Induction of Atrial Flutter by Pacing at a Long Cycle Length in a Patient with Unidirectional Isthmus Block

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Background: Unidirectional isthmus block (uni-IB) during ablation is a very rare phenomenon (Toluie, 2000). In the setting of uni-IB, pacing at a long cycle length (PLCL) (600 ms) from coronary sinus (CS) may readily induce counterclockwise (CCW) atrial flutter (AFL). Case Presentation: We performed isthmus ablation in an 82-year-old male with CCW-AFL. Intracardiac recordings were obtained from a 20-polar electrode catheter around the tricuspid annulus, a 10-polar catheter in the CS, and a His bundle catheter. Before the ablation, bidirectional conduction through the isthmus was demonstrated and CCW-AFL was induced by rapid pacing at 230 ms. Double potentials (DP) were recorded at the ablation site during the 5th application. The second potential of DP was gradually delayed until CCW-AFL was induced by PLCL. It was postulated that induction of CCW-AFL was made possible under the condition of clockwise uni-IB. Bidirectional isthmus block was obtained during the 8th RF application. Conclusion: An unusual patient was reported in whom CCW-AFL was readily induced with PLCL of 600 ms during isthmus ablation. Induction of CCW-AFL seemed to be dependent on the occurrence of unidirectional isthmus block.