What Do You Think 28

The ECG report stated: “Dual chamber pacing with Wenckebach AV block”.

What do you think?

An exercise in pacemaker ECG interpretation.

The report is correct in regard to dual chamber pacing.

The rhythm is sinus (rate 100bpm) with atrial sensing (As) and ventricular pacing (Vp). Although difficult to see, there is a ventricular stimulus artefact (red highlight).

There is also atrial pacing (Ap, yellow highlight).

Why Wenckebach AV block in the report?

As Vp is followed by a dropped sinus beat with no Vp (green highlight).

‍This is not Wenckebach AV block!

‍The AV delay (PR interval) with atrial sensing is constant.

Following the dropped beat, there is a pause of 1000 ms (60 bpm) before Ap Vp,  indicating the low rate has been set at 60bpm. Note the sensed (As Vp) and the paced (ApVp) AV delay are almost the same timing, if not identical. Because sensing of the sinus P wave takes time, the sensed AV delay is traditionally programmed shorter than the paced AV delay, although physiologically this is probably of little or no consequence.  

The timing of the atrial pacing suggests that the diagnosis is atrial undersensing. When atrial under sensing occurs, the atrial low rate is timed out and atrial pacing (Ap) occurs at 1000 ms after the previous As Vp.

Because of the discrepancy between the sinus rate (100 bpm) and atrial low rate for pacing (60 bpm), there is a significant pause between atrial undersensing and atrial pacing. Usually the atrial sensing rate and the programmed low rate are close resulting in an ineffective atrial stimulus artefact in the atrial refractory period.

A similar ECG appearance to our case study may occur with non-conducted atrial ectopics (blue highlight).

As with all non-conducted atrial ectopics, prematurity is important. Ventricular pacing does not follow the ectopic as the upper rate limit cannot be violated and thus this is not pacemaker malfunction.  

The diagnosis is usually obvious as the P wave is premature. Because the ectopic P wave is sensed, the sinus cycle is reset (red and blue arrows).  

Another differential diagnosis is ventricular exit block.

Although the appearances maybe similar, there are important diagnostic differences.

Once again there is As Vp.

• Because atrial sensing is normal, the sinus cycle length remains steady with regular P waves (red arrows).
• Ventricular pacing follows the sinus P wave, but is sub-threshold and the subtle bipolar stimulus artefact may deform the baseline (blue arrows).

Today, whenever we see dropped beats with cardiac pacing, the first thing we think of are ventricular pacing minimization algorithms. These algorithms are very popular in order to prevent ventricular pacing when not necessary. The ECGs will show ventricular sensing (Vs) and if there is a dropped beat often due to non-conducted atrial ectopics or Wenckebach AV block, the paced or sensed P wave does not conduct, and a pause occurs.

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This is the typical appearance with the Medtronic Managed Ventricular Pacing (MVP^TM) algorithm.

If failure of AV conduction occurs for one cycle (yellow highlight),then the next atrial paced event is followed by ventricular pacing with a very short80 ms AV delay. Ventricular pacing will occur, if two of four cycles show failure to conduct.

With our case study, there was ventricular pacing and no ventricular sensing. However, once ventricular pacing is established, scheduled conduction tests occur. With the Medtronic conduction test algorithm, AAI(R) pacing occurs for one cycle. Failure of conduction will result in an ECG similar to our case study, but once again the first AV delay is set at 80 ms.

Ap Vp with an AV delay of 300 ms. There is AAI (R) pacing (yellow highlight) for one beat with failed AV conduction. The next Ap Vp has an AV delay of 80 ms which is diagnostic of the algorithm and excludes a pacemaker complication.

There was another interesting finding in our case study, which is easily overlooked.

Prior to the dropped beat, the sensed AV delay was a constant 120 ms with a sinus rate of 100 bpm. During and immediately after the dropped beat, the sinus rate transiently dropped, and the sensed AV delay extended to 180 ms. Over the next two sinus beats, the AV delay shortened to 140 ms as the sinus rate returned to 100 bpm. 

This extension of the AV delay with rate slowing is referred to as rate adaptive or dynamic AV delay. A range of AV delays can be programmed in order to allow the upper rate to be extended during fast heart rates and optimal diastolic filling with slower heart rates.