What is the Jastrzębski differential output test?

By decreasing pacing output from high (non-selective LBBAP) to low (selective LBB capture), Stim-LVAT should remain constant if true LBB capture is present. A jump of ≥10–15 ms in Stim-LVAT marks the transition from conduction system to pure myocardial pacing.

ECG Criteria for Electrical Resynchronization with LBBAP

Q: What V1 morphology confirms LBBAP conduction system capture?

A right bundle branch block (RBBB)-like pattern in V1 (rsR', rSR') confirms conduction system capture in LBBAP. Loss of this pattern and transition to LBBB morphology indicates loss of LBB capture and reversion to pure myocardial pacing.

Q: What is the V6 RWPT cutoff for LBB capture?

A V6 R-wave peak time (RWPT) of ≤80 ms from the pacing spike to the R-wave peak is the Jastrzębski criterion for LBB capture, reflecting rapid conduction through the left bundle to the lateral LV wall.

Confirming true conduction system capture — and thus electrical resynchronization — in Left Bundle Branch Area Pacing (LBBAP) requires integrating several ECG parameters. No single finding is sufficient; the criteria below form a multilayered diagnostic framework endorsed by the EHRA 2023 consensus (Ponnusamy, Vijayaraman et al.).

Criterion 01

QRS Morphology: RBBB Pattern in V1

The hallmark of selective or non-selective LBBAP is a right bundle branch block (RBBB)-like morphology in V1 — typically rsR', rSR', or similar configurations. This pattern reflects activation originating in the left bundle branch or left ventricular conduction system and returning to the right ventricle via slower myocardial tissue.

Loss of this RBBB pattern with reversion to an LBBB-like morphology signals loss of conduction system capture and transition to pure myocardial pacing. A W-pattern or deep S in V1 at high output that transitions to a cleaner rSR' at lower output can indicate the non-selective → selective capture transition.

Criterion 02

V6 R-Wave Peak Time (RWPT)

Measured from the pacing spike to the peak of the R-wave in V6, the RWPT reflects how rapidly the left bundle branch activates the lateral LV wall via the conduction system.

Finding	Interpretation
RWPT ≤ 80 ms	LBB capture confirmed (Jastrzębski criterion)
RWPT > 80 ms	Suggests pure myocardial capture; slower lateral activation

This criterion may be confounded by pre-existing LBBB that normalizes with capture, and is more reliable in patients without baseline QRS abnormalities.

Criterion 03

Stimulus-to-LVAT (Stim-LVAT)

Stim-LVAT is measured from the pacing spike to the intrinsicoid deflection (peak R) in V5 or V6, serving as a proxy for left ventricular activation time from the pacing site.

▸ Key finding

      In LBBAP with conduction system capture, Stim-LVAT is short and stable across output changes. The target value is typically ≤ 80–90 ms; exact cutoffs vary by series. Constancy across differential output testing is the highest-confidence feature of true LBB trunk capture.

Criterion 04

Differential Output Testing (Jastrzębski Maneuver)

This is the most rigorous ECG-based confirmation of LBB capture. By stepwise reduction of pacing output, the clinician can identify the threshold at which myocardial capture is lost while LBB capture is preserved (non-selective → selective), and then the threshold at which LBB capture itself is lost.

High Output

Non-selective LBBAP: RBBB morphology in V1 with fusion component from local myocardium. Stim-LVAT at baseline.

Low Output
(above LBB threshold)

Selective LBBAP: Pure RBBB in V1. Stim-LVAT remains unchanged or minimally changed — this constancy is the key discriminator.

Below LBB threshold

Loss of RBBB pattern → transition to LBBB-like morphology. QRS widening. Stim-LVAT jumps ≥ 10–15 ms — marks transition to pure myocardial pacing.

The abrupt Stim-LVAT jump (≥ 10–15 ms) when crossing the LBB capture threshold is the defining ECG event of this maneuver.

Criterion 05

QRS Duration

Electrical resynchronization is confirmed by significant QRS narrowing relative to the intrinsic LBBB baseline. In patients paced for LBBB-mediated cardiomyopathy, successful LBBAP should produce a paced QRS substantially narrower than the native complex.

Parameter	Typical finding
Intrinsic LBBB	Often 150–220+ ms
Paced QRSd (good capture)	Typically < 130 ms, often 110–120 ms
Interpretation	QRS narrowing alone is insufficient; must be paired with RBBB morphology in V1

A significant left axis deviation in the paced QRS should prompt scrutiny — it may indicate preferential anterior or posterior fascicular capture rather than main LBB trunk activation.

Criterion 06

Frontal Axis: aVR / aVL Check

LBBAP capturing the left bundle proper should not produce significant left axis deviation. A normal or right-superior axis shift in the paced QRS is consistent with left bundle trunk activation rather than selective fascicular engagement. Persistent left axis deviation warrants re-evaluation of lead position and capture type.

Summary

Integrated Criteria Matrix

Criterion	Target Value	Significance
V1 morphology	RBBB pattern (rsR', rSR')	Conduction system capture confirmed
V6 RWPT	≤ 80 ms	Rapid LBB activation to lateral LV
Stim-LVAT	≤ 80–90 ms, stable across outputs	CS capture; not pure myocardial
QRS duration	< 130 ms vs. LBBB baseline	Electrical resynchronization achieved
Stim-LVAT jump at threshold	≥ 10–15 ms abrupt increase	Marks LBB capture boundary (Jastrzębski)
Frontal axis	No significant left axis deviation	LBB trunk capture (not fascicular)

FAQ

Frequently Asked Questions

What V1 morphology confirms LBBAP conduction system capture?

A right bundle branch block (RBBB)-like pattern in V1 — rsR', rSR' — confirms conduction system capture. Loss of this pattern and reversion to LBBB morphology indicates loss of LBB capture and transition to pure myocardial pacing.

What is the V6 RWPT cutoff for confirming LBB capture?

A V6 R-wave peak time (RWPT) of ≤ 80 ms from the pacing spike to the R-wave peak is the Jastrzębski criterion for LBB capture, reflecting rapid conduction through the left bundle to the lateral LV wall. Values > 80 ms suggest pure myocardial capture.

How does differential output testing confirm LBBAP capture?

By reducing pacing output from high (non-selective LBBAP) to low (selective LBB capture threshold), Stim-LVAT remains constant if true LBB capture is present. An abrupt jump of ≥ 10–15 ms in Stim-LVAT marks the transition from conduction system to pure myocardial pacing and identifies the LBB capture threshold.

What QRS duration is expected with successful LBBAP resynchronization?

In patients with baseline LBBB, successful LBBAP should produce a paced QRS substantially narrower than intrinsic — typically < 130 ms, often 110–120 ms with good capture. QRS duration must be interpreted alongside V1 morphology, as narrowing without the RBBB pattern does not confirm LBB trunk capture.

What does the EHRA 2023 consensus state about LBBAP capture criteria?

The EHRA 2023 consensus (Ponnusamy, Vijayaraman et al.) formalized a tiered diagnostic framework using V1 RBBB morphology, V6 RWPT ≤ 80 ms, and Stim-LVAT constancy across differential output testing. Stim-LVAT stability across output change is considered the highest-confidence criterion for true LBB trunk capture and electrical resynchronization.