A 72-year-old man is found to have a permanent pacemaker (DDD) implanted 3 years ago for complete heart block. He now presents with syncope. ECG shows a rate of 38 bpm with ventricular pacing spikes not followed by QRS complexes. The non-paced native rate is absent. What is the most likely pacemaker malfunction?
- A Failure to sense (undersensing)
- B Pacemaker-mediated tachycardia
- C Lead fracture causing oversensing
- D Failure to pace (loss of capture) ✓
Explanation
Failure to capture (loss of capture) is defined by the presence of pacemaker stimulation spikes on ECG that are NOT followed by a P wave (atrial) or QRS complex (ventricular). The pacemaker fires appropriately but the stimulus fails to depolarize the myocardium. Causes include lead dislodgement, lead fracture, elevated pacing threshold (from fibrosis, electrolyte abnormalities, antiarrhythmic drugs), or battery depletion. Failure to sense (undersensing) results in inappropriate pacing at fixed intervals regardless of native beats. The distinction is critical: sensing failure gives inappropriate output; capture failure gives output without effective depolarization.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.