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Etiology

Nurses who worked shifts had an increased risk for coronary heart disease

ACP J Club. 1996 May-June;124:78. doi:10.7326/ACPJC-1996-124-3-078


Source Citation

Kawachi I, Colditz GA, Stampfer MJ, et al. Prospective study of shift work and risk of coronary heart disease in women. Circulation. 1995 Dec 1; 92:3178-82.


Abstract

Objective

To determine the relation between shift work and the risk for coronary heart disease (CHD) in women.

Design

Population-based cohort study of women (Nurses' Health Study).

Setting

United States.

Participants

121 700 women who were nurses were enrolled in 1976 when they were between 30 and 55 years old. Exclusion criteria were death, myocardial infarction (MI), angina, or cerebrovascular disease before 1988. 79 109 women (72%) provided information on shift work and were included in the evaluation.

Assessment of Risk Factors

Questionnaires that were sent every 2 years were used to gather data on risk factors for CHD and other serious illnesses. In 1988, participants were asked to provide the total number of years they had worked shifts (≥ 3 nights/mo in addition to day and evening work). The data on shifts were reported as having ever worked shifts compared with having never worked shifts and were grouped into categories (1 to 2 y, 3 to 5 y, 6 to 9 y, 10 to 14 y, and ≥ 15 y). Follow-up was > 98%.

Main Outcome Measures

Total CHD defined as nonfatal MI and fatal CHD. Deaths were taken from national databases. Medical records, autopsy reports, and death certificates were requested after an event was reported and were reviewed by physicians blinded to exposure status.

Main Results

59% of the nurses reported having had worked shifts. During follow-up from 1988 to 1992, 292 incident cases of CHD occurred (248 were nonfatal MI and 44 were fatal CHD). A multivariate analysis was done that adjusted for smoking; body mass index; history of hypertension, diabetes, and hypercholesterolemia; past use of oral contraceptives; menopausal status; current use of postmenopausal hormones, aspirin, and vitamin E; alcohol intake; parental history of MI before age 60 years; and level of physical activity. Having ever worked shifts was associated with total CHD (relative risk [RR] 1.31, 95% CI 1.02 to 1.68) and nonfatal MI (RR 1.34, CI 1.02 to 1.75). A linear trend showed that increasing shift work was associated with increasing risk for CHD (P = 0.04 for the trend). The RR for CHD and shift work for 6 to 9 years was 1.60 (CI 1.05 to 2.42) and for shift work for 10 to 14 years was 1.66 (CI 1.05 to 2.64). Shift work was not associated with death from cancer or death caused by accidents and suicides.

Conclusion

Nurses who worked shifts for > 6 years had an increased risk for coronary heart disease and nonfatal myocardial infarction.

Source of funding: National Institutes of Health.

For article reprint: Dr. I. Kawachi, Channing Laboratory, 180 Longwood Avenue, Boston, MA 02115, USA. FAX 617-432-3755.


Commentary

This study by Kawachi and colleagues examines the relation between shift work and new-onset CHD in a cohort of women nurses during a 4-year follow-up. Internal validity (quality) of the study is high: Information about shift work was collected before cardiac events occurred; associations were adjusted not only by major cardiac risk factors but also by minor and potential risk factors; and CHD events were confirmed by medical record review with a > 98% follow-up rate. Thus, we may conclude that the association between shift work and CHD is valid. Because of its large population-based sample, study results can be generalized to most women with similar educational backgrounds.

The authors cite earlier studies that linked shift work with higher blood pressure and increased levels of serum cholesterol, glucose, and urinary adrenaline excretion, all potential mechanisms by which shift work could increase the risk for CHD. Shift work, however, may not be causally related to CHD but may be a marker for behavioral and psychosocial factors, which were not controlled for in this study. In a 20-year follow-up in the Framingham study (1), women with tension and lack of vacations were more likely to develop CHD. In a large case-control study in Sweden, job strain was significantly associated with CHD (2). Social isolation in animal models is also associated with a 4-fold increase in coronary artery atherosclerosis and has been associated with a worse prognosis in women after an MI (3, 4). Women who have little leisure time, have substantial job strain, and are socially isolated may also be more likely to do shift work.

This study adds important information about a modest association between shift work and CHD in women and should encourage future research in the complex relations between CHD risk and psychosocial and behavioral variables.

Sally E. McNagny, MD, MPH
Emory University School of MedicineAtlanta, Georgia, USA


References

1. Eaker ED, Pinsky J, Castelli WP. Am J Epidemiol. 1992;135:854-64.

2. Karasek R, Theorell T, eds. Healthy Work. New York: Basic Books; 1990.

3. Shively CA, Clarkson TB, Kaplan JR. Atherosclerosis. 1989;77:69-76.

4. Case RB, Moss AJ, Case N, McDermott M, Eberly S. JAMA. 1992;267:515-9.