Anatomic characteristics of the mitral isthmus region: The left atrial appendage isthmus as a possible ablation target

The mitral isthmus is a part of the postero-inferior area of the lateral left atrial wall located between the mitral annulus and the left inferior pulmonary vein ostium. Linear ablation lesions are created within the mitral isthmus for the invasive treatment of left atrial arrhythmias. However, the anatomy of this region is not fully understood. The aim of this study has been to provide a detailed morphometric description of the mitral isthmus region and to propose another possible isthmus within the investigated heart area that may serve as a potential new ablation target. Two hundred autopsied, non-atrial fibrillation hearts (23.5% deriving from females) whose donors were a mean of 47.6±17.6years old were investigated. We macroscopically assessed the anatomy of the postero-inferior area of the lateral left atrial wall. The mean mitral isthmus length was 28.8±7.0mm and was significantly longer than the left atrial appendage (LAA) isthmus (14.2±4.8mm) (p=.00). The distance between the LAA orifice and the left inferior pulmonary vein ostium (18.4±4.8mm) was longer than the LAA isthmus (p=.00) and shorter than the mitral isthmus (p=.00). The LAA isthmus was longer in hearts with a common left pulmonary vein (p=.037). In 65.5% of all cases the area between the right and left mitral isthmus lines was completely smooth. In the remaining hearts, crevices and diverticula (18.0%), intertrabecular recesses (7.0%), trabecular bridges (3.5%), or co-existence of these structures (6%) could be observed. The LAA isthmus line was smooth in 95.5% of all cases, with only small crevices in the remaining 4.5%. In conclusion, regardless of the anatomical variants of the left-sided pulmonary veins, the mitral isthmus area is quite uniform in size. The LAA isthmus is considerably shorter than the mitral isthmus. The mitral isthmus line has many unwanted structures that may entrap the catheter, which is not the case for the LAA isthmus. We proposed the LAA isthmus line for potential clinical use.