Adaptive Cardiac Resynchronization Therapy in Patients With RBBB

Inclusion Criteria: Included patients will have: * sinus rhythm or isolated atrial pacing * LV systolic dysfunction (EF \<35% at time of device implant) * AV conduction \<250 ms, RBBB, and * have an implanted CRT device based on the ACC/AHA/HRS guidelines (2, 18) or will have a CRT device implanted during the study enrollment per ACC/AHA/HRS guidelines (2). Exclusion Criteria: * atrial fibrillation * atrial flutter * atrial tachycardia * AV delay \> 250 ms * sinus tachycardia with resting heart rate at time of the study 100 bpm * frequent APCs (\> 25% of the total beats/min) or PVCs (\>20% of the total beats/min), or * patients with EF \> 40% at time of enrollment (if LV systolic function was found to be improved from time of implant).