Biventricular or Left Bundle Branch Pacing to Assess the Effect of Cardiac Resynchronization Therapy in Patients Undergoing Transcatheter Aortic Valve Implantation

Inclusion Criteria: * Age over 18 years * Successful and complication-free TAVI implantation for significant aortic stenosis * Signed informed consent after detailed patient information * New-onset LBBB wider than 130 ms after TAVI, persisting at the time of planned discharge (assessed postoperatively on or after day 5)(21) * Mildly reduced left ventricular systolic function on echocardiography at the planned time of discharge (at the earliest on postoperative day 5), with EF between 35% and 55% Exclusion Criteria: * Pre-existing right bundle branch block, or prior pacemaker/ICD implantation associated with significant (\>20%) ventricular pacing * Progression of LBBB during hospitalization to a degree indicating pacemaker implantation (e.g., Mobitz II or third-degree AV block, alternating bundle branch block) before the planned discharge date * Concomitant severe valvular disease of other valves (mitral, pulmonary, or tricuspid) * Severe infection and/or septic state * TAVI-associated stroke * More than moderate paravalvular leak of the TAVI valve on postoperative echocardiography * EF \< 35% or \> 55% on echocardiography performed at the planned time of discharge * GOLD stage IV COPD * Chronic renal failure requiring regular dialysis * Concurrent participation in another clinical trial