​Standard Cardiac Implantable Electronic Device Interrogation (Pacemaker / ICD / CRT)

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1) Patient & visit context

Indication for device (e.g., SSS/AVB, primary/secondary prevention ICD, CRT for HFrEF).

 

Reason for today’s interrogation (routine control / symptoms / post-procedure / shock / pre-op / advisory).

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Symptoms since last check (syncope/presyncope, palpitations, dyspnea, chest pain, shocks, HF symptoms).

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Current rhythm on presentation (sinus/AF/flutter/VT, paced rhythm) + heart rate.

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2) Device identification

Device type (Permanent pacemaker / ICD / CRT-P / CRT-D), manufacturer, model.

 

Implant date (and lead implant dates if different).

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System configuration (A/V/LV leads present; single/dual/biV; lead models if available).

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3) Programmed settings

Bradycardia settings

Mode (e.g., DDD/DDDR/VVI, etc.), lower rate limit, upper tracking/sensor rate.

 

Lower Rate Limit (LRL)

This is the minimum heart rate the pacemaker allows. If the device does not sense enough intrinsic beats, it will pace to keep the rate at or above this number.

Upper Tracking Rate (UTR)

Applies mainly to dual-chamber tracking modes like DDD/DDDR. It’s the fastest ventricular rate the pacemaker will produce by tracking the atrium.

If the atrium is beating faster than the UTR (e.g., during exercise), the pacemaker won’t pace the ventricle faster than this limit; it will start “dropping” some atrial events (e.g., Wenckebach-like behavior) to keep the ventricle at or below the UTR.

Upper Sensor Rate (USR)

Applies when rate response is ON (DDDR/VVIR/AAIR). It’s the maximum rate the pacemaker will pace based on sensor input (motion/accelerometer, minute ventilation, etc.).

 

AV delays (paced/sensed; dynamic if applicable), rate response on/off and sensor type.

 

AV delays refer to the time the pacemaker waits between an atrial event and a ventricular pace in a dual-chamber mode. It’s the device’s programmed version of the heart’s PR interval.

“Sensed AV” vs “Paced AV”

They’re separated because the timing and physiology differ depending on how the atrial event happened:

• Sensed AV delay (SAV): used after a native P-wave is sensed.
  It’s often programmed shorter because the atrium has already begun depolarizing before the device detected it.

• Paced AV delay (PAV): used after the device paces the atrium.
  It’s often programmed slightly longer because the atrial depolarization begins right at the pacing stimulus and the device wants to mimic normal conduction timing.

“Dynamic AV delay” or “rate-adaptive AV delay”

“Dynamic” means the AV delay changes automatically based on conditions, most commonly heart rate:

At higher rates, the device shortens the AV delay to mimic normal physiology (PR interval shortens with faster rates) and to maintain good timing.

At lower rates, it may allow a longer AV delay.

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Mode switch settings (on/off, detection criteria; AMS (automatic mode switch) rate).

 

AMS rate = when the device detects atrial rates above the AMS criteria, it mode switches to a non-tracking mode (e.g., DDI/DDIR or VVI, depending on programming) to prevent rapid ventricular pacing.

 

Tachycardia settings (ICD/CRT-D)

Detection zones (VT/VF rates), detection duration/counters.

 

In an ICD/CRT-D, “detection zones” and “detection duration/counters” describe how the device decides a fast rhythm is VT or VF before it delivers therapy.

Detection zones (VT/VF rates)

A zone is a programmed heart-rate range (usually expressed as bpm or cycle length in ms) that tells the ICD how to classify a tachycardia by speed.

An example setup:

• VT zone: 170–200 bpm

• Fast VT zone: 200–230 bpm

• VF zone: >230 bpm
  Different zones can have different therapies (ATP vs shock), and different discrimination rules (more SVT/VT discrimination in VT zones; less in VF zones to avoid missing VF).

Detection duration/counters

The ICD doesn’t treat just because it sees two fast beats. It requires the rhythm to be fast long enough to be confident it’s real and sustained. That “long enough” is defined by detection criteria, usually as counters and/or time.

