Two Lenses on Left Bundle Capture: the V6–V1 Interpeak Derivation vs QRS Transition Kinetics

Orientation

Cross-sectional fingerprint vs longitudinal kinetics

The two frameworks operate on different axes. One is cross-sectional and mechanistic: at a single moment, the combination of R-wave peak times in V1 (a surrogate for right ventricular activation delay) and V6 (a surrogate for left ventricular activation delay) produces a distinct signature for non-selective conduction-system capture, selective capture, and left ventricular septal myocardial capture. It tells you which type of capture you are looking at right now.

The other is longitudinal and diagnostic-yield-focused: the output-dependent QRS transition during a decremental threshold test is the accepted gold standard for confirming conduction-system capture, but its detectability changes over time. This second framework tracks that decay across three timepoints and reframes the disappearance of the transition not as loss of capture but as a signal to be classified.

The clinical payoff of combining them: the first supplies the instantaneous fingerprint whose stability the second then uses as the operational definition of maintained capture.

At a glance

Side-by-side summary

Framework one

The V6–V1 interpeak derivation, in full

The activation model

The derivation treats V1 and V6 intrinsicoid-deflection timing as independent readouts of the two ventricles. Four scenarios follow from whether the septal myocardium and the bundle are each captured:

• Non-selective capture — both the bundle and adjacent septal myocardium are captured. The right ventricle is reached partly by direct septal myocardial activation (short V1 delay) and the left ventricle by the conduction system (short V6 delay). Intermediate interpeak.
• Transition to selective capture (output decreased): direct septal myocardial capture is lost, so the right ventricle must now be reached by transseptal spread. V1 peak time lengthens; V6 peak time is unchanged; the interpeak widens.
• Transition to LV septal capture (bundle capture lost): right ventricular activation is unchanged because it still originates from the same septal site, but left ventricular activation is delayed by loss of the conduction system. V6 peak time lengthens; V1 peak time barely moves; the interpeak narrows.
• Pure LV septal capture — two oppositely directed wavefronts produce a relatively narrow QRS with substantial cancellation in V1 (no broad terminal R prime), and the V1 and V6 peaks fall at nearly the same time.

The measured interpeak values

The diagnostic thresholds

A cut-off greater than 33 ms (optimal near 32.5 ms) differentiates non-selective capture from LV septal capture with sensitivity 71.8 percent and specificity 90.0 percent (ROC AUC 84.7 percent). A cut-off greater than 44 ms is 100 percent specific for bundle capture, at the cost of low sensitivity — a rule-in threshold. The asymmetry is mechanistic: a short interpeak occurs with all three capture types, but a long interpeak (large right ventricular delay) is only physically possible when the conduction system is engaged. That is why long values are diagnostically decisive and short values are not.

The R-wave peak time shift criteria

Mean magnitudes: transition to selective capture raised the V1 peak time by 17.8 ± 10.0 ms (with the V6 value essentially flat); transition to LV septal capture raised the V6 peak time by 19.9 ± 6.7 ms (with the V1 value rising only 6.2 ± 6.3 ms). Loss of each capture component therefore delays its corresponding ventricle by roughly 20 ms.

Why the interpeak outperforms V6 peak time alone

The popular V6 peak time cut-off (under 75–80 ms) is confounded by initial latency, slow propagation through a diseased His–Purkinje system, and left ventricular dilatation — all of which prolong the V6 value even when the bundle is genuinely captured. The interpeak is largely immune because latency and slow myocardial conduction delay both the V1 and V6 peaks similarly, so their difference is preserved. Concretely, the interpeak correctly re-classified 63.8 percent of cases that a V6 peak time above 75 ms had labelled false-negative, and a combined rule (V6 peak time under 75 ms OR interpeak at least 33 ms) reached specificity 94.2 percent and sensitivity 78.2 percent. The interpeak was also unaffected by native QRS type, whereas both peak-time measures varied with the degree of underlying conduction disease.

