Pc-578-03 right atrial atypical flutter with 2:1 intra-atrial block mimicking left atrial focal and localised reentrant tachycardia

Atrial-level blocks are known confounders for diagnosis of SVTs and commonly occur after previous linear ablation lesions. 1. To demonstrate importance of recognition of atrial-level block in SVT diagnosis and treatment. 2. To review underlying assumptions in using entrainment to localize SVT origins. 3. To discuss the importance of thorough biatrial mapping in determination of unclear SVT mechanisms. N/A A 74y M with a history of bioprosthetic MV replacement, atypical RA flutter ablation (2018), and permanent pacemaker (2020) for sick sinus syndrome presented with fatigue and 76% AF/AT burden. For index RA ablation, both an intercaval line using bridging scar near the atriotomy site anterior to the appendage and a cavotricuspid isthmus line were made. Tachycardia was induced via burst pacing. Initial entrainment from CS suggested a L-sided tachycardia. A Pentaray map of the tachycardia showed LA anteroseptal reentry with entrainment at the site giving a PPI-TCL < 30ms. PVI and anteroseptal line were performed which changed cycle length but not CS activation. LA re-mapping demonstrated focal breakout near RIPV; ablation lengthened cycle length to 290ms without rhythm cessation. After PVI confirmation, mapping of the RA showed 2:1 intra-atrial conduction in the lateral RA with 1:1 conduction in the septal RA (fig A). Mapping 1:1 areas of RA conduction showed a gap in previous IVC line with intact CTI block with the majority of the cycle length at the posteroseptal floor (fig B), immediately opposite the site of initial LA anteroseptal reentry (fig C). During ablation of IVC line gap, progressive lengthening of the TCL was noted followed by termination of rhythm without inducibility (fig D). Response to entrainment assumes 1:1 conduction through the entire circuit, and contributed to the appearance of a left sided tachycardia upon initial CS entrainment. The finding of a LA RIPV tachycardia upon remapping the LA after PVI is consistent with a separate focal tachycardia but could represent epicardial connections from the CS to the posterior LA.