Irregular pulse in a pt with a cardiac resynchronization def
The present case appears in the recent issue of the journal Circulation.

An 83-year-old man presented to the emergency department with a 2-day history of palpitation, which he described as the intermittent sensation of his pulse feeling irregular. He did not complain of syncope, dyspnea, or chest pain. Past medical history was notable for myocardial infarction and heart failure, and a cardiac resynchronization defibrillator had been implanted >10 years previously. Further details were not immediately available.

On examination, the pulse was irregular at ≈100 beats per minute, blood pressure was 122/76 mm Hg, and other vital signs were normal. The device was palpable in the left prepectoral region, with no evidence of pocket infection. Cardiovascular, respiratory, and neurological examinations were otherwise unremarkable.

The 12-lead ECG recorded on admission is shown (Fig 1). What does this ECG show? What are the possible explanations for this phenomenon?

RESPONSE TO ECG CHALLENGE
This ECG contains no P waves, consistent with atrial fibrillation). Ventricular pacing stimuli are present at regular intervals of 920 ms (Figure 2, arrows) and result in myocardial depolarization, evidenced by the QRS complex immediately after each stimulus. These complexes do not exhibit the left-bundle-branch block morphology seen with right ventricular pacing; instead the small R wave in V1 and positive R wave in aVR suggest the merging wavefronts typical of biventricular pacing.

There are several intrinsic premature ventricular complexes in an alternating pattern (Figure 2, asterisks), each occurring at a constant interval from the previous biventricular paced beat. These complexes have right-bundle-branch block morphology, a predominantly superior axis, and positive deflections in the chest leads, consistent with origin near the inferior left ventricular base.

The most striking feature of this trace is that pacing stimuli occur at a fixed interval irrespective of premature ventricular complexes, indicating apparent undersensing. In summary, this ECG shows atrial fibrillation, biventricular pacing with normal capture but apparent undersensing, and ventricular ectopy.

Read more about the case here: https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.118.037792
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