QRS morphologies in V1 and V6 during left bundle branch area pacing: assessing the patterns

Without changing our pattern of thought, we will not be able to solve the problems we created with our current patterns of thought. --Albert Einstein

Since its original description as a means to capture the left bundle beyond the site of proximal block in 2017,¹ left bundle branch area pacing (LBBAP) has rapidly supplanted His bundle pacing (HBP) to become the preferred approach to deliver conduction system pacing (CSP). While there has been enthusiasm for reduced procedural times, lower pacing outputs, and more stable thresholds with LBBAP,² differentiating CSP from left ventricular septal pacing (LVSP) has been challenging. This is because, in contrast to HBP, the QRS morphologies generated by LBBAP are more variable.^3–5 Indeed, the consensus statement on CSP released by the European Heart Rhythm Association (EHRA) earlier this year recommends a seven-step algorithm to discern LBBAP from LVSP or deep septal pacing (DSP).⁶ The algorithm utilizes transitions in QRS morphology during pacing output decrement, evaluation of the time from left bundle branch (LBB) potential recording to R-wave peak time (LBBpo-RWPT) in V6 as compared to the time from pacing stimulus to the peak of the R-wave of that lead (V6RWPT), measurement of the V6-V1 interpeak interval, and use of extrastimulus testing, among others.

Central to differentiating CSP from right ventricular pacing (RVP) or DSP in the EHRA consensus statement has been the requirement of right ventricular (RV) conduction delay patterns (e.g. Qr, QR, and qR) in V1 with pacing. These ‘right bundaloid’ configurations are distinct from the rSR’ pattern of typical right bundle branch block (RBBB). While it has been noted that the terminal R-wave in V1 can be abrogated by fusion with intact right bundle conduction through adjustment of atrioventricular (AV) intervals,^1,7 there has been less attention on the morphology of the unipolar paced QRS in V1 and its implication for lead placement. Similarly, while an Rs or RS pattern in V6 may be expected in native RBBB, the significance of site-specific morphologies during LBBAP has only recently become a focus of attention. In studies utilizing duodecapolar catheters positioned on the LV septum, we observed that more distal pacing sites were associated with shorter V6RWPT.⁸ The concern this raised is that more apical lead positions might require shorter cut-offs for V6RWPT in order to guarantee CSP is present; how to practically implement this observation remained unclear, however.

In this issue of Europace, Sato and colleagues examined these questions with a real-world assessment of permanently paced leads. They prospectively enrolled patients undergoing pacemaker implant for third- or second-degree AV block at a single centre. The authors should be applauded for their systematic approach to CSP—all patients underwent a ‘dual-lead approach’ with a lead positioned for distal HBP and used as a landmark for attempted LBBAP. Left bundle branch area pacing was then determined through application of multiple criteria consistent with the most recent EHRA recommendations. Patients categorized as LVSP only demonstrated a terminal R-wave in lead V1 with a V6RWPT <90 ms. A diagnostic quadripolar catheter also utilized to record left ventricular electrogram (LV EGM) to assess activation patterns. With these measures in place, Sato and colleagues reported an overall success rate of 87% (71 of 82 patients) for LBBAP or LVSP and in whom LBBAP was confirmed in 50 (61%). Although numerically lower than prior studies, given the rigour with which lead placement was assessed, we feel that Sato and colleagues’ success rate represents a realistic figure.

In post-procedure, follow-up lead tips were visualized at the septum in the short-axis view—classified as anterior or inferior septum—and with penetration depth quantified using the ratio of the lead length in the septum to the overall wall thickness. In the long-axis view, distances from the lead tip to the mitral valve and to the LV apex were also measured. When correlating QRS morphology to echo location, Sato and colleagues observed that a higher ratio of R- to Q-wave amplitude was more commonly noted at the inferior vs. anterior septum. Specifically, they observed that a cut-off of 0.8 was associated with a specificity of 0.86, sensitivity of 0.69, and area under the curve of 0.79 for the inferior septum. Among all 12 patients with a lower RQ ratio (and, thus, a Qr complex) in lead V1 and an inferiorly directed axis, the lead was localized to the anterior septum. Furthermore, they observed that among patients achieving criteria for LBBAP lead tips were embedded deeper in the inferior septum than patients with LVSP only at this location and also deeper than patients with LBBAP at an anterior septal location.

The authors also examined the QRS morphology of LBBAP in lead V6. A deep S-wave was defined as an S-wave amplitude below the baseline of ≥20% of the QRS complex and was found in 15 of 71 patients (21%), and these leads were delivered closer to the LV apex. Importantly, while the V6RWPT was significantly shorter for these patients than the non-deep S-wave group, the stim-to-latest LV EGM interval recorded in the CS was longer.

What are the clinical implications of these findings? Simply put, they suggest that the variations in QRS morphology from LBBAP may be used to help predict lead tip location (see Figure 1). Right bundaloid configurations with a high RQ ratio (i.e. qR pattern) in V1 are more likely to reflect inferior septal locations. Importantly, at this location, LBBAP usually required lead delivery closer to the LV endocardial surface than LVSP (i.e. may require more lead rotations at implant). Low RQ ratio in V1 (i.e. Qr pattern) with an inferiorly directed axis was more likely to be in the anterior septum, and these leads were visualized further from the LV endocardial surface even when LBBAP was present, likely owing to the more oblique course of the leads as well as variations in left fascicular anatomy here. Perhaps most importantly, Sato and colleagues also draw attention to patients with RS complexes in V6, which were observed at more apical lead positions. Consistent with prior work, shorter V6RWPTs were observed at these more apical locations, but they had longer timing to the latest activated LV EGMs. The clinical translation of these findings is critical. These data suggest that for patients with an S-wave which is >20% of the QRS complex (i.e. RS complexes) the V6RWPT is shorter due to lead location and that assessment of V6RWPT may not be enough to ascertain conduction system capture.

Figure 1.

Evaluating paced QRS patterns in V1 and V6. Paced QRS morphology in V1 may be used to surmise lead tip location. Most lead tips in Sato and colleagues’ demonstrated lead tips at the inferior septum. A high RQ ratio (≥ 0.8) or “qR” pattern suggests that an inferior lead tip location is expected. If with Qr pattern and inferiorly directed axis, a more anterior location is expected. A deep S-wave (>20% of the total QRS) of the paced complex in V6 suggests a more apical lead location.

Sato and colleagues’ work is not without limitations. Most notably, inferences are largely limited to patients with normal ejection fraction and without underlying cardiomyopathy (present in 11% of their cohort) or LBBB (present in 6%). Importantly, left bundle capture was also assessed using currently accepted criteria but not invasively confirmed. Particularly as the indications for use of LBBAP have now formally expanded to patients with heart failure,⁹ expectations regarding paced QRS will need to be adjusted due to distorted chamber geometry from chamber dilation, hypertrophy, or scar.

With that said, the authors should be congratulated that their work invites operators to think more deeply about the patterns of QRS morphology associated with LBBAP and also appreciate where variations in those patterns (e.g. an RS in V6) reveal potential problems in applying empirical, timing-based criteria. If in seeing a pattern we can avoid a potential error of assessment, we will better be able to assess the performance of LBBAP between centres and across patient populations.