NID (Number of Intervals to Detect): e.g., “18/24” or “30/40”
Meaning: out of the last 24 (or 40) beats, at least 18 (or 30) must fall in that zone before the device declares VT/VF.

Duration (seconds): e.g., “VT must persist for 2.5 s”
Some devices express this as time, or combine time + counters.

Redetection counters: after a therapy, how quickly the device re-confirms ongoing VT/VF before giving another therapy.

Reconfirmation counters: checks just before shocking to ensure the rhythm is still present (avoids shocking after spontaneous termination).

Longer detection (more beats/seconds) reduces inappropriate therapy for brief SVT bursts or noise, and allows more self-termination—but may delay treatment of true VT/VF. Shorter detection treats faster but can increase inappropriate therapies.

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Therapies per zone: ATP schemes, shock energies, discriminators (SVT/VT).

 

4) Battery / generator status

Battery status: voltage (pacemakers report terminal (loaded) battery voltage) and estimated remaining longevity (months/years).

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“Estimated remaining longevity” is a model-based estimate that can change with:

• reprogramming outputs,

• pacing percentage,

• algorithm/diagnostic use,

• and manufacturer-specific estimation methods.

 

Voltage gives you a raw, comparable data point you can trend visit-to-visit, which is often more informative than a single “years remaining.”

As a device approaches end of life, the risk of rapid falls in battery voltage can become less predictable, and expert consensus highlights the need for more frequent monitoring in that phase. The battery’s “voltage vs time” curve can become steep, so the voltage can drop faster than it did earlier.

These possible rapid falls in voltage are because the voltage you see is “voltage under load” (aka terminal voltage) and these load conditions get worse near end-of-life. This “voltage under load” is the “true output” voltage (=terminal voltage) of the battery explained by equation: Terminal voltage = Open-circuit voltage − (Current × Internal resistance)

Open-circuit voltage: the battery’s voltage when nothing is drawing power (no load). Think “resting” voltage.

Internal resistance: the battery’s built-in opposition to current flow (from its chemistry/materials).

Current: how much current the device is drawing at that moment.

Terminal voltage: the voltage actually available at the battery’s terminals while delivering that current.

The above equation is describing that the voltage you “get” from a battery while it’s working is lower than the battery’s ideal chemical voltage, because some voltage is “used up” pushing current through the battery’s own internal resistance.

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The load conditions get worse because the battery’s internal resistance rises non-linearly as it gets older. Internal battery impedance (=battery's internal resistance) rises for a few related reasons. In classic lithium–iodine pacemaker batteries, the normal discharge reaction creates a lithium iodide layer between the anode and cathode. As that layer gradually thickens, it becomes a larger barrier to ion movement, so internal resistance increases over time.

In addition, as the battery’s active materials are progressively consumed, the cell becomes less able to deliver current without an internal voltage drop (polarization): ion transport and charge transfer become less efficient, which functionally behaves like higher internal resistance, particularly when the device is drawing current.

Some ICD/CRT-D batteries use different chemistries such as silver vanadium oxide, where manufacturers describe a characteristic mid-life rise in internal impedance that can lengthen capacitor charge times, sometimes followed by partial recovery before impedance rises again closer to end of life.

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As an example, a pacemaker battery starts at roughly 2.8 V and gradually declines over years as it supplies energy to the device. When the battery voltage reaches a manufacturer-defined alert point (often around 2.6–2.7 V), the device indicates ERI/RRT (elective/recommended replacement), prompting generator replacement planning. This alert threshold is not set because the myocardium “needs” 2.6–2.7 V to pace. The pacing amplitude you program (for example 1–2 V at a specified pulse width) is the delivered stimulus at the lead, and pacing output is not simply the battery voltage applied directly to the heart. Pacemakers create pacing pulses using internal output circuitry (typically involving charging and discharging capacitors), with the battery serving as the upstream power source. The device must maintain enough electrical “headroom” to run its electronics and reliably generate the programmed pulse across real-world conditions such as changing lead impedance and varying current demands. As batteries age, internal resistance rises, making voltage more prone to “sag” under load; therefore, the low-battery alert is deliberately conservative to ensure stable operation and reliable pacing even under worst-case demand, and to provide a safe time window to schedule replacement before end-of-service.

