This 2x2 randomized crossover feasibility study will evaluate the therapeutic value of adding pacing from the left ventricular lead to Left Bundle Branch Area Pacing (LBBAP) using a cardiac resynchronization therapy defibrillator (CRT-D). This combined therapy is known as Left Bundle Branch Optimized Cardiac Resynchronization Therapy (LOT-CRT) and will be compared to LBBAP therapy over sequential 6-month periods in CRT-indicated patients with LBBB. Primary and secondary outcomes are echocardiographic changes. Exploratory assessments will include NT-proBNP and subject-reported quality of life (KCCQ).

Inclusion Criteria:

• Subject meets current ACC/AHA/HRS guidelines for implantation of a CRT-D device.
• Subject has documented LVEF ≤ 35% within 60 days of enrollment.
• Subject meets the criteria for presence of Strauss-defined LBBB.
• Subject meets NYHA classification II or III.
• Subject is willing and able to provide written consent.
• Subject is at least 18 years of age at the time of consent.
• Subject is geographically stable and willing and able to complete study procedures, including follow-up visits.
• The subject's medical records must be accessible by the enrolling site over the follow-up period.

Exclusion Criteria:

• Subject has documented myocardial infarction before enrollment.
• Subject has non-LBBB conduction patterns such as Right Bundle Branch Block (RBBB) or non-specific IVCD.
• Subject has 2nd or 3rd degree AV block.
• Subject has persistent or permanent atrial fibrillation (AF) or atrial flutter (AFL).
• Subject has intrinsic (non-paced) QRS duration ≤ 120ms.
• Subject with previous or existing pacemaker (including transvenous and transcatheter pacing system), Implantable Cardioverter Defibrillator (ICD, transvenous) or CRT-D (transvenous) device or leads.
• Subject has contraindications for screw-in transvenous lead placement (e.g., mechanical right heart valve).
• Subject underwent valve surgery within 90 days prior to enrollment.
• Subject is unable or unwilling to undergo baseline CMR imaging, if previously collected CMR images are unavailable or do not meet minimum acceptable criteria.
• Subject is post-heart transplantation or is actively listed on the transplantation list.
• Subject is implanted with a left ventricular assist device (LVAD).
• Subject has severe renal disease.
• Subject is on continuous or uninterrupted infusion (inotropic) therapy for heart failure.
• Subject has complex and uncorrected congenital heart disease.
• Subject has life expectancy of less than 12 months.
• Subject is pregnant or breastfeeding.
• Subject is enrolled or planning to enroll in a concurrent clinical study that may confound the results of this study, without documented pre-approval from a Medtronic study manager.