What Do You Think 49

Last week I presented a series of ECGs on dual AV nodal pathway physiology.

In one ECG, I missed another form of dual AV nodal pathway physiology, which allows the creation of echo beats.  

Let us revisit the ECG as I presented it.

Junctional ectopic (red highlight).

The junctional focus does not penetrate the fast pathway allowing sinus conduction to recommence though the fast pathway, albeit with first degree AV block.

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This is NOT a junctional ectopic but rather an echo beat.

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Now let me present it again but this times with less trimming of the ECG.

Look at it carefully and try and find echo beats.

There are two different echo beats present.

• ‍The first (red highlight). The PR interval of the sinus beat before the premature complex is via the slow pathway (400 ms). This long PR interval allows the fast pathway to conduct retrograde to the atrium (light blue arrow) and then anterograde to the ventricle via the slow pathway and a long PR interval (purple arrow, 500 ms).                                
• The fast pathway now recovers, there is slow pathway offset and the PR interval shortens to 300 ms. ‍
• The second(yellow highlight). The PR interval of the sinus beat before the premature complex is now via the fast pathway (300 ms). Once again there is retrograde conduction to the atrium (light blue arrow) but this time via the slow pathway with a longer delay after the QRS. This longer delay allows anterograde to the ventricle again with a long PR interval (purple arrow, 500 ms).

Thus there is a complex interplay between the slow and past pathways, which I am going to call “pingpong in the AV node”.

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Let us review this interplay.

Follow the progression of slow(s) and fast (F) pathway conduction. There are three areas of interest.

• The first echo beat (red highlight). Fast pathway retrograde.
• Slow pathway offset (blue highlight).
• The second echo beat (yellow highlight). Slow pathway retrograde.

 Remember the slow pathway has a short refractory period which may allow two slow pathway conductions (both anterograde and retrograde/retrograde) to occur close to each other or more likely there is more than one slow pathway which has been reported during electrophysiology studies.

Can I hear you screaming to tell me more about echo beats?

Well you need to wait until the next fun with ECGs.

Harry Mond