Seasonal variation in the frequency of myocardial infarction diagnosed in a large emergency department of a European country with a temperate climate

Previous studies at different latitudes showed that acute myocardial infarction (AMI) exhibits a seasonal variation, with higher frequency in spring and winter. We conducted a retrospective analysis to verify whether the frequency of AMI cases diagnosed in the emergency department (ED) may follow a seasonal pattern in a European country with a temperate climate. A retrospective analysis was performed in the hospital database of the University Hospital of Parma (northwestern Italy), to retrieve the total number of AMI cases diagnosed in the ED during the entire year 2010. The search for AMI cases was conducted using both ICD-9 codes and related diagnostic terms. The seasonality was defined according to the typical equinoxes and solstices at the latitude of the study. A total of 83,919 patients visited the ED of the University Hospital of Parma during the year 2010, 502 (0.6%) of whom with a final diagnosis of AMI (mean age, 73±14 years; 188 women and 314 men). The largest frequency of AMIs was observed in autumn (n=148; 29%), followed by winter (n=136, 27%), whereas the lowest frequencies were recorded in spring (n=110; 22%) and summer (n=108; 22%). The difference in frequency distribution of AMI cases across the four seasons of the year was found to be statistically significant (P<0.001), and this trend was independent from sex and age. Compared to the summer period (i.e., the season with the lowest frequency of AMI cases), the relative risk (RR) for AMI was significantly higher in autumn (1.37; 95% CI, 1.15-1.63; P<0.001) and winter (1.26; 95% CI, 1.05-1.51; P=0.013), but not in spring (1.02; 95% CI, 0.83-1.24; P=0.857). Compared to the spring period, the RR for AMI was found also to be significantly higher in autumn (1.34; 95% CI, 1.13-1.60; P<0.001) and winter (1.24; 95% CI, 1.03-1.48; P=0.021).