Shift work linked to heart disease in nurses
Shift work linked to heart disease in nurses
Study suggests increased risk for women
Nurses who spend more than six years working rotating night shifts have a 51% greater risk of developing fatal coronary heart disease (CHD) and nonfatal myocardial infarction (MI), according to a recent study,1 but its authors and other experts caution that the findings -- although cause for concern -- are suggestive rather than definitive.
The CHD study was part of the larger ongoing Nurses' Health Study begun in 1976 at Harvard University in Boston. At that time, 121,700 female registered nurses 30 to 55 years old completed a mailed questionnaire requesting information about risk factors for cancer and heart disease, including current and past smoking habits (see related story in Hospital Employee Health, May 1993, pp. 69-70) and history of MI, angina, cancer, diabetes, hypertension, and high serum cholesterol levels. Follow-up questionnaires have been mailed every two years to the entire cohort to update information on cardiovascular risk factors and occurrence of major illnesses.
In 1988, the entire cohort was asked, "What is the total number of years during which you worked rotating night shifts (at least three nights per month in addition to days or evenings in that month)?" Responses were categorized as never, one to two, three to five, six to nine, 10 to 14, and 15 years or more. For the CHD study, the researchers further categorized the cohort into nurses who had never worked rotating night shifts vs. those who had ever done so.
Rotating night shifts have been associated with reduced job performance and higher levels of perceived stress.2 Several studies report a higher prevalence of coronary risk factors among rotating shift workers, including increased cigarette smoking,3 higher blood pressure,4 and increased cholesterol, glucose, and uric acid levels.5
Two previous prospective studies have examined the link between shift work and cardiovascular disease, with one finding no association,6 and the other reporting a relative risk of 1.4 among shift workers compared with day workers.7
New data on women found
The Harvard study's contribution to the literature lies not only in its significant risk finding, but also in that the cohort studied was female, says Meir J. Stampfer, MD, one of the study's researchers and a professor of epidemiology and nutrition at the Harvard School of Public Health.
"There have hardly been any data on women before," Stampfer tells Hospital Employee Health. "This adds to the accumulation of evidence. Only a few studies, which were quite limited in their design, have been done, and the results have been contradictory. This [study] suggests that rotating shift work could be an important contributor to heart disease."
The study end points comprised incident cases of nonfatal MI and fatal CHD occurring after return of the 1988 questionnaire and before June 1, 1992. Nurses who reported having a nonfatal MI were asked for permission to review their medical records for confirming symptoms plus cardiac enzyme level elevations or diagnostic electrocardiogram changes. When medical records were unavailable, an MI was considered probable if hospitalization was required and confirmatory information was obtained by letter or interview.
Fatal CHD was defined as fatal MI confirmed by hospital records, autopsy, or by CHD recorded on a death certificate.
Primary analysis of follow-up data from 1988 to 1992 used incidence rates with person-years of follow-up as the denominator. Relative risk was defined as the CHD incidence rate among women who had ever engaged in shift work divided by the corresponding rate among women who had never done shift work. Relative risks were adjusted for age and categorized in five-year age groups. Ninety-five percent confidence intervals (CI) were calculated.
The researchers also adjusted for multiple risk factors, such as smoking status; alcohol intake; body mass index; history of diabetes, hypertension, and high cholesterol; menopausal status; use of postmenopausal hormones; past use of oral contraceptives; levels of physical activity; vitamin E intake; average aspirin use; and parental history of MI before age 60.
No major risk factor differences
Of the total cohort, 110,141 eligible nurses were free of diagnosed CHD and cerebrovascular disease in 1988. Of those, 79,109 (71.8%) answered the shift-work question. The researchers compared the characteristics of those nurses to the 31,032 who did not.
Women who answered the question were less likely to have been current smokers (18.5%) in 1988 than women who did not answer the question (29.7%). Otherwise, the nonrespondents did not differ substantially from respondents in terms of major cardiovascular risk factors.
"Importantly, nonrespondents did not appear to be sicker than respondents," the study notes. Four-year CHD incidence among nonrespondents was similar to the incidence among respondents, causing the researchers to conclude that "the exclusion of women who did not respond to the 1988 question on shift work was unlikely to have introduced a major bias into our analyses."