 

If the battery terminal voltage falls below what the device’s electronics and output circuitry can reliably run on, the pacemaker doesn’t have a “graceful” way to keep behaving normally. It generally doesn’t just switch cleanly “off,” but it can drop into progressively more limited, pre-defined behaviors and, at the extreme, pacing output can become unreliable or absent.

 

ERI/RRT threshold (or “not at ERI”) + battery impedance (if available).

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ERI = Elective Replacement Indicator: the device has detected that the battery is approaching depletion and elective generator replacement should be planned.

RRT = Recommended Replacement Time: essentially the same concept; terminology varies by manufacturer (some show RRT, some ERI, some both/combined).

Battery impedance is a measure of the battery’s internal resistance as estimated by the device. As batteries deplete and age, impedance generally rises, and together with voltage it helps the device (and you) judge battery status and remaining service life.

In pacemakers, the power demand is low, so they can often operate even with very high internal resistance. In ICDs, impedance matters more because charging the shock capacitor needs high current; impedance rises can show up as longer charge times.

 

ICD/CRT-D only: capacitor charge time / last full-energy charge time (and trend if available).

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A key check that an ICD/CRT-D can deliver a shock promptly. It’s also a useful “summary” marker of battery condition and high-voltage system integrity, and it’s included in standard ICD follow-up content in HRS/EHRA consensus documents.

Capacitor charge time can change with time, there is gradual increase as the battery ages; some device chemistries show a temporary mid-life rise that can later improve. Concerning is a sudden jump or progressive prolongation that doesn’t match the battery stage, which can suggest battery/capacitor/HV circuit issues.

 

5) Leads & system integrity (include values and trends when possible)

For each lead (RA / RV / LV) and configuration (uni/bipolar):
Impedance (include trend).

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Lead impedance (Ohms, Ω) is the device’s estimate of the electrical resistance of the lead conductor (generator → lead conductor → electrode–tissue interface → return path). It’s mainly used as a lead integrity check.

Typical “normal” values:

Most transvenous brady pacing leads: commonly ~400–1200 Ω (always check the lead’s specified limits).

Active-fixation pacing leads: often ~300–1200 Ω

Passive-fixation pacing leads: can be higher, ~450–1800 Ω

What’s concerning:

Abrupt rise (especially a step change) or very high values (often >2000 Ω). This should lead to considering conductor fracture, header/connection issue, lead dislodgement/poor contact.

Abrupt fall or very low values (often <200 Ω) may be because of insulation breach or a short circuit.

Many device alert systems use <200 or >2000 Ω as conventional “out-of-range” impedance alerts.

Unipolar vs bipolar

Unipolar and bipolar describe how a pacemaker forms the electrical circuit it uses for pacing and sensing. In a bipolar configuration, both electrodes are located on the lead itself: the pacing impulse travels from the tip electrode to nearby heart tissue and returns to the ring electrode a few millimeters away. Because this circuit is small and confined to the heart, bipolar sensing is less susceptible to unwanted signals (oversensing/noise), producing cleaner sensing with less pickup of skeletal muscle activity or external electromagnetic interference. Bipolar configuration causes no/smaller pacing artifact on surface ECG.

In a unipolar configuration, the impulse still leaves the tip electrode, but it returns to the metal case of the pacemaker in the chest. This creates a much larger circuit through body tissues, which acts like a bigger antenna: unipolar systems are more prone to oversensing myopotentials or interference and typically produce a larger pacing spike on the surface ECG.

Bipolar = tip-to-ring (small loop, less noise, standard)

Unipolar = tip-to-can (large loop, more noise, bigger ECG spike, useful as a backup or for troubleshooting)

 

Sensing amplitude (P-wave / R-wave; note undersensing/oversensing concerns).

