The advent of devices for ASD closure without invasive surgery has prompted a need to more fully understand the anatomy of the atrial septum. Successful closure requires the creation of a "sandwich" wherein the left and right atrial components of the device (the bread) enclose the rims (the meat) surrounding the defect. The absence of a large segment of rim can be problematic. The purpose of this article is to describe how to echocardiographically visualize atrial septal anatomy and point out those conditions wherein a surgical approach is preferred.
The GORE HELEX Septal Occluder is pictured in Figure 1. Figure 2 shows its placement across a hypothetical ideal ASD with fully deployed right and left atrial disks.
The GORE HELEX Septal Occluder is designed for closure of small to medium sized holes usually less than 2 cm in diameter. Before attempting to close such defects, one must first determine existing ASD diameter, location, and rim anatomy. Figures 3 and 4 show 3D computed tomography images of normal human right atrial anatomy together with the superior vena cava, left innominate and azygous veins. Figure 5 shows adjoining left atrial anatomy (violet), tricuspid and mitral valves, and the position of the interatrial septum.
Figure 6 depicts intra-atrial anatomy with the anterior right atrial wall flapped open to the right. Displayed are the positions of the fossa ovalis (foramen ovale), eustachian valve (VE), and the superior and inferior vena cava. Note the position of the ascending aorta which is anterior and superior to the fossa ovalis, the location of most secundum ASDs.
Atrial Septal Rim Anatomy
The rims surrounding an ASD in both humans and canines can be divided arbitrarily into five zones or regions each of which is easily identified by trans-thoracic (TTE), trans-esophageal (TEE), and intracardiac echocardiographic (ICE) methods. Figure 7 is an artist's rendering of these five rims and their anatomic locations.
The locations of each rim can be conceptualized in terms of their anatomic positions relative to the anterior, posterior, superior, and inferior orientation of the heart as it sits in the chest. The S or superior rim is located just inferior to the entrance of the superior vena cava into the right atrium and below the right pulmonary artery as it passes behind the superior vena cava. The AS, or antero-superior rim, is located immediately behind the ascending aorta. The AI, or antero-inferior rim, is located just above the septal component of the tricuspid valve. The PI, or postero-inferior rim, is located along the back wall of the atrium just superior to the entrance of the inferior vena cava into the right atrium. The PS, or postero-superior rim, is also located along the posterior wall of the atrium but superiorly and just adjacent to the entrance of the right superior pulmonary vein into the left atrium.
Prior to contemplating ASD closure with any device, one must first identify these five rims to be sure there is adequate rim to retain the device. Because the most common imaging modalities used at or prior to the time of device placement are
trans-thoracic (TTE) and trans-esophageal echocardiography (TEE), this report will emphasize how each rim is identified using these two technologies.
For TTE, the pertinent views for seeing the five rims are apical four chamber (AI and PS rims), para-sternal short axis (AS rim), and sub-costal long axis (S and PI rims). Figure 8 shows a typical apical four chamber view which cuts the heart from the ventricular apex in front to the posterior superior atrial base at the level of insertion of the right and left upper pulmonary veins. The AI rim is adjacent to the AV valve insertion into the crux of the heart. The PS rim is just proximal to insertion of the right upper pulmonary vein into the left atrium.
The PI and S rims are identified from the sub-costal long axis view as seen in Figure 9. In this view, the superior and inferior vena cava are intersected together with the long axis of the right atrium. One identifies the superior rim just beneath the right pulmonary artery as it passes behind the superior vena cava. The postero-inferior rim is also identified as it joins the postero-inferior back wall of the left atrium.
The fifth rim, the AS rim, is located just behind the ascending aorta and is identified from standard para-sternal short axis views as depicted in Figure 10. It should be remembered that roughly 45% of all secundum ASDs are located anteriorly and superiorly behind the aorta rather than centrally near the fossa ovalis
In most children weighing 15 kg or more, a standard adult multi-plane probe may be safely inserted. For those weighing less than 15 kg, an 8 mm multi-plane pediatric probe is appropriate. The most important aspect of probe placement is pulling the lower mandible up and forward before the probe tip is inserted. The probe tip is then advanced to the level of the left atrium before proceeding. Ideally one should utilize a multi-plane probe which can rotate the beam from 0 degrees through +90 degrees. Limiting rotation to 0 degrees to +90 degrees ensures constant right-left orientation. Rotating beyond 90 degrees will reverse right-left relationships. In order to see the left and right atrial disks after deployment together with the sandwiched rims in between, one must see the disks perpendicular to the delivery sheath. This requires the ability to see the sheath at roughly 40 degrees to 80 degrees, the angle at which the sheath enters the heart from the inferior vena cava. Figure 11 is an artist's rendering of the delivery sheath and deployed left atrial disk within the heart. Note that the plane of the disk is not 90 degrees to the plane of the sheath (Figure 12). If a biplane probe is used, the sheath will not be visualized along its long axis obviating the ability to see the deployed disks perpendicular to the sheath and parallel to the enclosed septal rims.
