70 patients with heart failure, AF and CRT with BiV<95% will be randomised to either AF ablation or AV node ablation. Evaluation at 6 months with echocardiography and clinical assessment.

70 patients with heart failure, AF and CRT with BiV<95% will be recruited from outpatient clinic, pacing clinic or inpatient wards. Patients will be consented and will undergo baseline investigation with a transthoracic 2D echocardiogram, ECG, blood tests, cardiac CT with perfusion imaging, 6 minute walk test (6MWT), cardio-pulmonary exercise test (CPET) and Minnesota Living with Heart Failure Questionnaire. Patients who are successfully screened will be randomized 1:1 to receive AF catheter ablation rhythm control or AV-node ablation rate control.

Ablation Procedure:

Patients will receive either AF catheter ablation or AV-node ablation during the procedure. Patients randomised to AF catheter ablation will receive isolation of all four pulmonary veins and additional lesions, considered indicated by the operator. The procedure will be classified as successful if all four veins are isolated and the patient is in sinus rhythm. Patients may undergo DC-cardioversion under sedation if they remain in atrial fibrillation. AV-node ablation will be performed in line with current standard of care. Patients will also undergo additional research investigations at the time of ablation as follows:

Acute Haemodynamic Study:

All patients will also undergo an acute haemodynamic study with pressure wire assessment of both the right ventricle and the left ventricle at the start and end of the procedure. This involves inserting pressure wires into the RV via a vein in the groin and into the LV via an artery in the wrist or the groin. This will provide novel information on how AF catheter ablation and AV-node ablation affect cardiac contractility.

Invasive Electroanatomical Mapping (Optional):

This is an optional additional procedure that patients may opt into at the time of recruitment. This is performed during the ablation procedure with mapping catheters and software which are CE marked and already in clinical use. Mapping of all four cardiac chambers will be performed. This will be achieved via a vein and an artery in the groin. This will provide novel information on how AF catheter ablation and AV-node ablation affect electrical activation of the atria and ventricles.

Follow-up:

Patients will undergo a 2D transthoracic echocardiogram and ECG 1 week post ablation. Patients will then be followed up at 6 months with 2D transthoracic echocardiogram, ECG, clinical review, CPET, 6MWT, Minnesota Living with Heart Failure Questionnaire and pacing check.

Patients in the AF catheter ablation group who have a recurrence of AF after a 3 month blanking period will be permitted a re-do procedure. The follow-up investigations will take place 6 months after the re-do procedure. If a patient has recurrence of AF after a second ablation procedure and it is felt a third procedure is indicated, this will be performed after the six month follow-up period has ended and the patient is no longer in the study. They will be included in the intention-to-treat analysis.

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