Associations Between Physical Activity And Atrial Fibrillation: Is Resting Heart Rate A Mediator? The Tromso Study

High prevalences of atrial fibrillation (AF) have been reported among endurance athletes, whereas moderate physical activity has been shown to lower the risk of AF. Given that physical activity generally is reversely related to resting heart rate (RHR), RHR may be a mediator in the association between physical activity and AF. PURPOSE: To examine the role of RHR in the association between physical activity and hospital-diagnosed AF in a Norwegian cohort. METHODS: The study has a prospective design and 20 484 adults (50.3% men) who participated in the third Tromsø Study in 1986-1987 were included (mean baseline age 38 years). At baseline in 1986-1987, data were collected by questionnaire and physical examinations. Leisure time physical activity was assessed by a validated questionnaire and graded as Low, Moderate, High, and Vigorous activity. RHR was objectively measured after 2 minutes’ rest. Participants were followed from baseline through 2010 with respect to incident cases of hospital-diagnosed AF documented on an electrocardiogram. All analyses were adjusted for baseline age, sex, daily smoking, body mass index, height, cardiovascular disease, systolic and diastolic blood pressure, diabetes, and hypertension treatment. RESULTS: Physical activity was related to AF in a J-shaped pattern. Moderately active individuals had a 19% lower risk of AF (adjusted HR 0.81, 95% CI 0.68-0.97), whereas highly active had similar risk of AF as the low physical activity group. Vigorously active individuals showed a non-significantly higher risk of AF (HR 1.37, 95% CI 0.77-2.43). The estimates were virtually similar after additional adjustments for RHR. The association of RHR with AF showed a different pattern, as risk of AF increased with decreasing RHR (adjusted HR 0.92, 95% CI 0.86-0.98 for each 10 beats/minutes increase in RHR), independent of physical activity level. Among those reporting vigorous physical activity, 10% had an RHR <50 beats/minute. CONCLUSIONS: The relationship between leisure time physical activity and AF followed a J-shaped pattern, whereas RHR was inversely associated with risk of AF. The associations remained after mutual adjustment. RHR does not appear to be a mediator in the association between physical activity and AF.