Congenital Shunts

Overview

Secundum ASDs are the most common type of ASDs, accounting for approximately 75% of cases. The ostium secundum is a normal gap in septum that allows blood to bypass the lungs in the fetal circulation. During embryologic atrial septation, the ostium secundum is normally occluded by the septum secundum as it grows from the roof of the common atrium toward the crux of the heart. Secundum ASDs may result from inadequate growth of the septum secundum, which leads to persistence of the ostium secundum.

The size of secundum ASDs varies widely, from small fenestrations in the fossa ovalis to very large defects with minimal rims. They can be round or oval and may be single or multiple. Small defects typically do not result in any measurable degree of right heart enlargement or pulmonary hypertension and may not be readily apparent on TTE. Moderate to large defects can usually be identified on TTE. Dropout in the area of the fossa ovalis is common, and the presence of a shunt should be confirmed with color Doppler. Spectral Doppler should then be used to confirm the direction of flow across the shunt, which can be predominantly left-to-right, right-to-left, or bidirectional ( Fig. 41.3 ). The subcostal view is optimal for color imaging of flow across the ASD because the direction of flow is parallel to the ultrasound beam.

Agitated saline contrast studies can be used to confirm right-to-left interatrial shunting when there is uncertainty about the presence of an interatrial shunt on 2D imaging and color Doppler, but differentiating a patent foramen ovalis from a small ASD requires TEE for most adult patients. During an agitated saline contrast study, absence of agitated saline contrast adjacent to the interatrial septum due to inflow of blood without agitated saline contrast is called negative contrast . However, because superior vena cava (SVC) flow also streams along the interatrial septum, the presence or absence of negative contrast should be considered together with other echocardiographic findings to confirm an ASD.

Management

Transcatheter closure of secundum ASDs has replaced traditional surgical ASD closure in most cases. The feasibility of device closure depends on the size of the defect, adequacy of the tissue rims, proximity to valves and pulmonary veins, and whether any other CHD (e.g., anomalous pulmonary veins) is present ( Table 41.2 ). These data are readily obtained with TEE imaging. The echocardiographer should interrogate the ASD at multiple angles to assess all rims ( Fig. 41.4 ). ASDs are dynamic, and their dimensions vary throughout the cardiac cycle. The echocardiographer should report the maximal diameter of the defect in two orthogonal planes to assist with device selection.

Three-dimensional (3D) TEE imaging is an important adjunct to 2D imaging. Multiplanar reconstruction ensures that the defect is measured accurately. TEE images can be rotated to demonstrate the defect from the RA or LA side. If 3D images are being used to guide intervention, it is recommended that images be displayed in a standard format with the SVC at 12 o’clock from the RA view and the right upper pulmonary vein at 1 o’clock from the LA view to orient the image for the operator performing the transcatheter interventions ( Fig. 41.5 ).

Amplatzer septal occluders and Gore ASD occluders are approved by the US Food and Drug Administration (FDA) for percutaneous ASD closure ( Fig. 41.6 ). Previously, percutaneous ASD closure was limited to ASDs that are smaller than 30 mm in diameter and have at least 5 mm of circumferential rims. However, larger defects and defects with deficient rims have been closed successfully and with minimal complications. Potential complications of ASDs include arrhythmias, device erosion, perforation, pericardial effusion, thromboembolic events, device embolization, and allergic reactions to the nickel alloy contained in the device.

ASD closures can be performed under ultrasound guidance with TEE or ICE. The advantage of ICE is that a single operator can maneuver the ICE catheter and deploy the device with the patient under moderate sedation. Disadvantages are the learning curve associated with operating the ICE catheter, difficulty visualizing and assessing all rims and pulmonary venous connections, and limited sector width for guiding closure of larger defects. TEE guidance requires a second operator, and patients require deep sedation or general anesthesia for the duration of the procedure because of the discomfort of prolonged intubation with the TEE probe. However, TEE allows a more comprehensive evaluation of the rims in larger defects.

