A 72-year-old woman with a permanent pacemaker reports intermittent palpitations and lightheadedness after a recent generator replacement.
The pacemaker was originally implanted for symptomatic sinus node dysfunction.
She has no fever, pocket pain, or recent myocardial ischemic symptoms.
Her pulse varies between 58/min and 92/min, and her blood pressure is 126/70 mm Hg.
Cardiac examination shows occasional irregular beats without heart failure.
A rhythm strip recorded during symptoms is shown below.
Pacing spikes occur despite clearly visible spontaneous QRS complexes and sometimes fall within the ST segment or T wave.
Most pacing spikes are capable of producing depolarization when delivered outside the refractory period.
Which of the following is the most likely pacemaker malfunction?
Pacing artifacts occurring despite intrinsic QRS complexes indicate ventricular undersensing, or failure to sense intrinsic activity. Spikes may therefore fall during the ST segment or T wave. Preserved capture when pacing occurs outside the refractory period argues against failure to capture. Battery depletion would reduce output or alter pacing rate rather than generate inappropriately timed spikes. Appropriate rate response would coordinate pacing with demand. Oversensing causes inappropriate inhibition and absent pacing spikes, the opposite pattern.
