The PROTECT-HF multi-centre randomised controlled trial will compare two different pacing approaches for treating patients with slow heart rates. In it the investigators will compare a long-standing standard approach for pacing; right ventricular pacing, with a new form of pacing, physiological pacing (His and Left bundle area pacing) in 2600 patients.

Patients will be allocated at random to receive either right ventricular pacing or physiological pacing. Endpoint measurements will be undertaken at baseline, and at six-monthly intervals post-randomisation. Treatment allocation will be blinded to the endpoint assessor and the patient.

Recruitment and pacemaker implantation will be carried out at each participating centre. The primary analysis will be intention to treat. The investigators will also perform an on-treatment analysis.

2048 patients are needed to detect the expected effect size with 85% power. A total of 2600 patients will be recruited to allow for patient drop-out and crossover.

500-patient sub-study will assess within patient, and between groups, echocardiographic changes over a 24-month period to try and improve mechanistic understanding of PICM (Pacing Induced Cardiomyopathy).

Patients entering the study will attend for implantation of a pacemaker device and be randomised to either right ventricular pacing or physiological pacing.

Patients at sites participating in echo sub-study will be informed of and given opportunity to consent to echo sub-study, this will be optional to them, even if they have consented to the main study.

We will recruit patients who are referred for clinically indicated pacemaker implantation

Inclusion Criteria:

1. Adults aged over 18 with left ventricular ejection fraction >35% and one or more of the following guideline based ventricular pacing indications:

1. Permanent or intermittent 3rd degree AV block
2. Permanent or intermittent Mobitz type II AV block
3. First Degree AV block with a pacing indication
4. Slow chronic Atrial Fibrillation or Proposed AV node ablation
5. Bifascicular block with a pacing indication
6. Trifascicular block with a pacing indication
7. Wenckebach with a pacing indication

Exclusion Criteria:

1. Patients who are likely to only need occasional ventricular pacing, i.e. those with isolated sick sinus syndrome.
2. Pregnant women.
3. Unable to provide informed consent.
4. Those with comorbidity leading to a life expectancy <1year.