation in the onset of AMI with a morning peak (6 am to noon) (35%, chi 2 = 28.52, df = 3, p < 0.01) but no secondary late evening peak. The incidence of AMI was significantly lower on Sundays (9%) than on the other weekdays (chi 2 = 16.37, df = 6, p = 0.012). However, no seasonal variation (no winter or summer peaks) occurred in the incidence of AMI in this study (chi 2 = 0.77, df = 3, p = 0.99).Our results showed that there was a predominant morning peak in the onset of AMI. The low incidence of AMI cases on Sunday compared with other weekdays suggested that relief from tension or workload on Sundays might have an important role in this low percentage of AMI. Differing from other reports, there was no seasonal variation in the occurrence of AMI in our study, suggesting that the warm climate of a subtropical region does not provide an environment that is likely to increase the frequency of AMI.