THEORY AND METHODS

Job strain and ischaemic disease: does the inclusion of older employees in the cohort dilute the association? The WOLF Stockholm Study

M Kivimäki^1,2, T Theorell^3,4, H Westerlund⁴, J Vahtera², L Alfredsson^5,6

¹ International Institute for Society and Health, Department of Epidemiology and Public Health, University College London, London, UK
² Finnish Institute of Occupational Health, Helsinki, Finland
³ Karolinska Institutet, Stockholm, Sweden
⁴ National Institute for Psychosocial Medicine, Stockholm, Sweden
⁵ Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden
⁶ Stockholm Center of Public Health, Stockholm County Council, Stockholm, Sweden

Correspondence to:
Professor M Kivimäki, Department of Epidemiology and Public Health, University College London, 1–19 Torrington Place, London WC1E 6BT, UK; m.kivimaki{at}ucl.ac.uk

Objective: Evidence on the association between job strain and ischaemic disease is mixed. This study examined whether including older age individuals in a cohort used to assess job strain attenuates findings towards the null.

Design: A prospective cohort study with baseline screening in 1992–5. A follow-up for ischaemic disease until 2003 was based on linkages to national registries. Ischaemic disease was corroborated by objective criteria (hospitalisation as a result of acute myocardial infarction, unstable angina or ischaemic stroke; or death from ischaemic heart disease, cardiac arrest or ischaemic stroke).

Participants: 3160 male employees aged 19–65 years without pre-existing or current ischaemic disease.

Results: 93 cases of incident ischaemic disease were recorded. In the 19–55 year age group, the participants with job strain had a 1.76 (95% CI 1.05 to 2.95) times higher age-adjusted risk of incident ischaemic disease than those free of strain. Further adjustment for conventional cardiovascular risk factors had little effect on this association, but the impact of job strain was reduced by 70% to non-significant after the inclusion of employees older than 55 years in the cohort (hazard ratio 1.22, 95% CI 0.75 to 1.96).

Conclusions: Including older employees in a cohort may dilute the effect of job strain on cardiovascular disease and is a potential reason for inconsistent findings in previous studies.

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