Medtronic, Inc. is sponsoring the MIRACLE EF study, a prospective, randomized, controlled, double-blinded, global multi-center, Cardiac Resynchronization Therapy (CRT) in Heart Failure (HF) clinical study. The purpose of this study is to evaluate market released CRT pacemaker (CRT-P) devices in symptomatic HF patients with less severe left ventricular systolic dysfunction, specifically patients with reduced left ventricular ejection fraction (LVEF) in the range of 36% to 50%. This study will support expansion of indications for CRT worldwide. The outcome of this study is expected to support modification of existing U.S. and Japanese labeling for Medtronic's implantable CRT-P devices and to provide further evidence to support changes to cardiology practice guidelines (ACC/AHA, ESC guidelines) regarding the use of CRT in patients with mild to moderate HF.