A modern dual-chamber pacemaker can return a clean interrogation report — stable impedance, low capture threshold, comfortable safety margin, normal sensing — while the patient is not getting the therapy the implant was meant to deliver. The gap between "the lead works" and "the conduction system is being captured" is where Left Bundle Branch Area Pacing (LBBAP) lives, and it cannot be closed with telemetry alone.
What device interrogation can and cannot tell you
The interrogation report answers questions about the electrical contract between the lead tip and the local myocardium. It confirms that a stable, low-impedance circuit exists. It confirms that a pulse of a given output produces a depolarization the device can detect on the next cycle. It confirms mechanical stability — that the lead has not dislodged, that there are no short V-V intervals, that adaptive capture management is operating normally.
What it categorically cannot tell you is what tissue is being captured. The pulse generator sees a unipolar or bipolar capture confirmation signal — it does not distinguish between selective LBB capture, non-selective LBB capture, LV septal myocardial capture only, or RV septal capture from a screw that did not reach deep enough. All four scenarios will produce a "capture threshold" the device reports as normal. The device has no idea which one is happening. That is a 12-lead ECG question, and specifically a paced-QRS-morphology question.
Deep S waves in V5–V6: what they may signal
In a properly captured left bundle, the activation wavefront propagates from the LBB down through the Purkinje network and depolarizes the LV from endocardium outward in a near-physiologic sequence. The hallmarks on a paced 12-lead include a terminal R wave in V1 — reflecting late RV depolarization via transseptal spread — dominant R waves in V5–V6 with relatively narrow paced QRS, and a short V6 R-wave peak time (typically under 75–80 ms for non-selective LBB capture).
A morphology dominated by deep S waves in V5–V6, by contrast, can suggest that LV lateral wall depolarization is occurring late and from a direction away from those leads — a wavefront moving right-to-left across working myocardium rather than spreading rapidly down the left-sided conduction system. The differential for that pattern includes:
- LV septal capture without true LBB engagement. The screw is deep in the septum on the LV side but is not capturing the bundle itself. The QRS is narrower than RV apical pacing but wider than true LBBAP, and lateral precordial morphology can show persistent S waves.
- Deep septal myocardial capture only. Capture is purely myocardial, no conduction system recruitment, and the wavefront crawls through working tissue.
- Output-dependent loss of LBB capture. The lead may produce selective or non-selective capture at high output during implant, but at the programmed amplitude only the myocardium is being captured. This phenomenon is well described.
- Lead too proximal in the septum. The screw never reached the bundle.
The three thresholds problem
This is the crux of why interrogation can look perfect while LBB capture is suboptimal. The capture threshold the device reports is the myocardial capture threshold — the minimum output required to depolarize something near the electrode. The LBB capture threshold is a separate value, and it can only be determined by 12-lead ECG analysis during a threshold test, watching for the morphology transition where the V6 R-wave peak time abruptly shortens — or the QRS narrows, or the V1 terminal R appears or disappears — as output crosses the bundle's capture threshold.
In many LBBAP implants there are three thresholds to think about:
- Selective LBB capture threshold — typically the highest. Only the bundle is captured; the QRS shows discrete features distinct from the local myocardial electrogram.
- Non-selective LBB capture threshold — intermediate. Both the bundle and adjacent septal myocardium are captured simultaneously. This is the most common acceptable result.
- Myocardial-only capture threshold — typically the lowest. The wavefront propagates through working myocardium without conduction system recruitment. This is what the device reports.
If the programmed output sits above the myocardial threshold but below the LBB capture threshold, the device counter will read 100% ventricular pacing with normal-looking telemetry — while the patient is functionally receiving LV septal myocardial pacing without conduction system recruitment.
The terminal S-wave paradox
Here is where careful clinicians push back, and they are right to do so. In modern LBBAP, the presence of a terminal S wave in V5–V6 is often a marker of clinical success, not failure. The reasoning is sound:
When the lead successfully captures the left bundle branch, the LV activates rapidly via the Purkinje network. The right ventricle, however, is activated slightly later — because the impulse must travel back across the septum to reach RV myocardium, since the right bundle is not being captured by an LBBAP lead. That delay produces a late, rightward, anteriorly-directed vector. On the surface ECG, this late vector manifests as a terminal R wave in V1 (the classic "RBBB-like" or qR / rsR' pattern) and, simultaneously, as a terminal S wave (or terminal r') in lateral leads including I, V5, and V6.
A terminal S wave in V5–V6 is therefore part of the expected RBBB-like morphology of successful non-selective LBB capture. Treating any S wave in lateral leads as evidence of failure is an oversimplification — and a common one.
