| Scenario |
What the Device “Sees” |
Why It Withholds the Pulse |
| Normal demand-pacing behavior (VVI/VVIR mode) |
A native ventricular depolarization (R-wave) arrives before the lower-rate interval expires. |
The pacemaker is a ventricular-demand device; each sensed intrinsic beat restarts the timing circuit, so no stimulus is delivered until the next cycle length elapses. |
| Magnet placement or programmer command that sets pacing OFF |
Continuous magnetic field or explicit OFF programming. |
Magnet mode or programmer “ODO” disables output for troubleshooting or surgery. |
| Electromagnetic interference (EMI) |
Large, rapid voltage fluctuations on the sensing circuit that mimic heart signals (noise). |
The device interprets EMI as cardiac activity and inhibits until the noise stops. |
Physiological or mechanical oversensing
- Diaphragmatic myopotentials (deep breathing, coughing)
- Large T-waves or far-field atrial signals
- Helix micromotion with posture change
|
Artefacts exceed the programmed sensing threshold. |
Oversensing falsely resets the timer, producing inappropriate inhibition (e.g., pacing stops when lying prone and returns when supine). |
| Battery end-of-service programming or safety modes |
Internal battery voltage below the Recommended-Replacement-Time (RRT) threshold or firmware-initiated safe mode. |
The device may switch to low-output or asynchronous back-up modes; in some states it can inhibit until reprogrammed or replaced. |
| Noise-reversion algorithm (prolonged noise > 100 ms) |
Continuous noise on the sensing channel. |
Until the noise time-out expires, the pacemaker inhibits; afterward it may revert to asynchronous pacing (VOO) to guarantee output. |