The mitral isthmus (MI) or also known as the left atrial isthmus is a part of the posteroinferior site of the lateral left atria that positioned between the left inferior pulmonary vein (LIPV) ostium and mitral annulus (MA) [1]. MI is part of the reentrant circuit in which electrical impulses circulate in a loop through the atrium, continuously stimulating atrial tissue and thus causing arrhythmia [2]. The MI is a common target for interventional procedures aimed at restoring normal heart rhythm in some types of arrhythmias and atrial flutter. Based on the Cox-maze surgical procedure used in the 1980s to interrupt these circuits [3], the radiofrequency linear ablation method was developed [4]. Of the several linear lesions proposed for this method, only ablation of the MI and roof line continues to be widely used in the treatment of atrial fibrillation [5]. However, MI ablation is challenging technique [1,4] and may lead to significant complications such as cardiac tamponade [5], circumflex artery injury [6] and atrio-oesophageal fistula [7].

An ideal linear lesion should connect adjacent anatomical structures or areas of scar that interfere with electrical impulse propagation, be as short as possible to avoid the risk of “gaps,” and be testable for bidirectional block along the line [4]. However, some anatomical features of MI region significantly disrupt the use of ablation [1]. The presence of blood vessels within the MI (circumflex artery, great cardiac vein/coronary sinus, and vein of Marshall) may reduce the effectiveness of ablation: 1) blood flow within this vessel's walls could function as an epicardial "heat sink," drawing heat away from the ablation site through convective cooling and consequently, this may diminish the effectiveness of MI ablation. 2) myocardial sleeves surrounding this veins may establish an epicardial connection, that potentially bridging the lesion line 3) due to the proximity of blood vessels, complications such as thrombosis, acute spasm, or potential damage to the vessel walls may develop [8,9].

Thus understanding the cardiac anatomy of the MI may improve outcome or provide insight into the anatomical background of unwanted arrhythmia recurrence [10]. Therefore, this study aimed to examine in detail the features of the MI line and the topographic relationship of the blood vessels in this region.