The distinction that resolves the paradox
The critical distinction is between a terminal S wave — small, narrow, late-occurring negative deflection at the tail end of an otherwise dominant R wave — and a deep, broad, dominant S wave, where the S is the major deflection of the QRS complex. These can look superficially similar but mean very different things.
| Feature | Terminal S of successful LBBAP | Deep dominant S of failed LBB capture |
|---|---|---|
| R wave in V5/V6 | Tall, dominant R wave precedes the S | Small or absent R wave |
| Position of S | Terminal — late tail of QRS | Dominant body of QRS |
| R/S ratio | R ≫ S (R is dominant) | S ≥ R or S ≫ R |
| V1 morphology | qR, rsR', or terminal R wave present | QS or rS without terminal R |
| V6 R-wave peak time | Short (under 75–80 ms) | Prolonged (over 80–90 ms) |
| QRS duration | 110–130 ms typically | Often above 130 ms |
| Frontal axis | Variable, often near-normal | Often markedly leftward / superior |
The key insight: the terminal S wave of successful LBBAP rides on top of a dominant R wave in V5–V6. The ventricle has been activated quickly via the left-sided conduction system (producing the tall R), and the late RV activation produces a small terminal S at the end. The R is the main story; the terminal S is the punctuation.
In a failed LBBAP with LV septal capture only, the wavefront crawls through myocardium and the resulting morphology is a deep, dominant S wave with little or no preceding R — because the LV lateral wall is not being activated rapidly from a Purkinje-driven origin.
Putting morphology criteria together
No single ECG feature is fully diagnostic. Confirmation of LBB capture rests on a constellation of findings, weighted appropriately. The most useful surface criteria, in approximate order of specificity:
V6 R-wave peak time (RWPT)
Measured from the pacing spike to the peak of the R wave in V6. Under 75 ms strongly suggests LBB capture. 75–80 ms is borderline. Over 80 ms favors LV septal capture without bundle engagement. Over 90 ms essentially confirms myocardial-only capture. This is the single most quantitative criterion and should be measured precisely on a high-resolution tracing.
V1 terminal R wave
A qR, rsR', or unambiguous terminal R wave in V1 is the most specific surface ECG marker of LBB capture, reflecting late RV depolarization. A QS or rS pattern in V1 without terminal R argues against true LBB capture.
Output-dependent morphology change
The single most useful confirmatory test is decremental output testing during an in-person interrogation, recording a 12-lead simultaneously. As output crosses the LBB capture threshold from above, an abrupt morphology transition occurs — the QRS narrows, V6 RWPT shortens, or the V1 terminal R appears or disappears. The presence of this transition confirms the bundle was being captured at higher output. Its absence across the achievable output range suggests the bundle is not being captured at any setting.
Frontal axis
Successful LBBAP typically does not produce extreme leftward or superior axis deviation, because LBB capture preserves a relatively physiologic activation sequence. A markedly leftward axis is more consistent with deep septal capture activating the LV from a position that drives the wavefront superiorly.
QRS duration
Non-selective LBB capture typically produces a paced QRS of 110–130 ms. RV apical pacing typically produces 150–180 ms. A QRS of 130–145 ms sits in a gray zone where deep septal LV pacing is plausible — narrower than apical RV pacing but wider than true conduction system recruitment.
What actually resolves the question
When morphology is ambiguous — and it often is — the data that can definitively resolve "is the LBB being captured?" is not in the device interrogation report. Resolution requires:
- Intra-procedural 12-lead with output decremental testing. Did V6 R-wave peak time abruptly shorten at any output? Did QRS morphology transition? Did the V1 terminal R appear or disappear with output changes? The implanting operator's procedure note should document this.
- A current 12-lead at the programmed output, with precise measurement of paced QRS duration and V6 R-wave peak time. A bedside rhythm strip is not sufficient; a properly recorded 12-lead is.
- Threshold testing with simultaneous 12-lead recording. The gold standard for confirming LBB capture in the follow-up setting is watching the morphology change at the bundle's capture threshold, comparing it to the device-reported myocardial threshold.
- Programmed amplitude consideration. If true LBB capture requires a higher output than the standard programmed setting, the decision matrix becomes: reprogram to a higher output (with battery cost), accept LV septal capture (preserves longevity, less optimal physiology), or consider lead revision.
Clinical takeaways
Telemetry confirms the lead, not the capture target. A normal interrogation report — stable impedance, low threshold, healthy battery, no alerts — confirms the electrical integrity of the lead-myocardium contract. It does not confirm what tissue is being captured. That question requires surface ECG analysis.
The terminal S-wave in V5–V6 is not inherently a failure marker. A small terminal S riding on top of a dominant R wave is part of the expected RBBB-like morphology of successful non-selective LBB capture. The features that distinguish success from failure are the height of the preceding R, the V1 morphology, the V6 R-wave peak time, the frontal axis, and the QRS duration — taken together.
When the morphology is ambiguous, output decremental testing is the resolution. An abrupt morphology transition as output is reduced is the surface-ECG fingerprint of a real LBB capture threshold. Its presence confirms the bundle was being engaged at higher output. Its absence across the achievable range suggests deep septal capture without bundle recruitment, regardless of how clean the device telemetry looks.
The therapeutic benefit of LBBAP — reverse remodeling, narrower QRS, restored synchrony in pacing-dependent patients — depends on actually capturing the bundle, not merely on placing a lead in the septum. Confirming that capture is occurring, and at what output, is part of the procedure's clinical follow-through. The device cannot do this for the clinician. The 12-lead can, when read with the right framework.