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

Abstract: Socioeconomic differences in myocardial infarction risk Epidemiologicalanalyses of causes and mechanisms by Johan Hallqvist, Karolinska Institutet, Dept of Public Health Sciences, Division of Social Medicine, SE-172 83 Sundbyberg, Sweden This thesis investigates the seize of the socioeconomic differences in risk ofacute myocardial infarction (AMI), the time trends in incidence in different socioeconomicgroups, and explores some mechanisms generating the socioeconomic gradient. Threedifferent types of mechanisms were considered: causal chains, interaction or susceptibility,and triggering mechanisms. It analyses whether there is interaction between psychologicaldemands and decision latitude resulting in job strain and increased risk of AMI,whether physical exertion triggers AMI, and the contribution of job strain and tobaccosmoking to the increased risk of AMI in lower socioeconomic groups. The thesis is based on three studies. The first is a register cohort (the SocialEpidemiology Register) covering the population in Stockholm County during the period1971 to 1986, with 19513 cases of AMI. The second is a population-based case referentstudy (Stockholm Heart Epidemiology Program - SHEEP) with 2246 cases recruited 1992to 1994 in the same region. The third is a case-crossover study (ONSET) of 699 casesrecruited 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 females1.46 (95% CI: 1.13-1.88). When comparing low level employees, skilled workers andunskilled 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 amongmale high and middle level employees declined since 1976, and in male manual workerssince 1981. For female manual workers incidence increased steadily 1971 to 1986 andfor high and middle level employees it decreased. The increase over time of the relativerisk 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 amongmanual workers to job strain and its components explained about 25-50 % of theirrelative excess risk. The increased prevalence of low decision latitude explainedabout 30% of the socioeconomic difference in AMI. The increased prevalence of smoking explained about 20% of the relative excessrisk for male workers and 10% for female workers. The increased susceptibility tosmoking explained three quarters of the relative excess risk in both genders. Therelative risk from vigorous exertion was 4.5 (95% CI 3.1-6.6). The induction periodlasted from a few minutes to less then two hour. An increased risk remained for 45minutes after an episode of heavy exertion. Keywords: myocardial infarction, socioeconomic factors, job strain, smoking, physicalactivity, incidence, time trends, interaction, case-crossover.