Zipes with a Twist…

Initial “Big-Picture” overview:

We have a RBBB tachycardia with what looks to be a visually prolonged VA time of >70ms. This is in keeping with atypical AVJRT or AVRT. Could also be Focal AT though less likely, given it is a short RP tachycardia.

So already we have some suspected SVT’s. Lets look a little closer.

• His-Synchronous RV pacing performed from the RV electrode.
  • Note: The RV paced beat looks narrow-ish and so we may suspect fusion has ocurred between our paced beat and the tachycardia. In most circumstances, and even in this circumstance, we would suspect that this invalidates the Zipes (His-Synchronous RV pacing) manouevre. It certainly invalidates a negative result, but a positive result…one could argue that result remains valid so long as “His-Synchronicity” is proven. So, are we truly His-Synchronous?

• This IS probably His-synchronous, even though we don’t have measurements to confirm it, for two reasons:
  • Because you have a relatively narrow QRS complex, indicating that the RBB was “paced” at the same time as left bundle conduction occurred. So at the very least, we are “LBB-synchronous, which means we are also likely to be His Synchronous).
  • The pacing spike comes approximately 35-45ms after the expected His signal. Admittedly, our His signal here is pretty dodgy which seems to be an all too common theme in SVT manoeuvres, but with the eye of faith, we can perhaps just make it out.

• His Synchronous RV pacing advances the immediate AA interval. From this, we can tentatively deduce that there is an accessory pathway present, keeping in mind that our “His-Synchroncity” remains questionable without proper measurements.

• We can also see that the following VV interval accelerates. From this, we can conclude that IF a pathway is present, it also participates in tachycardia.

Lets go deeper…after pacing, something wonderful happens to the tachycardia.

• Firstly, after pacing, the tachycardia accelerates and maintains a faster cycle length post-pacing.

• Additionally the QRS narrows, and the VA time becomes shorter.
  • Shortening of the VA interval with the loss of a RBBB is strong evidence of a right sided accessory pathway which is participating in tachycardia, confirming orthodromic AVRT utilising a right sided AP.

• Moreover, acceleration of TCL with loss of bundle branch block is also a known phenomena during AVRT, often referred to as Coumel’s law.

So we have a diagnosis, but lets go even deeper. Why did the QRS become narrow complex after pacing?

My Two Cents:

• Probably His-Synchronous RV pacing with AA advancement & VV advancement indicating AVRT existance and participation in tachycardia.
  • Some people may be initially doubtfull in interpreting the validity of this example manouvre.

• However, a deeper dive into the post pacing behaviour of tachycardia demonstrates the following, which are strong indicators of AVRT
  • 1. Peeling back the refractoriness of the RBB resulting in
    2. VA shortening with loss of RBBB. 
    3. TCL acceleration with loss of RBBB. 

Strong evidence for Orthodromic AVRT utilising a right sided accessory pathway.

Thanks for tuning in
Cheers
Mitch & CPP Team

P.s: I’m happy to report that Beat Box will resume this year in 2025. It took a break while I and the team focused on completing the full course of EP in Practice, which is now available and already having a positive impact for deparments looking for EP education.

Thanks to everyone who contacted me asking if Beat Box will be continuing in 2025, and for suggesting future topics. If anyone has any suggestions for future Beat Box posts, feel free to make suggestions or even send in EGMs to info@cardiacphysinpractice.com.

Cheers
Mitch