Sub-threshold stimulation in variants of atrioventricular nodal re-entrant tachycardia: electrophysiological effects and impact for guidance of slow pathway ablation.

Aim: Sub-threshold stimulation (STS) applied during atrioventricular nodal re-entrant tachycardia (AVNRT) of the common (slow-fast) type has been shown to effectively characterise target sites suitable for slow pathway (SP) ablation but has not been investigated in the setting of fast-slow and slow-slow variants.
Methods and Results: Seventeen consecutive patients (52+/-16 years, 12 female) with sustained uncommon type AVNRT (fast-slow: n = 13, slow-slow: n = 4) were investigated. Mapping of the SP was started postero-septally close to the coronary sinus ostium and continued toward mid-septal sites, if required. Target sites for STS were selected according to established criteria including the recording of the earliest retrograde atrial activation during AVNRT. Long duration (5 s) constant current STS during AVNRT variants was performed in a stepwise manner (max 5 mA) at each site eligible for SP ablation until termination or capture occurred. Radiofrequency current (RFC) was delivered following successful STS termination of tachycardia (65 degrees C, 60 s) and exclusion of catheter dislodgement. Uncommon AVNRT with a mean cycle length of 405+/-70 ms was induced without spontaneous termination in all patients. Interruption of AVNRT variants due to selective STS-induced block of the retrograde (n = 12) or anterograde (n =2) SP occurred without capture in 14/17 (82%) patients. This was exclusively observed at sites with successful subsequent RFC application. AVNRT was rendered non-inducible in all patients after a median of 1 (1-11) RFC pulses without complications.
Conclusions: Uncommon AVNRT can be interrupted by STS delivered at subsequently successful target sites for SP ablation in most patients (82%). The high positive and negative concordance between the effects of STS and following RFC application indicates that STS-mapping is also useful in the setting of AVNRT variants.