Q> Any further insight from the attempted VOP about the mechanism whether it’s slow-fast, fast-slow, or slow-slow?
Ans. Let’s have a look at the 4 atrial activation sequences in CS catheter during:
a. Initial AVNRT (tachy1)
b. First VOP1
c. Second tachycardia and
d. Attempted VOP (VOP2).
There is very little doubt that during tachy1 the antegrade limb is SP and the retrograde limb is FP. The CS activation is sequentially ‘CS78CS12’. During VOP entrainment of tachy1 could be achieved with VAHV response. In the entrained beats also, the CS activation was similar i.e. ‘CS78CS12’; suggesting same VA route (FP) during entrainment which is the norm in typical AVNRT.
Now during the longer VA tachycardia (tachy2), the possibility of fast slow AVNRT is more likely as explained above (no critical AH jump noted, at or before induction). But more insights resurfaced when the atrial activation was looked into carefully. The CS activation was different there than tachy1. The timing of ‘A’ was near simultaneous in CS 78 and CS56 suggestive of a retrograde breakthrough at an equidistant point from either of the bipole (Fig 2B). Moreover, there was polarity reversal (between CS 78 and CS 56) of the initial segment of the triphasic/quadriphasic ‘A’ electrogram (EGM)(Fig 2B). In addition, the His proximal pole also had similar timing of ‘A’ signal compared to CS78/CS56. These unequivocally proved the retrograde circuit was different from FP (tachy1 and VOP1).
As described above, VOP2 repeatedly terminated the tachy2. During VOP the tachy2 had terminated without conduction to atrium (3rd beat in Fig 2C). The tachy2 continued for several beats after starting VOP (till 2nd beat of RV pacing in Fig 2C). In the 3rd beat captured ventricle has terminated the tachy2 by entering the excitable gap. From 3rd beat onwards it is merely a VA conduction happening after tachycardia termination, which is clear from AA interval measurements (Fig 2C). In addition, if we look at the EGM of CS, till the tachycardia was continuing, the ‘A’ EGM in CS 56 and CS78 was simultaneous (CS78=CS56 , first 2 beats in Fig2C). But as soon as tachy2 terminated (and there was VA conduction during continued RV pacing), the CS activation pattern was different, but was identical to tachy1and VOP1 (Fig 1B), hence now via FP (CS78CS56 ). This again proves that the retrograde limb of the tachy2 was anything other than FP and fast-slow AVNRT becomes the only possible mechanism. In fact, the EGM in CS78 itself changed significantly between tachy2 and VA conduction. The initial vector of +/- (marked as * in Fig 2C) became negative of higher amplitude (** in Fig 2C). This is also another kind of polarity reversal, and was due to change in the route of VA conduction.
In essence, this interesting case highlights importance of stepwise application of maneuvers to confirm or exclude SVT mechanisms. Minute EGM signal change and polarity reversal can also provide useful insights in determining exact AVNRT mechanism.