Figure 4. Diverticular Structures Surrounding the Left Atrial Appendage Ostium Left panel: Endocardial view of the atria showing the ostium of the left atrial appendage (LAA) and its surrounding structures. Of special note are the diverticular structures that are proximal to the LAA ostium. The relationship of the left superior and inferior pulmonary veins (LSPV and LIPV) is shown, as well as their separation by the vein/ligament of Marshal (LOM) ridge; which is an endocardial marker for the epicardial vein of Marshall (note that the course of the vein/ligament of Marshall running in between the Left pulmonary veins and the LAA, on its route to empty into the coronary sinus. The relationship of the LAA and the left circumflex artery is also illustrated. Right panel: After the left atrial appendage is occluded with a device (black circle represents the proximal end of the occlusion device), the surrounding pits/ divots that are proximal to the ostium remain “unprotected” and in contact with the left atrial circulation. In addition, note the relationship of the proximal end of the occlusion device with the left circumflex artery.