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.