LBBAP Capture Confirmation Framework: Evidence-Based ECG Criteria for Identifying Optimal vs. Suboptimal Left Bundle Branch Area Pacing

Why Capture Type Matters

Left bundle branch area pacing (LBBAP) encompasses a spectrum of capture types—from direct engagement of the left conduction system to pacing of adjacent septal myocardium only. The clinical distinction is consequential: true left bundle branch pacing (LBBP) recruits the native His-Purkinje network, producing rapid, coordinated left ventricular activation that closely approximates physiological contraction. Left ventricular septal pacing (LVSP), by contrast, generates a slower, cell-to-cell wavefront through the myocardium that, while substantially better than right ventricular apical pacing, does not provide the same degree of electrical synchrony. Over months and years, this difference may affect ventricular remodeling, ejection fraction trajectory, and the risk of pacing-induced cardiomyopathy (PICM).

Confirming which type of capture has been achieved—and detecting when that capture degrades—is therefore among the most important tasks in LBBAP follow-up. Yet the ECG criteria for this determination remain nuanced, and widely circulated AI-generated summaries frequently confuse LBBP with His-bundle pacing, misidentify the expected morphology, or omit the quantitative measurements that actually drive the diagnosis. This article provides a structured framework, grounded in published evidence, for clinicians and informed patients navigating this landscape.

The Three Capture Types: Getting the Physiology Right

Before any ECG criterion makes sense, the three capture types during LBBAP must be clearly understood. They represent a continuum determined by which tissues the pacing stimulus reaches at a given output.

Nonselective Left Bundle Branch Pacing (ns-LBBP)

At outputs above both the conduction system threshold and the local myocardial threshold, the pacing stimulus captures the LBB and the adjacent interventricular septal myocardium simultaneously. On the surface ECG, this produces what is classified as an atypical RBBB pattern in lead V1—typically a Qr, qR, or QR morphology. The QRS onset begins immediately from the pacing spike (no isoelectric interval) because local myocardium depolarizes at once, while the conduction system component produces rapid activation of the left ventricle, resulting in a relatively narrow overall QRS (typically 110–130 ms). On the intracardiac electrogram (EGM), the local ventricular signal is fused with the pacing artifact because both myocardium and conduction tissue are captured together.

Selective Left Bundle Branch Pacing (s-LBBP)

When output drops below the myocardial threshold but remains above the conduction system threshold, only the LBB itself is captured. The surface ECG now shows a typical RBBB pattern in V1—classically an M-shaped QRS (rsR′) with a prominent, wide terminal R′—and a characteristic isoelectric interval (stimulus-to-QRS delay) because there is a latency period while the impulse travels from the LBB to the surrounding myocardium before surface depolarization begins. On the EGM, a discrete local potential appears separated from the pacing spike. The QRS is often slightly narrower than in ns-LBBP because the activation wavefront originates purely from the conduction system.

Left Ventricular Septal Pacing (LVSP)

When output falls below the LBB threshold, only the local septal myocardium is captured. The conduction system is not engaged. The surface ECG shows a wider QRS (often >140–150 ms), loss of the terminal R′ in V1, and a morphology that may approach a nonspecific intraventricular conduction delay or, depending on the lead position, a pattern that superficially resembles LBBB. The V6 R-wave peak time (RWPT) prolongs abruptly because left ventricular activation now depends on slow myocardial cell-to-cell conduction rather than the rapid Purkinje network.

The Capture Confirmation Criteria: A Systematic Approach

The EHRA 2023 clinical consensus statement on conduction system pacing implantation established a hierarchical framework for confirming LBB capture. Subsequent studies in 2024 and 2025 have refined the quantitative cutoffs, introduced the concept of a "global RWPT," and proposed new morphologic markers. Below, each criterion is presented in order of diagnostic weight.

Criterion 1: QRS Morphology Transition During Differential Output Pacing (Gold Standard)

The most definitive non-invasive evidence of conduction system capture is demonstrating a transition in QRS morphology as pacing output is decremented. During unipolar threshold testing, voltage is progressively reduced while recording a 12-lead ECG. If the lead is truly engaging the LBB, the clinician should observe an abrupt change at the point where LBB capture is lost: the QRS widens, the terminal R′ in V1 disappears or diminishes, and V6 RWPT prolongs by ≥10 ms. The transition may also occur in the opposite direction—from LVSP to ns-LBBP—during lead screwing-in, as the helix penetrates deeper into the septum and reaches the conduction tissue.