The retrograde right-bundle caveat

The model assumes right ventricular activation during non-selective capture is dominated by intraseptal spread rather than retrograde right-bundle conduction. This holds in the study population (about 64 percent had right bundle branch block, left bundle branch block, or complete atrioventricular block, precluding retrograde conduction), and supporting observations reinforce it. The exception is a very proximal lead near the His bifurcation (bundle-potential-to-QRS at least 30–35 ms), where retrograde conduction contributes more and transition to selective capture may produce only minimal V1 peak-time prolongation — the scenario where the V1 criterion quietly under-performs.

Framework two

The QRS transition-kinetics and microdislodgement framework

What was tracked

The output-dependent transition (non-selective capture to selective capture or to LV septal capture as unipolar output is decremented) is the consensus-endorsed gold standard for confirming direct conduction-system capture, and its diagnostic yield is highest immediately after fixation. The study performed the threshold test at three timepoints — intraoperative, 16–72 hours postoperative, and roughly three-month follow-up — and asked whether the transition was still detectable.

The decay curve

The reframing that matters

The transition disappears in most patients, yet bundle capture itself remained in 92.0 percent. True microdislodgement occurred in only 5.0 percent and macrodislodgement in 3.0 percent of the confirmed-capture subset. In other words, loss of the confirmatory transition is common and mostly benign — it reflects maturation of the lead–tissue interface and a narrowing gap between conduction-system and myocardial thresholds, not loss of physiological capture. Preserved capture, when the transition was gone, was defined operationally as unchanged paced QRS morphology, V6 peak time, and V6–V1 interpeak interval versus intraoperative values — precisely where the interpeak fingerprint becomes the maintenance criterion.

The classification logic

Microdislodgement is characterised by comparing transition kinetics across the three timepoints and inferring where the lead functionally sits, using a regional scheme that distinguishes the deep septal region from the right septal region. Subtypes are distinguished according to whether the lead has functionally retreated from the deep septal region toward the right septum and how the transition threshold shifted between timepoints. The granular per-subtype numeric definitions reside in the full-text tables of the primary study; the regional framework itself is the conceptual contribution.

Field context: the literature is not settled

Detectability of the transition over time is genuinely disputed. One series reported selective transition in 75.4 percent acutely but only 30.9 percent at three months — closely mirroring the decay above — whereas another found the transition consistently reproducible at follow-up. This is best presented as an open methodological question, since threshold-test technique, timing, and definitions differ across centres, rather than a resolved point.

Synthesis

How the two frameworks interlock

1. Confirm at implant with the gold-standard transition, and simultaneously record the interpeak fingerprint — interpeak, V1 peak time, V6 peak time — as the baseline.
2. Recognise that the transition will fade (86 percent → 33 percent). Its disappearance is expected and, on its own, not evidence of failure.
3. Judge maintenance by fingerprint stability, not by the transition. A flat V6 peak time and a preserved interpeak of at least 33–44 ms across visits is the positive evidence of maintained capture; the directional peak-time shifts (V6 up ≥15 ms means bundle loss; V1 up ≥15 ms means septal loss) are the alarms.
4. The interpeak is the linchpin because it is latency- and conduction-disease-independent, making it the one parameter fairly compared across timepoints without correcting for lead maturation or myocardial slowing.

Caveats

Honest evidence limitations

• Neither study is a true long-horizon durability study. The interpeak work is single-instant; the kinetics work extends only to about three months. Claims about years-long maintenance remain inferential.
• The interpeak thresholds (33/44 ms) were derived in a population enriched for conduction disease and atrioventricular block; applicability to preserved-conduction patients, where retrograde right-bundle activation is possible, is explicitly limited by the authors.
• Measurement precision is method-dependent (high-sweep-speed digital calipers vs standard printouts); borderline interpeak values are fragile, which should temper any single-cut-off framing.