Bachmann bundle potential during atrial lead placement: A case series

Introduction

In 1914 Sir Thomas Lewis¹ observed that differences in the speed of atrial impulse propagation are influenced by myo-fiber orientation. Two years later Jean George Bachmann² reported that left atrial (LA) activation occurs primarily over a muscular band of longitudinally arranged fibers immediately adjacent to the crista terminalis (CT), which became known as Bachmann bundle (BB). Damage to BB results in interatrial conduction delay (IACD) manifesting as prolonged P-wave duration (PWD) > 120 ms on the surface electrocar-diogram.^2,3 Prolonged PWD is associated with atrial electro-mechanical dysfunction⁴ and the development of atrial fibrillation (AF).⁵ BB pacing (BBp) shortens interatrial conduction time⁶ and reduces atrial dyssynchrony by restoring atrial activation sequences elicited by the sinus node.^1–4 In 2001 Bailin et al⁷ published the only multicenter randomized controlled trial specifically focused on BBp, comparing it with traditional right atrial appendage pacing (RAAp). In that study, BBp was associated with a significant reduction in the rate of progression to chronic AF.⁷

While alternative site atrial pacing studies have been performed, few focused on BBp specifically. This may be due to the lack of a uniform approach to and/or definition of BBp. We propose a novel integrative approach to BBp that combines a local electrogram signature at or near BB and characteristic BBp P-wave morphology.^7,8 The following case series of 24 consecutive patients demonstrates the consistent appearance of endocardial local BB area potentials (LBBAPs). Pacing at the location where LBBAPs are recorded results in BBp P-wave morphology, which recapitulates normal sinus P-wave morphology and axis and corrects baseline IACD. To our knowledge, this is the first description of using local electrograms with paced P-wave morphology to define BBp.

Case series

Patient selection and characteristics

Searching the Paceart Optima™ system (Medtronic, Inc, Minneapolis, MN) for 3830 SelectSecure™ leads (Medtronic, Inc, Minneapolis, MN) implanted between January 1, 2017, and December 30, 2020, and reviewing the medical record, we identified 75 patients with attempted BB lead placement who also had Bard LABSYSTEM™ PRO EP Recording System (Boston Scientific, Marlborough, MA) procedural intracardiac electrograms available for review. Of these, 24 patients had intracardiac electrogram recordings during atrial lead placement (in addition to ventricular lead placement) and are included in this series. A prospective activation mapping study was also performed in 3 patients and is detailed in the Online Supplement. This study was approved by the University of Vermont’s institutional review board and adhered to the Helsinki Declaration as revised in 2013. Sinus and paced P-wave indices, lead parameters, and electrograms for all patients are included in Online Supplemental Tables 1 and 2 and Online Supplemental Figures 1–5.

Results

We observed 3 distinct interindividually reproducible characteristics of the BB area pacing site.

Local BB area potentials

A typical example of LBBAPs is shown in Figure 1. A 77-year-old man (patient 1) with symptomatic sinus node dysfunction underwent dual-chamber pacemaker placement with BBp and right ventricular septal pacing. Figure 1A shows fluoroscopic views of the atrial lead in the anterosuperior right atrial septum (RAS). In Figure 1B, the bipolar recording from the atrial lead channel (labeled “lead”) shows 2 distinct potentials separated by 38 ms. The surface electrocardiogram leads show a wide, low-amplitude, bifid P wave during sinus rhythm. With high-output pacing (10 V at 0.5 ms), both LBBAPs are captured and the PWD shortens. With decrementing output, capture of 1 of the 2 LBBAPs is lost, coincident with the appearance of an isoelectric stimulus-to-P-wave (S-P) interval with onset of an even narrower PWD. Baseline IACD is corrected at all pacing outputs. Local unipolar tip and ring electrograms are shown in Figure 1C. Two distinct signals separated by 38 ms are notable in the bipolar recording (Figure 1B), while the unipolar recording from the ring electrode shows only the first potential (Figure 1C): we call this electrogram “CT” because the first area to be activated by the sinus node is typically the CT and because the ring electrode is relatively posterior where the CT would be expected to be located anatomically. The unipolar electrogram recorded from the more anterior and septal tip electrode shows a far-field early potential that times with CT, separated by 38 ms from a high-frequency near-field potential, which we call “BB.” Figure 1D shows normalization of the BBp P wave compared with the sinus P wave.