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Sensing amplitude (millivolts, mV) is the size of the intrinsic intracardiac electrogram that the lead records:

• Atrial lead: P-wave amplitude (atrial depolarization)

• Ventricular lead: R-wave amplitude (ventricular depolarization)

This is important because the device must reliably detect (sense) native beats to avoid undersensing (missing beats leads to inappropriate pacing or missed arrhythmia detection), or oversensing (counting noise/T-waves as beats which leads to inappropriate inhibition or inappropriate ICD therapy).

“Normal” values

European (EHRA) implantation targets:

• Aim for atrial sensing ≥1.5 mV

• Aim for ventricular sensing ≥4 mV

Typical ranges in practice

• Atrial signals often 1.5–5 mV

• Ventricular signals often 5–25 mV
   

Very low atrial sensing can drive mode-switch/AF detection issues; very low ventricular sensing can be dangerous in ICDs (risk of VF undersensing).

What’s concerning

A sudden drop from baseline (even if still “above minimum”) can be an early clue to lead micro-dislodgement, fracture, insulation issues, perforation, or myocardial changes.

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Capture threshold (V @ ms) and whether stable vs prior.

 

Capture threshold (=pacing threshold)

The capture threshold is the minimum pacing output that consistently depolarizes the myocardium (“captures” the chamber). It’s recorded as:

Voltage (V) @ pulse width (ms), e.g., 0.75 V @ 0.4 ms. This is the actual delivered energy to the myocardium (given lead impedance stays constant).

It’s the key determinant of:

• safety margin (reliable capture), and

• battery longevity (higher thresholds drain battery faster).

“Normal” values

At implantation (acute targets for standard transvenous RA/RV PPM leads)

• A commonly cited target is ≤1.5 V @ 0.5 ms for transvenous leads. Often the target threshold is well below this (around 0.5–1.0 V @ 0.4–0.5 ms).

Chronic follow-up (what’s commonly seen in clinic for standard PPM RA/RV leads)

• Often around ~0.5–1.0 V @ 0.4–0.5 ms (atrial and ventricular), with RV values frequently in the lower end of that range in well-functioning systems.

LV/CRT lead practical benchmark (commonly used in trials and practice)

• LV (coronary sinus) leads more often have higher thresholds; ≤2.5 V @ 0.5 ms is commonly used as an “acceptable” criterion in trials and clinical discussions, and higher values may still be clinically workable depending on phrenic stimulation and safety margin.

What’s concerning

Acute high threshold right after implant can suggest suboptimal contact; but it may improve after a short wait/retest.

Rising threshold later (especially if abrupt) raises concern for lead dislodgement, micro-perforation, exit block/scar, medications/electrolytes, or lead failure. It also forces higher programmed outputs which increases battery drain.

 

ICD only: HV/shock impedance (and any lead integrity alerts/noise episodes).

 

This is impedance of the high-voltage shocking circuit (coils), not the pacing tip-ring circuit.

Common alert thresholds: <20 Ω or >125 Ω (device/brand dependent).

 

Any alerts (lead integrity, impedance out of range, noise, insulation/conductor concerns).

 

6) Pacing dependency / underlying rhythm

Underlying rhythm assessment (how tested) and pacemaker dependence (yes/no/uncertain).

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Pacemaker dependence means that if pacing stops (or is set well below the intrinsic rate), does the patient reliably generate an adequate native rhythm without dangerous pauses or severe symptoms.

In practice you combine interrogation data + a supervised intrinsic rhythm test + clinical context.

A patient is pacemaker-dependent if loss of pacing would likely cause clinically significant bradycardia/asystole with symptoms or hemodynamic compromise.

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There isn’t a single universal cutoff, but many centers use a pragmatic standard like:

• No intrinsic rhythm (or very slow/unreliable escape, often <30–40 bpm), or

• asystolic pauses when pacing is withheld.
   

