M-Beat — A Novel Marker for Selective Left Bundle Branch Capture

Authors, Journal, Affiliations, Type, DOI

• Authors: Shunmuga Sundaram Ponnusamy (corresponding); William Basil; Pugazhendhi Vijayaraman
• Journal: Journal of Cardiovascular Electrophysiology 2022;33(8):1888–1892 — Brief Communications. Received 23 January 2022; accepted 5 June 2022. © 2022 Wiley Periodicals
• Affiliations: Department of Cardiology, Velammal Medical College, Madurai, India; Medtronic India Private Limited, Mumbai; Geisinger Heart Institute, Wilkes-Barre
• Type: Retrospective single-centre observational study — 217 consecutive attempted LBBAP, 210 successful (96.7%), mean follow-up 11.1 ± 7.1 months
• DOI: 10.1111/jce.15597
• Disclosures: Ponnusamy and Basil — consultants to Medtronic; Vijayaraman — speaker, consultant, research and fellowship support from Medtronic, consultant to Abbott, Biotronik and Boston Scientific, and a patent for a His delivery tool

Overview

During rapid lead deployment, mechanical irritation generates premature ventricular complexes (PVCs) whose morphology tracks the helix position; a PVC with an RBBB-delay pattern is the "template" or "fixation beat" that identifies the LBB area. This brief report describes a subtype of template beat — the M-beat — defined by an M-pattern (rsR′) in V1, a slurred S wave in V6 with >50% amplitude increase, and a rounded rather than peaked terminal R/R′ in aVR — which mimics the selective LBB-paced beat. In 210 successful implants, a template beat occurred in 90.4% and an M-beat in 32.8%; among the 69 patients with an M-beat, 68 (98.5%) also demonstrated a nonselective-to-selective transition at near-threshold output. As a predictor of selective capture, the M-beat had specificity 96.8%, sensitivity 58.6% and PPV 98.6% — a rule-in marker, not a rule-out. Patients with an M-beat had shorter fluoroscopy (13.1 vs 16.8 min) and fewer deployment attempts (2.2 vs 2.8), and none of them lost the V1 R wave or had a threshold rise >1 V during follow-up, whereas six and two patients respectively did in the non-M-beat group.

Keywords

Left bundle branch pacing; M-beat; Premature ventricular complexes; Selective LBB capture; Template beats

Key Takeaways

The definition

• M-beat = a template/fixation beat with all of:
  1. M-pattern (rsR′) in lead V1
  2. Slurred S wave in V6 with >50% increase in amplitude
  3. Terminal R/R′ in aVR that is rounded rather than peaked (reflecting RBB delay)
• The claim: the M-beat morphology mimics the selective LBB-captured paced beat, so its appearance during rotation predicts that the final position will capture the LBB selectively
• Template beat (the broader category) = a PVC during deployment with RBBB-delay pattern; it predicts having reached the LBB area
• The distinction the authors emphasise: an M-beat is generated mechanically, without any pacing manoeuvre, so it is independent of the relative excitability of septal myocardium and LBB — unlike the nonselective-to-selective transition at threshold testing, which requires a threshold difference and can be falsely negative when the two tissues have similar excitability

Cohort and results

• 217 attempted, 210 successful (96.7%)
• Indications: AV block 97 (46.2%), CRT 84 (40%), sinus node dysfunction 22 (10.5%), AV-junction ablation 7 (3.3%)
• Age 61.6 ± 11.1 years; septal thickness 10.5 ± 1.4 mm; mean V6 RWPT 68.2 ± 12.9 ms
• QRS 142.9 ± 32.6 → 114.1 ± 10.6 ms (P < .001)
• Template beat: 190 (90.4%); nonselective → selective transition: 116 (55.2%); M-beat: 69 (32.8%)
• M-beat QRS duration 129.3 ± 13.1 ms — longer than the template beat in the group's prior study (121.1 ± 3.7 ms), attributed to the delayed RV activation of selective capture

Group comparison — M-beat versus no M-beat

• Both groups improved: QRS to 114.1 ± 9.3 (group I) and 114.2 ± 11.3 ms (group II); LVEF 46.1 → 53.2% (P = .002) and 47.9 → 54.5% (P = .01)
• Follow-up events occurred only in the non-M-beat group: 6 patients lost the R wave in V1 and 2 had a threshold rise >1 V; because local myocardial capture remained <1 V, no reinterventions were performed
• At follow-up (11.1 ± 7.1 months): threshold unchanged 0.5 ± 0.1 V, sensed R wave improved to 12.4 ± 6.7 mV (P < .001), impedance fell to 573 ± 84 Ω (P = .001)