Of the nurses who answered the shift-work question, 32,153 (40.6%) reported they had never done shift work, while 46,956 (59.4%) reported having done some shift work.
Comparing across nurses reporting different durations of rotating night shift work, the researchers found that longer shift-work durations were associated with higher age-adjusted prevalence of current smoking, hypertension, diabetes, body mass index, and physical activity levels. An inverse relation also was found between mean daily alcohol intake and duration of rotating night shifts.
A total of 292 incident cases of CHD (248 nonfatal MI and 44 fatal CHD) during 302,964 person-years of follow-up were observed. Having ever worked rotating night shifts was associated with an approximate 1.4-fold increase in the age-adjusted risk of total incident CHD compared with never doing shift work.
Higher risk after six years
Most importantly, increasing durations of shift work were associated with higher relative risks of CHD, says Stampfer, who adds he was surprised by the finding.
"I didn't think [longer shift work durations] would turn out to be very important, but I was wrong," he says.
Categorizing shift work durations into "less than six years" and "six or more years," the researchers found that the risk of CHD increased after six or more years of rotating night shifts. Compared with nurses who had never done shift work, the multivariate-adjusted relative risks of total CHD were 1.21 (95% CI, 0.92 to 1.59) among nurses reporting less than six years (not statistically significant because of the CI range) and 1.51 (95% CI, 1.12 to 2.03) among those reporting six or more years of rotating night shifts.
The age-adjusted relative risk of CHD was 1.38% (95% CI, 1.08 to 1.76) in nurses who reported ever doing shift work compared with those who had never done so, meaning that overall, nurses who had ever done shift work had a 38% higher risk of CHD than nurses who had never done shift work.
Despite the significant findings related to CHD and longer durations of rotating night shift work, as well as the overall greater risk associated with ever having performed shift work, Stampfer says he does not regard the study's results as definitive proof of a link.
"This is suggestive and supports the need for more study," he states. "We need to find out what the mechanisms are -- why this happens, why this is getting translated to a higher risk. Maybe there's something we can do to make rotating shift work less dangerous."
Stampfer suggests one possible remedy is for nurses to work on more "stable" shifts instead of quick rotations. He suggests rotating on a longer cycle of at least a month or staying on the same shift consistently.
Anna Gilmore, RN, director of labor relations and workplace advocacy for the American Nurses Association in Washington, DC, says the professional organization has long been concerned about the hazards of shift work. She agrees with Stampfer's recommendation.
"Obviously, the health care industry has to have workers who work around the clock, and if the employer and the employee can figure out reasonable schedules to accommodate that need, as well as make sure the nurse has adequate time off and isn't rotating shifts, that decreases the hazards as much as possible," Gilmore says.
Having a routine schedule of either all night, evening, or day work decreases nurses' health hazards because their systems can adapt to the routine.
"Some nurses love the night shift and stay on it for years and their body adapts very well, while some work nights only until they can get transferred to a more pleasurable shift that meets their personal preference. We encourage employers to let that happen as often and as quickly as possible," she states.
Gilmore says the Harvard study has "legitimacy," but "it is only one study." Nevertheless, nurses should consider its findings as an addition to a growing body of literature about the hazards of shift work when they are planning their schedules, she adds.
Some limitations present
But Eyal Shahar, MD, an assistant professor of epidemiology with the University of Minnesota School of Public Health in Minneapolis, says the study's findings should not prompt nurses to change schedules or careers.
The study is a "terrific work analytically and in careful discussion of the results," but the problem with any observational study is the lack of scientific experimentation that is present in randomized clinical trials, says Shahar, who also is a member of the American Heart Association's Epidemiology and Prevention Council.
While noting the difficulty of doing a randomized clinical trial of shift workers, Shahar says that in an experimental study, the two cohorts (shift workers and non-shift workers) would be comparable in every characteristic that could affect coronary heart disease. Both groups would have the same proportion of smokers, the same proportion of older nurses, and so on.
