The purpose of this study is to provide evidence that the Refined Ventricular Pacing Algorithm leads to clinically relevant reduction (at least 50% reduction) of the incidence of ventricular pacing.

Electrical stimulation in the apex of the right ventricle ( ventricular pacing) usually improves the heart function of patients with a pacemaker and can even be life-saving. However, evidence is accumulating that ventricular pacing may also have undesired long-term cardiac effects. Therefore, it makes sense to limit ventricular pacing to the absolute required minimum. The functionality RVP (Refined Ventricular Pacing) in the C-series 2nd generation pacemakers of Vitatron B.V. Arnhem, the Netherlands is designed to reduce ventricular pacing.

After implantation of the Vitatron C50 D model C50A2 (pacemaker) or Vitatron C60 DR model C60A2 (pacemaker) and a 4-6 weeks stabilization period, proper functioning of pacemaker and leads (stimulation- and sensing parameters) is checked. The pacemakers will be programmed according to predefined settings.

In the following 4-weeks Baseline period diagnostic data (atrial fibrillation burden and percentage of ventricular pacing (% VP)) are collected in the pacemaker memory. Based on these data, patients will be excluded from further participation (patients with more than 15% atrial fibrillation) or subdivided into three groups: (a) < 30% VP (30- VP group), (b) >30% VP, Sick Sinus Syndrome and normal conductivity (SSS group), (c) >30% VP, 1st or 2nd degree AV block. Patients in these three groups will be treated for 4 weeks alternatively with the RVP functionality switched ON or OFF. The order will be determined by randomization. At the end of these two cross-over periods the % VP and the judgment of the patients of the last period will be assessed. Adverse events will be recorded from the moment of study enrolment.

Inclusion Criteria:

• Patients shall be willing to sign the Patient Informed Consent for this study
• Patients shall have at least one of the following indications for a pacemaker: - Sick Sinus Syndrome with normal QRS complexes
• First degree AV block with a PR interval <_220 ms for patients <_ 70 years of age, or <_ 260 for patients over 70 years
• Second-degree AV block, mobitz I (wenckebach) or mobitz II
• Patients shall be available for follow-up for the duration of their participation.

Exclusion Criteria:

• Patients involved in another investigation study conducted in parallel to this study
• Patients younger than 18 years of age and/or patients that do NOT meet other local requirements for participation
• Pregnant patients
• Patients with lead integrity problems (and the lead is not being replaced)
• Patients with persistant AF
• Patients with a complete AV block
• Patients with NYHA (New York Heart Association0 class III and IV
• Patients who underwent thoracic surgery in the last three months or are expected to have in the near future
• Patients with a 2:1 block
• Patients with a life expectancy less than half a year