Anatomy of the True Interatrial Septum for Transseptal Access to the Left Atrium

Abstract Clinical anatomy of the interatrial septum is treacherous, difficult and its unfamiliarity can cause many serious complications. This work aims to create an anatomical map of the true interatrial septum. An appreciation of the anatomical situation is essential for safe and efficacious transseptal access from the right atrium to the left heart chambers. Examination of 135 autopsied human hearts (Caucasian) of both sexes (28% females) aged from 19 to 94 years old (47.0 ± 18.2) with BMI = 27.1 ± 6.0 kg/m2 was conducted. Focus was specifically targeted on the assessment of the fossa ovalis, patent foramen ovale (PFO), and right-sided septal pouch (RSP) morphology. Mean values of cranio-caudal and antero-posterior fossa ovalis diameters were 12.1 ± 3.6 and 14.1 ± 3.6 mm, respectively. The fossa ovalis was situated an average of 10.1 ± 4.4 mm above the inferior vena cava ostium, 20.7 ± 5.2 mm from the right atrioventricular ring, and 12.6 ± 5.2 mm under the right atrium roof. Four types of fossa ovalis anatomy have been observed (smooth-56.3%, PFO-24.4%, RSP-11.9%, net-like formation-7.4%). The PFO mean channel length was 10.5 ± 5.2 mm. The tunnel-like PFO (channel length ≥12 mm) was observed in 8.9% of specimens. The RSP was observed in 11.9% of specimens (with mean depth = 6.3 ± 3.8 mm) and was directed apex upward in all observed specimens (may imitate the PFO channel). The fossa ovalis/interatrial septum surface area ratio was 18.3 ± 9.0%. In conclusion: (1) An anatomical map of the interatrial septum from the right atrial side was presented. (2) The RSP may imitate the PFO channel. (3) The “true” interatrial septum represents only about 20% of the whole interatrial septum area. (4) There is wide variation in the location and geometry of the fossa ovalis. (5) We could distinguish four different types of the fossa ovalis area.