Day of week, monthly and seasonal variation of acute myocardial infarction

The chronobiology of the acute myocardial infarction (AMI) is an interesting matter for recognition of the biological mechanism that might trigger AMI. The present study was aimed to investigate the circadian, seasonal and circannual variation of the AMI occurrence with solar, lunar, and Christian months. A retrospective study of 1174 cases with diagnosis of AMI was conducted. All cases included in our study were admitted to the coronary care unit of Shahid Beheshti and Yahyanejad Hospitals in Babol, the north of Iran, between 1990 and 2000. The data was extracted by the day and month of diagnosis based on the solar, lunar and Christian calendar from hospital charts. The chi-square test was used to test the uniformity of the observed frequencies of AMI onset in relation of the time factors based on solar, lunar, Christian and the days of week. The results showed that there was a significant trend toward higher rate of MI at the beginning of the week, on the Saturday (18.1%) and the least (12.3%) on Friday. Although, the observed frequency of AMI tended to be higher month than other months, the monthly variation was not statistically significant (P>0.05). However, there was significant difference in MI occurrence in Christian months with high peak in November (P<0.05). Also, we did not found as significant seasonal variation of AMI were 26.2%, 23.1%, 26.8% and 23.9% of MI occurred in the spring, summer, fall, and winter respectively (P>0.05). The average number of AMI occurrence, was significantly greater in three days of full moon phase in lunar months than other days (P<0.05). Our results indicated a significant day of week variation in AMI occurrence with higher peak on Saturday and full moon phase of lunar month. With regard the lunar and solar month variation, more investigation, with larger sample sizes is still necessary but in Christian months, MI occurrence was significantly increased in November.