This is a multi-site, non-randomized, data collection study. The purpose of this study is to investigate the cardiac electrical and mechanical function during LBBAP with different AV intervals.

Left bundle branch pacing (LBBP), achieved via trans-septal approach with the pacing tip at the left side of the ventricular septum, has recently initiated and been well practiced in China. LBBP is characterized with relatively ease of implantation (usually <5 minutes fluoroscopic use time), narrow paced QRS duration (similar to physiological pacing), low and stable pacing threshold (usually around 0.5V @0.5ms, good for device longevity and long-term stability without need of back-up pacing lead), high R wave amplitude for pacing management, and LBBB (left bundle branch block) correction by a low pacing output.

LBBAP (left bundle branch area pacing) can generate a relatively narrow QRSd and fast left ventricular activation. As AV interval is an important pacing parameter relevant to cardiac function. This study is to investigate the cardiac electrical and mechanical function during LBBAP with different AV intervals.

Inclusion Criteria:

• Patients aged from 18 to 80 years old
• Patients who are willing to provide Informed Consent
• Patients who received LBBAP for at least 3 months
• Patients' implanted device can provide intracardiac electrograms
• Bradycardia patients with AVB or heart failure patients with LBBB

Exclusion Criteria:

• Patients who are pregnant or have a plan for pregnancy during the study
• Patients with persistent AF
• Patients who have medical conditions that would limit study participation
• Patients who were already enrolled in other clinical trial which would impact his/her participation

Patients who received LBBAP for at least 3 months and come back to hospital for regular follow-up will be considered to be recruited voluntarily.