Geographical differences in the incidence of acute myocardial infarction in Sweden. Analyses of possible causes using two parallel case-control studies

Objectives. To analyse differences in myocardial infarction incidence betwe en two Swedish counties and to evaluate the importance of major risk factor s for the observed differences. Design. The incidence of first myocardial infarction was studied using info rmation from registers. For a number of risk factors of myocardial infarcti on, the prevalence as well as the relative risk was estimated from populati on controls of case-control studies in the two areas. Subjects. Men and women aged 45-64 years in Stockholm and Vasternorrland Co unty 1993-94. Main outcome measures. Relative risks (RRs) and impact fractions were used to evaluate the importance of differences in risk factor prevalence for dif ferences in myocardial infarction incidence between the two areas. Results. The incidence of first myocardial infarction was higher in Vastern orrland than in Stockholm amongst both men (RR = 1.23; 95% CI = 1.08-1.40) and women (RR = 1.41; 95% CI = 1.11-1.79). Obesity and increased levels of blood serum lipids were more prevalent in Vasternorrland than in Stockholm amongst men with impact fractions of 6 and 9-11%, respectively. Amongst wom en, corresponding differences were not seen, but job strain and shift work tended to be more common in the more northern area. Current smelting was mo re frequent in Stockholm, particularly for women. Conclusions. The incidence of first myocardial infarction was higher in Vas ternorrland than in Stockholm in both genders. A higher prevalence of obesi ty and elevated blood serum lipids may explain, in part, this excess incide nce amongst men, but amongst women the causes of the higher incidence in th e more northern area remain largely unclear.