There is growing awareness of the importance of electrical rotors to the maintenance of atrial fibrillation (AF). Recent work in our laboratory has found that AF evolves over time, from rapid, focal activation, next to transitional rotors, and finally to stable, long duration rotors, whose locations are frequently separate from the transitional rotor sites. This project will test the hypothesis that mapping and ablation of the transitional rotors sites may prevent atrial fibrillation from progressing to sustained atrial fibrillation, and therefore increase the AF initiation threshold. The investigators will test this hypothesis during clinically-indicated electrophysiology study prior to ablation of symptomatic AF.