A Crushing Heartbreak #ECGChallenge
The present case appears in the journal Circulation. A 58-year-old woman with a dual-chamber pacemaker presented to the emergency department for sudden dizziness and fatigue after lying down on her left side.

On arrival, her heart rate was 45 beats/min, and her other vital signs were normal. Cardiac examination revealed a regular, bradycardic rhythm with an S1 of variable intensity. Cannon A waves were present on neck examination.

Her pulse generator site had no evidence of infection. The pulmonary, vascular, and neurological examinations were normal. Her 12-lead ECG (figure) is shown. What pacemaker malfunctions are present?

This 12-lead ECG shows sinus rhythm with more P waves than QRS complexes (Figure). The PP intervals have a regular cycle length of ≈520 ms, whereas the RR intervals have a regular cycle length of ≈1280 ms. Thus, there is evidence of complete atrioventricular block.

In addition, there are pacing stimuli 160 ms after each P wave, indicating that the ventricular stimuli are tracking the atrial rhythm. However, these pacing stimuli do not cause myocardial depolarization (ie, capture) of the ventricle, because the QRS complexes are narrow and have an incomplete right bundle-branch block morphology, rather than the usual left bundle-branch block morphology seen with right ventricular (RV) pacing, reflecting a junctional escape rhythm at a rate of ≈47 beats/min.

In addition, the ventricular pacing stimuli occur at regular intervals regardless of the presence of a preceding native QRS complex, suggesting ventricular undersensing.

In sum, this ECG revealed underlying sinus rhythm with complete atrioventricular block and a dual-chamber pacemaker in DDD mode with an appropriately sensing right atrial lead and an undersensing, noncapturing RV lead.

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