The purpose of this clinical study is to test the hypothesis that market released Cardiac Resynchronization Therapy (CRT) devices which contain the AdaptivCRT® (aCRT) algorithm have a superior outcome compared to standard CRT devices in CRT indicated patients with normal atrio-ventricular (AV) conduction and left bundle branch block (LBBB).

Inclusion Criteria:

• Subject is willing to sign and date the study Patient Informed Consent Form.
• Subject is indicated for a CRT device according to local guidelines.
• Sinus Rhythm at time of enrollment
• Complete Left Bundle Branch Block (LBBB) as documented on an ECG (within 30 days prior to enrollment). Strauss Criteria used for LBBB.
• Intrinsic, normal AV conduction as documented on an ECG by a PR interval less than or equal to 200ms (within 30 days prior to enrollment).
• Left ventricular ejection fraction less than or equal to 35% (documented within 180 days prior to enrollment).
• NYHA class II, III or IV (documented within 30 days prior to enrollment) despite optimal medical therapy. Optimal medical therapy is defined as maximal tolerated dose of Beta-blockers and a therapeutic dose of ACE-I, ARB or Aldosterone Antagonist.

Exclusion Criteria:

• Subject is less than 18 years of age (or has not reached minimum age per local law).
• Subject is not expected to remain available for at least 2 years of follow-up visits.
• Subject has permanent atrial arrhythmias for which pharmacological therapy and/or cardioversion have been unsuccessful or have not been attempted
• Subject is, or previously has been, receiving cardiac resynchronization therapy.
• Subject is currently enrolled or planning to participate in a potentially confounding drug or device trial during the course of this study. Co-enrollment in concurrent trials is only allowed when documented pre-approval is obtained from the Medtronic study manager.
• Subject has unstable angina, or experienced an acute myocardial infarction (MI) or received coronary artery revascularization (CABG) or coronary angioplasty (PTCA) within 30 days prior to enrollment.
• Subject has a mechanical tricuspid heart valve or is scheduled to undergo valve repair or valve replacement during the course of the study.
• Subject is post heart transplant (subjects on the heart transplant list for the first time are not excluded).
• Subject has a limited life expectancy due to non-cardiac causes that would not allow completion of the study.
• Subject is pregnant (if required by local law, women of child-bearing potential must undergo a pregnancy test within seven days prior to device implant).
• Subject meets any exclusion criteria required by local law.