The purpose of this clinical project is to evaluate the efficacy of the presence of Right Ventricular Apical (RVA) pacing induced ventricular dyssynchrony as a guiding parameter for bi-ventricular pacing in patients with bradycardia and normal left ventricular ejection fraction (LVEF). The results of this project may provide with the evidence based medicine for guidelines expansion of using Cardiac resynchronization therapy (CRT) in patients with Heart Block and normal LVEF (LVEF >45%).

All eligible patients willing to provide written informed consent were invited to participate in the study, and then randomized 1:2 to either the Control Group or Echo-guided Group. Patients randomized into the Control Group were implanted with a dual chamber pacemaker (DDDR) device (St. Jude Medical) standard therapy. Patients randomized into the Echo-guided Group were implanted with a cardiac resynchronization therapy pacemaker (CRT-P) device (St. Jude Medical), which was programmed to DDDR pacing mode (CRT-P OFF) at implant. For each Echo-guided Group patient, presence of RVA pacing induced ventricular dyssynchrony was assessed within 24-hour of implant using the standardized criteria of Doppler echocardiography. The Echocardiography Core Laboratory (Echo Core Lab) then analyzed each image and the treatment allocation of the Echo-guided Group was done within 72-hour post implant based on the Echo Core Lab outcomes:

• Patients with the presence of RVA pacing induced ventricular dyssynchrony were allocated to the CRT-P standard therapy with CRT-P turned ON
• Patients with the absence of RVA pacing induced ventricular dyssynchrony were allocated to the DDDR standard therapy with CRT-P remained OFF

Patients meeting all the inclusion criteria and not meeting any of the exclusion criteria were eligible for the study. Data was collected at the following study visits:Enrollment, Implant & Randomization, Pre-discharge (PDH) (≤ 72 hours post implant), Month 1 follow-up visit: 30 ± 14 days post implant, Month 3 follow-up visit: 90 ± 14 days post implant, Month 6 follow-up visit: 180 ± 14 days post implant and Month 12 follow-up visit: 365 ± 14 days post implant

Inclusion Criteria:

• Patient meets current indications for implantation of a DDDR pacemaker
• Patient is geographically stable and willing to comply with the required follow-up schedule
• Patient has LVEF >45%
• Patient has atrioventricular block (IIo or above)
• Patient has adequate echocardiographic images to measure LV volumes and LVEF

Exclusion Criteria:

• Patient has permanent atrial fibrillation (AF)
• Patient has unstable angina or an acute coronary syndrome
• Patient has undergone percutaneous coronary intervention or coronary artery bypass surgery within the previous 3 months
• Patient's life expectancy is less than 1 year
• Patient is less than 18 years old
• Patient is pregnant
• Patient has received a heart transplant