Socioeconomic differences in myocardial infarction risk : epidemiological analyses of causes and mechanisms

This thesis investigates the seize of the socioeconomic differences in risk of acute myocardial infarction (AMI), the time trends in incidence in different socioeconomic groups, and explores some mechanisms generating the socioeconomic gradient. Three different types of mechanisms were considered: causal chains, interaction or susceptibility, and triggering mechanisms. It analyses whether there is interaction between psychological demands and decision latitude resulting in job strain and increased risk of AMI, whether physical exertion triggers AMI, and the contribution of job strain and tobacco smoking to the increased risk of AMI in lower socioeconomic groups.

The thesis is based on three studies. The first is a register cohort (the Social Epidemiology Register) covering the population in Stockholm County during the period 1971 to 1986, with 19513 cases of AMI. The second is a population-based case referent study (Stockholm Heart Epidemiology Program - SHEEP) with 2246 cases recruited 1992 to 1994 in the same region. The third is a case-crossover study (ONSET) of 699 cases recruited from SHEEP.

The relative risk of non-fatal and fatal first events of AMI for male manual workers (compared with non-manual employees) was 1.65 (95% CI: 1.38-1.97), and for females 1.46 (95% CI: 1.13-1.88). When comparing low level employees, skilled workers and unskilled workers with high and middle level employees, the relative risks were: 1.21, 1.66, and 1.77 for men, and 1.73, 1.84, and 2.05 for women. Incidence among male high and middle level employees declined since 1976, and in male manual workers since 1981. For female manual workers incidence increased steadily 1971 to 1986 and for high and middle level employees it decreased. The increase over time of the relative risk in low socioeconomic positions continued into the 1990s.

High demands and low decision latitude interact with a synergy index of 7.5 (95% C.I.: 1.8-30.6) among occupied men 45-64 years old. An increased susceptibility among manual workers to job strain and its components explained about 25-50 % of their relative excess risk. The increased prevalence of low decision latitude explained about 30% of the socioeconomic difference in AMI.

The increased prevalence of smoking explained about 20% of the relative excess risk for male workers and 10% for female workers. The increased susceptibility to smoking explained three quarters of the relative excess risk in both genders. The relative risk from vigorous exertion was 4.5 (95% CI 3.1-6.6). The induction period lasted from a few minutes to less then two hour. An increased risk remained for 45 minutes after an episode of heavy exertion.