Catheter ablation of cavotricuspid isthmus dependent flutter has become a commonly performed procedure with high success rate and is a prototype of macro–re-entrant arrhythmia. Although catheter ablation of isthmus-dependent flutter has become a routine procedure, not infrequently, its behavior in the electrophysiology laboratory is unusual. We present one such case highlighting some of the principles of electrophysiology of re-entrant arrhythmia and atrial flutter. See Editor’s Perspective p e71 A 69-year-old woman with history of prior hospital admission for congestive heart failure with preserved left ventricular systolic function in the setting of atrial tachyarrhythmia presented for atrial flutter ablation. Atrial flutter appeared to be typical on surface ECG and intracardiac electrogram recordings through 20-electrode halo catheter placed along the tricuspid annulus. Recordings through the halo catheter showed counterclockwise activation sequence, and entrainment from the cavotricuspid isthmus confirmed isthmus-dependent flutter. Catheter ablation was done with 8 mm nonirrigated radiofrequency ablation catheter to create a line of block in the cavotricuspid isthmus from tricuspid annulus to the inferior vena cava. During the ablation, the activation pattern changed from counterclockwise to clockwise in direction (Figure 1). What is the mechanism of this change in activation? Figure 1. Surface ECG and atrial electrogram recorded through duodecapolar halo catheter in the left half of the figure suggests counterclockwise flutter with atrial activation up the interatrial septum (dashed red arrow) and down the lateral right atrial wall (solid blue arrow). The upper 3 channels show the surface ECG with negative P waves in leads III and aVF and positive P waves in V1 typical of counterclockwise flutter (orange arrows ). The right half of the figure shows reversed activation pattern on the electrodes of halo catheter with ablation of cavotricuspid isthmus, suggestive of clockwise flutter with activation up the lateral wall of the right atrium and down the …