Retrograde Right Bundle Branch Block

Recently, I've been trying out a SVT manoeuvre I had learned about during the Mayo EP course, but never really used. It was a quiet afternoon, and I was reviewing some old SVT procedures. I came across something like this a few times:

Figure 1. Is there (1) a slow pathway or (2) a septal accessory pathway?

Here, the measured interval shows that the VA time "jumps" from 148 to 202ms with a 40ms (360 to 320ms) shortening of the S1S2 coupling interval.

What does this indicate? In the antegrade direction, an increase in the AV conduction time with a small decrease in coupling interval is usually taken to indicate a switch from fast to slow pathway conduction - this is confirmed by showing that the increase in conduction time is related to an increase in the AH interval ("AH jump").

What about in the retrograde direction? Here, the His signal, which in Panel A must have been buried in the ventricular electrogram, has been released, and is now apparent (blue arrow in Panel B). The increase in VA time must therefore have been related to an increase in VH time. Therefore, this cannot imply the presence of a slow pathway, but instead suggests retrograde right bundle branch block. Assuming the His was buried in the ventricular electrogram but is now released, it looks to have shifted about 40 or 50ms to the right, which is roughly the amount of time one would expect for conduction to cross the ventricular septum and reach the left bundle to go up to the His.

Why is this observation of use? Well, the first point is that this occurs fairly frequently (about 80% of the time). It is not hard to induce retrograde right bundle branch block using RV extrastimulus testing. The second point is that this manoeuvre is more or less analogous to a para-Hisian pacing!

The idea of para-Hisian pacing is basically to compare VA time on loss of His capture. If the VA time remains similar whether the His is captured or not, it suggests there must be an accessory pathway used to conduct from the ventricle to atrium. In the absence of such a pathway, one would expect the VA time to prolong because on loss of His capture, locally captured ventricular myocardium would need to conduct down to the terminus of the right bundle branch before conducting retrogradely up to the atrium - this clearly will take a significantly longer time than if a (septal) accessory pathway is available for retrograde conduction.

In the case of retrograde right bundle branch block, one would observe the same phenomenon. If VA time is significantly prolonged on induction of retrograde right bundle branch block (marked by release of the His i.e. a VH "jump"), then this suggests that there isn't a (septal) accessory pathway available for retrograde conduction. If however VA time is increased minimally, then there is a strong suggestion of a septal accessory pathway (able to conduct retrogradely).

This maneuver was, I believe, first described by Sam Asirvatham's group in the Mayo clinic. In a retrospective study of 105 patients undergoing EPS and ablation for AVNRT or AVRT, the average V-H interval increase with induction of RBBB was 53.7 ms for patients with AVRT and 54.4 ms for patients with AVNRT (P = NS). However, the average V-A interval increase with induction of RBBB was 13.6 ms with AVRT and 70.1 ms with AVNRT (P < 0.001). All patients with a greater V-H than V-A interval change had AVRT, and those with a smaller had AVNRT.

Turning back to the example tracing in Figure 1, the VA interval increased significantly, suggesting absence of septal accessory pathway conduction. (No comment can be made about presence or absence of a slow pathway).