PROJECT SUMMARY Atrial fibrillation is a highly prevalent condition resulting in disordered contraction of the atria (top chambers of the heart). It affects one in four individuals during their lifetime. It is known that the arrhythmia has a bidirectional relationship with dysfunctional atrial contraction, a condition which itself is becoming understood as a means by which individuals may develop heart failure. However, the majority of published literature to date involves examining atrial function while lying supine at rest, thus incompletely characterizing the varying states of exertion that one experiences during activities of daily living. Exercise-based cardiovascular magnetic resonance imaging is a powerful non-invasive diagnostic modality that can measure heart function during exercise and has the potential to uncover key mechanisms by which patients with atrial fibrillation may progress to overt heart failure – and whether current treatment techniques (namely, catheter ablation) may alter that trajectory. The broad goal of this application is to facilitate training of the principal investigator, Dr. Cory Trankle, MD, for a career as a patient-centered independent researcher who uses exercise-based diagnostic modalities (including magnetic resonance imaging), with a specific interest in characterizing mechanisms of exercise limitations such as left atrial dysfunction and identifying them as potential biomarkers and targets for therapeutic interventions. In addition to didactic coursework and subspecialty seminars, Dr. Trankle will lead a multidisciplinary team conducting a prospective clinical study investigating the effects that catheter ablation for atrial fibrillation has on atrial function during exercise. He will address three specific aims in patients with atrial fibrillation undergoing catheter ablation: (1) determine the effects of ablation on the ability of the left atrium to augment its function during exercise, (2) determine the impacts ablation on left ventricular performance during exercise, and (3) determine whether those changes experienced in left ventricular performance are indeed associated with changes in left atrial function. A better understanding of the impact of catheter ablation, a common treatment for atrial fibrillation, has on these cardiac structures during exercise has the potential to identify those at highest risk for heart failure incidence and lead to more targeted treatment in this highly prevalent disease.