How to determine pacemaker dependence

From interrogation look at:

• Pacing percentages that match the indication: atrial pacing % is more informative in sinus node dysfunction (SSS), while ventricular pacing % is more informative in AV conduction disease. Very high pacing percentages can suggest dependence, but they are also strongly influenced by device programming, so they should be confirmed with an underlying rhythm test.

• Rate histograms / intrinsic rate distribution.

• Evidence of underlying AV block (e.g., episodes of ventricular pacing at low atrial rates).

• Any “no intrinsic conduction” flags / AV conduction check results.
  Do an intrinsic rhythm test

Safest common method:
Lower the Lower Rate Limit (LRL) stepwise while on continuous ECG/telemetry.

Example protocol:

• Reduce LRL: 60 → 50 → 40 bpm (sometimes 30 bpm).

• Pause at each step long enough to see whether intrinsic beats appear.

• Observe:

  • Is there a native ventricular rhythm?

  • What is the lowest intrinsic rate?

  • Are there pauses?

  • Is there AV conduction?

Interpretation:

• Not dependent: intrinsic rhythm appears promptly and is stable at a reasonable rate (e.g., sinus/escape >40–50 bpm) with no long pauses and the patient tolerates it.

• Dependent: no intrinsic rhythm emerges, or only a very slow/unreliable escape rhythm appears (often <30–40 bpm) and/or the patient becomes symptomatic or hypotensive.

• Uncertain: intrinsic rhythm appears intermittently, conduction is inconsistent (e.g., intermittent high-grade AV block).

 

Another common method (higher risk):

A brief pacing inhibition / “pause” test can be more definitive. This should only be done in a controlled setting (especially if dependence is likely). Use the shortest pause needed and abort immediately if there’s no escape rhythm (basically means prolonged asystole).

Examples on how to document

“Pacemaker dependence: NO.” Underlying rhythm present: sinus bradycardia 42 bpm with intact AV conduction when LRL decreased to 40 bpm; patient asymptomatic.
“Pacemaker dependence: YES.” No intrinsic ventricular activity observed during stepwise LRL reduction to 30 bpm (brief pause test performed under monitoring); pacing required to maintain perfusing rhythm.

“Pacemaker dependence: UNCERTAIN.” Intermittent intrinsic conduction; history of high-grade AV block—treat as potentially dependent; conservative programming and follow-up.

 

If tested: lowest intrinsic rate observed and any AV conduction noted.

 

7) Percent pacing and key performance metrics

Atrial pacing %, ventricular pacing % (and atrial sensing % / ventricular sensing % if reported).

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CRT devices: % biventricular or LV pacing.

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Practical target used in most clinics:

Aim for the highest achievable BiV% — typically ≥98% (ideally as close to 100% as possible).
This is explicitly recommended in the HRS/EHRA/APHRS/SOLAECE ICD programming consensus: “preferably >98%” to improve survival and reduce HF hospitalization.

Minimum targets (especially highlighted in AF)

The 2021 ESC Pacing/CRT guideline uses <90–95% as “incomplete” BiV capture (often due to conducted AF) and recommends adding AV junction ablation when effective BiV pacing cannot be achieved.

 

8) Arrhythmia diagnostics & stored episodes (report burden, not just counts)

AT/AF: mode switch episodes plus AF/AT burden (%, total time), longest episode, date of most recent.

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VT/VF: number of episodes, most recent date/time, durations/cycle length.

 

Therapies delivered: ATP/shocks—appropriate vs inappropriate + EGMs reviewed (yes/no).

 

Any non-sustained arrhythmias of clinical relevance.

 

9) Sensing and output programming (with safety margins)

Sensitivity settings / sensing threshold (atrial/ventricular; note if unusually high/low vs typical and why).

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Sensitivity is the programmed voltage threshold for sensing/minimum signal size (mV) the device will count as a real electrical event on that channel outside blanking/refractory periods ​the device will count as a real electrical event on that channel. If the signal is bigger than the sensitivity setting it will be sensed, if it’s smaller it will be disregarded.

