13.3 Interpreting & Conveying Interrogated Data: Diagnostics, Counters & Stored Episodes
Key Takeaways
- Task B7 is separate from task B6: the specialist must interpret interrogated data and convey it accurately to the operator, not merely retrieve it.
- Marker channels annotate the device's own interpretation, so a marker that disagrees with the electrogram identifies a sensing problem rather than an arrhythmia.
- Atrial high-rate episodes lasting 24 hours or more carry meaningfully elevated stroke risk and should be reported explicitly with their burden.
- Right ventricular pacing above roughly 40 percent is associated with pacing-induced cardiomyopathy and heart failure hospitalization.
- A CRT device delivering less than about 90-95 percent biventricular pacing is under-delivering therapy, and the cause must be identified.
13.3 Interpreting & Conveying Interrogated Data: Diagnostics, Counters & Stored Episodes
CCI splits device work into two consecutive intra-procedural tasks: B6 — Interrogate CIEDs and B7 — Interpret and convey interrogated data from CIEDs. The separation is deliberate. Pulling a report is mechanical; reading it correctly and telling the operator what matters, in the middle of a case, is the skill.
1. Reading a Stored Episode
Every stored episode has three synchronized layers, and interpretation means reading them together.
| Layer | What it is |
|---|---|
| Electrogram (EGM) | The actual recorded signal — near-field (bipolar tip-to-ring, local myocardium), far-field (shock coil to can, a wide "surface-like" view), or atrial |
| Marker channel | The device's annotation of what it thought each signal was: AS/AP (atrial sensed/paced), VS/VP, TS/TF (tachy sensed/fibrillation), and therapy markers |
| Interval plot | Measured cycle lengths, plotted against the programmed detection zones |
The single most useful diagnostic principle: when the marker channel disagrees with the electrogram, the problem is sensing, not rhythm. If the ventricular electrogram shows a regular sinus rhythm at 80 bpm while the markers show alternating VS-VS pairs at intervals implying 160 bpm, the device is double-counting — not observing a tachycardia.
Near-field versus far-field electrograms
- Near-field (bipolar) shows sharp, local, high-frequency signals and is used for timing and rate.
- Far-field (can-to-coil) produces a broad, surface-ECG-like tracing and is used for morphology — including QRS width and the morphology-discrimination algorithms that separate SVT from VT.
Reading both together answers the discrimination question: a wide, distorted far-field morphology different from the stored sinus template supports VT, while a preserved sinus morphology at a fast rate supports SVT with a rapid conducted response.
2. Counters and Diagnostics
| Diagnostic | Interpretation |
|---|---|
| Percentage atrial paced | High values suggest sinus node dysfunction or an aggressively programmed lower rate |
| Percentage ventricular paced | > ~40% RV pacing is associated with pacing-induced cardiomyopathy and heart failure hospitalization |
| Percentage biventricular paced (CRT) | Target ≥ 95%; below ~90% the patient is under-treated |
| Rate histogram | Distribution of heart rates; a histogram crowded at the lower rate limit suggests chronotropic incompetence or an over-aggressive lower rate |
| Mode switch episodes | Count and total duration = atrial fibrillation burden |
| Atrial high-rate episodes (AHRE) | Device-detected atrial tachyarrhythmia, often subclinical |
| Sensor / activity trend | Patient activity over time; a falling trend can precede heart failure decompensation |
| Thoracic impedance | Falls as pulmonary fluid accumulates; used in heart failure alerts |
| Non-sustained VT episodes | Count and cycle length; may precede clinical events |
Atrial high-rate episodes
Device-detected AHRE are frequently the first evidence of atrial fibrillation in a patient with no symptoms, and their duration is what matters:
- Episodes lasting ≥ 24 hours are associated with a meaningfully increased stroke risk and generally prompt consideration of anticoagulation in a patient with elevated CHA₂DS₂-VASc.
- Very brief episodes (< 5-6 minutes) carry much lower risk, and the evidence for anticoagulating them is weak.
- Every AHRE must be confirmed on the electrogram, because far-field R-wave oversensing, atrial lead noise, and repetitive non-reentrant ventriculoatrial synchrony all masquerade as AHRE.
This is exactly the kind of finding task B7 exists for: the number in the summary panel is a hypothesis, and the electrogram is the evidence.
Pacing-induced cardiomyopathy
Chronic right ventricular apical pacing produces an LBBB-like activation pattern with mechanical dyssynchrony. A patient with a new drop in ejection fraction, high RV pacing percentage, and no other explanation should be flagged for consideration of upgrade to biventricular or conduction system pacing. Prevention uses algorithms that minimize unnecessary ventricular pacing by extending the AV delay or switching between AAI and DDD modes.
