The relation of exposure to shift work with atherosclerosis and myocardial infarction in a general population☆
Introduction
Characteristics of the work environment are associated with cardiovascular morbidity. Virtanen and Notkola [1] studied the influence of workload, control, noise and shift work and found the adjusted rate ratio for shift work to be not associated with cardiovascular diseases but related to cerebrovascular diseases. Currently, there is no information on possible associations between shift work and myocardial infarction (MI).
Further it remains unclear whether shift work is associated with early manifestations of cardiovascular disease [2]. Stressful working conditions seem to contribute to a higher tendency of unhealthy lifestyle behaviours [3] such as poor diet and smoking, but findings are controversial. Substantial unfavourable changes in the cardiovascular risk profile in shift workers are only proved for smoking [4]. Continued exposure to day–night shift work also gives rise to an increased body mass index (BMI) above the normal effect of ageing on BMI [5], whereas in other studies shift workers did not differ with respect to BMI [6]. The prevalence of diabetes increases with exposure to shift work [7], but these findings have not always been confirmed [8], [9].
Currently, few data address the role of workplace factors in the underlying process of atherosclerosis itself. Shift workers more often report higher exposure to physical workload and work environment factors related to heart disease than day workers [10]. For example chronic exposure to noise levels at work is associated with an excess risk of MI and death, but results indicate that noise-related cardiovascular disease effects are reversible [10]. Air pollution and dust may be another interesting factor [11].
Shift work is at least an occupational and social stressor, and shift schedules including night shifts are among the most common cumbering work forms and continuously increasing.
Former studies that investigated possible associations between work conditions and cardiovascular disease included only few atherosclerotic risk factors. Often such factors were measured only by interview with potential for considerable residual confounding [4], [12], [13], [14] or showed significant small but unadjusted relative risks [15]. In our study, we measured many important risk factors by medical examination in an epidemiologic setting. At the same time we recorded results independent from different types of shift schedules and specific working conditions to be able to make a general statement.
The association between shift work and cardiovascular disorders might arise from both direct mechanisms as well as indirectly through effects connected to more stressful working and living conditions. We also considered that health disorders among shift workers could have a multifactorial origin. But there are no proved, irreversible effects of other working conditions such as noise levels [10]. This study is, to the best of our knowledge, the first to examine the association between shift work and atherosclerosis and MI in a general population sample by hypothesizing that shift work is a chronic stressor. Moreover, MI among shift workers is expected to occur at a younger age compared to subjects who never worked in shift schedules. Studies about occupational stress show that the age of 45 and older is of special interest in this context [16].
Section snippets
Study population
Subjects were participants in the Study of Health in Pomerania (SHIP), which is a cross-sectional survey of the adult population in West Pomerania, the north-eastern area of Germany [17]. In brief, SHIP is a population-based study with a broad range of health indicators, diseases, risk and protective factors. The central objective of SHIP is to assess prevalence and incidence of common risk factors and diseases and to investigate their complex interactions. A representative sample of the
Results
The study population comprised 1242 women and 1268 men of whom 190 women (15.3%) and 508 men (40.1%) were previously exposed to shift work. In total 27.8% were former shift workers with a mean duration of shift work of 13.2 years (±11.0 years). Altogether, 35.9% of the shift workers quitted work in the first 5 years, 8.6% after 6–10 years, 23.3% after 11–20 years, and 22.2% after more than 20 years.
Table 1 presents atherosclerotic risk factors and MI frequencies in subjects exposed and
Discussion
The aim of the present study was to assess the association between work-related stress as measured by history of shift work and the prevalence of subclinical atherosclerosis as measured by carotid IMT and the risk of MI. The present study indicates occupational exposure to shift work to be a risk factor for CHD that is independent of age, sex and further confounding risk factors. For this association the duration of shift work is important: subjects who reported a shift work exposure of 20
Acknowledgements
SHIP is part of the Community Medicine Net (http://www.medizin.uni-greifswald.de/cm) of the University of Greifswald. It is funded by grants from the German Federal Ministry of Education and Research (BMBF, grant 01ZZ96030), the Ministry for Education, Research, and Cultural Affairs and the Ministry for Social Affairs of the Federal State of Mecklenburg-West Pomerania. The contribution to the data collection made by the field workers, study physicians, ultrasound technicians, interviewers, and
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This research work was funded in parts by grants from the German Federal Ministry for Education and Research (BMBF, grant no. 01ZZ96030), from the Ministry for Education, Research, and Cultural Affairs and the Ministry for Social Affairs of the Federal State of Mecklenburg-West Pomerania.