What Do You Think 15

32-year old with an implanted Medtronic dual chamber pacemaker for sick sinus syndrome. ECG reported as malfunction with a ventricular escape rate of 30 bpm. All bradycardia pacemakers have a programmable low rate of 30 bpm.

Rhythm strips: Look carefully

What do you think?

Sick sinus syndrome usually means atrial pacing so look for it!

Bipolar atrial pacing (Ap, red arrows), rate 50 bpm with Wenckebach AV block (red highlight)and a dropped beat (yellow highlight).

But we saw ventricular pacing at 30 bpm.

Following the atrial pacing and dropped beat, there is both atrial and ventricular pacing with an 80 msec delay. Then follows atrial pacing with a long AV delay. 

This is not pacemaker malfunction, but rather inappropriate use of a very successful ventricular pacing minimization algorithm manufactured by Medtronic ( Minneapolis MN, USA) called Managed Ventricular Pacing (MVP^TM).To prevent left ventricular dysfunction, such algorithms are designed to avoid ventricular pacing in patients when it is not necessary.The pacemaker effectively paces AAI(R), but in the event of failed AV conduction, will mode convert to dual chamber-ventricular pacing. I call this algorithm-offset.

With MVP, failure of AV conduction during AAI(R) pacing requires a specific number of events before conversion to DDD(R) pacing. If failure of AV conduction occurs for one cycle, then the next atrial paced event is followed by ventricular pacing with a very short 80 ms AV delay.

Wenckebach AV block. Atrial pacing (Ap) with ventricular sensing (Vs) and a dropped atrial paced beat (red highlight). The next Ap Vp has an 80 ms delay (yellow highlight). There is mode conversion for one cycle, but no algorithm-offset.

Algorithm-offset to DDD(R) pacing will occur if two of four consecutive cycles have failure of AV conduction (red highlight and numbers 1 and 4).

Two short 80 ms AV delays (yellow highlight) followed by AP VP with the programmed AV delay (blue highlight) after a ventricular ectopic.

Never diagnose single chamber ventricular pacing until you have excluded atrial pacing, which can be very subtle.

Unusual pacing ECGs are frequently due to algorithms.