A Bradycardia with Pacing

This ECG was reported as sinus bradycardia 50 bpm, left anterior hemiblock, Wenckebach AV block and ventricular pacing after a 2 second pause (30 beats per minute).

Programming a pacemaker at such a low rate is unusual and it is important to be sure that there is no atrial pacing.The ECG was reproduced as two rhythm strips.

Atrial pacing is present, but no ventricular pacing after a long AV delay (PR interval) of almost half a second.What is going on here?This is called MVP (Managed Ventricular Pacing) by Medtronic. Although rarely seen on ECGs, it is common with Holter monitor tracings and is frequently interpreted as pacemaker malfunction.All pacemaker companies have programmable algorithms to prevent ventricular pacing, when it is not required as ongoing right ventricular pacing may result in left ventricular dysfunction. The actual mode is atrial pacing (AAI) with continuing evaluation of AV conduction. If there is a dropped beat, then ventricular pacing occurs after the next atrial paced beat with an 80 ms AV delay. In the above ECG, lead II is magnified to show atrial pacing and then ventricular pacing with a very short AV delay (red vertical arrows).To prevent this happening all the time after two dropped beats in 4 cycles, the mode switches to dual chamber pacing (DDD) with ventricular pacing.

Once again, the ECG pattern is “chaotic” and doesn’t conform to normal pacemaker function. The dropped beats are numbered in red and the rhythm goes from atrial paced (Ap), ventricular sensed (Vs) to atrial paced, ventricular paced (Vp) with an intervening ventricular ectopic to complicate things. Note the the AV delay of Ap Vs (280 ms) is longer than with Ap Vp (200 ms), which is a violation of normal AV relationships and is the clue that this is a ventricular minimization algorithm.With normal dual chamber pacing, the AV delay is always shorter with ventricular sensing than with ventricular pacing.With these ventricular minimization algorithms, the tracings are unique to the actual company. Even more confusing is that algorithm iteration is ongoing and Medtronic have recently made a number of software tweaks to minimize the pauses and confuse the issue even more.After a period, a scheduled conduction test is performed and if conduction is present then the pacemaker switches back to AAI. Once again, if there is no conduction, then the ECG appearances are unique for each company. To prevent oscillations between AAI and DDD pacing, the companies use artificial intelligence to delay the conduction tests from every few minutes to up to 24 hours.

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Assoc Prof Harry Mond