Diagnostic performance

• Among 69 M-beat patients, 68 (98.5%) had a documented nonselective→selective transition
• Overall: specificity 96.77%, sensitivity 58.62%, PPV 98.55%
• Baseline LBBB (84/210, 40%): nonselective→selective transition in 53 (63.1%), M-beat in 36 (42.8%); specificity 96.7%, sensitivity 66.04%, PPV 97.22%
• Baseline non-LBBB (126/210, 60%): transition in 63 (50%), M-beat in 33 (26.2%); specificity 100%, sensitivity 52.38%, PPV 100%
• The clinical framing: the M-beat is useful precisely where capture confirmation is hardest — baseline LBBB — because it is independent of the baseline QRS morphology and of the septal myocardial refractory period
• Why sensitivity is low, in the authors' account: the retrospective design and the fact that once rotation is interrupted for testing, further PVCs may not be generated

Limitations of the document

Authors' own stated limitations

• Programmed deep septal stimulation was not used to demonstrate selective capture
• During follow-up, LBB capture was confirmed by surface ECG only
• Group I had fewer attempts and shorter procedure time, so the association with fewer follow-up threshold events may be confounded
• Larger studies are required to confirm the role of the M-beat in predicting loss or rise of capture threshold

Additional limitations evident in the study

• It is a brief, retrospective, single-centre report with 69 M-beat patients and no prospective validation. The performance figures are in-sample
• The M-beat is defined by three subjective morphological features — slurred S, >50% amplitude increase, rounded aVR terminal deflection — with no interobserver reproducibility data and no blinded adjudication. A morphology-based marker without agreement statistics is difficult to adopt
• The reference standard is the nonselective→selective transition, which the paper itself argues is unreliable when myocardial and LBB excitability are similar. Validating a new marker against an imperfect standard, then claiming the new marker is independent of that standard's limitation, is circular unless the two are compared against a third, direct standard — which is not done
• Sensitivity is only 58.6%. The M-beat is absent in two thirds of patients and in 41% of those with a proven selective transition, so it cannot be used to exclude selective capture or to decide that a lead is nonselective
• The follow-up comparison is uncontrolled and confounded. M-beat patients had fewer deployment attempts and shorter fluoroscopy, so less tissue trauma may explain their better electrical follow-up independently of any capture state. The event numbers (6 and 2) are too small for statistical inference and are reported without a test
• The cohort is unusual: 40% CRT, mean LVEF ~47%, and a mean QRS of 143 ms — a different case mix from most LBBP series, which may affect the incidence of both template beats and selective capture
• No clinical outcomes — no heart failure events, hospitalisation, mortality, or lead revision beyond the small electrical events
• The claim that the M-beat is "independent of local myocardial refractoriness" is a mechanistic assertion; the study does not measure refractoriness or excitability, and the independence is inferred from the absence of a pacing protocol rather than demonstrated
• The template-beat incidence rose from 58% in the group's prior study to 90.4% here, attributed to the learning curve and technique adoption — which also means incidence figures are operator-dependent and not transferable
• No comparison with other selective-capture markers (fixation beats, V6–V1, programmed stimulation) in the same patients, so the incremental value of the M-beat over existing signs is unquantified
• Rating rationale: rated medium. It is a reasonably sized single-centre cohort with a simple, mechanistically motivated observation and a striking PPV, and it adds a marker that may help in baseline LBBB. But it is a brief retrospective report, with a subjective definition and no reproducibility data, low sensitivity, a circular validation strategy, confounded follow-up comparisons, and no clinical outcomes

Key Concepts Mentioned

• concepts/Left-Bundle-Branch-Area-Pacing — the technique; this paper adds an implantation marker
• concepts/Conduction-System-Pacing — the umbrella category
• M-beat — a template beat with M-pattern in V1, slurred S in V6, rounded aVR terminal deflection; predicts selective LBB capture
• Template beat / fixation beat — a mechanically induced PVC with RBBB-delay pattern, marking the LBB area
• PVC-guided LBBAP — rapid lead deployment while observing mechanically generated ectopy
• Selective vs nonselective LBB capture — the state the M-beat predicts
• Nonselective → selective transition — the threshold-test reference standard, dependent on differential tissue excitability
• Myocardial refractoriness / excitability — the confounder the M-beat is proposed to bypass
• Lead deployment attempts and fluoroscopy time — the procedural variables associated with M-beat presence

Key Entities Mentioned

• concepts/Left-Bundle-Branch-Area-Pacing — primary concept page
• Velammal Medical College, Madurai, India — the investigating centre
• Shunmuga Sundaram Ponnusamy; Pugazhendhi Vijayaraman — the investigators
• Medtronic — the 3830 lead and C315 sheath used, the affiliation of co-author William Basil, and the disclosed relationship of the authors
• Jastrzębski et al. — the fixation-beat study whose diagnostic metrics are cited for comparison
• Su et al. — the long-term study whose 74.4% selective-capture rate is cited
• Wiley Periodicals / Journal of Cardiovascular Electrophysiology — publisher

Wiki Pages Updated

• Created: wiki/sources/m-beat-jce-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