Achieving Conduction System Activation With Leadless Left Ventricular Endocardial Pacing (ACCESS-CRT)

Inclusion Criteria: Age over 18 years Ability to provide informed consent to participate and willing to comply with the clinical investigation plan and follow-up schedule. Patients meeting European Society of Cardiology Criteria for Cardiac Resynchronization Therapy (CRT) with Class 1 or 2a recommendation: Dyssynchronous Heart Failure: Left Ventricular Ejection Fraction (LVEF) ≤35%, QRS duration >150ms or >130ms in presence of left bundle branch block (LBBB). Also included here are patients with EF ≤35%, an existing pacing system and an RV pacing burden of >20%. BLOCK HF population: Patients requiring pacing for atrioventricular (AV) block with an LVEF ≤50% AV node ablation (AVNA) population: Patients planned for AVNA with EF ≤50%. Exclusion Criteria: Any contraindication to LV endocardial pacing. LV thrombus Contra-indication to heparin Contra-indication to anti-platelet agents Failure of acoustic window screening Septal wall thickness <5mm (minimum required wall thickness at any target implant site) Myocardial infarction within 40 days prior to enrolment. Cardiac surgery or coronary revascularisation procedure within 3 months prior to enrolment or to be scheduled for such procedures within the following 7 months. Participation in other studies with active treatment/investigational arm. Pregnant or planning to become pregnant in the next 7 months.