Step one is to identify the AI and PS rims. This is done by visualizing the heart from directly behind the left atrium at 0 degrees. A standard four chamber view is produced which orients the left heart structures to the observer's right and the right sided structures on the observer's left. This view cuts the heart from the posterior superior aspect where the right and left upper pulmonary veins enter the left atrium to the apex of the ventricles which is anterior and inferior as the heart sits in the chest (Figures 13 - 16). This view is analogous (although the antero-posterior reverse) to the standard trans-thoracic four chamber view.
Figure 15 shows a pathology specimen with severe left ventricular hypertrophy with the heart cut at 0 degrees exactly along the same plane subtended by the TEE probe. The cut is from the posterior superior aspect of the left atrium near the entrance of the right upper pulmonary vein to the apex of the left ventricle anteriorly and inferiorly, hence the designation of the postero-superior and antero-inferior rims of the atrial septum. Figure 17 shows absence of the PS rim. Such defects if larger than about 2 cm require surgical closure. The largest GORE HELEX Septal Occluder available is too small to close defects of this size, and closure with other commercially available devices may not be suitable due to the risk of erosion through the posterior atrial wall from an obligatorily oversized device.
Step two is to identify the S and PI rims. Figure 18 shows that a vertically oriented plane will cut through both rims. By rotating the probe to roughly +90 degrees (Figure 19), a plane will be created that will cut through the superior vena cava, right atrium, and inferior vena cava. This view is analogous to the trans-thoracic sub-coastal long axis view. Figure 19 (inset) shows the probe oriented in this fashion. By limiting rotation to +90 degrees, one will obligatorily orient the superior vena cava to the right with the inferior vena cava to the left (Figures 19, 20). This 90 degree view is the most important of all views because it is the only way to visualize the postero-inferior rim. Figure 21 demonstrates absence of the postero-inferior rim which results in an inferior sinus venosus ASD which cannot be closed with any device now available. Such defects often include absence of portions of the postero-superior rim near the entrance of the right upper pulmonary vein into the left atrium as well as the antero-superior rim behind the aorta. These ASDs often exceed 2.5 cm in diameter. If one looks only in the horizontal plane, the ASD may appear deceptively central in location with a diameter that may appear ideal for GORE HELEX Septal Occluder closure. But like an iceberg, the bulk of the opening is below the four chamber plane, and deployment and subsequent release of a device may lead to embolization.
The last step is to identify the AS rim behind the aorta by rotating the probe to +15 degrees to 65 degrees which orients the beam parallel to the aortic valve. Figure 22 displays the probe orientation and the resultant artist's rendering of the view obtained. One notes that this image is analogous to the trans-thoracic para-sternal short axis view. Figure 23 shows a centrally located ASD with present AS rim. Figure 24, in contrast, shows a more typical secundum ASD with complete absence of the AS rim.
Is Rim Documentation Necessary?
Why is it necessary to identify and measure all five rims before attempting to insert and release the GORE HELEX Septal Occluder? If adequate rim is present completely surrounding the ASD and provided the diameter is less than 18 mm, the opening can almost always be closed using the GORE HELEX Septal Occluder. Absence of the superior rim results in a high sinus venosus ASD which is commonly associated with partial anomalous pulmonary venous connection of the right upper and/or middle pulmonary veins to the superior vena cava. Obviously, such holes require a surgical approach. If the AS rim is absent, the hole can still be closed provided the other four rims are present. The device is sized and positioned such that the AS portions of the right and left atrial disks hug the posterior wall of the ascending aorta as if one is surrounding a tree trunk with one's arms. Absence of the AI rim results in a form of endocardial cushion defect known as an ostium primum ASD. Since the hole is located immediately above the AV valves with no intervening tissue, a deployed device will obligatorily be in contact with AV valve tissue. Closure with any available device is currently not possible. Holes in the region inferiorly between the AI and PI rims are termed coronary sinus ASDs and are not closeable using interventional devices. Absence of the postero-inferior rim results in an inferior type sinus venosus atrial septal defect. As noted, such openings are often very large sometimes exceeding 3 cm in diameter and require surgery to safely close. Absence of the postero-superior rim alone is problematic. If the diameter of the hole is less than about 16 - 20 mm, the hole may be closeable. Larger diameter holes are not closeable with the GORE HELEX Septal Occluder. Holes that are more or less centrally located or positioned anteriorly and superiorly behind the aorta with adequate surrounding rims are best suited to be closed safely with the GORE HELEX Septal Occluder.