ECG: Atrial Flutter; likely typical cavotricuspid isthmus dependent.

EGM: Atrial activation pattern is from proximal to distal halo consistent with counter-clockwise typical CTI flutter.

Entrainment from Halo 5-6 -- concealed entrainment with relatively short post-pacing interval.

Entrainment from CS 1-2 -- manifested entrainment with relatively long post-pacing interval.

Double-potential signal on ablation catheter is noted while ablating cavotricuspid isthmus
Interval between the split of >90ms along the line is highly specific for complete isthmus block.

Termination while ablating cavotricuspid isthmus. Noted with upstream block at distal halo before termination.

Incomplete isthmus block -- pacing from proximal CS should have reached distal halo last; not first.

Incomplete isthmus block to complete block.

Bidirectional cavotricuspid isthmus block -- pacing from distal halo reached a more proximal poles of halo before reaching proximal CS and pacing from proximal CS reached proximal halo before reaching distal halo.
Average tranisthmus conduction time for complete block = 140ms or at least 50% more than the time before ablation.

Difficult anatomy in CTI-dependent AFL ablation. Asirvatham SJ. J Cardiovasc Electrophysiol. 2009.