The principal objective is to test non-inferiority of the CRT-DX system as compared to a conventional CRT-D system, in terms of the combined endpoint of mortality, hospitalizations due cardiovascular causes, any complication leading to loss of lead functionality, in the subset of patients without evidence of sinus dysfunction on optimal therapy.

Resting heart rate is strongly associated with incident worsening Heart Failure (HF) and mortality. Current devices for cardiac resynchronization (CRT-D) normally provide atrio-ventricular (AV) sequential pacing modes during resynchronization, but the best pacing programming strategy is not clear.

On the one hand a basic rate of 50 to 70 bpm (optionally with some rate-responsive function) could be considered for therapy up-titration, specifically betablockers; on the other hand, increasing pacing rates may partially reduce benefits from resynchronization, reducing filling time and contractility reservoir.

The Pegasus investigation is the only large randomized investigation comparing DDD with 70 bpm basic rate to DDD(R) @40 bpm. Results showed no difference in investigation endpoints, including mortality and HF-hospitalization.

These results may support the use of a device implementing both a CRT function and a right ventricular single-lead with and an atrial sensing dipole (CRT-DX system). This system can track ventricular pacing and resynchronization following atrial sensing, even if it cannot provide atrial pacing support. It should be assessed whether such limitation is counterbalanced by the advantages related to the reduced number of necessary leads, with simplified implantation and less complications.

The objective of the investigation is to assess whether atrial pacing support is really necessary in the subset of patients with indication to CRT-D and no evidence of sinus dysfunction on optimal therapy. The investigation will test the hypothesis that a CRT-DX system is not inferior to a conventional CRT-D system in this class of subjects.

Inclusion Criteria:

1. Male and female patients ≥ 18 years old
2. Patients willing to participate in the investigation and signing the Patient Informed Consent (PIC) Form;
3. Indication to CRT-D implant according to current ESC Guidelines on cardiac pacing and cardiac resynchronization therapy
4. Sinus rhythm at time of implant;
5. Optimized medical therapy according to current ESC Guidelines
6. Rest heart rate (HR) >45 bpm at baseline ECG, with betablocker therapy at optimal medical dosage; or, if rest HR<45 bpm: maximum heart rate at the 6- minute walking test >85 bpm.

Exclusion Criteria:

1. Any indication to atrial pacing according to current guidelines;
2. Both: resting heart rate <45 bpm at baseline ECG, with betablocker therapy at optimal medical dosage and maximum heart rate at the 6-minute walking test <85 bpm;
3. NYHA Class IV;
4. Permanent Atrial Fibrillation
5. Replacement of/upgrading from previously implanted pacing system;
6. Dialysis patients;
7. Pregnant or breast-feeding women.