13.4 Pacing & Sensing Malfunctions, Electromagnetic Interference & Temporary Pacing
Key Takeaways
- Failure to capture shows a pacing spike with no depolarization; failure to output shows no spike at all where one was expected.
- Undersensing produces inappropriate pacing spikes that fall too early, while oversensing produces inappropriate pauses with no spike at all.
- A higher programmed sensitivity number means a less sensitive device, so raising the millivolt value corrects oversensing and lowering it corrects undersensing.
- Hyperkalemia, antiarrhythmic drugs, myocardial infarction at the lead tip, and acidosis all raise the capture threshold and can cause acute loss of capture.
- Temporary transvenous pacing output is set at two to three times threshold, with sensitivity set to reliably sense the intrinsic R wave in demand mode.
13.4 Pacing & Sensing Malfunctions, Electromagnetic Interference & Temporary Pacing
CCI lists "Evaluation of temporary and permanent pacing malfunctions", "CIED troubleshooting", and "Temporary pacemaker operation" as three separate knowledge areas. Troubleshooting is a small, closed set of problems, and the fastest route to the answer is a fixed framework.
1. The Four Malfunctions
| Malfunction | ECG appearance | Meaning |
|---|---|---|
| Failure to capture | Spike present, no depolarization follows | The stimulus reached the tissue but did not depolarize it |
| Failure to output | No spike where one was expected | The device did not deliver a stimulus |
| Undersensing | Inappropriate spikes appearing too soon after an intrinsic beat | The device did not see an intrinsic event |
| Oversensing | Inappropriate pauses with no spike | The device saw something that was not an intrinsic cardiac event and inhibited |
The two most reliable discriminators: failure to capture has a spike, failure to output does not; and undersensing adds spikes, oversensing removes them.
Failure to capture — causes
| Category | Examples |
|---|---|
| Lead | Dislodgement, micro-dislodgement, perforation, poor contact |
| Threshold rise | Hyperkalemia, acidosis, antiarrhythmic drugs (class IC especially), myocardial infarction at the lead tip, exit block from fibrosis |
| Device | Output programmed too low, battery depletion, header connection problem |
| Circuit | Conductor fracture with intermittent contact, loose set screw |
Hyperkalemia is the classic acute answer because it raises the resting membrane potential, inactivates sodium channels, and both raises threshold and widens the QRS at the same time.
Failure to output — causes
Complete conductor fracture (open circuit, very high impedance), a loose or disconnected set screw, battery at end of life, oversensing that inhibits output (which is really a sensing problem presenting as no output), or the device being programmed to a rate below the intrinsic rate so no output was ever expected — the last of which is not a malfunction at all.
Undersensing — causes
Sensitivity set too low (numerically too high a millivolt threshold), lead dislodgement, insulation or conductor failure, a low-amplitude intrinsic signal (as in the substrate change of infarction), a signal falling within a refractory or blanking period, or a change in the intrinsic rhythm's morphology such as new bundle branch block or ventricular ectopy.
A specific and testable point: an intrinsic beat that falls inside a programmed refractory period is not undersensing. It is normal, designed behavior, and correcting it means adjusting the refractory period, not the sensitivity.
Oversensing — causes
| Source | Typical signature |
|---|---|
| T-wave oversensing | Counted rate is exactly double the true rate |
| Myopotential oversensing | Pectoral muscle activity or deep breathing; more common with unipolar sensing |
| Far-field R-wave sensing on the atrial channel | Ventricular events counted as atrial events |
| Lead fracture / insulation failure noise | Non-physiologic, very short intervals; impedance abnormality |
| Crosstalk | Ventricular channel senses the atrial output |
| Electromagnetic interference | Correlates with an external source |
2. The Sensitivity Convention
This is the single most frequently inverted concept in device troubleshooting.
Sensitivity is programmed as a voltage threshold in millivolts, and the device only counts signals larger than that threshold.
| Programmed value | Device behavior | Corrects |
|---|---|---|
| Lower number (e.g. 0.5 mV) | More sensitive — detects smaller signals | Undersensing |
| Higher number (e.g. 5.0 mV) | Less sensitive — ignores smaller signals | Oversensing |
So the counterintuitive rule is: to make a device more sensitive, program a smaller number. A patient with T-wave oversensing needs the sensitivity value raised (made numerically larger, so the smaller T wave is ignored); a patient undersensing a small R wave needs it lowered.
