• The IAS has two components. The septum primum(SP) and the septum secundum(SS).
• The SP is a membrane-like structure of myocardium with fibrous and elastic tissue that corresponds to the floor of the FO. The FO covers 20% area of the IAS. The FO is the “true” septum. On trans-esophageal echocardiography the FO appears as a crater like depression from the right atrial perspective. The only safe site for TSP is the floor of the FO.
• The SS is an extensive muscular rim that surrounds the FO. The SS is actually an enfolding of the atrial wall, leaving in between an extracardiac space filled with adipose tissue. The SS is the “false” septum. A TSP performed in this region during a SHI would result in exiting the heart cavity into the epicardial adipose tissue (EAT) causing a hematoma in the SS or a hemopericardium. Hence option “a” is incorrect.
• The “aortic” mound is the anterosuperior margin of the FO. It abuts the transverse pericardial sinus and through it the non-coronary aortic sinus. Performing a TSP at this area results in puncture of the aortic root, which is the most feared complication of a TSP. Hence option “c” is incorrect.
• The Waterstone’s interatrial groove is a fold filled with EAT on the external surface of the heart, located between the SVC and the right pulmonary veins. It is otherwise known as the Sondergaard’s groove or the posterosuperior interatrial sulcus. It contains vascular fibrofatty tissue. A TSP at this site allows blood to dissect between the atrial layers leading to a hemopericardium. Hence option “d” is incorrect.

 Reference:

1.  Faltera FF, Leo LL, Paiocchi V, et al. Multimodality imaging anatomy of interatrial septum and mitral annulus. Heart 2021;107:277-81.