Lowering the Threshold for Left Bundle Branch Area Pacing
Lukasz P. Cerbin, MD1 and Lohit Garg,
MD1
1 Division of Cardiac Electrophysiology, University of
Colorado Anschutz Medical Campus, Aurora, Colorado, USA.
Relevant Disclosures: None
Funding: None
Corresponding Author: Lohit Garg, MD
Division of Cardiac Electrophysiology, University of Colorado Anschutz
Medical Campus
12401, E. 17th Ave, Aurora, Colorado-80045
Email: Lohit.garg@cuanschutz.edu
Cardiac pacing remains the mainstay of therapy for conduction system
disease and irreversible bradyarrhythmias. Due to known complications
from chronic RV pacing and electromechanical
dissociation1,2, there has been a growing interest in
physiologic pacing. While cardiac resynchronization therapy (CRT) with
LV pacing from a lead in the coronary sinus has been shown to ameliorate
some of the morbidity from RV septal pacing3,
activation is still via myocyte to myocyte conduction rather than
engaging the specialized conduction system. Furthermore, LV lead
placement can be challenging and response to CRT is heterogeneous and
difficult to predict4. And finally, CRT’s role in
patients with preserved LV systolic function has not been established.
In this light, there has recently been a substantial growth in
conduction system pacing directly engaging the native His-Purkinje
conduction system. This began with a case series published in 2000
demonstrating selective his bundle capture and subsequent permanent His
bundle pacing (HBP) lead placement in patients with heart failure and
tachycardia induced cardiomyopathy secondary to atrial
fibrillation5. Additional studies demonstrated the
feasibility and clinical benefit of HBP as a strategy for
CRT6,7. However, continued research revealed high
capture thresholds and low R wave amplitudes, calling this technique
into question8. Furthermore, HBP failed to demonstrate
improved outcomes in patients with underlying LBBB, likely due to the
presence of conduction disease distal to the site of
capture9. Due to these concerns, the enthusiasm for
HBP gradually waned, with a subsequent rise in interest in left bundle
branch area pacing (LBBAP). This approach utilizes the same lead and
implant technique to fix a lumenless pacing lead approximately 1.5 cm
distal to the anatomical His bundle within the interventricular septum,
directly engaging the left bundle.
In this issue, Mehta et al. describe a single center’s experience of
implantation of left bundle branch area pacing leads, complete with one
year follow-up data. Their retrospective study included 65 patients over
an 18 month period who received a 3830 Medtronic lead for LBBAP. Only
patients with one year of follow up data were included. Importantly,
they excluded 7 patients in whom LBBAP implant was unsuccessful,
resulting in a reported implant success rate of 91.8%. The reported
procedure time was 72.7 +/- 28.8 minutes. The mean QRS duration
decreased significantly in patients with pre-existing LBBB, was
unchanged in those with pre-existing RBBB, and increased significantly
in patients without pre-exiting bundle branch block. At one year
follow-up, they found a significantly higher capture threshold compared
to threshold at implant (p<0.0001), although they noted that
the absolute increase (0.2-0.3 V) was relatively modest. No short or
long-term complications related to device implant were identified.
This study underscores several important points regarding LBBAP.
Firstly, procedural success is high, with the authors reporting an
implant success rate of 91.8% in this study. Reasons for implant
failure included unacceptably high pacing thresholds, inability to meet
parameters used to ensure capture of the left bundle, and anatomic
constraints in one patient that prohibited lead placement. This success
rate is similar to that reported in other studies10.
Other studies have found higher implant success rate with LBBAP as
compared to HBP11. Secondly, this success rate is
likely easily reproducible as the implant technique does not require
additional or specialized training, only a long delivery sheath and some
familiarity with the mechanism to fix the 3830 lead to the
interventricular septum. With preliminary data suggesting improved
outcomes compared to traditional RV septal pacing12and the relative ease of procedural proficiency, it would seem that this
technique could rapidly become the gold standard for pacemaker
implantation.
While there is great enthusiasm for LBBAP, several key questions remain.
Firstly, this study, along with a recent publication from the
Geisinger-Rush Conduction System Pacing Registry12,
are the first to publish one year data following LBBAP lead implant.
While lead parameters were reported to be stable at one year in this
study, longer term data are needed. Furthermore, in addition to data on
long term lead parameters and clinical efficacy, long term safety
outcomes will be important. Particularly, subclinical perforation of the
helix into the LV may carry thrombotic risk and long term data on
whether this increases stroke risk will be
important13. One prior study found two cases of
subclinical septal perforation occurring after LBBAP lead
placement14. Another case report described a patient
presenting with presyncope two weeks after device implant with
perforation of the lead into the LV and associated lack of capture up to
3.5 V @ 0.4 ms (bipolar)15. The incidence and risk
factors for these events has yet to be established and the associated
morbidity, including stroke risk from thromboembolism, remains
uncharacterized. Another question that needs to be addressed is that of
extractability of LBBAP leads. While there have been few published
reports of successful extraction of these leads16,
these lumen-less leads represent unique challenges in terms of
extraction tools and technique. More data is needed to characterize the
approach and outcomes for extraction of LBBAP leads.
Finally, the criteria for left bundle capture, as opposed to left septal
pacing, has yet to be firmly established, with both being included in
the catch-all term “left bundle branch area pacing.” A recent study by
Wu et al. utilized temporary HBP leads and LV septal mapping with
multielectrode catheters during LBBAP lead implant to identify markers
of selective LBBP17. They identified several useful
parameters, including paced RBBB pattern (100% sensitive), LBB
potential on the lead electrogram, abrupt shortening of stim-LV
activation time (LVAT) and certain stim-LVAT times (which varied based
on the presence or absence of an underlying LBBB). Another study from
the same group utilized a decapolar coronary sinus catheter to examine
differences in LV activation time and depolarization with left bundle
capture versus left septal pacing, finding that the a model combining
the presence of a LBBB potential on the lead coupled with a validated
stim-LVAT had excellent test characteristics (AUC = 0.985) to
differentiate conduction system from septal pacing18.
In the current study, the authors reported 67.7% of patients with
selective left bundle capture and 32.3% with left septal pacing when
applying this algorithm. A scientific consensus on how to identify left
bundle capture is essential to both study the long term clinical
benefits in patients undergoing LBBAP implantation as well as
establishing criteria to guide implanting physicians attempting to
achieve selective left bundle capture.
In summary, left bundle branch area pacing remains an exciting and
burgeoning area within cardiac electrophysiology. This study by Mehta et
al. demonstrated stable and acceptable lead parameters one year after
LBBAP implantation, adding evidence of the long term safety and efficacy
of this technique. Several questions remain, including longer term
efficacy, the incidence of subacute and chronic complications, the
ability to extract these leads when indicated, and the effect of LBBAP
clinical outcomes. Furthermore, rigorous definitions for left bundle
capture and left septal pacing need to be agreed upon. However, if these
data can be replicated on a larger scale and longer term data shows
similar safety and efficacy, LBBAP may likely represent the gold
standard for pacing for bradyarrhythmia indications in the near future.