u t o H t p f T i p a A 50-year-old woman, who had previously undergone miral valve repair for rheumatic mitral valve disease, underwent radiofrequency catheter ablation procedure for permanent trial fibrillation (AF). The procedure involved pulmonary vein PV) antral isolation with creation of additional linear lesions long the left atrial septum between the right-sided PVs and the itral annulus. Five months later, a repeat procedure was performed for ecurrent, paroxysmal, atypical atrial flutter (AFL). At the ime of the second procedure, the patient developed a left FL with a cycle length of 290 ms, consistent with her linical arrhythmia. A catheter was placed in the coronary inus (CS). After double transseptal catheterization was erformed, a 4-mm-tip ablation catheter (Biosense Webster, iamond Bar, CA) was placed in the left atrium for maping, and a duo-decapolar, circular mapping catheter was laced near the right superior PV ostium (Biosense Webter). Using the ablation catheter, an electroanatomic actiation map of the left atrium was created during AFL Figure 1). The map revealed a macroreentrant mechanism. he reentrant circuit involved the atrial septum with caudalo-cranial activation of the left atrial septum through a arrow isthmus that was bounded by scar. An attempt at ntrainment mapping was made at a site near the mid left trial septum (green dot with arrow in Figure 1). Pacing was elivered from the ablation catheter at a cycle length of 270 s using a pacing output of 10.0 mA and a pulse width of ms (Figure 2). Is the site where pacing was delivered a ritical part of the reentrant circuit?