Q: After initial SP modification, why the initial tachy1 became non-inducible, and tachy2 got induced?
Ans. The exact mechanism is difficult to predict, like various other mysteries of AVNRT [1]. We hypothesize, with SP modification the ERP of SP and/or FP had probably changed such that fast-slow/ slow-slow AVNRT was easily induced without AH jump.
Is it fast-slow AVNRT?
Yes. Most likely as the retrograde A was earliest near CS os. A possible reason is increase in effective refractory period (ERP) of SP after ablation. This is known after SP modification [2]. Hence, with APD, ERP of SP was reached earlier than FP (unlike pre-ablation, when normally ERP of FP is reached before SP, producing unidirectional block in FP leading to jump to SP and typical echo; finally perpetuating slow-fast AVNRT). We hypothesize, now, with the unidirectional block happening earlier in SP, it was still conducted via FP (ERP of FP is also known to shorten with SP modification) [2]. By that time SP recovered and was available to conduct retrogradely leading to fast-slow AVNRT.
Can it be slow-slow AVNRT?
There was no AH jump and only modest PR interval (PR = 190 ms). So antegrade limb is very unlikely to be SP.
Can it be still be slow-fast AVNRT?
No, very unlikely. The retrograde limb is definitely SP as the earliest ‘A’ in tachy2 was near CS ostium [3].