"If one group is not the same as the other, then who knows what the cause is of the excess disease in one group? In observational studies like this, we do not assign nurses to be shift workers; they assign themselves. That means the group of shift workers is not the same as the group of non-shift workers," he explains.
The presence of multiple variables in observational study cohorts requires researchers to control for those variables statistically, which the Harvard study authors did "elegantly," Shahar says.
Some risks factors unmeasured?
Still, "you can't measure every single behavior and trait of these nurses. [The authors] measured many important ones and indeed found that women who worked night shifts were less healthy than women who did not. That is a finding, but does it explain the excess coronary heart disease in them? It does not explain it completely according to their statistical analysis," he states. "There are some excess rates we cannot explain by controlling analytically for all of these variables."
Shahar says the question then becomes, "Have [the researchers] done a good enough job of eliminating all of the differences in the two groups? Have they done a good enough job statistically of controlling everything they could have controlled for? The answer to that is a question mark. I don't know if they have done this, nor do the authors know. The problem is not unique to this study. It is the way one should look at epidemiological studies vs. clinical trials. There is a huge difference."
Shahar also points out that epidemiologically, it is often true that groups with excess risks, such as shift workers, often tend to also have less favorable risk profiles. According to the Harvard study, nurses who worked longer durations of rotating night shifts also were more likely to be smokers and to have a history of early MI in their family, as well as other risk factors.
The researchers have controlled statistically for that, "but it generates kind of an uneasy feeling if the whole risk profile is greater," he says. Perhaps there were additional risk factors that were never considered or measured.
Another potential problem is the lack of a "graded relationship" in the data on the association between shift-work durations and CHD, Shahar says. Given that the data suggest that CHD risk increases after six years of shift work, one would expect to see the risk increase even more with longer durations of shift work.
"That is to say people with six to nine years would have some risk, and people with 10 to 14 years would have some more risk, and people with 15 or more years would have more risk. But with six and [more years of shift work], I don't see any graded relationship there," he notes. "When I don't see any graded relationship for something like this, I say to myself, are we dealing with a group that has some characteristics that we have not controlled for very accurately? Is up to six [years] okay, and above six it doesn't matter how much?"
Possible explanations are that there were not enough cases in the study, or that the CIs were unstable, he adds.
Also, Shahar questions why six years is the "magic cut point" for CHD risk.
"Behaviors or exposures that are detrimental do not have a magic cut-point number," he states. "Think about smoking. If you smoke one cigarette a day, that's your risk. If you smoke five, that's your risk. If you smoke 10, that's your risk. It's not that if you smoke up to 10 that's safe and more than 10 is harmful. When I see things that say up to six years is okay and above six years is harmful, I feel uncomfortable about that."
Roger Rosa, PhD, research psychologist with the National Institute for Occupational Safety and Health in Cincinnati, says no good estimate exists of when to cease working rotating or night shifts.
"When shift workers grow in seniority on the job, they often are able to select a better schedule, and that often is a day schedule," Rosa explains. "Younger workers often have to do the night shifts, and younger people tend to be more resilient, so it's hard to get a good estimate of when to get off rotating shift work."
There are not enough studies to make a "hard-and-fast statement," he says. In addition, prospective studies are rare.
References
1. Kawachi I, Colditz GA, Stampfer MJ, et al. Prospective study of shift work and risk of coronary heart disease in women. Circulation 1995; 92:3178-3182.
2. Coffey LC, Skipper JK, Jung FD. Nurses and shift work: Effects on job performance and job-related stress. J Adv Nursing 1988; 13:245-254.
3. Knutsson A, Akerstedt T, Jonsson B. Prevalence of risk factors for coronary artery disease among day and shift workers. Scand J Work Environ Health 1988; 14:317-321.
4. Thelle DS, Forde OH, Try K, et al. The Tromso Heart Study. Acta Med Scand 1976; 200:107-118.
5. Theorell T, Akerstedt E. Day and night work: Changes in cholesterol, uric acid, glucose and potassium in serum and circadian patterns of urinary catecholamine excretion. Acta Med Scand 1976; 200:47-53.
