A 65-year-old man has a 12-lead ECG showing fixed heart rate of 72 bpm with paced spikes preceding each QRS complex. On Holter monitoring, native P waves are present but not sensed by the device, and there are competitive paced beats. Which pacemaker malfunction is described?
- A Failure to sense (undersensing) ✓
- B Failure to capture
- C Failure to pace (pacemaker inhibition)
- D Pacemaker-mediated tachycardia
Correct answer: A. Failure to sense (undersensing)
Explanation
Undersensing (failure to sense) occurs when the pacemaker fails to detect native cardiac activity (P waves or QRS complexes), resulting in competitive pacing — pacemaker stimuli fire regardless of intrinsic beats. This can occur if sensing threshold is set too high or if lead dislodgement has occurred. Failure to capture means pacemaker spikes are present but no cardiac depolarisation follows. Failure to pace means no spikes are delivered. Pacemaker-mediated tachycardia (PMT) is a re-entry tachycardia involving the pacemaker as the retrograde limb.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.