Introduction <p>High density 3D mapping for typical atrial flutter (AFL) ablation can provide a highly detailed voltage and activation map. Post-ablation mapping is typically used to identify cavotricuspid isthmus (CTI). Low voltage areas in the right atrium may yield misleading mapping results.</p> Methods and results <p>3D electroanatomic voltage and activation mapping of the right atrium (RA) was performed in 15 patients with typical AFL, using 3D high density mapping with a Halo or Pentaray catheter pre- and post-ablation. Activation and entrainment mapping confirmed CTI dependent AFL in all patients. Mean number of map points was 611 ± 312. Lateral low voltage areas were seen in 11 (73%) patients. Post-ablation activation map during CS pacing (<i>n</i> = 8) demonstrated latest activation on the lateral wall aligned with the low voltage areas (in 6 of 8 patients), and in 3 (27.3%) patients this was later than just lateral to the CTI ablation line, masquerading as a gap in the ablation line. However, bidirectional block was confirmed by differential pacing, widely split double potentials on the ablation line and non-inducibility. Failure to recognize this misleading activation map in 2 patients resulted in delivery of significantly more ablation lesions (34 vs. 19, <i>p</i> = 0.0007).</p> Conclusion <p>Areas of low voltage in the low lateral right atrium may lead to slow conduction and delayed activation in this area, in some cases even greater delay than just lateral to the CTI ablation line mimicking a gap in the ablation line. Comparing pre-ablation voltage to post-ablation activation map can identify areas of low voltage with slow conduction. The use of other maneuvers can prove bidirectional block and avoid further unnecessary RF delivery.</p> Graphical Abstract <p></p>