Cardiac pacemaker (PM) implantation is the established treatment for relevant bradyarrhythmias. Conventional PMs require 1-3 pacing leads to register the heart's intrinsic activity ("sensing") and to deliver the electrical stimuli to the heart ("pacing"). These leads are responsible for the vast majority of morbidity after implantation and PM failures. Therefore, a leadless PM system (Micra TPS™, Medtronic, United States) has been introduced a few years ago. This system overcomes the limitations of leads, however, the first generation of the Micra TPS™ only allowed sensing and pacing in the right ventricle. More recently, an upgraded version has been introduced and gained market approval (Micra AV, Medtronic, United States). According to published results from several clinical trials, this device allows sensing the atrial activity and, thus, timing the delivery of the ventricular pacing impulse in a physiological manner similar to a conventional dual-chamber PM with two leads. Clinical feasibility and safety for this concept have been established already. However, it is unclear if this translates into a direct clinical benefit for patients in comparison to conventional PM systems.

The aim of this trial is to compare the therapeutic efficacy of the Micra AV™ PM and conventional dual-chamber PM systems in patients with intermittent or permanent atrioventricular conduction block and a PM indication according to the latest European guidelines. Thus, patients will be randomized to either a conventional dual-chamber PM implantation or the implantation of a leadless Micra AV™ system. Patients will be stratified for gender (female/male) and a priori estimated physical exercise capacity ("fit"/"unfit"). The primary outcome will be the physical exercise capacity of the patients.

The null hypothesis with regards to the primary endpoint is that the leadless pacemaker arm shows an inferior VO2 anaerobic threshold than the conventional pacemaker arm. Hence the alternative hypothesis postulates that the leadless pacemaker arm shows a non-inferior VO2 anaerobic threshold compared to the conventional pacemaker arm. Rejection of the null hypothesis is needed to conclude non-inferiority.

Inclusion Criteria:

• Patients (≥70y) undergoing a de-novo pacemaker implantation due to intermittent or permanent AV block, qualifying for a conventional or leadless pacemaker
• Written informed consent

Exclusion Criteria:

• Permanent atrial fibrillation or atrial standstill
• Evidence of sinus node disease and need for right atrial pacing (not possible with Micra AV)
• LVEF <50% and permanent high-degree or total AVB (requiring CRT/His-Bundle/CSP pacing)
• Preoperative E/A ratio >1.5 in the echocardiography
• Any co-existing ICD indications (no leadless ICD systems available)
• Hemodialysis
• Presence of a mechanical tricuspid valve prosthesis
• Unwilling or unable to comply fully with study procedures and follow-up