6. Taylor PJ, Pocock SJ. Mortality of shift and day workers, 1956-68. Br J Ind Med 1972; 29:201-207.
7. Knutsson A, Akerstedt T, Jonsson BG, et al. Increased risk of ischaemic heart disease in shift workers. Lancet 1986; 2:89-92. *
Infection Control for the Health Care Worker is a free booklet providing health care workers with a user-friendly, ready reference on workplace infection transmission and prevention. Chapters include: how pathogens are spread, fundamentals of infection protection, and employee health (high-risk employees, employee education, immunization recommendations, and exposure plans). Another chapter focuses on the CDC's new isolation guidelines, outlining pathogen- and syndrome-specific precautions for airborne, droplet, and contact transmission. Contact Kimberly-Clark's Partners in Quality helpline at (800) 324-3577.
Street Smart From 9 to 5 is a new training program on how to deal with hostile, agitated, or physically assaultive employees, clients, or visitors. It is based on the National Crisis Prevention Institute's (CPI) nonviolent crisis intervention training program. A variety of options is offered, ranging from videotapes to on-site trainings, depending on an organization's needs and budget. For a free brochure describing program options and content, contact CPI at 3315-K North 124th St., Brookfield, WI 53005. Telephone: (800) 558-8976. E-mail: [email protected].
Smoking cessation information and materials are available from a number of sources. For pamphlets, posters, and scientific reports, contact the Office on Smoking and Health, Centers for Disease Control and Prevention, Mailstop K-50, 4770 Buford Highway, NE, Atlanta, GA 30341-3724. Telephone: (800) CDC-1311 (copies of action guide on secondhand smoke) or (404) 488-5705 (other information). For information on individual and group stop-smoking programs, contact the American Cancer Society, 1599 Clifton Road NE, Atlanta, GA 30329. Telephone: (800) ACS-2345. For information and education programs for health care facilities, contact the American Heart Association, 7272 Greenville Ave., Dallas, TX 75231. Telephone: (214) 373-6300. For information on smoking and health and stop-smoking telephone counseling, contact the National Cancer Institute, Building 31, Room 10A24, 9000 Rockville Pike, Bethesda, MD 20892. Telephone: (800) 4-CANCER.
Emerging Infectious Diseases is a new quarterly journal published by the federal Centers for Disease Control and Prevention. Available both electronically and in hard copy, it provides information in three sections: factors underlying disease emergence, summaries of specific diseases or syndromes, and brief laboratory or epidemiologic reports. For more information, send e-mail to [email protected], or contact Editor, Mailstop C-12, National Center for Infectious Diseases, CDC, 1600 Clifton Road, Atlanta, GA 30333. Telephone: (404) 639-3967. Fax: (404) 639-3039.
A specially designed workers' compensation program for hospitals claims to help reduce workers' compensation costs by providing services such as modified duty programs, drug-free workplace credits, claims management, and loss-control services that include an anti-fraud hotline, slip and fall elimination, ergonomics analysis, and 24-hour emergency claims service. The program is for single hospitals and health care delivery networks with traditional or self-funded workers' compensation plans. For information and brochures, contact: Brian Miles, Spain Agency Inc., Hospital Workers' Compensation Department, 625 Route 6, Mahopac, NY 10541. Telephone: (800) 247-5521. Fax: (914) 628-1804.
The Pocket Guide to Protection from Bloodborne Pathogens explains how the viruses are transmitted and what steps can be taken to lower the risk of accidental infection. It also includes information on universal precautions, housekeeping procedures, and handling and disposal of blood and other potentially infectious materials. Guides can be personalized with a hospital's name on the cover. A free examination copy is available, as are quantity discount prices. Contact: Business & Legal Reports Inc., 39 Academy St., Madison, CT 06443-1513. Telephone: (800) 727-5257. Fax: (203) 245-2559. *
Subscribe Now for Access
You have reached your article limit for the month. We hope you found our articles both enjoyable and insightful. For information on new subscriptions, product trials, alternative billing arrangements or group and site discounts please call 800-688-2421. We look forward to having you as a long-term member of the Relias Media community.