Left atrium isthmus

The left atrium isthmus (mitral isthmus) is the area of atrial myocardium that lies between the mitral annulus and the left inferior pulmonary vein. It is typically a narrow band of tissue, averaging 3–7 mm in thickness and 30–50 mm in length, although considerable anatomical variability exists among individuals.The mitral isthmus is clinically significant because it serves as a critical substrate for macroreentrant arrhythmias, particularly perimitral atrial flutter, and is a common target for linear ablation during catheter-based treatment of atrial fibrillation and atypical atrial flutters.

The region is closely associated with several important vascular structures, including the great cardiac vein (GCV), the vein of Marshall (VOM), and the left circumflex artery (LCx), which may complicate ablation procedures due to their proximity and variable anatomical relationships.