Rationale: Permanent cardiac pacing is the only available therapy in patients with atrioventricular (AV) conduction disorders and can be life-saving. Right ventricular pacing (RVP), the routine clinical practice for decades in these patients, is non-physiologic, leads to dyssynchronous electrical and mechanical activation of the ventricles, and may cause pacing-induced cardiomyopathy and heart failure. Left ventricular septal pacing (LVSP) is an emerging form of physiologic pacing that can possibly overcome the adverse effects of RVP. Study design and hypotheses: The LEAP trial is a multi-center investigator-initiated, prospective, randomized controlled, open label, blinded endpoint evaluation (PROBE) study that compares LVSP with conventional RVP. A total of four hundred seventy patients with a class I or IIa indication for pacemaker implantation due to AV conduction disorders and an expected ventricular pacing percentage \>20% will be randomized 1:1 to LVSP or RVP. The primary endpoint is a composite endpoint of all-cause mortality, hospitalization for heart failure and a more than 10% decrease in left ventricular ejection fraction (LVEF) in absolute terms leading to a LVEF below 50% at one year follow-up. LVSP is anticipated to result in improved outcomes. Secondary objectives are to evaluate whether LVSP is cost-effective and associated with an improved quality of life (QOL) as compared to RVP. Quality of life is expected to improve with LVSP and reduced healthcare resource utilizations are expected to ensure lower costs in the LVSP group during follow-up, despite initial higher costs of the implantation. Study design: Multi-center investigator-initiated, prospective, randomized controlled, open label, blinded endpoint evaluation (PROBE) study. Study population: Adult patients with a bradycardia-pacing indication because of AV conduction disorders with an expected ventricular pacing percentage of ≥ 20% and a left ventricular ejection fraction (LVEF) \>/= 40%. Four hundred seventy patients will be randomized 1:1 to LVSP or RVP. Intervention: LVSP vs RVP. Main study parameters/endpoints: The primary endpoint is a composite of all-cause mortality, hospitalization for heart failure, and a more than 10% point decrease in left ventricular ejection fraction (LVEF) leading to an LVEF below 50%, which as a binary combined endpoint will be determined at one year follow-up. Secondary endpoints are: * Time to first occurrence of all cause mortality or hospitalization for heart failure. * Time to first occurrence of all cause mortality. * Time to first occurrence of hospitalization for heart failure. * Time to first occurrence of atrial fibrillation (AF) de novo. * The echocardiographic changes in LVEF at one year. * The echocardiographic changes in diastolic (dys-)function at one year. * The occurrence of pacemaker related complications. * Quality of life (QOL), cost-effectiveness analyses (CEA) and budget impact analysis (BIA). The secondary endpoints (other than echocardiographic LVEF change) will be determined at the end of the follow-up period, when the last included patient has reached one year follow-up. The individual follow-up time for patients at this time point will vary with a minimum of one year.