Electrophysiological Characteristics and Possible Mechanism of Bipolar Pacing in Left Bundle Branch Pacing

Authors, Journal, Affiliations, Type, DOI

• Authors: Hao Wu; Longfu Jiang (corresponding); Jiabo Shen; Lu Zhang; Jinyan Zhong; Shanshan Zhuo
• Journal: Heart Rhythm 2022;19(12):2019–2026. © 2022 Heart Rhythm Society
• Affiliation: Department of Cardiology, HwaMei Hospital, University of Chinese Academy of Sciences, Ningbo, Zhejiang, China — the group that developed the continuous-pacing technique
• Type: Single-centre prospective mechanistic study — 75 patients screened, 65 included who strictly met LBB-capture criteria; threshold testing in unipolar and bipolar configurations
• DOI: 10.1016/j.hrthm.2022.06.022
• Funding: Zhejiang Provincial Public Service and Application Research Foundation (LGF22H020009); Ningbo Health Branding Subject Fund (PPXK2018-01); Medical Scientific Research Foundation of Zhejiang (2022KY1122). Disclosures: none declared

Overview

Left bundle branch pacing is programmed bipolar as often as unipolar, yet its capture physiology had been described almost entirely in unipolar terms. This study performs a decremental bipolar threshold test in 65 patients with strictly confirmed LBB capture and finds that bipolar pacing passes through two transitions, not one. At high output, the cathode captures the LBB while the anode captures the right ventricular septum — a state the authors call NS-bipolar-LB — producing the shortest paced QRS (118.2 ms) and shortest V1 RWPT (95.6 ms) of any capture type. As output falls, anodal capture is lost, giving NS-cathodal-LB (QRS 133.8 ms), and then either selective cathodal LBB capture (S-cathodal-LB) or LVS-only capture depending on the patient. V6 RWPT stayed short and constant across all three LBB-capturing states (64.8–65.7 ms) and lengthened only with LVS-cathodal capture (82.8 ms). The clinical speculation is that high-output bipolar pacing's simultaneous right and left septal capture may be a more synchronous mode than conventional unipolar LBBP — and that bipolar should be preferred for LBBP because its low-output behaviour is identical to unipolar while its high-output QRS is narrower.

Keywords

Physiological pacing; Left bundle branch pacing; Bipolar pacing; Intracardiac electrogram; Continuous pacing technique

Key Takeaways

Population and capture verification

• 75 screened, 65 included — inclusion required RBBB-delay paced morphology plus (1) a documented NS-LB → S-LB or NS-LB → LVS transition during unipolar threshold testing and (2) short and constant V6 RWPT (preferably <70 ms) at 8 V and 2 V
• LBB potential recorded in 58/65 (89.2%)
• Unipolar transitions: NS-LB → S-LB in 46 (70.8%), NS-LB → LVS in 19 (29.2%)
• Unipolar V6 RWPT at 8 V vs 2 V: 65.2 ± 7.8 vs 66.5 ± 8.0 ms — constant, as required
• Baseline: age 73.3 ± 9.3; male 64.6%; AV block 60.0%, SSS 30.8%; LVEF 60.1 ± 6.2%; native QRS 105.3 ± 31.7 ms (narrow 69.2%, RBBB 23.1%, LBBB 10.7%, NIVCD 4.6%); lead depth 14.8 ± 2.6 mm; unipolar impedance 700.6 ± 163.5 Ω; sensing 11.3 ± 5.7 mV

The two bipolar transitions

• Every one of the 65 patients had two morphology transitions as bipolar output fell from 8 V to near threshold
• First transition (higher output): V1 changes from "QS or Qr" to "rSr′ or QR"; the LBB lead EGM changes from a complex mixed wave to two connected individual waves
• Second transition (near threshold), in one of two forms:
  • 46 patients (70.8%): V1 changes to "rSR, RSR, or M" and the LB lead develops an isoelectric interval between the pacing artefact and the local ventricular EGM — the bipolar analogue of NS → selective
  • 19 patients (29.2%): V6 RWPT prolongs with no obvious change in the LB lead EGM — the analogue of NS → LVS

The four bipolar capture types

• V6 RWPT is identical across the three LBB-capturing states and across unipolar NS-LB and S-LB (64.8, 65.7, 65.7, 65.2, 65.9 ms; all NS), and equally prolonged in LVS-cathodal and unipolar LVS (82.8 vs 83.1 ms)
• V1 RWPT and paced QRS step up at each transition: V1 RWPT 95.6 → 107.5 → 120.5 ms (each P < .001); QRS 118.2 → 133.8 → 145.3 ms
• The threshold sequence is the reverse: 2.5 V → 0.8 V → 0.5 V (all P < .001), with no difference between S-cathodal-LB and LVS-cathodal (0.5 vs 0.6 V)
• Bipolar and unipolar thresholds match at low output — NS-cathodal vs unipolar NS-LB 0.8 vs 0.8 V, S-cathodal vs unipolar S-LB 0.5 vs 0.5 V, LVS-cathodal vs unipolar LVS 0.6 vs 0.5 V

The proposed mechanism

• Bipolar EGM is the synthesis of two unipolar recordings from adjacent electrodes; after septal placement the cathode lies on the left side of the septum and the anode on the right
• NS-bipolar-LB: the anode instantly captures and senses right ventricular septal myocardium, while the cathode captures LBB and left septal myocardium; the two wavefronts fuse, giving the shortest V1 RWPT and narrowest QRS. The authors argue the right bundle branch is not captured, because if it were the QRS would resemble the intrinsic beat
• NS-cathodal-LB: loss of anodal capture delays RV activation (V1 RWPT rises), and the EGM splits into EGM1 (cathodal local capture + artefact) and EGM2 (delayed anodal-region activation); V6 RWPT is unchanged because LV activation still proceeds through the LBB
• S-cathodal-LB: with local myocardial capture gone, activation reaches the LV only through the Purkinje system, producing an isoelectric interval and a further V1 RWPT rise; the EGM splits into EGM3 (left septal activation via LBB) and EGM4 (delayed anodal region)
• LVS-cathodal: loss of LBB capture prolongs V6 RWPT, while V1 RWPT changes little; the mechanism mirrors unipolar LVS

