Cycle length dependent block in the "mitral-pulmonary vein" isthmus.

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.