7. Left bundle branch area pacing: just an alternative to HBP or an independent pacing method?
LBBAP is considered an alternative, so far, to HBP, with both these methods constituting the two components of CSP. LBBAP includes LV septal pacing and LBBP. There are no studies to date for LV septal pacing, having a clinical follow-up, as opposed to RV septal pacing. Left bundle branch pacing, compared to HBP, presents several differentiations.(Figure 5) Mostly this is due to anatomical reasons. As opposed to the small size of the His bundle, the left bundle branch extends comparatively to a much larger area in the interventricular septum, which has two anatomical explanations. That is because, on the one hand, the left bundle branch subdivides into two main fascicles, the left anterior and the left posterior fascicle. At the same time, other sources divide the left bundle branch into three fascicles concluding with the left septal fascicle. On the other hand, there is a rich network of fibers among these structures, creating a wide range of variations. As a result, this part of the conduction system can be stimulated in an easier way successfully than the His bundle area.10, 57
In addition to the anatomical ones, there are technical reasons making LBBP easier. The infrahisian conduction system and the proximal right and left bundle branches are enveloped in an insulating sheath of lesser thickness than the penetrating bundle of His. In contrast, most peripheral branches are located in the myocardium, thus requiring a much smaller threshold for selective stimulation, with a consequent significant impact on battery longevity. Also, for LBBP, as in HBP, the method used for the pacing lead implantation is fluoroscopy. The only difference is the wider target area in the interventricular septum and, therefore, the fewer requirements for lead placement accuracy.58 Other technical issues differentiate the LBBP implantation procedure from HBP’s. In challenging cases, fixing a lead into the His bundle as an anatomical landmark may be helpful.15 Special technical feature of the implantation technique of LBBP is the fulcrum sign of the pacing lead, which makes the lead and its subsequent parameters more stable. Sheath angiography is also helpful by gently pulling back the catheter and administering a contrast agent to establish the correlation of lead and septum positions and identify the depth penetrated by the bipolar lead with its active helix. The most commonly used lead is Select Secure™ 3830, as in HBP, with 9 mm tip to ring spacing and 1.8 mm the exposed helix. The w-shaped QRS configuration of pacing the right ventricular septum at the initial fixation site indicates LBBP.
This technique is also demanding to program. Apart from the threshold, better parameters are recorded regarding sensing, which is also maintained in the follow-up, especially concerning atrial oversensing and ventricular undersensing, in contrast to HBP. In addition, LBBP offers ease in the need for future AV node ablation in heart failure patients with refractory atrial fibrillation and a rapid average heart rate despite optimal medical therapy in contrast with HBP that this may not be possible. LBBP, as an alternative method, may be successful in blocks that are too distal to be treated with HBP, although these blocks are not that frequent. A recent publication, in a series of 333 consecutive patients with AV block referred for pacemaker implantation, recorded the prevalence of the site of conduction block.59 The study found that the exact site of conduction block was nodal in 55% and infranodal in 45% of the cases, which in turn, distributed in intra-Hisian 89%, infra-Hisian 4%, and indeterminate 7% of the patients. Therefore, although infra-Hisian blocks are rare, LBBP is an alternative pacing modality for these patients.
Compared to HBP, where complications are minimal, the LBBP technique’s complications may be life-threatening to the patient. There have been reported cases of septal perforation to the LV chamber (during the procedure, but also post-procedure in case of excessive slack of the pacing lead), LBBP lead dislocation, especially when there is not adequate slack of the lead, right bundle branch injury, septal injury or hematoma, and coronary artery injury, especially when the pacing lead is placed deep in the proximal septum, causing even myocardial infarction.60 Moreover, LBBP lead fracture may occur, causing loss of pacing, with unpleasant consequences, especially for pacemaker-dependent patients. However, the rate of all these procedure-related complications of LBBP is still low.
Therefore, LBBP is currently being tested not only as an alternative method to HBP in case of technical difficulties, such as a high HBP threshold at implant, but also as a primary pacing method, with all the corresponding indications described for HBP.61 Success rate is very high (>90 %), and the achieved capture thresholds are low (< 1 Volt at 0.4 msec), which maintain in the follow-up. At the same time, beneficial results have also been found in echocardiographic parameters, such as LVEF and left ventricular volume measurements. Various studies have pointed out this in the last few years. The main indications for LBBP concern bradycardia and CRT. Zhang et al. (2019) in 11 patients, Wu et al. (2020) in 32 patients, Huang et al. (2020) in 61 patients, and Vijayaraman et al. (2021) in 277 patients with impaired LVEF <50% and LBBB found a notable improvement in LVEF, NYHA class, BNP plasma level, but also lower capture threshold and higher R-wave sensing amplitude in comparison to HBP patients.62-65 Padala et al. (2020) in 305 patients showed low rates of lead dislodgement over two weeks with LBBP, while similar findings of complications were also recorded by Su et al. (2021) in 632 patients, with a follow-up of 8.6 months.66, 67 Finally, LBBP was even tested in cases after unsuccessful or suboptimal HBP but also after TAVR implantation in a few patients, with encouraging results regarding improvement in LVEF and reduction in QRS duration over follow-up, respectively.35, 61
All the above results, as also the recorded post-procedure stability of electrical parameters, suggest that LBBP is a feasible and effective pacing method with promising results for the near future. Nevertheless, the fact that these data come from observational, single-center studies with a small number of patients and a short follow-up explains why LBBP is not included as an official indication in the latest 2021 ESC Guidelines on cardiac pacing. That’s why it’s highlighted, as for HBP, the need to conduct large RCTs, which will correlate these findings with clinical endpoints such as heart failure hospitalizations and mortality.