Rate or Rhythm Control in CRT: the RHYTHMIC Study

Inclusion Criteria: * Ability to provide informed consent to participate and willing to comply with the clinical investigation plan and follow-up schedule. * QRS duration \>120ms on surface ECG, severe left ventricular systolic impairment (EF≤35%) and clinical symptoms of heart failure despite optimum medical therapy (NYHA class II-IV) at time of CRT implant or upgrade * Successful CRT implant or upgrade including atrial lead * Biventricular pacing percentage \<95% secondary to atrial fibrillation at least 3 months post implant or upgrade * Clinically indicated for AV node ablation Exclusion Criteria: * Life expectancy \<1 year * Presence of atrial or ventricular thrombus * Permanent atrial fibrillation * Mechanical aortic valve replacement * Severe peripheral vascular disease * Female participants who are pregnant, lactating or planning pregnancy during the course of the study. * Participation in other studies with active treatment / investigational arm