Lower mV means the pacemaker is MORE sensitive as the device will count smaller signals as real events.

If the pacemaker is programmed too sensitive (mV set too low) it increases the risk of oversensing: The device may mistake noise / muscle signals / far-field signals / T-waves for heartbeats. This may lead to pacemaker inhibiting pacing and causing pauses/symptoms in dependent patients. In ICDs it can contribute to inappropriate detection/therapy.

If the pacemaker is not programmed sensitive enough (mV set too high) it increases the risk of undersensing. This might lead the pacemaker to pace when it shouldn’t, or fail to mode switch accurately. In ICDs the worst-case risk is missing VF/VT signals, especially because VF signals can be low amplitude.

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There isn’t one universal normal sensitivity treshold because manufacturers use different sensing/filtering and ICDs often use dynamic sensitivity.  The recommended approach is to program sensitivity with a safety margin: set it to roughly less than half of the smallest reliably sensed intrinsic P-wave/R-wave (about a 2:1 sensing margin), then confirm you’re not oversensing.

In practice, common starting points (manufacturer-nominal, bipolar) are around 0.4–0.6 mV in the atrium and 1.5–2.5 mV in the ventricle, then adjusted based on the patient’s signals and any oversensing/undersensing. Ventricular sensitivity in ICDs is often much more sensitive, commonly ~0.3–0.6 mV, because it must reliably sense low-amplitude VF

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Output settings (amplitude and pulse width for each chamber) and capture management/autocapture on/off.

 

Pacing output is the stimulus the device delivers to depolarize the chamber (“capture”), and it’s specified as an amplitude (V) and pulse width (ms), for example 2.5 V @ 0.4 ms. For a given (constant) lead impedance, the energy delivered per pulse—and therefore the battery cost—increases in proportion to this programmed output.

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Higher output improves safety margin but costs battery. If capture is marginal at a low voltage, increasing pulse width sometimes achieves capture without needing very high voltage.
 

Higher voltage has downsides compared with solving “too low output” problem by increasing pulse width. The main issues are battery life, device electronics efficiency, and side effects like extracardiac stimulation. Battery drain rises fast with voltage.

With typical constant-voltage pacing, energy delivered per pulse scales roughly like:

 

 

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So if you double voltage, energy goes up about 4× (for the same impedance and pulse width). That can shorten generator longevity much more than a modest increase in pulse width.

Also high amplitudes can be disproportionately inefficient. Many generators use internal circuits (often described as voltage “doublers/triplers”) to produce higher outputs than the battery’s nominal voltage. When those stages are engaged, the device may draw significantly more current than you’d expect, so “turning up voltage” can have an outsized effect on longevity.

Higher voltage increases the risk of stimulating non-cardiac tissue. This matters especially with LV leads (CRT) and sometimes RV/RA leads. Higher output can capture the phrenic nerve and cause diaphragmatic twitching/hiccups.

For many leads, a small increase in pulse width (e.g., 0.4 → 0.6 ms) can restore capture with a smaller battery drain penalty than a large voltage jump.

 

How outputs are chosen

Clinically, the goal is reliable capture with a safety margin above the measured capture threshold. A common approach is ~2× the measured threshold (for amplitude at a given pulse width), but the exact rule varies. If the patient is pacemaker-dependent, you keep a more conservative margin.

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Capture management / autocapture means that an algorithm in the device tests the capture threshold periodically (or beat-to-beat in some systems) and then automatically sets output to the lowest safe value (plus a margin).

 

10) Summary, changes, and plan

Overall impression: normal function / abnormal findings (battery nearing ERI, lead issue suspected, inadequate sensing/capture, high RV pacing, low BiV%, recurrent AF/VT, etc.).

 

Changes made today (exact programming changes) + rationale.

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Follow-up plan: remote monitoring on/off, next remote transmission, next in-clinic check.

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Safety notes (e.g., pacemaker-dependent—avoid prolonged pacing suspension; ICD therapies temporarily disabled/enabled and confirmed).

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