Discussion
The main findings of this study
were that narrower QRS complexes and better pacing outcomes (threshold
and detection) were obtained at implantation and at three months using
LBBAP in comparison to HBP.
The success rate was similar between LBBAP and HBP, which achieved
comparable outcomes to those previously described in studies on each
technique.8 No variable was related to a lower success
rate with either technique. The presence of a previously implanted
cardiac device negatively affected the success of the implant with both
techniques and in the group as a whole, as previously
reported.15 Problems related to the design and
functioning were responsible for LBBAP failures, with four cases in
which the interventricular septum could not be penetrated, and for HBP
failures, with six cases in which the His electrogram was not recorded
or the threshold was exceeded; in another three patients, HBP failure
was due to the impossibility of narrowing the QRS duration. The
percentage of lead-related complications was comparable to that observed
by other authors16 and was higher with the HBP
procedure.
To date, no study has directly compared the two techniques. Wang et
al.17 published comparable results (for threshold and
detection) to the present findings in 44 patients after HBP and in 8
after LBBAP, but they did not analyze differences between the techniques
and did not compare data on QRS duration. In the present series, energy
(threshold) requirements were decreased and the sensed R-wave amplitude
was increased with LBBAP in comparison to HBP, both immediately
post-implantation and at three months. The His bundle was separated from
the membranous septum and insulated by thick fibers in a substantial
proportion (32%) of cases,18 signifying that high
energy is needed to activate the conduction system at this site, a
situation that does not arise in left branch pacing. The radiological
exposure time was shorter with the LBBAP technique because it does not
require a search for the His-bundle electrogram, which is essential in
HBP.
Among the successful procedures, the paced QRS duration was shorter with
LBBAP than with HBP, and the values for each technique were in line with
previous reports.8 In cases with a narrow baseline
QRS, the QRS interval lengthening after LBBAP was greater than after
selective HBP and lesser than after non-selective HBP (Figure 4).
Non-selective HBP gives rise to wider QRS intervals in comparison to
selective HBP because it captures muscle cells that are closer to the
His bundle stem, although this increased QRS duration has not proven to
be clinically relevant.19 Any position in the RV
endocardium (apex, outflow tract, septum) produces wider paced QRS
complexes, explaining the not inconsiderable incidence of post-pacing
dysfunction. For this reason, HBP or LBBAP is the procedure of choice in
patients needing ABT, especially if a pacing percentage of
> 20% is anticipated. According to our experience, LBBAP
achieves the same success rate as HBP but with better pacing values and
a lower radiological exposure time.
Outcomes were more favorable with LBBAP than with HBP in patients with
intraventricular conduction disturbance. The anatomical localization of
this disorder crucially influences the possibility of BBB correction.
According to Upadhyay et al.20, the likelihood of a
reversal using HBP is lower with a more distal localization of this
anomaly. A more distal pacing was achieved with LBBAP (Figure 2) at the
basal interventricular septum, where left branch fibers of the left
branch are present in the subendocardium with a high degree of
ramification, likely improving contact between the lead and the septal
Purkinje system.6 In our patients with LBBB, a success
rate of 100% was obtained with LBBAP and 66.7% with HBP was
(P < .1), although the techniques obtained a similar
reduction in QRS width among successful cases. However, the percentage
of cases with paced QRS of ≥ 120 ms was close-to-significantly higher in
the HBP group (75% vs. 27.3%; P = .07).
Successful RBBB correction is also likely to depend on the anatomical
site of the disorder, but scant data are available. Sharma et
al.21 reported that HRB achieved a success rate of
95% (with no control group) in patients with RBBB and a baseline
QRS of ≥120 ms and LVEF of ≤ 50, reducing the mean QRS width from 158 to
127 ms and achieving correction in 78% of cases. In the present study,
the success rate was 87.5% with HBP and a similar reduction in QRS
width was achieved (159 to 122 ms), although it remained at ≥120 ms in
50% of the patients. In contrast, only one patient (9.1%) in our LBBAP
group had a paced QRS of 120 ms after the procedure. and they showed a
greater reduction in the QRS complex (P = .09) that reached
statistical significance (P < .05) in those with a
baseline QRS of ≥150 ms.
The degree of narrowing of the QRS complex by conventional CRT is
associated with a higher response rate and has been described as an
independent predictor by some authors.14 In our
series, LBBAP achieved a higher degree of narrowing in patients with a
QRS duration of ≥130 ms than was obtained with HBP or with conventional
CRT in patients with previous failure of HBP/ LBBAP pacing.