Clinical implications the authors propose

• NS-bipolar-LB may be an "ideal synchronization pacing model" — it shortens both RV and LV activation, addressing the interventricular dyssynchrony that conventional LBBP produces (visible even as an incomplete RBBB pattern)
• The cost is a higher output: the anodal capture threshold averaged 2.5 V/0.5 ms, which the authors consider within an acceptable range; they propose it as an additional programmable feature for future CRT devices
• Bipolar may be preferable to unipolar in LBBP — at high output it gives a shorter paced QRS than unipolar, and at low output it performs identically

Limitations of the document

Authors' own stated limitations

• Single centre, small sample, and four capture types were defined from that small sample — other types may exist
• Too few patients with LV dilatation and LBBB, and no analysis of the electrophysiology in those special conduction substrates
• Difficulty assigning the R-wave peak in a few patients, especially in V1, which may cause measurement deviation
• No long-term follow-up, so the stability of thresholds across bipolar capture types is unknown
• The mechanisms are inferred from ECG and EGM morphology; activation mapping of both ventricles at each pacing site would be necessary to prove them

Additional limitations evident in the study

• The entire mechanistic account is an inference from waveform shapes. The paper offers no direct recording from the anode, no activation map, and no proof that the anode is capturing RV septal myocardium — the anodal-capture explanation is plausible and internally consistent, but it is a hypothesis
• The headline clinical claim — that bipolar is "more appropriate" than unipolar — rests on an acute QRS surrogate. A paced QRS 16 ms narrower at high output is not evidence of better synchrony, better haemodynamics, or better outcomes, and the authors do not measure any of these
• The high-output state may not be clinically usable. NS-bipolar-LB requires a mean 2.5 V, roughly five times the selective capture threshold; programming at that output would accelerate battery depletion, and the paper does not discuss device longevity, safety margin, or whether the state is stable over time
• The cohort is highly selected and mostly non-failing — LVEF 60%, only 2 patients with heart failure as the pacing indication and 10.7% with LBBB — which is the opposite of the CRT population in which the "ideal synchronization model" claim would matter
• Only 65 of 75 screened patients were included, and inclusion required both a documented unipolar transition and constant V6 RWPT — the strictest possible capture definition. The findings may not apply to the many implants where capture is less certain
• Subgroup sizes for the second transition are small (46 versus 19), and the LVS-cathodal group in particular is too small to characterise reliably
• No reproducibility data for the four-type classification; the EGM labels (EGM1–4) are interpretive, and no second blinded observer is reported
• The threshold comparison with unipolar is not the same measurement. The bipolar "anode capture threshold" is inferred from the point at which V1 RWPT lengthens, not measured directly as an anodal capture threshold
• The paper does not test whether the two transitions are clinically actionable — no protocol for selecting an output, and no comparison of outcomes between programming strategies
• Rating rationale: rated medium. It is a carefully performed, well-illustrated mechanistic study that explains a real phenomenon — the second transition in bipolar LBBP — and provides a coherent account of anodal capture. But it is small, single-centre, hypothesis-generating, its mechanism is not directly demonstrated, and its therapeutic proposal (high-output bipolar as an "ideal" CRT mode) is unsupported by any outcome or haemodynamic data

Key Concepts Mentioned

• concepts/Left-Bundle-Branch-Area-Pacing — the technique; this paper describes its bipolar capture physiology
• concepts/Conduction-System-Pacing — the umbrella category
• Anodal capture — the anode's capture of right ventricular septal myocardium, the defining feature of NS-bipolar-LB
• NS-bipolar-LB / NS-cathodal-LB / S-cathodal-LB / LVS-cathodal — the four bipolar capture states
• Double transition — the two output-dependent morphology changes during bipolar threshold testing
• V6 RWPT and V1 RWPT — LV and RV activation surrogates; V6 stays constant with LBB capture and lengthens with LVS capture, while V1 lengthens with loss of anodal then local myocardial capture
• Paced QRS duration — narrowest in NS-bipolar-LB (118.2 ms), longest in S-cathodal-LB (145.3 ms)
• EGM1–EGM4 — the split local electrogram components proposed to represent cathodal and delayed anodal-region activation
• Interventricular dyssynchrony — the problem the anodal-capture state is proposed to solve

Key Entities Mentioned

• concepts/Left-Bundle-Branch-Area-Pacing — primary concept page
• HwaMei Hospital, University of Chinese Academy of Sciences, Ningbo — the investigating centre and the origin of the continuous-pacing technique
• Hao Wu; Longfu Jiang; Jiabo Shen — the investigators
• Abbott Laboratories — the digital EP recording system used for measurement
• Ponnusamy — prior description of the "double transition sign" in a case report
• Jastrzębski; Burri — the V6–V1 and physiology-based criteria used to define capture
• Heart Rhythm Society / Elsevier — publisher

Wiki Pages Updated

• Created: wiki/sources/bipolar-lbbp-hrs-2022.md
• Updated: wiki/concepts/Left-Bundle-Branch-Area-Pacing.md
• Updated: wiki/concepts/Conduction-System-Pacing.md
• Updated: wiki/sourceindex.md
• Updated: wiki/wikiindex.md