Muscular architecture of the mitral isthmus: anatomical determinants for catheter ablation

Abstract

This editorial refers to ‘The anatomical characteristics of three different endocardial lines in the left atrium: evaluation by computed tomography prior to mitral isthmus block attempt’ by Y. Cho et al ., on page 1104 Early electrophysiological studies demonstrated the presence of an isthmus of conductive tissue in the low lateral left atrium (LA) during ablation of left free-wall accessory pathways.1 These studies introduced the LA ‘isthmus’ concept—a site of intra-atrial conduction block that shows the latest atrial activation during sinus rhythm. Although this posteroinferior area of the lateral LA wall between the orifice of the left inferior pulmonary vein (PV) and the mitral annulus cannot be considered an anatomic entity, it is now termed by electrophysiologists as the LA isthmus, or mitral isthmus. Linear ablation connecting the inferior margin of the ostium of the left inferior PV and the mitral annulus, particularly when complete linear block is achieved, appears to increase the success rate of catheter ablation in patients with atrial fibrillation (AF) and prevent macro-reentry around the mitral annulus or the left PVs.2,3 However, the creation of mitral isthmus lesions by catheter ablation is technically challenging and may be associated with significant complications. Factors that make obtaining a complete, transmural, and permanent ablation line across the mitral isthmus difficult may be electrical as well as anatomical because of the variable and complex endocardial geometry of the LA posterolateral region. Other factors include the unpredictable content of atrial myocardium and the cooling effect of the circumflex artery at different locations of this atrial territory. A better understanding of the anatomo-functional substrate is essential for developing a proper LA isthmus ablation strategy that can contribute to successful outcome. In a valuable anatomic study in 20 hearts, Anton Becker4 showed marked variability in the dimensions of the …