[Implantation]
The left axillary vein was punctured to obtain venous access, an incision was made 4 cm below the left clavicle to make a capsular bag. the C315 HIS sheath was advanced over a long guide-wire towards the tricuspid annulus, the 3830 lead was then advanced to the tip of the sheath towards His bundle region, HB region was mapped in the unipolar configuration. His potential can be detected[Fig1①] , His lead was fixed by sightly screw followed by HBP, however HBP showed a unsatisfactory sensing, a high and unstable capture threshold, then His lead was used as a protective lead in case of temporary pacing, and the X-ray reference image of location of His lead was routinely regard as a marker for LBBP lead implantation.
Another 3830 lead was advanced to tip of the sheath towards surface of right ventricular septum, the initial site for LBBP, was approximately 1cm distal to the HBP lead position on the surface of right ventricular septum along the line between HBP site and right ventricular apex in the fluoroscopy view of right anterior oblique (RAO 30°)[Fig1④] , At this site, the paced QRS complex morphology usually demonstrates a “w” pattern with a notch at the nadir of the QRS in lead V1 [Fig1②][5] .
Then, the sheath was rotated anticlockwise to maintain the orientation of lead tip perpendicular to septal surface and provide adequate support to screw the lead into the septum. As lead tip screwing into inter-ventricular septum, lead parameters (R wave sensing and LBB capture morphologies and thresholds) were assessed synchronously in both unipolar and bipolar configurations. The following signs will appear: 1) the notch of paced QRS complex in lead V1 will move from nadir to the end [Fig1③] . 2) First increase then reduction of pacing impedance in unipolar. 3) the position of lead tip advances with fulcrum sign left behind on fluoroscopy [Fig1④] .
The lead tip advanced cautiously until satisfactory parameters were achieved, the latter included lead tip impedance 560Ω, ring impedance 650Ω, bipolar impedance 735Ω, threshold 0.7V, QRS amplitude 28.5mV, left branch bundle potential cannot be detected during the procedure.
The right ventricular defibrillation lead 6947 was implanted in right ventricle (RV) apex in a standard fashion (rotation, fixation et al.). Parameters of RV lead were identified with threshold 0.7v, the QRS amplitude 5.4mV, and impedance 399Ω.
The system (C315 HIS sheath and 3830 lead) located in His bundle region was then withdrawn to inferior right atrial septum. Parameters of right atrial lead were assessed with threshold 0.4V, P amplitude 2.7mV, and impedance 574Ω.