Ilyas K. Colombowala, MD, FACC, FHRS
Cardiac Electrophysiology · Houston, TX · colombowala.com
EP Staff Education

Device

CRT Interrogation (Optimization)

CRT-P and CRT-D interrogation focused on the unique parameters of biventricular pacing — BiV pacing percentage, AV/VV optimization, LV vector selection, and the phrenic surveillance that protects this therapy.

Interrogation walk-through

  • ▸P — Presenting rhythm: live strip, intrinsic vs paced, BiV vs RV-only beats, current AV/V-V on display.
  • ▸B — Battery: voltage, % remaining, longevity factoring three-lead pacing load and any shocks if CRT-D.
  • ▸L — Lead impedance: RA, RV (+ HV if CRT-D), LV in the programmed vector with per-vector survey.
  • ▸S — Sensing: P-wave, R-wave, LV sensing where available, far-field and phrenic screen.
  • ▸T — Threshold: per-lead capture threshold at 0.5 ms; phrenic capture screen at 10 V in every LV vector.
  • ▸O — Observations: BiV %, AT/AF, PVC burden, mode-switch, HF diagnostics, fusion/pseudofusion histograms, VT/VF if CRT-D.
  • ▸P — Program & Print: AV/V-V optimization, LV vector, MultiPoint Pacing, adaptive CRT, therapy programming if CRT-D, report saved and printed.

Common issues / troubleshooting

  • ▸BiV pacing <90% from breakthrough AF or frequent PVCs
  • ▸Phrenic nerve capture emerging months after implant
  • ▸Rising LV capture threshold suggesting lead maturation or microdislodgement
  • ▸CRT non-response despite high BiV percentage — opportunity for AV/V-V optimization
  • ▸Loss of LV capture intermittently with beat-to-beat fusion
Heart Generator RA RV LV (via CS)
CRT — three leads to coordinate atria, RV, and LV

We work through every interrogation using the PBL-STOP framework: Presenting rhythm, Battery, Lead impedance, Sensing, Threshold, Observations, Program & Print. On a CRT device the O and final P sections do the heavy lifting — Observations to diagnose non-response, Program & Print to fix it.

Device overview

CRT-P (pacemaker) and CRT-D (defibrillator) systems have the standard atrial and RV leads plus a third lead pacing the LV through a coronary sinus branch. On top of standard pacemaker/ICD checks, every visit confirms the LV is being captured, RV-LV timing is appropriate, and the patient is actually receiving the therapy that was prescribed. CRT therapy that is not being delivered — breakthrough AF, frequent PVCs, loss of LV capture, excessive AV delay — is the dominant cause of CRT non-response.

PBL-STOP walkthrough

P — Presenting Rhythm

The live BiV picture before anything else.

  • Intrinsic vs paced on the surface ECG and EGM
  • BiV beats vs RV-only vs LV-only vs fusion/pseudofusion in this snapshot
  • Snapshot %A-paced and %V-paced for the session
  • Programmed AV delay, V-V offset, and LV vector currently in use
  • Any ongoing AF, frequent PVCs, or NSVT visible on arrival

B — Battery Status

  • Voltage compared to model RRT/ERI cutoff
  • Percent remaining, projected months of service
  • CRT-specific drivers of accelerated drain:
    • Three-lead pacing load (RA + RV + LV continuously)
    • High LV output with a borderline threshold
    • MultiPoint Pacing or multi-site LV pacing
    • Frequent shocks if CRT-D
    • High RV-only pacing burden in atypical configurations

L — Lead Impedance

Three pacing impedances every visit; four numbers if CRT-D.

  • RA pacing impedance: 400–1200 ohms
  • RV pacing impedance: 400–1200 ohms
  • RV HV (shock) impedance: 30–80 ohms if CRT-D — a >20-ohm swing is the lead red flag
  • LV pacing impedance in the programmed vector: typically 400–1500 ohms; CS-lead vectors run higher than RV/RA values, so know the vector-specific baseline
  • Sudden swing in LV impedance is the most common phrenic/dislodgement clue and warrants re-checking every available vector
  • Trend each value across at least three prior visits

S — Sensing

  • P-wave amplitude: typically >1.5 mV
  • R-wave amplitude: typically >5 mV on the RV channel
  • LV sensing where the platform exposes it — useful for diagnosing intermittent capture loss vs sensed beats
  • Far-field R-wave on the atrial channel can drive inappropriate mode switches and rob BiV pacing
  • Phrenic-related sensing artifacts on the LV channel are uncommon but worth a look in patients reporting hiccup-like symptoms

T — Threshold

  • Capture threshold per lead at 0.5 ms — RA, RV, and LV
  • LV deserves a beat-by-beat manual confirmation; auto-tests can be fooled by fusion in conducted patients
  • Phrenic capture screen at 10 V in every LV vector — phrenic capture can emerge months after implant and must be re-checked every visit
  • Document a vector / threshold / phrenic table for the chart even if you do not change the vector
  • Safety margin programmed at 2× threshold (LV often programmed slightly higher to absorb day-to-day variation)

O — Observations

The single most important section for CRT — this is where non-response is diagnosed.

