The Effects of Daily Exercise Duration on Cardiac Responses and Atrial Fibrillation

Atrial fibrillation (AF) is a supraventricular tachyarrhythmia strongly associated with cardiovascular disease (CVD) and sedentary lifestyles. Despite the abundant benefits of regular exercise, AF incidence for professional endurance athletes is proportionate to CVD patients. To assess exercise dose and AF, we compared the effects of strenuous endurance training on mice by varying daily swim durations (120, 180 and 240 minutes). After receiving the same cumulative work while swimming (estimated from O2 consumption), all exercised groups showed similar elevations (P<0.04) in skeletal muscle mitochondria content and ventricular hypertrophy (P<0.02). By contrast, inducible AF increased (P<0.04) progressively with daily swim dose without markedly affecting atrial refractoriness (P>0.05). Associated with a dose dependency is pronounced (P<0.0001) bradycardia, (P<0.003) hypertrophy, (P<0.0007) fibrosis and (P<0.0001) macrophage accumulation in the atria, that is not observed in the ventricles. Our results demonstrate that prolonging daily swim exercise promotes progressively adverse atrial-specific remodelling leading to increased AF susceptibility.