Longer working hours among patients returning to work after a first myocardial infarction are associated with increased risk of coronary heart disease (CHD) events, according to a new prospective Canadian study.
The findings were published online Monday by Université Laval's Xavier Trudel, PhD, and colleagues, and will appear in the April 6 issue of the Journal of the American College of Cardiology.
Working long hours appears to be common, according to the researchers, who cited International Labour Office figures suggesting approximately 1 in 5 workers worldwide work over 48 hours per week, representing more than 614 million people.
Previous prospective studies suggest long working hours have potentially harmful effects on cardiovascular health, including CHD and stroke, the researchers said.
The researchers hypothesized that cardiovascular effects of long working hours may be higher in patients with pre-existing cardiovascular conditions, although there have been no previous studies into this phenomenon.
The current prospective study's aim was, therefore, to test the effects of longer working hours in 967 people – aged 35 to 59 years, recruited across 30 hospitals in Quebec province between November 1995 and October 1997 – who had returned to work after a first myocardial infarction.
At baseline, men were over-represented in the category working 55 hours or more per week, the highest category of working hours, accounting for 10.7% of all men vs. 1.9% of all women. Younger people who perceived their economic situation to be financially comfortable were also over-represented in the highest working hours category, as were those with other CHD risk factors including smokers, those with higher alcohol intake and less psychical activity, those with higher job strain and low support, and those with hypertension and diabetes.
Long working hours were assessed on average 6 weeks after return to work with a mean follow-up duration of 5.9 years, and incident CHD events – defined as either fatal or nonfatal myocardial infarction and unstable angina – were determined on the basis of the patients' medical files.
The researchers found that CHD events occurred among 205 patients (21.1%), an overall incidence rate of 3.60 cases per 100 person-years. Unadjusted analyses show those working 55 hours or more per week vs. those working 35 to 40 hours per week had a twofold increase in the risk of recurrent CHD (hazard ratio [HR]: 2.00; 95% confident interval [CI]: 1.36 to 2.95).
In the fully adjusted model, controlling for sociodemographic, lifestyle-related risk, clinical risk, work environment and personality factors, long working hours remained associated with the risk of recurrent CHD (HR: 1.67; 95% CI: 1.10 to 2.53).
Longer working hours combined with job strain was also associated with a stronger risk. Job strain was measured by psychological demand and decision latitude – a worker's potential control over tasks during working hours – using elements from the Job Content Questionnaire.
Participants working 55 hours or more per week that were simultaneously exposed to job strain had the highest risk when compared with workers working 35 to 40 hours per week who were not exposed to job strain, the researchers said, although they stressed that “the estimate was imprecise” (HR: 2.55; 95% CI: 1.30 to 4.98).
A linear risk increase was also seen after 40 working hours per week, with a stronger effect after the first 4 years of follow-up.
Using a spline regression model to estimate the effect of weekly working hours in its continuous form on the risk of recurrent CHD events, the researchers found a constant risk increase for each 10-hour increase after the reference 40 hours per week. The fully adjusted HRs were;
- 50h/week - HR: 1.20; 95% CI: 1.02 to 1.39
- 60h/week - HR: 1.50; 95% CI: 1.07 to 2.09
- 70h/week - HR: 1.92; 95% CI: 1.06 to 2.88
- 80h/week - HR: 2.46; 95% CI: 1.41 to 4.29
- 90h/week - HR: 3.10; 95% CI: 1.19 to 5.30
- 100h/week - HR: 3.78; 95% CI: 1.31 to 10.9
“Our findings further suggest that participants exposed to both long hours and job strain could be at particular risk,” the researchers said. “This could be attributable to the deleterious effect of prolonged exposure to work stressors among those working longer hours. This result should, however, be interpreted with caution given the reduced statistical power.”
They added that other effects of long working hours could have an adverse effect on cardiovascular health, including changing in lifestyle habits, with studies suggesting those working longer tend to smoke more, be less psychically active and consume more alcohol.
“Secondary prevention interventions aiming to reduce the number of working hours among these patients may lower the risk of CHD recurrence,” the researchers concluded. “Long working hours should be assessed as part of early and subsequent routine clinical follow-up to improve the prognosis of post-MI patients.”
In an accompanying editorial, University of California's Jian Li, MD, PhD, and Johannes Siegrist, PhD, from the Heinrich-Heine-University of Düsseldorf, said the study provides new research evidence that work-related factors play an important role in CHD prognosis.
“Occupational health services are urgently needed to be incorporated into cardiac rehabilitation programs and secondary prevention of CHD,” they said, noting: “This cooperation, however, is still poorly developed within cardiac rehabilitation programs.”
Trudel X, Brisson C, Talbot D, et al. Long Working Hours and Risk of Recurrent Coronary Events. J Am Coll Cardiol 2021;77:1616-25.
Li J, Siegrist J. Occupational Risks of Recurrent Coronary Heart Disease. J Am Coll Cardiol 2021;77:1626-8.