Post-market Prospective, Multicenter, Randomized and Single-blinded Clinical Investigation to Evaluate Whether Cardiac Re-Synchronization Therapy (CRT) Using Automatic Continuous Atrioventricular (AV) Delay Optimization is Superior to CRT With Conventional Biventricular Stimulation (BiV). (CRUSTY +)

Inclusion Criteria: More than 18 years of age; Have signed the Informed Consent form. have been implanted with an Abbott CRT-D with the Sync-AV dynamic function under the current Class I or Class IIa ESC indications (2021 guidelines) for CRT implantation (including upgrades from single or dual chamber pacemakers or ICDs); Have to be in sinus rhythm at the Baseline visit and with Left Branch Block (LBBB) (2021 ESC/REVERSE): LVEF needs to be <35% while under optimal medical treatment to be willing to meet all study requirements and have the ability to do participate to this study. Exclusion Criteria: Inclusion / Exclusion criteria: Inclusion criteria: To participate in this clinical study, patients must meet ALL of the following inclusion criteria: More than 18 years of age; Have signed the Informed Consent form. And have been implanted with an Abbott CRT-D with the Sync-AV dynamic function under the current Class I or Class IIa ESC indications (2021 guidelines) for CRT implantation (including upgrades from single or dual chamber pacemakers or ICDs); Have to be in sinus rhythm at the Baseline visit and with Left Branch Block (LBBB) (2021 ESC/REVERSE): LVEF needs to be <35% while under optimal medical treatment In addition, patients need to be willing to meet all study requirements and have the ability to do participate to this study. Exclusion criteria: Patients who meet any of the following exclusion criteria are NOT eligible to participate in the clinical study: Having suffered a myocardial infarction or unstable angina in the 40 days prior to inclusion. Having undergone coronary revascularization (PTCA, Stent or CABG) in the 4 weeks prior to inclusion. Having suffered a Cerebrovascular Accident (CVA) or a Transient Ischemic Accident (TIA) in the 3 months prior to inclusion. NYHA Class IV. Having received a heart transplant or being waiting to receive it (status I classification); Suffering from primary valve disease requiring surgical intervention. Having long-lasting or permanent atrial fibrillation Showing inadequate transthoracic echocardiographic images which will not allow to establish cardiac output and LV volumes. Having a life expectancy <12months. Being pregnant or planning to become pregnant during the clinical investigation. Inability to meet the monitoring schedule. Currently participating in any other clinical interventional research. Having a permanent high degree AV block