BiV delivery analytics

  • BiV pacing percentage — the headline metric

    • 95%: excellent

    • 90–95%: investigate PVCs or AT burden
    • <90%: action required
  • Fusion / pseudofusion histograms where the platform provides them — high fusion burden steals the resynchronisation benefit even when BiV % looks fine
  • MV/AV conduction patterns — long native PR letting intrinsic conduction win, or short PR producing fusion beats
  • Beat-classification breakdown: BiV vs RV-only vs LV-only vs intrinsic vs fusion

Arrhythmia and rate counters

  • AT/AF episodes — count, duration, longest, fastest atrial rate
  • PVC burden — frequent PVCs steal BiV beats one at a time
  • Mode-switch frequency and behavior
  • VT/VF episode log with full ICD-style EGM walk if CRT-D (see ICD walkthrough)

HF diagnostics

  • OptiVol / thoracic impedance trends
  • HeartLogic or equivalent composite HF alerts
  • Activity hours, night heart rate, heart rate variability
  • Cross-reference with clinical status, weight, diuretic changes

P — Program & Print

This is where the CRT visit earns its keep.

BiV recovery first

If BiV % is below target, fix the cause before tweaking anything else:

  • Rate control or AVN ablation referral for AF with rapid conduction
  • Antiarrhythmic or PVC ablation referral for high PVC burden
  • AV delay adjustment if intrinsic conduction is breaking through
  • Vector or threshold fix if LV capture is intermittent
  • Mode-switch criteria recalibrated to keep BiV pacing during AT

AV delay

  • Sensed AV typical starting point 100–120 ms; paced AV 150–180 ms
  • Shorten if intrinsic conduction is breaking through
  • Lengthen modestly if A-wave truncation is hurting filling
  • Echo-guided iterative optimization in non-responders; device-based algorithms (SmartDelay, AdaptivCRT, QuickOpt) as second line

V-V offset

  • Default 0 ms at implant; most responders settle on LV-first by 0–40 ms
  • Adjust in non-responders, ideally with imaging or device algorithm guidance

LV vector selection

  • Lowest stable capture threshold
  • No phrenic capture at 10 V
  • Longest QLV (RV-to-LV interval) for best electrical synchrony
  • Be willing to accept a slightly higher threshold for cleaner synchrony or phrenic avoidance

Advanced features

  • MultiPoint Pacing toggle where the lead supports it — extra battery cost, real synchrony gain in selected non-responders
  • Adaptive CRT — LV-only vs BiV switching by intrinsic conduction; on by default in most LBBB patients with intact AV conduction
  • Manufacturer-specific LV-RV timing algorithms

Therapy programming if CRT-D

  • Detection zones, ATP, shock energies, post-shock pacing, SVT discriminators — per the ICD walkthrough

Wrap-up

  • Document each change with prior value and rationale
  • Re-interrogate after every change — confirm BiV % recovery in real time when possible
  • Save and print the summary report plus any clinically relevant episode EGMs
  • Set next remote and in-office visit
  • Escalate to the EP attending for: BiV <90% with new HF symptoms, AF unresponsive to rate control, loss of LV capture not solved by vector change, new phrenic capture in all available vectors, HF decompensation signal on device diagnostics, any shock on CRT-D

Reference

Framework reference: “Keeping the Pace using PBL-STOP” — Chart Healthcare Academy

Watch

Short videos to help illustrate this topic. Embedded from the original channels — content belongs to them.

PBL-STOP walkthrough

Last reviewed by Dr. Colombowala on May 22, 2026.

Clinical-reference content, not medical advice. This page is written for EP staff and does not create a doctor-patient relationship. It does not replace institutional policy, current device manuals, or attending direction during a case. See the full disclaimer.

© 2026 Ilyas K. Colombowala, MD. All rights reserved. Reproduction, redistribution, or republication of this content in any form without written permission is prohibited.

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