Criterion 2: V6 R-Wave Peak Time (Stim-LVAT)

The R-wave peak time in lead V6—measured from the pacing stimulus to the peak of the R wave in V6 (also called Stim-LVAT, stimulus to left ventricular activation time)—is the single most useful quantitative ECG measurement for LBBAP capture assessment. It reflects the time required for the electrical impulse to reach the lateral left ventricular wall. When the conduction system is engaged, this interval is short and stable because the Purkinje network provides rapid impulse delivery. When only myocardium is captured, the impulse must travel cell-to-cell, and RWPT prolongs.

Criterion 3: V6-V1 Interpeak Interval

The interval between the R-wave peak in V6 and the R-wave peak in V1 reflects the relative timing of left ventricular versus right ventricular activation. During ns-LBBP, the left ventricle activates rapidly via the conduction system while the right ventricle activates later through the septum and/or retrograde right bundle, creating a positive (V6 before V1) interpeak interval. During LVSP, the left ventricle activates slowly via myocardium, narrowing or reversing this difference.

Criterion 4: Terminal R′ in Lead V1

The presence of a terminal R′ deflection in V1 is a prerequisite morphological feature of LBBAP capture (both selective and nonselective). It reflects delayed right ventricular activation relative to the left ventricle—the hallmark of direct left conduction system engagement. The EHRA consensus notes that correct V1 electrode placement is essential, as positioning the electrode too high on the chest can obscure the terminal R′.

During s-LBBP, lead V1 shows a classic M-shaped or rsR′ morphology with a wide, prominent R′. During ns-LBBP, the R′ is present but the morphology is modified by the simultaneous myocardial activation, producing a Qr or qR pattern. During LVSP, the R′ is typically absent—this is the most visible morphological marker of LBB capture loss on a standard ECG.

Criterion 5: The LBB Potential

A sharp, high-frequency deflection recorded on the unipolar EGM from the pacing lead tip, preceding the ventricular EGM by approximately 15–35 ms, represents direct recording of the left bundle branch potential. Its presence is strong evidence that the lead tip is in close proximity to the LBB. During selective LBBP, this potential appears as a discrete signal separated from the pacing artifact by the isoelectric interval. The EHRA consensus documents that the LBB potential-to-QRS interval is typically in the range of 25–34 ms for trunk capture, varying by fascicular capture site.

However, the LBB potential is identifiable in only approximately 40–50% of LBBAP cases, so its absence does not exclude LBB capture. It is also not part of the standard remote monitoring data available from CareLink or other telemonitoring platforms, limiting its utility to in-office interrogation with an EP recording system.

Criterion 6: The LBBP Score (Composite Criterion)

Recognizing that no single ECG criterion has both high sensitivity and specificity, Briongos-Figuero and colleagues proposed a composite "LBBP score" that integrates V6 RWPT, V6-V1 interpeak interval, and aVL RWPT into a ranked scoring system. This score was validated in a cohort of 174 patients with confirmed LBBAP and demonstrated superior discrimination between LBB capture and LVSP compared to any individual criterion alone.

The score is particularly useful in the clinical scenarios where differential output testing fails to produce a visible transition—which occurs in up to half of LBBAP procedures due to overlapping thresholds between the conduction system and adjacent myocardium.

Criterion 7: Emerging Morphologic Markers (2025)

Recent work published in 2025 in JACC: Clinical Electrophysiology has identified two additional morphologic ECG criteria for LBB capture identification. The first is the V6 upstroke/downstroke pattern—a characteristic waveform signature that differs between LBB capture and LVSP. The second is the appearance of QRS downstroke slurring or notching in any lead not previously present during lead penetration, which was found to be highly specific for LBB capture. An "M" QRS morphology in lead II or III was also described as a new marker. These criteria are being incorporated into multi-parameter diagnostic approaches but are not yet part of the formal EHRA consensus framework.

