The primitive atrium is divided into right and left atria by two septae: the thin, membrane-like septum primum and the thicker septum secundum. The septum primum grows caudally from the roof of the atrium to fuse with the endocardial cushions and close the foramen primum. Coalescing perforations and resorption of the cranial portion of the septum primum creates the foramen secundum. The septum secundum then grows caudally from the roof of the atrium to the right of the septum primum and closes the foramen secundum. The slit-like recess that forms between the two septae tucked behind the thick inferior muscular rim of the septum secundum is the foramen ovale, which is incompletely fused in 25% (patent foramen ovale).⁶ The fossa ovalis is a central depression in the right face of the septum primum inferior to the foramen ovale and in situ lying obliquely off the coronal plane (Fig. 4-2). It is bounded superiorly by the thick septum secundum (superior limbus)—the most caudal portion of which is the “limbic ledge.”⁷

Critical structures surrounding the fossa ovalis include the aortic root/mound (anterior and superior), coronary sinus (CS) (anterior and inferior), and posterior right atrium. The posterior atrium is created by an infolding of the right and left atrial tissue between layers of epicardial fat where transeptal puncture would initially be epicardial before entering the left atrium. Late tamponade can occur once the sheath is removed from the left atrium.

Because critical structures neighboring the fossa ovalis are fluoroscopically invisible, it is important to understand their location in order to avoid inadvertent puncture.¹⁴,¹⁵,¹⁶ The aortic root anterior to the fossa ovalis can be marked by a pigtail catheter placed in the noncoronary cusp (NCC) of the aortic valve or a properly positioned His bundle catheter because the His bundle penetrates the membranous septum beneath the commissure of the NCC and right coronary cusp (RCC). A deflectable catheter within the CS inferior to the fossa ovalis outlines its course along the left atrio-ventricular groove.

Rather than a direct approach, the most common transeptal technique is the pull-down or drag approach that takes advantage not only of the coaxial relationship of the superior vena cava (SVC), interatrial septum, and inferior vena cava but also the movement of the TN over specific landmarks giving the operator an understanding of the TN tip location relative to the fossa ovalis.⁷ The initial setup requires that the TN/TD/TS assembly is meticulously flushed with heparinized saline. Over a 0.032-inch guidewire, the TD/TS is advanced to the SVC (level of the carina). The TN is advanced into the TD/TS unit so that the TN tip is just within (but not beyond) the tip of the TD under high magnification fluoroscopy (a packaged stylet can also be used with the Brockenbrough TN to guide delivery of the TN into the TD/TS and avoid puncturing the wall of the TD/TS assembly). Once the TN tip is just within the tip of the TD, it is important to keep the distance between the hubs of the TN/TD fixed to avoid the advancement of the TN out of the TD. The arrow of the TN should be aligned with the side arm of the TS and hub mark of the TD so that the curves of all three (TN/TD/TS) are aligned and not fighting each other.¹⁴ Using the arrow of the TD, the assembly is then rotated so that the TN tip points posteromedially (4-5 o’clock) when looking down the barrel of the needle (horizontal plane) and in the same direction as the intracardiac echocardiography (ICE) beam visualizing the fossa ovalis.¹⁴ Clockwise and counterclockwise torque directs the needle more posteriorly (toward the posterior atrial wall) and anteriorly (toward the aortic root), respectively.