BACKGROUND: High-density three-dimensional (3D) mapping in the Triangle of Koch during sinus rhythm (SR) has demonstrated low amplitude, late activating, multi-component signals suggestive of the AV nodal (AVN) right inferior extension (RIE). Ablation at the site of these signals has been successful for the treatment of typical AV nodal reentrant tachycardia (AVNRT). The leftward inferior extension (LIE) of the AVN has been proposed as a participatory limb in AVNRT and described in the inferoseptal region of the left atrium (LA). We present the use of Ripple mapping of the LA in sinus rhythm to identify low amplitude, late-activating multi-component atrial electrograms similar to those seen at the RIE. METHODS AND RESULTS: Patients undergoing first-time ablation for paroxysmal atrial fibrillation at one institution over a 1-year period were included. High-density mapping of the entire LA was performed in SR using multi-spline mapping catheter & CARTO-3 (Biosense Webster Inc.). Left atrial activation was recorded and displayed using CARTO Ripple mapping, and local electrograms manually reviewed in areas with delayed and multi-component activation. Thirty-two patients underwent LA mapping; in 26 patients, low amplitude, late activating, multi-component electrograms were recorded in the inferoseptal region with discrete, late far-field signal similar in profile to previously described RIE slow pathway potential. Of the six remaining cases, the annotated LA activation was posterolateral in five and in the superior septum in one case. The mapping technique was also utilized in a case of "slow/fast" typical AVNRT in which ablation at the typical slow pathway site in the Triangle of Koch and within the coronary sinus did not elicit junctional response and tachycardia remained inducible. Mapping the LA septum in SR was performed. Ablation at the inferoseptal site with late-activating, multi-component electrogram resulted in junctional beats with non-inducibility of tachycardia without recurrence through 31 months post-procedure. CONCLUSION: Using high-density 3D mapping in SR, multi-component far-field electrograms can be identified at sites of late LA activation. In one case of AVNRT requiring LA ablation, ablation at this site was successful. Further study may provide additional information regarding the relationship of these signals and the LIE of the AVN.