Editor’s Perspective: The Isthmus of Uncertainty

Abstract

Whether in the atrium or the ventricle, macro–reentrant tachyarrhythmias are among the most challenging arrhythmias to eliminate with ablation. Namdar et al,1 in this installment of Teaching Rounds in Cardiac Electrophysiology, present and explain insightful findings from a patient with a complex macro–reentrant circuit encountered after prior cardiac surgery. They take us through a step-by-step approach and their thought process in identifying the eventual curative ablation target. Article see p 171 In addition to the outstanding educational points specific to this case, several important principles for mapping and ablating macro–reentrant arrhythmia when multiple boundaries and isthmuses are possible can be gleaned from their report. Does identifying a site of early activation from an activation map of the arrhythmia facilitate ablation? Unlike focal tachycardias, reentrant arrhythmias have no true early or late sites of activation. Timing is based completely on the empirical choice of a reference electrogram and the setting of the mapping window.2 Although this principle is apparent and most experienced electrophysiologists will either map and define the entire circuit of tachycardia or use entrainment mapping to identify appropriate sites for ablation, several corollaries to this principle are equally important but not as readily recognized.3,4 ### P-Wave Morphology P-wave vector and morphology (or QRS morphology in the case of ventricular tachycardia) can be invaluable in estimating location of origin for a focal arrhythmia. With macro–reentry however, there is not necessarily a true onset for the P wave (or QRS). Thus, although patterns that have been identified as characteristic for cavotricuspid isthmus–dependent flutter are suggestive of the arrhythmia mechanism, general analysis of the flutter-wave morphology for other macro–reentrant arrhythmias and even common cavotricuspid isthmus–dependent arrhythmias in the presence of extensive atrial disease or prior surgery or ablation is often of little value. When an isoelectric segment is noted in …