The PACE-FIB trial is a multicentre, randomised, open-label clinical trial. Patients older than 18 years, with permanent AF, LVEF>40%, average resting heart rate ≤ 110 beats per minute (bpm), at least one hospitalisation due to HF in the previous year and basal NT-proBNP level>900 pg/ml will be randomised to either CSP and subsequent AV node ablation (intervention group) vs. pharmacologic rate control optimised according to clinical practice guidelines. The impact of both strategies on a composite primary endpoint of all-cause mortality, HF hospitalisation and worsening HF will be evaluated during a 36-month follow-up.

Permanent atrial fibrillation (AF) causes beat-to-beat heart rate irregularity, which has shown to decrease cardiac output. Observational data suggest that heart rate regularization through atrioventricular (AV) node ablation and pacemaker implantation improves outcomes in heart failure (HF) patients. However, no trials have been conducted to assess its potential benefit in HF and left ventricular ejection fraction (LVEF)>40%, a population in whom treatment strategies effectively improving outcomes are scarce.

The goal of this trial is to assess the benefit of heart rate regularization through AV node ablation and conduction system pacing (CSP) in patients with permanent AF and HF with preserved or mildly reduced systolic function. The investigators hypothesize that heart rate regularization added to physiological pacing - preventing the deleterious effect of right apical pacing - reduces mortality, HF hospitalisations or worsening HF in these patients.

Inclusion Criteria:

• Permanent atrial fibrillation
• At least one episode of hospitalisation due to heart failure in the previous 12 months.
• Left ventricular ejection fraction > 40%
• Average resting heart rate ≤ 110 beats per minute
• NT-proBNP ≥ 900 pg/ml in the 30 days prior to enrollment
• Age ≥ 18 years
• Capacity to understand the nature of the study, legal ability and willingness to give informed consent.

Exclusion Criteria:

• Severe frailty (Clinical Frailty Scale ≥ 7) or comorbidity reducing life expectancy to < 12 months.
• Acute heart failure at the time of enrollment or systolic blood pressure < 80 mmHg in the absence of inotropic agents.
• Severe chronic kidney disease (estimated Glomerular Filtration Rate ≤ 20 ml/1,73 m2
• Severe mitral or aortic valvular heart disease
• Anaemia (Haemoglobin < 10 g/dl)
• Morbid obesity (BMI ≥ 35)
• Severe Chronic Obstructive Pulmonary Disease (Gold ≥ 3)
• Presence of a different indication for pacing of implantable cardioverter-defibrillator (ICD)
• Obstructive hypertrophic cardiomyopathy
• Infiltrative cardiomyopathy (amyloidosis, sarcoidosis, Fabry disease, others)
• Simultaneous participation in a different trial

Clinical outcomes will be adjudicated by the investigator and reviewed by the Clinical Events Committee, that will be blind to treatment received by the patient.