The patients are randomized to EITHER imgaging guided placement of the LV lead targeting the myocardial region with the latest mechanical activation and VV-interval settings programmed with simultaneous biventricular pacing (routine CRT-strategy arm) OR LV lead placement guided by procedural electrical mapping of all available cardiac veins targeting the myocardial region with the latest electrical activation combined with VV electrical optimization the day after implantation to achieve the narrowest paced QRS width (intervention arm).