Practical Framework: How to Assess LBBAP Capture in Clinical Follow-Up

The following systematic approach applies to in-office interrogation, post-implant ECG review, or evaluation of a patient with suspected loss of optimal capture. It integrates the criteria above into a workflow ordered by clinical utility and ease of measurement.

Summary Table: LBBAP Capture Types and Their ECG Signatures

What a 6-Lead ECG Can and Cannot Tell You

Many clinical settings—including remote monitoring transmissions, emergency departments, and some outpatient offices—record only a limited lead set (typically leads I, II, III, aVR, aVL, aVF). This creates specific limitations and opportunities for LBBAP capture assessment.

The 6-lead limb set does include aVL, which has been validated as an independent criterion for LBB capture discrimination. The aVL RWPT is part of the LBBP composite score. It also includes lead I, which contributes to the "global RWPT" criterion proposed in 2024 that combines leads I and V6 for the highest area under the ROC curve (97.1%) in distinguishing ns-LBBP from LVSP. However, without V1 through V6, the most visually distinctive feature—the terminal R′ in V1—is invisible, V6 RWPT cannot be measured, and the V6-V1 interpeak interval cannot be calculated.

In practical terms, a 6-lead limb-only ECG can raise suspicion of capture change (QRS widening, axis shift, morphology change in inferior or lateral leads compared to baseline), but it cannot definitively confirm or exclude LBB capture. Any suspicion on 6-lead review should trigger a full 12-lead ECG and device interrogation.

Special Considerations: Non-Basal Lead Positions

The EHRA consensus and most early LBBAP studies assumed a basal septal lead position, characterized by a monophasic R wave in V6. In clinical practice, leads are sometimes implanted in more distal (non-basal) positions, recognizable by an Rs or rS pattern in V6 with more than 20% of the QRS amplitude in the terminal S wave.

This matters because non-basal positions systematically shorten V6 RWPT and prolong V6-V1 interpeak time compared to basal positions, meaning the standard cutoff values may misclassify capture type. The 2024 analysis by Rijks and colleagues established that for non-basally positioned leads, the optimal V6 RWPT cutoff shifts to <70 ms, with 100% specificity only below 55 ms. The V6-V1 interpeak cutoff shifts to >47 ms, with 100% specificity above 73 ms. These adjusted values should be used whenever the paced V6 morphology shows an Rs or rS configuration.

Monitoring for Capture Degradation Over Time

Post-implant threshold maturation follows a well-characterized trajectory: acute thresholds at implant are typically low (0.5–0.7 V at 0.4–0.5 ms), with a transient rise over the first 4–8 weeks as the inflammatory response at the electrode-tissue interface evolves, followed by a return to stable chronic values. Published registries report chronic LBBAP thresholds that remain stable or decrease slightly over 18+ months of follow-up.

During the acute maturation period, if the programmed output is close to the LBB threshold, the patient may transiently shift from LBB capture to LVSP as the threshold rises. This is usually self-limiting, but it underscores the importance of programming an adequate safety margin above the measured threshold—the EHRA consensus recommends thresholds <1.5 V at 0.5 ms (ideally <1.0 V) and suggests programming output with at least a 2:1 voltage safety margin.

Serial ECG comparison is the most practical surveillance tool. A remote monitoring transmission that shows QRS widening or morphology change compared to the post-implant baseline warrants in-office interrogation with a full 12-lead ECG and threshold testing. Lead impedance changes, sensing amplitude drops, or increased pacing thresholds on remote reports provide corroborating device-side data.

Clinical Decision Points: When Suboptimal Capture Is Confirmed

If assessment confirms that LBB capture has been lost and the patient is now in LVSP, the clinical response depends on context. For patients with preserved LV function and low pacing burden, LVSP may be clinically acceptable—the EHRA consensus explicitly notes that "it is unclear at this point if conduction system capture with LBBAP is necessary to achieve good clinical outcome." For patients with heart failure, CRT indications, or high pacing burden (>40%), the distinction may have greater hemodynamic relevance, and output reprogramming (increasing voltage or pulse width to recapture the LBB), AV interval optimization, or in some cases lead revision should be considered.