Sensing configuration matters too. Bipolar sensing uses the closely spaced tip and ring, producing a small antenna that rejects far-field signals and myopotentials. Unipolar sensing uses the tip against the generator can, producing a large antenna that is far more vulnerable to myopotential and electromagnetic oversensing. Converting a unipolar system to bipolar is a standard fix for myopotential oversensing.
3. Electromagnetic Interference
| Source | Risk | Practical guidance |
|---|---|---|
| Monopolar electrocautery | High — oversensing, inhibition, inappropriate shocks, power-on reset | Bipolar cautery, short bursts, asynchronous programming, therapy suspension |
| MRI | High if the system is not conditional | Conditional systems only, MRI mode, monitored scan |
| External defibrillation | High | Pads ≥ 8 cm from the generator, anteroposterior orientation, interrogate afterward |
| Radiation therapy | Cumulative dose damage and single-event upsets | Dose limits; relocate the generator if it is in the field |
| Radiofrequency ablation | Moderate | Program asynchronously if dependent, suspend tachytherapy, position the dispersive pad away from the device |
| TENS units, electrolysis, electroconvulsive therapy | Moderate | Case-specific evaluation |
| Airport / retail security gates | Low | Walk through at a normal pace; do not linger |
| Cell phones | Low | Keep ≥ 15 cm (6 inches) from the generator; use the opposite ear |
| Consumer devices with strong magnets | Low but real | Keep certain phones, phone cases, wireless chargers, and smart-watch bands ≥ 15 cm from the generator |
| Household appliances, microwaves, induction hobs | Negligible to low | Normal use is safe; keep induction cooktops at arm's length |
| Arc welding, large industrial motors, high-power radio transmitters | High | Occupational assessment required |
4. Temporary Pacing
Temporary pacing appears in the knowledge list explicitly and in task D8 as "emergency pacing."
Modalities
| Modality | Speed | Notes |
|---|---|---|
| Transcutaneous | Fastest | Pads on the chest; painful, requires sedation; verify mechanical capture with a pulse, not just electrical capture on the monitor |
| Transvenous | Minutes | Balloon-tipped or standard wire to the RV apex; reliable and tolerated |
| Epicardial | Post-cardiac-surgery | Wires already in place |
| Through an existing EP catheter | Immediate in the lab | Connect the RV diagnostic catheter to the stimulator or a temporary generator |
Operating a temporary transvenous generator
Three controls, each with a rule:
- Rate — set above the intrinsic rate to achieve consistent pacing, typically 60-80 bpm, or higher for overdrive suppression of a pause-dependent arrhythmia.
- Output (mA) — determine the threshold by decrementing until capture is lost, then set the output at 2 to 3 times threshold. A typical acute ventricular threshold is under 1 mA, so a typical setting is 2-5 mA.
- Sensitivity (mV) — in demand (VVI) mode, set low enough (a smaller number) to sense the intrinsic R wave reliably. Turning the dial fully to asynchronous disables sensing and paces at a fixed rate regardless of intrinsic activity — appropriate only for specific situations such as cautery interference, and hazardous otherwise because of R-on-T risk.
Safety practice: insulate all exposed wire terminals (a glove over the connector block is the classic method) so that stray current cannot reach the endocardium — a microshock hazard at as little as 10-100 µA. Secure the wire, obtain a chest radiograph to confirm position, check thresholds at least daily because they rise over the first days, and monitor continuously.
Complications: RV perforation (thresholds rise, a pericardial rub or effusion appears, and pacing may capture the diaphragm), lead dislodgement with loss of capture, infection at the access site, arrhythmia during placement, and thrombosis. Thresholds that rise steadily over hours to days are expected; an abrupt rise with new diaphragmatic pacing suggests perforation.
A patient with a VVI pacemaker programmed to a sensitivity of 4.0 mV shows intermittent pacing spikes falling shortly after clearly visible intrinsic QRS complexes. Interrogation shows a sensed R wave amplitude of 3.2 mV and a stable lead impedance of 510 ohms. What is the malfunction and the correct programming change?
A dialysis patient with a permanent pacemaker presents with pacing spikes that are not followed by ventricular depolarization, a widened intrinsic QRS, and peaked T waves. Lead impedance and the chest radiograph are unchanged from six months ago. What is the most likely cause?
After placing a temporary transvenous pacing wire in the right ventricle, the specialist finds the capture threshold is 0.8 mA. What output should be programmed, and what additional safety step is required at the connector block?