by Todd Neale

British civil servants working three to four hours longer than usual per day appear to have an increased risk of having a coronary heart disease event, a prospective cohort study showed.

After accounting for traditional cardiovascular risk factors and other variables, those working 11 to 12 hours per day had a 56 percent greater risk of coronary death, nonfatal myocardial infarction, or angina than those who worked normal hours, according to Marianna Virtanen, PhD, of the Finnish Institute of Occupational Health in Helsinki, and colleagues.

Workers who put in just one or two extra hours a day did not appear to have an elevated risk of heart disease events, the researchers reported online in the European Heart Journal.

In an accompanying editorial, Gordon McInnes, MD, of the University of Glasgow, said that overtime-induced work stress might contribute to a substantial portion of cardiovascular disease.

“If the effect is truly causal, the importance is much greater than commonly recognized,” he wrote.

Although there is little evidence that easing stress reduces cardiovascular risk, “further research should examine whether interventions designed to reduce overtime work could alter the risk of cardiovascular disease.”

Overtime work has increased steadily in recent years, according to Virtanen and her colleagues, and there is mounting evidence that working extra hours is linked to poorer physical and mental health.

To examine the association between overtime work and incident coronary heart disease events, the researchers turned to the Whitehall II Study, which prospectively follows British civil servants.

The current analysis included 6,014 men and women ages 39 to 61 who were free from coronary heart disease and worked full-time at baseline.

More than half (54%) reported a normal working day of seven to eight hours at baseline. Another 21% said they worked one hour of overtime, 15% said they worked two, and 10% said they put in an extra three or four hours.

Through an average follow-up of 11.2 years, there were 369 cases of coronary death, clinically verified nonfatal MI, or definite angina.

After adjustment for sociodemographic characteristics, individuals who worked three to four hours of overtime per day had a 1.6-fold (95% CI 1.15 to 2.23) increased risk of having an event compared with those who did not work overtime.

Adjustment for numerous cardiovascular risk factors, sleep deprivation, sick days, psychological distress, job demands, decision-making ability at work, and type A behavior pattern only slightly attenuated the magnitude of the association.

The results were similar when angina was excluded from the outcome (HR 1.67, 95% CI 1.02 to 2.76).

The amount of decision-making ability that participants had at work significantly modified the association when angina was included in the outcome.

Individuals with low decision latitude had a 1.78-fold (95% CI 1.10 to 2.89) increased risk, whereas those with high decision latitude did not have an elevated risk.

There was no significant association between any amount of overtime work and all-cause mortality.

Although the study could not explain the link between overtime and cardiovascular events, Virtanen and her colleagues offered several possible mechanisms, including hidden hypertension, type A behavior patterns, difficulties in unwinding after work, and a willingness to go to work sick.

They also listed some limitations, including possible residual confounding from unmeasured factors, the inability to assess changes in risk factors, the possibility that work hours changed during follow-up, the lack of assessment for depressive and anxiety disorders, reduced statistical power for subgroup analyses, and the lack of blue-collar workers.

McInnes noted that the lack of information on cardioprotective medication use was another limitation.

Todd Neale is a MedPage Today Staff Writer.

Originally published in MedPage Today. Visit MedPageToday.com for more cardiovascular news.