EP Staff Education Clinical Staff Ilyas Colombowala, MD, FACC, FHRS →
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Device

ICD Interrogation Walkthrough

Clinic and remote ICD interrogation workflow with emphasis on shock review, detection programming, lead diagnostics, and the high-stakes decisions an ICD interrogation can trigger.

Interrogation walk-through

  • ▸P — Pull the chart: indication (primary vs secondary), model, last visit, any alerts or shocks since.
  • ▸B — Battery voltage, % remaining, RRT/ERI/EOL status, longevity projection (factor in shock count).
  • ▸L — Pacing impedance and high-voltage shock impedance per lead, thresholds, sensing amplitudes.
  • ▸S — Episode log: VT/VF, AT/AF burden, NSVT in monitor zone, AHRE, patient-triggered transmissions.
  • ▸T — Therapy zones, ATP and shock deliveries, EGM-by-EGM appropriateness review, post-shock pacing setup.
  • ▸O — Pacing optimization, % RV pacing minimization, SVT discriminators, detection duration tuning.
  • ▸P — Programming changes, re-interrogation, patient education after any shock, follow-up plan, attending notified.

Common issues / troubleshooting

  • ▸Inappropriate shock for AF with rapid ventricular response
  • ▸T-wave oversensing causing double-counting in the VF zone
  • ▸Lead noise from header connection or fracture mimicking VT
  • ▸Failed ATP with acceleration into VF
  • ▸Phantom shock reports without device-recorded therapy

We work through every interrogation using the PBL-STOP framework — a quick mental checklist so nothing gets missed. On an ICD the T section carries the most weight, because the device’s job is therapy decisions and every one of those decisions deserves an EGM review.

Device overview

An ICD does everything a pacemaker does, plus it monitors for ventricular tachycardia and fibrillation and delivers ATP or shocks when criteria are met. Every clinic visit answers two questions: is the device working, and did the device make any decisions since the last visit — and were they correct?

PBL-STOP walkthrough

P — Presentation / Pre-interrogation review

  • Manufacturer, model, serial, implant date
  • Indication: primary prevention (low EF, ischemic vs non-ischemic) vs secondary prevention (prior VT/VF arrest)
  • Programmed zones at last visit (VT monitor / VT / VF cutoffs)
  • Shock history since implant — count, last shock date
  • Remote alerts since last in-person check (high-voltage impedance, RV lead noise, AHRE)
  • Current antiarrhythmics and beta-blocker dosing
  • Patient-reported symptoms: palpitations, near-syncope, perceived shocks

B — Battery / longevity

  • Voltage compared against model-specific RRT/ERI threshold
  • Percent remaining, projected months of service
  • BOL / MOL / ERI / EOS status
  • Drivers of accelerated depletion specific to ICDs:
    • Frequent shock deliveries (each charge is expensive)
    • Frequent capacitor reforming
    • High RV pacing burden — bad for the patient and the battery
    • High pacing outputs in CRT-D configurations

Devices with multiple recent shocks may have a markedly shorter projected longevity than the prior visit indicated — always recompute after a shock cluster.

L — Lead measurements

Per lead, with trends:

  • RA and RV pacing impedance: 400–1200 ohms
  • RV shock (high-voltage) impedance: 30–80 ohms
    • Change >20 ohms suggests a coil or HV conductor issue
  • Capture thresholds at 0.5 ms
  • Sensing amplitudes:
    • R-wave typically >5 mV
    • Drop >50% from baseline warrants EGM review for undersensing
  • Sudden swing in any of these is more important than the absolute number

S — Stored events / Sensed rhythm

  • Episode log organized by zone and chronology
  • For each episode: stored EGM, intervals, classification
  • AT/AF burden — episode count, total duration, fastest atrial rate
  • AHRE >6 minutes triggers an anticoagulation discussion
  • NSVT in the monitor zone — frequency, fastest rate
  • Pacing percentages and rate histograms
  • Patient-triggered transmissions and surrounding rhythm

T — Therapy

This is the heart of an ICD interrogation. Slow down here.

Detection zones — confirm and rationalize

  • VT monitor zone: counts only, no therapy
  • VT zone: cutoff typically 170–188 bpm, ATP then shocks
  • VF zone: cutoff typically 188–220 bpm, shocks (with ATP during charging on many platforms)
  • Long detection (30/40 intervals or ~12 seconds) reduces both inappropriate shocks and avoidable shocks for self-terminating runs

SVT discriminators — confirm enabled

  • Onset (sudden vs gradual)
  • Stability (regular vs irregular RR)
  • Morphology (template match against stored sinus QRS)
  • A:V relationship in dual-chamber/CRT-D devices

Episode-by-episode appropriateness review

For every therapy delivered since the last visit, walk the EGM:

  • Was the rhythm truly VT or VF?
  • Were the intervals consistent with the classification?
  • For ATP: did it terminate, was it ineffective, or did it accelerate to VF?
  • For shock: did it convert? On the first shock or after redetect?
  • Was sensing clean — no T-wave oversensing, no double-counting, no lead noise masquerading as VF?
  • For inappropriate shocks, classify the cause:
    • AF with rapid conduction
    • Sinus tachycardia tracking through discriminators
    • SVT (AVNRT, AVRT, AT)
    • Lead noise or fracture
    • T-wave oversensing
    • EMI

Post-shock pacing

  • Confirm a post-shock pacing rate (often 90 bpm) is programmed to bridge bradyasystole during stunned myocardium
  • Confirm post-shock outputs are set with adequate safety margin — captures matter more than battery here
  • Duration of post-shock pacing (often 30 seconds to several minutes)

Manual testing as indicated

  • Capture threshold at 0.5 ms
  • Sensing test in VVI
  • Morphology template re-acquisition if the QRS has changed since implant
  • DFT testing is rarely repeated in clinic — flag to the attending if indicated

O — Optimization

  • Minimize RV pacing in primary-prevention ICDs — this device is for shocks, not pacing
  • AV delay set to favor intrinsic conduction
  • Mode switch sensitivity calibrated to real AT, not far-field oversensing
  • Rate response off unless chronotropic incompetence is documented
  • Detection duration lengthened where appropriate to reduce avoidable shocks
  • SVT discriminators tightened or loosened based on the appropriateness review

P — Plan / Programming changes

  • Document every change with the prior value and the rationale
  • Re-interrogate after the change
  • Reset counters per institutional policy
  • Schedule next remote transmission and in-office visit
  • Patient education after any shock — appropriate or not. Confirm action plan: when to call, when to go to the ED, what one shock vs multiple shocks means.
  • Escalate to the EP attending for:
    • Any shock (same-day or next-day call)
    • Inappropriate detection from oversensing
    • HV impedance alert or lead fracture pattern
    • VT cluster (≥3 episodes in 24 hours) — VT storm protocol
    • Failed shock — vector, position, energy review
    • Phantom shock with no device-recorded therapy — exclude noise first

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.