During transcatheter ASD closure, a right heart catheterization is performed, and measurements of hemodynamics and saturations are obtained. A wire is advanced from a femoral vein, across the ASD, and into the left upper pulmonary vein under TEE/ICE and fluoroscopic guidance. A sizing balloon is advanced over the wire across the defect and inflated with a mixture of saline and iodinated contrast until flow across the ASD is completely obstructed by the inflated balloon and a discrete narrowing (i.e., waist) is seen in the balloon. The diameter of the waist is referred to as the stretched diameter of the defect; it is measured on both TEE and fluoroscopic images to guide device selection. The stretched diameter is typically 10% to 20% larger than the corresponding diameter on baseline images.

The echocardiographer must ensure that there is no residual shunting at this point. If flow continues to occur across the septum, a second defect could be present, or the balloon may need to be inflated further to seal the defect completely and give the most accurate measurement. The balloon is then slowly deflated until a small amount of flow is observed and then reinflated another few milliliters until cessation of flow is again seen; the balloon waist at this point is called the stop flow diameter . The stop flow diameter is typically used to guide selection of the device size.

The delivery sheath is advanced over a wire, across the defect, and into the LA, and the closure device is then advanced through the sheath into the LA. The LA portion of the device is deployed with the catheter in the LA, and the catheter and device are then retracted together until the LA disk is flush with the LA aspect of the interatrial septum. The RA disk is then deployed. Before the device is released from the catheter, appropriate positioning is confirmed by interrogating all rims with 2D and color Doppler, and push/pull maneuvers are performed to confirm device stability. After the device is released, trace residual flow is expected and normal.

Repeat TTE is recommended within the first week after the procedure to ensure that a pericardial effusion has not developed and that the device is well seated. Agitated saline studies may remain positive at this first follow-up study, but residual shunting across the device typically resolves after the device is endothelialized, several months after the procedure.

Overview

Primum ASDs are part of the spectrum of atrioventricular septal defects (AVSDs), which are commonly called atrioventricular canal defects or endocardial cushion defects . AVSDs are caused by deficient formation of the atrioventricular (AV) septum from embryonic endocardial cushions, which results in a distinct malformation of the central fibrous cardiac crux.

AVSDs are characterized as partial, transitional, or complete. Isolated primum ASDs and isolated inlet VSDs are considered to be partial AVSDs. A transitional AVSD has a primum ASD and two AV valve annuli but abnormal left AV valve morphology. There is often a small inlet VSD that may be partly or completed closed by the chordal tissue of the AV valves. In the complete form of AVSD, a primum ASD and an inlet VSD exist, and there is one common AV valve. AVSDs are associated with trisomy 21 (i.e., Down syndrome), DiGeorge syndrome, Holt-Oram syndrome, and Ellis-van Creveld syndrome. Secundum-type ASDs and persistent left SVC are the most common anomalies associated with AVSD.

Unlike secundum ASDs, which are commonly diagnosed in adulthood, primum ASDs are more commonly diagnosed early in life because the atrial defect is usually large and the associated malformation of the AV valves often results in significant left AV valve regurgitation and heart failure symptoms. However, patients may be asymptomatic until adulthood if the degree of AV valve regurgitation is mild.

The cardiac crux and primum ASDs are well visualized in the apical 4-chamber view with TTE imaging. TEE is rarely necessary to establish the diagnosis, although it may be useful for further characterization of the AV valve. In AVSD, there is a common AV valve with five leaflets, rather than distinct mitral and tricuspid valves ( Fig. 41.7 ). The common AV valve is divided into left and right AV valves by the atrial or ventricular septum ( Table 41.3 ). The common AV valve may override both ventricles, or it may be unbalanced, predominantly draining into one fully formed ventricle and draining only partially into a hypoplastic second ventricle. Abnormal septal attachments are common.

The common AV valve results in anterior displacement of the aortic valve and elongation of the LVOT, and distortion of the LVOT may result in tunnel-like subaortic stenosis ( Fig. 41.8 ). Pulsed-wave Doppler imaging of the LVOT demonstrates flow acceleration in these cases. The term gooseneck deformity has been used to describe the angiographic appearance of the LVOT in patients with AVSD.