ECG rhythms Pacing
Ventricular pacing with loss of capture ECG
Intermittent loss of capture. Pacing spikes arrive on time, but some are not followed by a paced QRS; slow narrow escape beats fill the gaps.
A 10-second recording of the heart model, played in real time. Run live starts the engine itself with this rhythm.
Computed by the TracingsMD heart model (Junkers engine): simulated for education, not a patient recording and not for diagnosis.
Measured on this recording
| Ventricular rate | 52/min |
|---|---|
| PR | Not assessable in this rhythm |
| QRS | 84 ms; wide beats 156 ms |
| QT | Not assessable in this rhythm |
| QTc (Bazett) | Not assessable in this rhythm |
| QTc (Fridericia) | Not assessable in this rhythm |
What it is
The pacemaker fires — a pacing spike appears — but the ventricle does not depolarize: some spikes are not followed by a paced QRS, and the heart depends on its own rhythm in those gaps. This recording shows fixed-rate (asynchronous) ventricular pacing, as with transcutaneous pacing.
How to read this strip
| Rate | Slower than the set pacing rate. |
|---|---|
| Rhythm | Irregular: paced beats mixed with escape beats. A fixed-rate spike that lands just after an escape beat falls in the refractory period and cannot capture; spikes that fail at other times are true loss of capture. |
| P waves | None seen. |
| PR | None. |
| QRS | Paced beats wide (≥120 ms); escape beats narrow. |
| QT | Not the focus here. |
Causes
- Transcutaneous pacing: current (mA) too low, or poor pad position or skin contact.
- Temporary or implanted leads: dislodgement or fracture, a loose connection, or a migrated temporary wire.
- A rise in the capture threshold: hyperkalemia, acidosis, ischemia or infarction at the lead tip, some antiarrhythmic drugs (for example flecainide), fibrosis early after implant.
- Battery depletion in an implanted device.
What to consider
- With transcutaneous pacing: increase the current until every spike outside the refractory period captures, check pad position and contact, and confirm a pulse at the paced rate (a femoral pulse is less confused by muscle twitching).
- With a transvenous or implanted system: check connections, potassium and drugs, obtain a chest X-ray for lead position and urgent device interrogation; prepare transcutaneous pacing if capture is not restored.
- Tell true non-capture from functional non-capture: a spike that falls in refractory tissue just after a native beat cannot capture. This happens with fixed-rate pacing, as here, or when a demand pacemaker undersenses.
- A magnet switches a pacemaker to asynchronous fixed-rate pacing; it overrides oversensing but does not fix loss of capture.
Key point
A spike shows output, a wide QRS after it shows electrical capture, and only a pulse at the paced rate shows mechanical capture. Fixed-rate pacing competes with the patient's own beats and can fire on a T wave; use demand mode when the device offers it.
Not to be confused with
- Ventricular paced rhythm with consistent capture — Several spikes are not followed by a wide QRS.
- Failure to pace (no output) — Spikes are present at the set interval, so the pacer is firing.
- Ventricular tachycardia — The rate is slow and the wide complexes follow pacing spikes.