Figure 1.

A: Fluoroscopic images of the atrial lead in right anterior oblique (upper) and left anterior oblique (lower) views for patient 1. B: Bipolar recordings of LBBAPs from the atrial lead channel and P-wave duration and morphology during sinus rhythm and BBp with decrementing output. C: Unipolar recordings from the atrial lead ring and tip electrodes. D: P-wave duration and morphology at 25 mm/s during sinus rhythm and BBp. Panels B and C show LBBAPs and P waves at 100 mm/s, P waves amplified 32×, and LBBAPs amplified 4×. BBp = Bachmann bundle pacing; LBBAPs = local Bachmann bundle area potentials.

Output-dependent capture of sensed electrograms: High-output response

In most patients, at high output, local potentials are captured, the P-wave deflection begins immediately after the pacing stimulus, and PWD shortens relative to the sinus P wave (Figures 1–3; Online Supplemental Figures 1–5).

Figure 3.

LBBAPs and BBp P-wave morphology for patients 6–9. Intracardiac recording at 100 mm/s demonstrates LBBAPs on the atrial lead channel (far left of each panel) and P-wave morphology during sinus rhythm and BBp at decrementing output. A component of the LBBAPs not captured at lower outputs correlates with the PTF in lead V₁ timing (blue arrows). P-wave morphology during sinus rhythm and BBp at 25 mm/s is shown (far right of each panel). Abbreviations as in Figures 1 and 2.

Output-dependent capture of sensed electrograms: Low-output response

In a subset of patients (n = 11; patients 2–5 in Figure 2 and patients 11–17 in Online Supplemental Figures 2 and 3), as capture of LBBAPs is lost with decrementing pacing output, time from stimulus to P-wave peak increases and PWD prolongs. In a second subset of patients (n = 10, patients 1 and 6–9 in Figures 2 and 3, respectively, and patients 18–22 in Online Supplemental Figure 4), as capture of the LBBAPs is lost with decrementing output, an isoelectric S-P interval is observed and PWD remains constant or shortens further. The local potential that times (approximately) with the onset of the narrowed paced P wave (red dashed line) and that tracks with the timing of the P-terminal force (PTF) in lead V₁ (blue arrows) in Figures 2 and 3 we postulate to be the BB potential (see Discussion).

Figure 2.

LBBAPs and BBp P-wave morphology for patients 2–5. Intracardiac recording at 100 mm/s demonstrates LBBAPs on the atrial lead channel (far left of each panel) and P-wave morphology during sinus rhythm and BBp at decrementing output. A component of the LBBAPs not captured at lower outputs correlates with the PTF in lead V₁ timing (blue arrows). P-wave morphology during sinus rhythm and BBp at 25 mm/s is shown (far right of each panel). PTF = P-terminal force; other abbreviations as in Figure 1.

Discussion

Site-selective atrial pacing, including BBp, has been studied with the goal to correct IACD and reduce the risk of AF.⁹ Distinct epicardial BB potentials have been mapped since the 1960s.^8,10–12 In this case series we describe the right atrial endocardial signature of the BB area, recognition of which can aid in atrial lead placement at a site where pacing results in atrial activation similar or identical to normal sinus node activation. The characteristics of the endocardial electrogram signature of the BB area include (1) dual or multicomponent electrograms at the anterosuperior RAS aspect of the cavoatrial junction, (2) recruitment of these potentials at high-output pacing results in immediate P-wave onset and PWD narrowing, and (3) loss of capture of far-field components with lower-output pacing results in latency in paced P-wave peak time with or without an isoelectric S-P interval. Correction of PWD and morphology in patients with baseline IACD occurs uniformly at sites demonstrating characteristics 1–3 in our series of 24 patients.

We propose that the LBBAPs reported in this series are attributable to or are near BB because pacing at this site consistently reproduces characteristic BBp P waves demonstrated experimentally by MacLean et al⁸ using implanted electrodes at the visualized epicardial BB site: tall, peaked, and narrowed P waves with an inferior and leftward axis and a biphasic or negative P wave in lead V₁.

