A 12-lead ECG obtained on postoperative day 1 after upgrade from a leadless right ventricular pacemaker to a dual-chamber DDD-LBBAP system offers an early window into capture quality. The combination of QRS duration, V1 morphology, frontal plane axis, and lateral precordial behavior allows a structured assessment of whether conduction system region capture has been achieved — and helps frame what to monitor during the 4 to 6 week reassessment window.
Rhythm and Intervals
The tracing demonstrates AV dual-paced rhythm at 60 bpm. Atrial pacing artifact is followed by an appropriately timed ventricular pacing artifact, confirming both lead sense-and-pace integrity on POD #1.
| Parameter | Value | Interpretation |
|---|---|---|
| Ventricular rate | 60 bpm | Programmed lower rate |
| AV interval | 184 ms | Reasonable programmed AVD for atrial sensing-paced ventricular function |
| Paced QRS duration | 126 ms | Substantial narrowing versus typical RV-only paced QRS (140–180 ms) |
| QTc | 462 ms | Borderline; expected with paced ventricular activation |
| Frontal plane axis | ≈ −52° | Superior left axis — typical for septal LBB-area exit |
The most clinically meaningful finding is the paced QRS duration of 126 ms. When prior RV-only pacing produced QRS in the 140–160 ms range, narrowing into the 120s ms range supports successful conduction system region capture rather than non-physiologic myocardial-only capture.
V1 Morphology — The RBBB-Like Pattern
Lead V1 demonstrates a small r' or qR-type morphology with a terminal R component. This is the classic RBBB-like pattern expected from LBBAP, reflecting earlier left ventricular septal activation followed by later RV free wall depolarization. The presence of this pattern supports proper septal lead position and argues against pure RV myocardial capture.
Capture of the left-sided conduction system causes the LV to depolarize before the RV, mimicking right bundle branch block on the surface ECG. Loss of the V1 r' or transition to a pure LBBB pattern would suggest lead retraction toward the RV side of the septum or capture of working myocardium without conduction system engagement.
Frontal Plane Axis and Inferior Leads
The superior axis near −52° aligns with a basal-septal exit point and apical-to-basal septal activation. Deep QS complexes in the inferior leads (II, III, aVF) are concordant with this axis and reflect the wavefront direction away from the inferior wall — an expected pattern, not a signal of ischemia, for a basal LBB-area pacing site.
Lateral Precordial Leads — The Capture Type Question
Lateral precordial morphology (V5–V6) is where the capture type discussion becomes nuanced. With unequivocal selective LBB capture, V5 and V6 typically show:
- Dominant R wave morphology
- Crisp, early V6 R-wave peak time (RWPT typically < 75–80 ms)
- Smooth transition without prominent terminal S
Deep S waves in V5–V6 raise the differential between non-selective LBB capture, LV septal myocardial capture (LVSP), and deep septal myocardial capture without true conduction system engagement. The surface 12-lead alone cannot reliably resolve these — that is the role of intraprocedural criteria.
What the Surface ECG Cannot Tell You
A definitive determination of capture type requires data not available on the postoperative tracing alone:
- V6 R-wave peak time measured during programmed stimulation
- Output-dependent QRS transition during decremental output testing
- V6–V1 interpeak interval as a discriminator between selective LBB, non-selective LBB, and LVSP
- Procedural fluoroscopic position and intracardiac electrogram morphology at implant
Even when selective LBB capture cannot be confirmed, LV septal pacing (LVSP) produces meaningful resynchronization compared with RV apical pacing. The MELOS registry and related cohorts (Jastrzębski et al.) have shown comparable clinical outcomes between non-selective LBB capture and LVSP in most patient populations. Functionally, conduction system region pacing is markedly superior to chronic RV apical pacing regardless of which precise capture variant is achieved.
Comparison Against Prior Tracing
The automated comparator note ("no significant change") between the immediate post-implant ECG and POD #1 reflects stability of capture and lead position over the first 24 hours — the desired finding. Acute lead micro-displacement or capture loss would manifest as QRS widening, loss of the V1 r', or an axis shift.
What to Monitor Going Forward
Immediate Postoperative Period (POD #1 to Week 2)
- Atrial lead-related symptoms — pleuritic-type pain, hiccups, diaphragmatic twitch, or shoulder-tip pain may suggest phrenic nerve interaction or pericardial irritation; warrants device interrogation
- Pocket signs — expected ecchymosis and tenderness should diminish progressively from POD #2 onward; expanding hematoma, increasing pain, or systemic signs require evaluation
- Supine positional discomfort typically peaks at 48–72 hours and improves through the first week
4 to 6 Week Reassessment Window
- Repeat 12-lead ECG with attention to QRS stability, V1 morphology, and any axis shift
- Device interrogation: capture threshold trend, lead impedance, sensing amplitudes
- Programmed AV delay sweep to assess whether QRS narrows further with optimization
- Threshold rise of more than approximately 1V from the implant value warrants attention
3 to 6 Month Echocardiographic Reassessment
- LV dimensions and ejection fraction trend, particularly in patients with prior pacing-induced cardiomyopathy
- LA volume index — reverse atrial remodeling is a recognized benefit of restoring AV synchrony
- Diastolic function indices (E/e′, E/A)
- Mitral and tricuspid regurgitation severity, particularly when prior RV pacing contributed to functional regurgitation
Summary
This POD #1 12-lead is consistent with a successful conduction system region pacing result: paced QRS of 126 ms (a substantial reduction from typical RV-only pacing), V1 RBBB-like morphology, and a superior axis appropriate for a basal-septal LBB-area exit. The deep S waves in V5–V6 leave open whether the capture is non-selective LBB, LV septal, or deep septal myocardial — a question the surface ECG alone cannot resolve and that the intraprocedural data are designed to answer. Functionally, the patient is in a markedly better hemodynamic position than under chronic RV apical pacing.
Frequently Asked Questions
Paced QRS duration after successful LBBAP typically ranges from 110 to 130 ms, substantially narrower than RV apical pacing (commonly 140 to 180 ms). A reduction from the prior RV-paced QRS into the 120s ms range supports successful conduction system region capture.
LBBAP captures the left-sided conduction system first, so left ventricular activation precedes right ventricular activation. The terminal r' or qR notch in V1 reflects later RV free wall depolarization, mimicking right bundle branch block morphology. This pattern is expected and supports proper septal lead position.
Surface ECG alone cannot reliably separate non-selective LBB capture from LV septal myocardial capture. Intraprocedural criteria are required: V6 R-wave peak time below 75 to 80 ms, output-dependent QRS transition during programmed stimulation, and the V6–V1 interpeak interval. Functionally, both produce meaningful resynchronization compared with RV apical pacing.
Deep S waves in lateral precordial leads on a paced ECG can suggest deep septal myocardial capture rather than unequivocal selective LBB capture. With true LBB capture, V5 and V6 typically show dominant R waves with a crisp early R-wave peak time. The finding does not exclude meaningful clinical benefit but argues for procedural confirmation of capture type.
A standard reassessment window is 4 to 6 weeks post-implant, when acute tissue inflammation around the lead has resolved and chronic capture thresholds stabilize. Repeat 12-lead ECG, device interrogation, and AV delay optimization are typically performed at this visit.