Inappropriate heart rate counting and pacing system pseudomalfunction

A patient with AV nodal disease and a Medtronic A2DR01 magnetic resonance conditional dual-chamber pacemaker set to MVP mode (mode designed to limit right ventricular pacing) was being monitored in the ICU with an ECG monitor configured to display pacemaker artifacts. The top trace is ECG lead II (bipolar, white-to-red lead). The middle trace is lead V5, which is a unipolar lead. The bottom trace is from an invasive intra-arterial catheter. No EMI was present.
The strip recorded a bradycardia alarm at 35 bpm, despite effective atrial pacing at 70 bpm with an effective ventricular rate of 54 bpm. Using the arrhythmia/rhythm "relearn" function on the monitor may correct such miscounting. However, as pacing activity changes, miscounting can recur.
The blue down-arrows point to atrial pacing artifacts (AAI pacing) until a QRS complex was dropped (ie, two atrial pacing events without an intervening QRS complex). After the third atrial pace, the pacemaker in the MVP mode delivered a ventricular-paced beat (red up-arrow) on the S-T segment 60 msec after the atrial pace, generating an effective QRS complex. Note the two other dropped QRS complexes that were followed by this 60-msec AV ventricular pacing sequence. The MVP is typically programmed to deliver such a 60-msec ventricular pacing sequence when QRS complexes are dropped.

HR: heart rate; AV: atrioventricular; MVP: managed ventricular pacing; ICU: intensive care unit; ECG: electrocardiography; EMI: electromagnetic interference; AAI: atrial chamber spacing, atrial chamber sensing, atrial chamber inhibited; bpm: beats/minute; msec: millisecond.

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Inappropriate heart rate counting and pacing system pseudomalfunction