Atrial fibrillation (A-fib) is a heart condition characterized by a fast, irregular heart rate. The Centers for Disease Control and Prevention (CDC) estimate that between 2.7 and 6.1 million people in the United States have been diagnosed with A-fib.
While there are no statistics available for the global incidence of this heart condition, studies predict that the number of A-fib patients worldwide is on the rise.
The CDC have identified a series of risk factors for A-fib, which include old age, hypertension, diabetes, and heavy drinking. New research led by Prof. Mika Kivimaki, from the Department of Epidemiology at University College London in the United Kingdom, has identified another risk factor: working long hours.
The findings are published in the European Heart Journal.
Large population sample
The study was undertaken by Prof. Kivimaki in collaboration with other researchers belonging to the Individual-Participant-Data Meta-analysis in Working Populations Consortium, which brings together specialists from a number of European countries.
They collected and examined data from 85, 494 participants from Denmark, Finland, Sweden, and the U.K., all of whom were recruited for studies in their countries between 1991 and 2004.
"The great strength of our study was its size, with nearly 85,000 participants, which makes it large by the standard of any study in this field," says Prof. Kivimaki.
The researchers established five categories for working hours: under 35 hours per week, 35 to 40 hours each week (the standard commitment for full-time employees), 41 to 48 hours per week, 49 to 54 hours every week, and 55 hours or more.
Of the total number of participants, 4,484 people qualified as working for at least 55 hours every week.
Long hours raise A-fib risk
None of the participants had A-fib at the beginning of the study. They were followed up for a period of 10 years, during which time 1,061 people developed the heart condition.
This means that the incidence rate of A-fib among the total number of participants was 12.4 per 1,000 people, and that among those working 55 hours or more each week, it was 17.6 per 1,000 people.
The study concluded that, compared with working regular hours (typically defined as between 35 and 40 hours each week), those with very long work commitments (55 hours or more) are around 40 percent likelier to be diagnosed with A-fib in time.
The findings remained after making adjustments for relevant factors, such as old age, sex, obesity, physical exercise, socioeconomic status, smoking status, and heavy drinking.
Some limitations to the study have also been reported, however. Working hours were only assessed at the beginning of the study, and they were subsequently not confirmed at any point during the 10-year follow-up period. Additionally, the type of work done by the participants was not taken into account.
Prof. Kivimaki dismisses the impact of these setbacks, relying on the impressive size of the population sample. "I do not think the results would have been dramatically different with repeat measurements of working hours because people tend to keep their working patterns," he adds.
Yet the published article is accompanied by an editorial authored by Dr. Bakhtawar Mahmoodi and Dr. Lucas Boersma, both from the St. Antonius Hospital in Nieuwegein, the Netherlands, which highlights the importance of these shortcomings.
"The authors should be congratulated for the impressive collaborative effort required to integrate patient level data from multiple studies to increase the power. However, [...] there are many inherent limitations of the data that preclude from definite conclusions on acknowledging long working hours as an independent risk factor for atrial fibrillation," they caution.