New-Onset AF Risk Seen to Rise With Longer Work Hours

July 17, 2017

LONDON, UK — People who keep especially long hours at work, 55 hours/week or more, have about a 40% increased long-term risk of developing atrial fibrillation (AF) compared with those working 35 to 40 hours/week, suggest patient-level data from more than 85,000 people across eight European observational cohorts[1] .

There was a suggestion that the association between weekly work hours and incident AF may be "dose-dependent" but reached significance only for those with the longest work hours, Prof Mika Kivimäki (University College London, UK) told theheart.org | Medscape Cardiology by email.

Importantly, the mean age in the combined cohorts was a young 43 years. Fewer than 1% had cardiovascular disease (CVD) at the start, and only 2.4% had it at baseline or any point during follow-up, said Kivimäki, lead author on the study published July 14, 2017 in the European Heart Journal.

Even among those who went on to develop AF, only 10% had CVD at baseline.

That they observed an association between long working hours and AF even in people with no apparent CVD before AF diagnosis suggests the association is not attributable to CVD and "that there is a more direct link between long working hours and AF," said Kivimäki.

All participants had been free of AF at the beginning, and the analysis adjusted for a range of socioeconomic and lifestyle AF risk factors, including age, sex, smoking, exercise level, and alcohol intake.

The findings do suggest a relationship between long work hours and AF, but not necessarily a causative one, and don't shed light on the mechanism for any potential causative effect, write Drs Bakhtawar K Mahmoodi and Lucas V Boersma (St Antonius Hospital, Nieuwegein, the Netherlands) in an accompanying editorial[2] .

"Intuitively, one would suspect the usual pathways causing changes in autonomic tone and increasing atrial fibrosis resulting in AF, but no evidence yet exists to prove such a mechanism," they write.

The cohort of 85,494 "working men and women" was put together from eight population-based studies in Denmark, Finland, Sweden, and the United Kingdom that were part of the Individual-Participant-Data Meta-analysis in Working Populations (IPD-Work) consortium. Women made up 65% of the population; 5.2% worked at least 55 hours/week, and 62.5% worked 35 to 40 hours/week at baseline.

Over the average follow-up of 10 years, there were 1061 incident cases of AF for a cumulative 12.4 cases per 1000.

The cases, of which 71% were identified before age 65, were defined by "electrocardiograms, hospital records, drug reimbursement registers, and death certificates,” the report states.

"So the cases were quite severe and not only those with small irregularities in heartbeats," Kivimäki noted.

The hazard ratio (HR) for incident AF for those working the longest hours vs 35 to 40 hours/week was 1.42 (95% CI 1.13–1.80, P=0.003), after adjustment for age, sex, and socioeconomic status.

It was similar at 1.41 (95% CI 1.10–1.80, P=0.006), after further adjustment for "lifestyle factors."

And it was similar after further adjustment for CVD documented at the time of AF diagnosis, solely among patients with CVD at baseline, and also among those with CVD diagnosed at baseline or follow-up.

There were no significant associations between longer working hours and sex, age, or socioeconomic group.

"In absolute terms, the increased risk of AF among individuals with long working hours is relatively modest," the authors note. "The number of cases varied between 13 and 449 in the included studies."

The decade-long follow-up "is not necessarily a strength but rather a limitation of the analysis," according to the editorialists.

"All analyses used baseline measures of working hours and adjustment variables. Besides the fact that people may have changed their job and working hours during the follow-up, the baseline adjustment variables are also subject to change during follow-up... Therefore, we cannot exclude the possibility of relevant residual confounding."

The group of participants who worked the longest at baseline "is likely to include more people who continued to work long hours compared with the reference group," Kivimäki said.

The latter group that reported more conventional work hours "in turn, is likely to include more people who always work standard hours," she said. "And it is these long-term working patterns that are probably driving the observed associations."

Kivimäki, the other authors, and the editorialists reported that they have no relevant financial relationships.  

Follow Steve Stiles on Twitter: @SteveStiles2. For more from theheart.org | Medscape Cardiology, follow us on Twitter and Facebook.

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