ABC Farma · Cardiac Electrophysiology
Clinical education at peer level — bilingual EP content

LBBAP QRS Morphology Evolution: Reading Capture Quality on Serial 6-Lead ECG

Cardiac Electrophysiology Conduction System Pacing Published May 2, 2026

Serial single-channel and 6-lead ambulatory tracings can offer surprisingly granular insight into how a left bundle branch area pacing (LBBAP) lead is behaving over time. A side-by-side comparison of two 6-lead Kardia recordings several months apart in a DDD-LBBAP system illustrates the morphologic features that distinguish selective from non-selective capture — and the differential of why a paced QRS may narrow over time.

Tracing 1: Earlier Recording (HR 50 bpm)

Per-Lead Morphology

Filter: Enhanced · 60 Hz · 25 mm/s · 10 mm/mV · 6L
I
Small, slurred, low-amplitude R wave with a notched/rounded contour. The upstroke is not crisp.
II
Deep, wide-looking QS-type complex with a slow terminal component.
III
Negative, broad.
aVR
Discrete negative deflection.
aVL
Low-amplitude, broad, somewhat notched R.
aVF
Predominantly negative, wide.
Impression: Paced QRS appears wider and more notched, with a sluggish intrinsicoid deflection in the lateral leads. The morphology is closer to what is expected from left-sided septal/myocardial capture (LVSP-leaning) rather than crisp left bundle capture — or possibly fusion at this slower rate. There is also more LV activation delay signature in lead I (slurred upstroke).

Tracing 2: Follow-up Recording (HR 60 bpm)

Per-Lead Morphology

Filter: Enhanced · 60 Hz · 25 mm/s · 10 mm/mV · 6L
I
Tall, narrow, sharply peaked R wave with a clean rapid upstroke.
II
Well-formed QS/rS with a discrete, narrow negative deflection — much tighter than the prior recording.
III
Discrete narrow negative.
aVR
Discrete narrow negative.
aVL
Tall, narrow, peaked R; very rapid intrinsicoid.
aVF
Deep, narrow QS — discrete, not slurred.
Impression: The paced QRS is clearly narrower and more discrete, with a rapid R-wave peak time in lateral leads (I, aVL). This is the morphology pattern more consistent with selective or near-selective LBB capture — fast lateral LV activation, tight terminal forces, and a clean frontal-plane axis.

Frontal-Plane Axis: Preserved Across Both Tracings

Both tracings show the expected LBBAP signature in the limb leads: positive I/aVL, negative II/III/aVF — a left/superior axis from septal pacing exiting toward the LV lateral wall. That vector is preserved between the two recordings. What has changed is the width and crispness of the complex, not the direction of the resultant.

Differential for the Morphology Change

Several non-exclusive mechanisms can account for the transition from a wider, slurred paced complex to a narrower, more discrete one in serial recordings:

  1. Capture-type transition. The earlier tracing may represent non-selective LBB capture with a larger LV septal myocardial component (or even LVSP-dominant capture); the later tracing represents a shift toward more selective LBB capture — or at least non-selective capture with a much smaller myocardial component. The narrower QRS, faster R-peak time in I/aVL, and discrete terminal forces all favor better engagement of the conduction system in the follow-up tracing.
  2. Output or programming change. A change in LV1 output, pulse width, or pacing vector — or a threshold-driven transition between selective and non-selective at a given output — can flip morphology between these two patterns at the same lead position. This warrants correlation with the most recent device interrogation.
  3. Rate-related conduction change. At a slower paced rate, more fusion with a slow escape rhythm or with residual His–Purkinje activity can distort the paced complex. At a faster rate the pacing is "cleaner" with less fusion. This is the simplest explanation if programmed output and lead position are unchanged between the two recordings.
  4. Lead maturation and resolution of acute peri-implant changes. Several months further out from a DDD-LBBAP upgrade, local edema and acute inflammatory changes around the helix have typically resolved, often producing a tighter capture morphology and lower thresholds. This is consistent with the typical post-implant trajectory.

Bottom Line

The follow-up tracing's QRS is the better-looking conduction-system-paced complex — narrower, more discrete, with a faster lateral R-peak time. The earlier tracing looks like either a more myocardial-dominant capture or a fusion-distorted complex. If output and programmed mode are unchanged between the two recordings, the most parsimonious read is maturation plus a shift toward more selective LBB capture — the favorable direction.

Recommended correlation at next interrogation

  • Paced QRS duration measured on a contemporaneous 12-lead.
  • V6 R-wave peak time (V6RWPT) — a key selective vs non-selective discriminator.
  • Documentation of selective vs non-selective capture at the current programmed output and pulse width.
  • Threshold and impedance trends since implant to confirm lead stability.

If the follow-up Kardia tracing reflects a true narrowing on the 12-lead as well, that is a favorable electrical and remodeling signal.

This article is intended for medical education and is not a substitute for individualized clinical evaluation. Interpretation of paced ECG morphology — particularly distinctions between selective LBB capture, non-selective LBB capture, and LV septal pacing — should always be correlated with the device interrogation, threshold testing, and a contemporaneous 12-lead ECG. Single-channel or 6-lead ambulatory tracings have intrinsic limitations and cannot replace formal 12-lead assessment.