The investigators sought to evaluate the morphological and functional changes, risk stratification and prognosis of patients of participants with compete left bundle branch block (CLBBB).

The conduction of this study was largely due to the increased clinical requirement, which reflected the increased awareness among physicians of heart failure due to asynchronous cardiac function caused by CLBBB. The investigators also aim to figure out the time point or CMR parameters for cardiac resynchronization therapy in patients with CLBBB.

The effect of cardiac resynchronization therapy (CRT) for heart failure patients was heterogeneous. Candidate selection was important before intervention. The underlying mechanical dyssynchrony of left ventricular bundle branch block was insufficiently descripted. Earlier study of investigators found novel imaging methods such as cardiovascular magnetic resonance imaging including T1 Mapping and feature tracking imaging can provide more detailed information about regional and global LV function in patients. While the role of new cardiac MR imaging techniques in predicting CRT responses, especially in LBBB patients, is still insufficient. Z Chen et al used T1 mapping technique to quantitatively assess the diffuse fibrosis burden of myocardial in heart failure patients. But they found focal fibrosis burden, not diffuse burden, is associated with a poor response to CRT. Other cardiac MR imaging parameters also showed potential predictors of CRT, such as 16 segment time-to-maximum radial wall thickness , scar locations and RV septal lead placement.In this study cardiovascular magnetic resonance imaging (including T1 Mapping combined with feature tracking imaging ) will be applied to follow up LV function in LBBB patients (with or without intervention) in 10 years to find out prognostic predictors and time point or CMR parameters for cardiac resynchronization therapy in patients with CLBBB.

Inclusion Criteria:

• Subject is 18 years or older and able and willing to consent.
• The patient should present a complete left bundle branch block (LBBB) with QRS duration of >120ms
• The patients should be in NYHA functional class I, II or III.

Exclusion Criteria:

• No informed consent
• Permanent atrial fibrillation, flutter or tachycardia (>100 bpm).
• Right bundle branch block
• Recent myocardial infarction, within 40 days prior to enrolment.
• Subject underwent coronary artery bypass graft (CABG) or valve surgery, within 90 days.
• Implanted with a LV assist device (LVAD), or has reasonable probability (per investigator's discretion) of receiving a LVAD in the next year.
• Severe aortic stenosis (with a valve area of <1.0 cm2 or significant valve disease expected to be operated on within study period).
• Complex and uncorrected congenital heart disease.
• Claustrophobia or devices

subjects with complete left bundle branch block