Why CRT pacing percentage falls
| Cause | Recognition |
|---|---|
| Atrial fibrillation with rapid conduction | Intrinsic conduction outpaces the device; may need rate control or AV junction ablation |
| Frequent premature ventricular complexes | High PVC counts in diagnostics; consider PVC ablation |
| Loss of LV capture | Rising LV threshold or impedance change |
| Phrenic nerve stimulation forcing low output | Documented diaphragmatic stimulation; reprogram the pacing vector |
| Programmed AV delay too long | Intrinsic conduction pre-empts the paced beat |
3. ICD Therapy Episodes
An ICD episode report shows the detected rate, the zone entered, the discrimination decision, and the therapy delivered.
Detection zones are programmed by rate: a VT monitor zone (detect only), a VT zone (usually anti-tachycardia pacing first, then shocks), and a VF zone (shock, sometimes with ATP delivered during charging). Contemporary programming favors higher rate cut-offs and longer detection durations, because many fast rhythms terminate spontaneously and unnecessary shocks are harmful.
Anti-tachycardia pacing (ATP) delivers a burst or ramp of pacing faster than the tachycardia to penetrate the excitable gap and terminate reentrant monomorphic VT painlessly. It is effective in a large majority of monomorphic VT episodes and is attempted first whenever the rhythm permits.
Appropriate versus inappropriate therapy is the central interpretive question:
| Appropriate shock | Inappropriate shock |
|---|---|
| True VT or VF on the electrogram | SVT with rapid conduction (AF, flutter, sinus tachycardia) |
| Rate in zone, morphology distinct from sinus, AV dissociation | T-wave oversensing — alternating large/small deflections, rate exactly double the true rate |
| Lead fracture / noise — non-physiologic, very short intervals; abrupt impedance rise | |
| Diaphragmatic myopotential oversensing — noise with deep breathing or on integrated bipolar leads | |
| Double counting of the R wave |
Discrimination algorithms separate SVT from VT in the VT zone: onset (gradual for sinus tachycardia, abrupt for VT), stability (irregular for AF, regular for VT), morphology compared with a stored sinus template, and, in dual-chamber devices, AV relationship (more V than A events supports VT; more A than V supports atrial flutter or fibrillation). These algorithms are not applied in the VF zone, where the device shocks without discrimination — the deliberate safety trade-off.
Electrical storm is defined as three or more separate ventricular arrhythmia episodes requiring therapy within 24 hours and is a medical emergency requiring antiarrhythmic therapy, sedation, treatment of the trigger, and consideration of urgent ablation.
4. Conveying the Data
Task B7 explicitly includes conveying the information. During an active case the operator needs a short, structured, unambiguous statement rather than a printout. An effective format:
- Device and dependence. "Medtronic dual-chamber ICD, implanted 2021. Patient is pacemaker-dependent, no escape below 30."
- Battery and leads. "Battery normal, estimated 4 years. RV lead impedance 480 ohms and stable, threshold 0.75 volts at 0.4 milliseconds, R wave 9 millivolts. Atrial lead unchanged."
- The finding that changes management. "Mode switch burden is 22 percent, with an atrial high-rate episode of 31 hours last week, confirmed on the electrogram as true atrial fibrillation."
- Therapies. "Two episodes in the past month, both terminated by ATP, no shocks. Morphology consistent with monomorphic VT."
- What has been changed. "Tachytherapy is now suspended and the device is programmed DOO at 70. It must be restored before the patient leaves the room."
Two communication rules apply. Never report a device-generated conclusion as fact without checking the electrogram — the correct phrasing is "the device counted 14 atrial high-rate episodes; I reviewed the electrograms and 11 are true atrial fibrillation and 3 are far-field R-wave oversensing." And always close the loop on reprogramming, because the most common device-related adverse event in a procedural setting is a device left in its procedural configuration.
An ICD episode shows a marker channel counting ventricular events at intervals of 300 ms, entering the VF zone and delivering a shock. The stored ventricular electrogram shows a regular rhythm at 100 beats per minute with alternating tall and small deflections of consistent morphology. What happened?
A CRT-D interrogation shows biventricular pacing at 81 percent, a high premature ventricular complex count, and a stable left ventricular threshold and impedance. What is the significance and the most likely explanation?
A pacemaker interrogation reports 46 atrial high-rate episodes in the past six months, including one lasting 31 hours. The patient has a CHA2DS2-VASc score of 4 and takes no anticoagulant. What is the correct handling of this finding?