Enrollment: 590 subjects, with 295 subjects in the AV node ablation arm and 295 subjects in the control arm, will be enrolled. Study patients will undergo stratified randomization at ≥ 30 days after CRT implant. Participants in will sign informed consent and be screened prior to randomisation. After CRT implant, patients will have at least 30 days for optimisation of heart failure therapy, prior to randomisation.