Atrial extrastimuli, atrial overdrive pacing for SVT: When is it useful?

Today, I thought I’d write a bit about atrial extra-stimuli and atrial overdrive pacing during SVT since these manoeuvres aren’t used so often. Personally, I often take a little time to think through what they mean in real-life cases. Also, there are some interesting responses and tips I learned about recently.

First, when are they useful? Atrial extra-stimuli  (or PACs) and atrial overdrive pacing (AOD) are basically used to differentiate a junctional tachycardia from a slow-fast AVN reentry tachycardia. In clinical practice, AVNRT is much more common than JT, so one would think this isn’t much of an issue. Mostly this is true, but occasionally, it can be a problem post ablation of a AVNRT slow pathway, when a slower narrow complex rhythm with a short or zero VA time is seen. This is particularly common during isuprel infusion towards the end of the case:

18 year old female with recurrent SVT. AVNRT was diagnosed at initial EP study and the slow pathway was ablated. Following ablation, this tachycardia (which was a little slower than the original tachy) remained inducible.
Is this still AVNRT?

So how can one resolve this problem?

• Just wait! Often, the VA relationship becomes variable. This doesn’t occur with AVNRT. If the A moves in and out of the V electrograms, then this can only be JT.
• Place a His-refractory PAC. PACs timed to His-refractoriness, or early PACs are both useful. In the case of a PAC timed to His-refractoriness during a JT, the JT will not be affected (because the PAC cannot have travelled down to affect the tachycardia). So no reset of the tachycardia occurs. However, if the rhythm is a AVNRT and the PAC falls during His refractoriness, the PAC can still engage the slow pathway early and advance the subsequent His. As an example of this, look back to Figure 1. Here, we have placed a PAC slightly earlier than the His. The His electrogram arrives on time and is of similar morphology as previously, indicating the PAC cannot have travelled down to reset the tachycardia. However, the subsequent His is advanced (HH 322ms decreased to 295ms) - this must have been through early engagement of the slow pathway. A schematic of the response to a His-refractory PAC is shown in this diagram (taken from the original paper describing this manoeuvre by Padaniliam et al. JACC 2008):

• Place a early (pre-His) PAC. As mentioned earlier, placing an early PAC (before the His signal) can also be useful. Look at this example:

Is this tachycardia AVNRT or JT?

Consider what would occur if the rhythm were a JT. The early PAC would be able to advance the immediate His, but would not terminate the tachycardia. With AVNRT, an early PAC should often be able to engage the fast (and possibly the slow) pathway, leading to refractoriness and thereby terminating the tachycardia. Hence a early PAC which consistently terminates the tachycardia indicates AVNRT and not JT. Schematically, one can show this as below.

(Note that if the tachycardia is not terminated, then one needs to look at the position of the His to determine whether the rhythm is JT or AVNRT - absence of termination of the tachycardia does not mean JT. I think this is quite a subtle point - I didn't appreciate this immediately anyway).

• Atrial overdrive (AOD) pacing. This is really just a special case of a PAC but it is worth describing separately. Look at this example:

Atrial overdrive pacing from proximal CS. Is this AVNRT or JT?

In many ways, AOD here is analogous to using a VOD to distinguish between AT and AVNRT/AVRT. With AOD, one would look for either a A-H-A response (consistent with AVNRT) or A-H-H-A (consistent with JT):

To correctly interpret AOD, (just as in VOD), it is is absolutely essential to measure the intervals to ensure one is not caught out by "pseudo" responses. First, check that the tachycardia is accelerated to the pacing cycle length and not terminated by the AOD. Next, check which is the last entrained beat:

The reason why it is crucial to check which is the last entrained beat is apparent after measuring out the intervals. As the red arrows show, it is the fifth ventricular electrogram which is the last entrained beat and not the fourth. Therefore, this is a A-H-A response and not a A-H-H-A response. Hence, the rhythm is AVNRT and not JT.

(In fact, all of these manoeuvres were done on the same patient, so it is good that they all agree!)

I'd like to close with some EGMs from George Klein's excellent book, "Electrophysiological Manoeuvres for Arrhythmia Analysis":

• It is quite frequent to get one or two extra beats like this following atrial extra-stimulus testing after slow pathway ablation. Again, the usual question is, echo beat (i.e. persistent AVNRT) versus junctional beat. The timing of the His can be useful, but often, putting in a S3 will also sort things out immediately. A S3 will be able to advance the next QRS over a wide range of coupling intervals for JT, whereas it would only rarely be able to do so with AVNRT:

In this case, comparing panels A and B, S3 advances the subsequent QRS (587ms versus 677ms), hence this is likely to be a junctional beat.

•  What about this interesting EGM? Here, a early PAC is able to advance two subsequent beats! In this case, the most likely explanation is a 2 for 1 response, where the early PAC is able to engage both the fast and slow pathways and give rise to two QRS complexes. This implies the slow pathway is still intact.