His Bundle Pacing in the Era of Left Bundle Branch Pacing

Soon after the rapid growth in popularity of His bundle pacing (HBP) over the period 2016–20, this conduction system pacing (CSP) modality was overshadowed by left bundle branch area pacing (LBBAP). The LBBAP technique was widely adopted by clinicians due to better pacing parameters, higher efficacy in patients with distal conduction system abnormalities, a broader pacing target and more lenient success criteria, resulting in an apparently faster learning curve. Given this situation, there are several relevant questions regarding HBP:

• Are there any advantages of HBP over LBBAP?
• What could the current uses of HBP be?
• Where are we today regarding the development of the HBP technique?
• Can we overcome the known limitations of HBP with better tools, improved implantation techniques and/or patient selection?

This focused review addresses these questions.

Milestones in His Bundle Pacing

The feasibility of temporary pacing of the His bundle (HB) in humans was first demonstrated by Narula et al. in 1970.¹ It took three decades before these observations led to clinical implementation: Deshumukh et al. reported the outcomes of permanent HBP in a series of 12 patients with narrow QRS in 2000, and a similar study was published by Moriña Vázquez et al. in 2001.^2,3 These very first reports of HBP pacemaker implantation addressed the potentially most important clinical indication for HBP, namely heart failure with permanent AF and the need for regular and synchronous activation of the left ventricle (LV). These reports were followed by studies from pioneering centres in Europe.^4–10 The feasibility of HBP in routine clinical practice was first reported by Sharma et al., and Keene et al. published the first large multicentre study on learning curves and mid-term outcomes.^11,12

The feasibility of HBP-CRT in patients with left bundle branch block (LBBB) is based on the concept introduced by James and Sherf of the longitudinal separation (by collagen and sparse transverse junctions) of Purkinje fibres within the HB, making them predestined for the left bundle branch (LBB) or right bundle branch.^13,14 This concept was validated by Narula in 1977, who demonstrated that LBBB may be due to a focal lesion in the HB, a lesion that causes block or block-equivalent delay in LBB-predestined fibres.¹⁵ Narula showed that pacing the HB proximal to the lesion reproduces the LBBB QRS morphology, whereas pacing the HB distal to the lesion restores a narrow, fully physiological QRS.¹⁵ Recently, this concept was further substantiated by Upadhyay et al., who demonstrated the absence of pre-QRS conduction system potentials in the septal portion of the LBB, which could be restored by pacing the HB distal to the lesion in LBB-predestined fibres (if QRS normalisation was achieved).¹⁶ The clinical use of HBP in patients with LBBB and CRT indications dates back to the first case report by Moriña-Vázquez et al. in 2005, followed by another study by the same group.^17,18 The practical use of HBP to correct LBBB dyssynchrony was further expanded by Vijayaraman et al. by combining HBP with coronary venous pacing, resulting in a hybrid pacing approach labelled His-OpTimised (HOT)-CRT.¹⁹ This pacing modality can be used when HBP results in only partial correction of LBBB. Recently, the first multicentre observational studies and randomised trials evaluating HBP-CRT and HOT-CRT were published, with favourable results.^20–22

Development and Limitations of the His Bundle Pacing Technique

Initially, the lack of dedicated equipment for HBP necessitated manual reshaping of the lead stylet to reach the HB area; often the HB region was delineated with a diagnostic catheter introduced via femoral access, adding to the complexity of the procedure. The first reports of HBP used this method, coupled with the use of an active helix lead. Relatively low success rates, high capture thresholds, long procedure/fluoroscopy times and sensing issues limited the use of permanent HBP to a few pioneering centres.² At that time, the reported HB capture thresholds ranged from values of 1.5–3.2 V in narrow QRS to 3.7 V in LBBB patients.^{2,4–6,9,10,18} The success rate at that time is difficult to assess reliably because of the known under-reporting of failures in observational studies, the high likelihood of the preselection of patients, the lack of established criteria to confirm HB capture during presumed non-selective pacing (common use of the term ‘para-Hisian pacing’) and the acceptance of excessively high (by current standards) acute capture thresholds. Nevertheless, the success rate appears to be less than 70% and 30% in patients with narrow QRS and LBBB, respectively.^{2,4–6,9,10,18}

An evident increase in the success rate and an improvement in pacing parameters were observed with the use of the SelectSecure 3830 fixed-helix, lumenless, thin (4.1 Fr) lead, together with the dedicated C315HIS delivery catheter with a preformed septal curve (Medtronic).¹⁰ It was widely accepted that a delivery catheter offered more support during lead fixation and facilitated localisation of the HB region compared with the manually shaped stylet-based lead delivery technique. Using a delivery catheter soon became the dominant approach for HBP worldwide, resulting in an improvement in acute and chronic HB capture thresholds to approximately 1.6–1.8 V (already in the early experience phase) and a success rate of approximately 80–90%.^23–26 Another step forward was the use of an electrophysiological system for HBP; this approach facilitated confirmation of HB capture, visualisation of HB potential during mapping and visualisation of HB current of injury (COI) with appropriate filter settings (0.1–250 Hz).²⁷

Further research to improve the HBP technique focused on achieving a lower acute capture threshold and higher success rates. This included the introduction of the dual-lead technique, reports on the importance of the HB COI and the development of the negative HB potential as an intraprocedural marker of the optimal HB pacing site and good lead contact, and the use of contrast to delineate the summit of the tricuspid annulus and distal HBP region.^27–32 However, the extent to which these seminal observations and recommendations have changed real-world clinical practice is not known. Of note are advances in the delineation of HB physiology, including the development of several HB capture criteria beyond the simple threshold test and the determination of HB chronaxie and strength–duration curves to optimise battery longevity and facilitate selective or non-selective HB capture.^33–36

Important clues for the future use of HBP (for patient selection and improvement of implantation tools) came from studies that delineated the limitations of HBP, namely a relatively low success rate in LBBB as well as in distal atrioventricular block.^16,37–39 Since then, the only progress in the HBP implantation technique was the introduction of the steerable 3D catheter, which provides better reach to superior sites and facilitates localisation of the HB region, especially for beginners. The use of steerable catheters probably results in a higher success rate and a steeper learning curve, but this has never been formally investigated because research interests have already shifted to LBBAP. One of the authors (MJ) believes that the use of a continuous pacing and recording technique for HBP is also a valuable step forward (Figure 1). This technique places more emphasis on constant monitoring of HB signals and pace mapping rather than activation mapping, potentially facilitating the localisation of low-threshold areas and helping with lead fixation, because loss of HB capture during lead rotation due to unfavourable lead trajectory is then immediately apparent.