EP Staff Education Clinical Staff Ilyas Colombowala, MD, FACC, FHRS →
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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 — Chart review: HF etiology, LBBB status, QRS duration, NYHA class, echo response, last visit findings.
  • ▸B — Battery voltage, % remaining, longevity factoring in three-lead pacing load.
  • ▸L — Three leads: RA, RV (with shock impedance if CRT-D), LV in programmed vector — plus full LV vector survey.
  • ▸S — AT/AF burden, PVC burden, BiV%, mode-switch behavior, HF diagnostics (OptiVol/HeartLogic), VT/VF if CRT-D.
  • ▸T — Threshold confirmation per lead; therapy review if CRT-D (see ICD walkthrough).
  • ▸O — AV delay, V-V offset, LV vector selection, MultiPoint/adaptive CRT, RV-LV timing — the biggest section in CRT.
  • ▸P — Programming changes, AVN ablation discussion if AF-driven loss of BiV, follow-up, attending escalation as needed.

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

We work through every interrogation using the PBL-STOP framework — a quick mental checklist so nothing gets missed. On a CRT device the O section is where most of the value comes from, because optimized timing is what turns a delivered therapy into a clinical response.

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 — Presentation / Pre-interrogation review

  • Manufacturer, model, serial, implant date
  • HF etiology (ischemic vs non-ischemic), baseline EF, current EF if known
  • Baseline QRS duration and morphology (LBBB vs non-LBBB)
  • NYHA class trend
  • Echo response status at 3 and 6 months — responder vs non-responder
  • Last visit BiV percentage, LV vector, AV/V-V settings
  • Remote alerts since last check: HF diagnostics, AT/AF, lead alerts
  • Diuretic regimen, beta-blocker dose, antiarrhythmics

B — Battery / longevity

  • Voltage compared to model RRT/ERI cutoff
  • Percent remaining, projected months
  • CRT-specific depletion drivers:
    • Three-lead pacing load (RA + RV + LV continuously)
    • High LV output if threshold is borderline
    • Frequent shocks if CRT-D
    • High RV pacing burden in atypical CRT configurations

L — Lead measurements

Three leads, every visit:

  • RA: impedance, capture threshold, P-wave amplitude
  • RV: impedance, threshold, R-wave amplitude — plus shock impedance if CRT-D (30–80 ohms)
  • LV in programmed vector: impedance, threshold at 0.5 ms, sensing amplitude
  • LV vector survey — even when the programmed vector is fine:
    • Threshold per vector at 0.5 ms
    • Phrenic capture screen at 10 V in every vector
    • Document a vector / threshold / phrenic table for the chart
  • Sudden trend changes in any lead beat the absolute value as a red flag

S — Stored events / Sensed rhythm

  • BiV pacing percentage — the single most important CRT metric:

    • 95%: excellent

    • 90–95%: investigate PVCs or AT burden
    • <90%: action required
  • AT/AF episodes — count, total duration, longest, fastest atrial rate
  • PVC burden — frequent PVCs steal BiV beats
  • Mode-switch percentage and behavior
  • HF diagnostics where available:
    • OptiVol / thoracic impedance trends
    • HeartLogic composite alerts
    • Activity hours, night heart rate, heart rate variability
  • VT/VF episode log if CRT-D

T — Threshold testing / Therapy

  • Manual capture threshold confirmation per lead at 0.5 ms when auto-thresholds are rising or borderline
  • Special attention to the LV — confirm capture beat-by-beat on the EGM, not just the auto-test
  • If CRT-D, full therapy review per the ICD walkthrough (zones, ATP, shocks, post-shock pacing, appropriateness review)
  • If CRT-P, threshold confirmation is the main T deliverable

O — Optimization

This is where the CRT visit earns its keep.

BiV percentage forensics

If BiV% is below target, find out why before changing anything else:

  • AT/AF with rapid conduction — rate control vs AVN ablation
  • Frequent PVCs — antiarrhythmic, PVC ablation referral
  • AV delay too long, letting intrinsic conduction win
  • LV undersensing or capture loss
  • Mode-switch episodes leaving the device in DDI/VVI

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 optimization (iterative mitral inflow Doppler) in non-responders
  • Device-based algorithms (SmartDelay, AdaptivCRT, QuickOpt) as a second line

V-V offset (interventricular delay)

  • Default 0 ms at implant
  • Most responders settle on LV-first by 0–40 ms
  • Some patients prefer simultaneous or RV-first
  • Adjust in non-responders, ideally with imaging or device algorithm guidance

LV vector selection

The optimization decision that pays the biggest dividends.

  • Choose for the lowest stable capture threshold
  • No phrenic capture at 10 V
  • Best electrical synchrony — longest QLV (RV-to-LV interval) from the local LV EGM
  • Be willing to accept a slightly higher threshold for cleaner synchrony or phrenic avoidance

Advanced features

  • MultiPoint Pacing (multi-site LV stimulation) where the lead supports it
  • Adaptive CRT — LV-only vs BiV switching by intrinsic conduction
  • LV-RV timing algorithms specific to the manufacturer

Rate response and mode switch

  • Rate response off unless chronotropic incompetence is documented
  • Mode-switch settings calibrated to keep BiV pacing during AT — aggressive switching loses CRT

P — Plan / Programming changes

  • Document each parameter changed with prior value and rationale
  • Re-interrogate after every change — confirm BiV% recovery in real time when possible
  • Reset counters per policy
  • Set next remote and in-office visit
  • Escalate to the EP attending for:
    • BiV pacing <90% with new HF symptoms
    • AF with rapid conduction unresponsive to rate control — AV node ablation discussion
    • Loss of LV capture or rising threshold not solved by vector change
    • New phrenic capture in all available vectors
    • HF decompensation signal on device diagnostics with clinical correlation
    • Any shock on CRT-D — same protocol as the ICD walkthrough
    • Patient symptoms without device explanation — loop in HF team

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

Not medical advice. This page is educational. Your situation may differ — discuss it with Dr. Colombowala or your treating physician before making decisions.