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.