Role of Left Bundle Branch Pacing: will it replace other pacing modality for CRT?
LVSP has emerged as an alternative method for delivering physiological pacing particularly in patients with infranodal atrioventricular block and left bundle branch block (LBBB). Though HBP can correct classic LBBB in 97% of patients60, the site of block is often located within the His or proximal left bundle61. The proximal left bundle branches run through the LV septum and fan out proving a wider target for pacing. Huang et al have reported a technique for left bundle pacing using a transseptal approach62. Left bundle pacing has been reported to have low pacing thresholds, larger R waves and, by targeting the distal conduction system, a lower theoretical risk for developing distal conduction block63. The LVSP could be an attractive way to overcome the majority of the limitations of HBP. These include difficulty in identifying the adequate location of the His bundle, high and unstable threshold in up to 10% of patients, damage to the His bundle during implantation, heart block distal to the His bundle, undersensing R wave amplitude and oversensing atrial signals and lower success rate in patients with left bundle branch block (LBBB) and heart failure due to high pacing output required to correct LBBB58,60,64–68.  The rationale behind correcting LBBB by HBP is that increased pacing output captures the left bundle branch (LBB) beyond the area of block. El-Sherif et al.69 showed that HBP required 20 V to capture the LBB. Huang et al.62 first reported direct left bundle branch pacing in patients with LBBB and heart failure. HBP at 10 V failed to correct LBBB but after advancing the pacing tip towards the left ventricle LBBB resolved at a low pacing capture threshold (0.5V).  The heart failure symptoms improved, LV ejection fraction improved and adjustment of pacing parameters led to normalization of QRS. Chen et al.70 used a transventricular septal approach for left bundle pacing in patients with bradycardia. In their study, ECG configuration changing from LBBB pattern to RBBB pattern was observed as the pacing tip was advanced from the right interventricular septum to capture the left bundle branch. Demonstration of left bundle potential during intrinsic rhythm provided strong evidence of successful left bundle pacing. Notably, paced ECG QRS duration was significantly shorter (111.85±10.77 ms) compared with either RV septal pacing (154.80±9.85 ms) or RV apical pacing (165.50±17.80 ms) with comparable capture threshold. Interestingly, both left and right bundle branch block could be corrected by left bundle pacing at low capture threshold. Interestingly, both left and right bundle branch block could be corrected by LVSP at a low capture threshold.  Additionally, the QRS in LVSP is typically <130ms instead of complete RBBB. It is also significantly shorter than during RV pacing70,71. This may be explained by retrograde activation of the right bundle during pacing or connection between the main right and left bundle branches72,73 or cell to cell conduction74. The LVSP procedural success is 80.5-93% in small studies71,75,76. Vijayaraman et al76. reported periprocedural lead dislodgement in 3 of 97 patients who underwent LVSP. Potential complications for left bundle pacing procedure include right bundle branch (RBB) injury during the procedure, lead dislodgement, septal perforation and coronary artery injury especially the septal perforators77.