A failed SVT pacing manoeuvre clinches the diagnosis.

The Big Picture:

• Initially, a narrow complex “tachycardia” with a short VA time (-ve VA time in fact) is giving me slow/fast AVJRT vibes.

• RV burst pacing successfully captured the ventricle, but failed to accelerate the Atrium (AV dissociation).

• Tachycardia continued at the SAME CL after RV burst pacing but with a drastically different VA time & an upright P wave in the inferior leads.

Conclusions?

• AV dissociation without termination makes AVRT unlikely. Upright inferior P waves makes fast/slow AVJRT unlikely. Sooooo….Focal AT?

Diving deeper

Which tachycardias can we exclude?

• AVRT: RV burst pacing dissociates the ventricles from the atria without terminating SVT, arguing against AVRT (this is a useful and largely universal tenant of EP – AV dissociation without tachycardia termination almost always argues AGAINST AVRT). So AVRT is off the table of possibilities.

• • There may be an argument for nodofascicular tachycardia, which often presents with VA block, however, the upright P wave in II/III/aVF in the second half of the screen argues against this, since nodofascicular tachycardia usually produces an inferiorly negative P wave as atrial depolarisation originates from the posteroseptal RA (near the slow pathway region).

•  AVJRT: Slow/fast AVJRT being changed into fast/slow AVJRT with RV burst pacing is I suppose possible, especially given the change in VA time pre and post RV pacing. In my mind, two things make this unlikely. Firstly, the CL does not change by even 1ms. Secondly, the P wave is upright in II/III/aVF – this is not consistent with fast/slow AVJRT where atrial depolarisation occurs from the RA posteroseptal region of the slow pathway.

• Junctional Ectopic Tachycardia: Although JET classically presents with VA block/dissociation, I think JET is also unlikely because the change in VA time after RV burst pacing implies that our pacing stimulus “disrupted” the AV node and Bundle of His (since these structures are responsible for AV and VA times) WITHOUT disrupting tachycardia. There was no overdrive suppression response, which is what you’d expect if the tachycardia was JET. In my mind, this implies that the focus is “supranodal” or within the atrium.

• Focal AT – our last remaining tachycardia differential?

So if Focal AT is the tachycardia, how do we explain the change in VA time after RV burst pacing?

• Remember, we did not accelerate the atrium here. But we can assume that our pacing stimulus reached as far as the AV node, since the VA time was disrupted.

• The change in VA time was due to “Peeling back the refractoriness of the Fast Pathway”

My Two cents:

• In this example, RV burst pacing failed to either terminate or entrain tachycardia, yet still provided the best single piece evidence of the case for the diagnosis. Again, this stresses the need to examine all your manoeuvres, even if they did not have the intended effect.

• Dissociating the atria and ventricles without terminating tachycardia, with an upright P wave in the inferior leads made AVRT of any form, and fast/slow AVJRT very unlikely.

• The change in VA time without an overdrive suppression response made JET also unlikely.

• So initially, we had a Focal AT with antegrade conduction down a slow pathway producing a very long PR interval and producing a pseudo short VA time.

• RV overdrive pacing peeled back the refractoriness of the fast pathway, allowing subsequent antegrade conduction down the fast pathway, accounting for the change in VA time post RV burst pacing.

Cheers
Mitch & CPP Team

For proof, here is the final map of the Superior Crista Focal AT ablation location. You will notice, that this is consistent with the P wave morphology.

SVT differentiation and pacing manoeuvres are taught in more depth in the SVT program of EP in Practice.