Correction of IACD

IACD is often mediated by conduction delay over BB or delayed activation of BB and manifests as prolonged, low-amplitude, bifid, or biphasic P waves in inferior leads and a broadnegativePTFinleadV₁.^3,12,13 Without direct visualization and/or high resolution mapping, we cannot prove which electrogram comes from where, presuming the earliest to be at or near the superior CT, and the later potentials to represent activation of the rightward extension of BB. Regardless, pacing in this region (Figures 1B and 1D) normalizes the P-wave vector, morphology, and duration, thus correcting IACD via restoration of the normal atrial activation sequence, which others have shown to be dependent on early activation of BB relative to total atrial activation.^3,12 In a separate retrospective analysis, we demonstrate that fluoroscopically guided RAS lead placement is not sufficient to normalize the paced P wave and the absence of P-wave normalization is associated with higher AF incidence.¹⁴

The terminal portion of the surface P wave correlates with zones of latest activation, most commonly the posteroinferior region adjacent to the left inferior pulmonary vein and/or the tip of the LA appendage.¹⁵ During sinus rhythm, the LA insertion of BB is the most common breakthrough site initiating LA activation, which results in an anterior-to-posterior vector of activation and a negative PTF in lead V₁.^1,3,15 The PTF in lead V₁ is classically attributed to LA activation, and a larger PTF in lead V₁ is a sign of slower LA activation.¹⁵ BBp results in earlier timing of the postulated BB potential within the inscription of the P wave (shown by red dashed lines in Figures 2 and 3) and correlates with the timing of the PTF in lead V₁ (blue arrows in Figures 2 and 3), and this corresponds to shorter paced PWD and lower PTF in lead V₁ (Online Supplemental Table 1). In the 3 patients with prospective activation mapping, BBp advances activation of proximal and distal coronary sinus catheter electrodes and advances zones of late LA activation on electroanatomic maps showing local activation time compared with sinus rhythm and RAAp (Online Supplemental Figures 6–8). In our series, pacing in the region of BB resulted in atrial resynchronization and correction of IACD by advancing latest zones of atrial activation.

Isoelectric S-P interval

BBp is often described as having a lack of paced P-wave latency,^6–8 presumably because of fast interatrial conduction almost immediately after BB capture. This contrasts with our observation that the presence of an isoelectric S-P interval is output dependent. The isoelectric S-P interval in our series could be due to an unmasked virtual electrode effect at lower outputs, particularly in the case of anisotropy in the region. Additional theories about the isoelectric S-P interval are detailed in the Online Supplement.

Clinical implications

In this series, we demonstrate the feasibility and reproducibility of identifying distinct LBBAPs using a standard atrial lead and electrogram recording system. Guiding lead placement to endocardial LBBAPs is a novel approach that aims to define the site of BB more accurately and can be used in future studies to compare electrically guided BBp (using LBBAPs and paced P-wave criteria) with RAAp on clinical outcomes.

Limitations

The absence of high-resolution mapping and/or other imaging modalities beyond electrograms, fluoroscopy, and the paced P-wave morphology limits our interpretation of the electrograms discussed herein to inference. There are other bundles nearby such as the septopulmonary bundle, not to mention the CT itself, which can demonstrate complex electrograms. Countering this concern is the notable similarity between the electrograms we observed and those seen during epicardial and plunge electrode mapping studies.^3,8,10,11 We cannot definitively state that one must demonstrate these findings to obtain an ideal BBp site. The critical observation likely is the paced P-wave morphology in its totality. Further prospective studies including clinical outcomes will be needed to determine an ideal BBp site.

Conclusion

Herein we report a novel approach that combines fluoroscopic and electrical mapping to achieve successful BBp. We demonstrate a localized endocardial electrogram signature that approximates BB and is readily apparent on a standard electrogram recording system. Pacing from an actively fixed lead guided by LBBAPs generates a normalized P-wave axis and morphology and corrects baseline IACD. To our knowledge, this case series is the first to combine right atrial endocardial potential mapping and pacing response to guide permanent BBp site selection.