This prospective observational study is designed to describe the factors determining the success of pulmonary vein isolation (PVI) for patients with persistent atrial fibrillation (AF). These factors - described as left atrial (LA) function - are simultaneously studied by multi-modal techniques in patients undergoing radiofrequency ablation. The simultaneous measurements are conducted before the PVI in AF and after the sinus rhythm converted due to PVI. These measurements include invasive LA pressure monitoring, LA voltage map creation, LA strain evaluation by transthoracal echocardiography, LA scar-mapping by cardiac MRI, defining biomarkers from blood samples. During the one year follow-up period rhythm monitoring is conducted by regular Holter-monitoring. Our aim is to define the predictive values of the above mentioned factors and to create a new score system for predicting PVI success in persistent AF population.