Non-invasive Characterization of the Mechanisms of Atrial Fibrillation Maintenance

Inclusion Criteria: * Patients with paroxysmal AF symptomatic and refractory to at least one antiarrhythmic medication. * Patients with persistent AF symptomatic and refractory to at least one antiarrhythmic medication. * Patients with severe mitral stenosis (mitral valve area ≤1.5 cm2, stage D) and favorable valve morphology in the absence of left atrial thrombus or moderate-to-severe mitral regurgitation. * Patients must be able and willing to provide written informed consent to participate in the study. * Prior anticoagulation for\> 4 weeks or transesophageal echocardiogram excluding intracardiac thrombi (in patients with paroxysmal AF). Exclusion Criteria: * Patients with inadequate anticoagulation levels. * Patients with left atrial thrombus, tumor, or another abnormality which precludes catheter introduction on TEE prior to the procedure. * Patients with moderate-to-severe mitral regurgitation. * Patients with contraindications to systemic anticoagulation with heparin or coumadin. * Prior atrial fibrillation ablation. * Patients who are or may potentially be pregnant. * Contraindication for adenosine administration; * Current enrollment in another investigational drug or device study. * Pacemaker or Implantable Cardioverter Defibrillator.