Cycle Length Dependent Block in the “Mitral–Pulmonary Vein” Isthmus

Abstract
We report a case of cycle length dependent activation sequence in the coronary sinus catheter during assessment of mitral‐pulmonary vein isthmus block. A 61‐year‐old patient presented with atrial tachycardia following a recent pulmonary vein isolation for paroxysmal atrial fibrillation. A perimitral macroreentrant atrial tachycardia was demonstrated during mapping. The isthmus block observed following initial ablation of the mitral‐pulmonary vein appeared to be pacing cycle dependent and to our knowledge has not been previously described.