A 78-year-old man with complete heart block (CHB) who had a permanent pacemaker implanted 3 years ago presents with pre-syncope. Pacemaker check shows it is programmed VVI at 60 ppm. His intrinsic rate is 38 bpm. ECG shows pacing spikes not consistently followed by QRS complexes. What is the most likely cause?
- A Pacemaker failure to capture (exit block) due to elevated pacing threshold ✓
- B Failure to sense (undersensing) — the device fails to detect intrinsic rhythm and delivers competitive pacing
- C Lead fracture causing failure to pace (no output)
- D Pacemaker syndrome due to AV dyssynchrony in VVI pacing
Explanation
When pacing spikes are present but are not consistently followed by myocardial depolarisation (QRS complex), this represents failure to capture (exit block). The pacing impulse is delivered at the programmed rate and voltage, but the myocardium does not respond — usually because the stimulation threshold has risen above the output energy (pacing threshold increases with fibrosis at the electrode tip, electrolyte disturbances particularly hyperkalaemia, certain drugs, or lead dislodgement). Failure to sense (undersensing) presents as inappropriate pacing during intrinsic rhythm (competitive pacing) — pacing spikes would appear at a different inappropriate time relative to intrinsic QRS. Lead fracture causes intermittent or absent output (no spikes). Pacemaker syndrome relates to haemodynamic deterioration from loss of AV synchrony, not to capture failure.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.