Conversion to partial isthmus-dependent flutter during cavotricuspid isthmus ablation: The utility of Advisor HD grid in identifying local fragment electrograms.

Complete block creation by radiofrequency (RF) ablation at the cavotricuspid isthmus (CTI) is a highly successful procedure for the treatment of typical atrial flutter (AFL). Occasionally, a rare type of AFL, such as lower or upper loop reentry, or partial isthmus-dependent flutter, can coexist with typical right AFL. A 73-year-old man underwent CTI ablation for a clockwise CTI-dependent typical atrial flutter. During the ablation procedure, the morphology of the flutter wave changed in the surface electrocardiogram and endocardial atrial activation sequence, suggesting that the typical AFL had converted to another AFL (AFL2). High-density mapping using the HD grid catheter could not reveal the reentrant circuit of AFL2 but detected a critical conduction gap at the boundary between the inferior vena cava and CTI. There was also an impulse collision in the remaining CTI. The RF application at the gap terminated the AFL2 and completed the block line of the CTI. Based on these findings, AFL2 was comparable with partial isthmus-dependent flutter. The present case demonstrates the utility of high-density mapping with a HD grid for the identification of small amplitude high-frequency electrograms at critical sites of the arrhythmia. Learning objective: A rare type of atrial flutter (AFL) can coexist with typical AFL. In such cases, a high-density mapping is useful to identify the critical portion of the reentrant circuit. The Advisor HD grid multipolar catheter (Abbott, St Paul, MN, USA) is unique in that it allows bipolar recording perpendicular and parallel to the splines via 16 electrodes. In this case report, high density mapping using HD grid catheter identified small amplitude high-frequency electrograms at critical sites of the arrhythmia.