Achieving Conduction System Activation With Leadless Left Ventricular Endocardial Pacing

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.