Shift work and 20-year incidence of acute myocardial infarction: results from the Kuopio Ischemic Heart Disease Risk Factor Study

Objectives It remains unclear whether different types of shift work impose similar risks for cardiovascular events in middle-aged workers, especially those with pre-existing ischaemic heart disease (IHD). This study investigated the relations between different shift types and incident acute myocardial infarction (AMI) among men with and without pre-existing IHD, respectively. Methods We analysed data on 1891 men, aged 42-60years at baseline, in the prospective Kuopio Ischemic Heart Disease Risk Factor Study cohort, using Cox proportional hazard models with adjustment for demographic, biological, behavioural and psychosocial job factors. We evaluated the associations of baseline shift work with 20-year incidence of AMI, and their modification by pre-existing IHD, using both stratified analysis and models with product terms between shift work and IHD. Results Travelling work (at least 3 nights per week away from home) was strongly positively associated with AMI among men with IHD (HR=2.45, 95% CI 1. 08 to 5.59) but not among men without (HR=0.93, 95% CI 0.43 to 2.00). No clear associations were found between other types of shift work and AMI for both men with and without IHD. On both additive and multiplicative scales, baseline IHD status positively modified the association of travelling work with AMI (relative excess risk for interaction=3.23, 95% CI -0.50 to 6.97, p for multiplicative interaction=0.044). Conclusions We found mixed results for the associations between different types of shift work and AMI among those with and without pre-existing IHD. Future research should investigate these associations and effect modification for a broad spectrum of work schedules.