Abstract
Atrial level defects are reviewed distinguishing between true atrial septal defects and venous septation defects. Transthoracic and transesophageal approach to imaging these defects is reviewed with image and video examples.
Keywords
atrial septal defect, congenital heart disease, device closure, sinus venosus defect
Introduction
Atrial septal defects (ASDs) occur in 0.1% of the population and represent the largest group of congenital defects in the adult population. Echocardiographic evaluation of ASDs should include characterization of the defect, evaluation for additional associated lesions, and description of the physiologic effect of the ASD. Here we characterize a standardized approach as well as highlight some common pitfalls.
Anatomy/Embryology
Atrial septation occurs early in embryologic formation and is nearly complete by 2 months’ gestation. Embryologic formation of the atrial septum is a complex series of changes but is simplified here. Septation occurs with the formation of two membranes. The first membrane that occurs on the left atrium (LA) side is called the septum primum, and the second membrane is the septum secundum. Initially, these membranes function to allow for continuous inferior vena cava (IVC)-LA flow in utero (via the foramen ovale). After birth, the membranes fuse (in the majority of patients) to form the fossa ovalis.
Patent Foramen Ovale
In most patients, the foramen ovale closes by the second month of life. In adulthood, persistence of a patent foramen ovale (PFO) is a variant existing in 20%–25% of the general population. Although the clinical importance of PFO is unclear, there are some specific circumstances when diagnosis is important. Examples include patients with right atrial hypertension (e.g., pulmonary hypertension), pre-cardiopulmonary bypass, in anticipation of procedures that require interatrial access (such as left atrial ablation), and in patients with recurrent embolic events. On transthoracic echocardiogram (TTE), diagnosis can be made with either color Doppler or agitated saline contrast.
- 1.
Color Doppler: On TTE, the interatrial septum is best imaged in the apical four-chamber view, subcostal four-chamber, and the parasternal short-axis view at the aortic valve level. However, because of the frequently poor color Doppler signal in the subcostal four-chamber position in adults, this image can have low sensitivity, particularly in the setting of abdominal obesity. Color flow typically has a tunnel-like appearance. On transesophageal imaging, the atrial septum is best imaged in the bicaval view. Typically, a clockwise rotation of the probe, which scans the septum and fossa ovalis from left to right, is needed to detect and identify the exact location of the PFO. In both TTE and transesophageal echocardiography (TEE), a low Nyquist limit with a high frame rate is required to detect the low velocity, intermittent flow of a PFO.
shows an example on TTE. (A TEE example is shown in Chapter 17 , Fig. 17.8 , and 
.)
- 2.
Agitated saline contrast: using the same views for color Doppler, saline contrast can be used to determine the presence of an interatrial connections. Imaging should begin prior to the opacification of the right atrium (RA). PFO flow typically occurs 3–5 beats after right ventricle (RV) opacification. In contrast, ASD flow is even faster, almost instantaneous (
). In contrast, extracardiac shunting occurs after five beats. In patients with very low right atrial pressure, additional maneuvers such as Valsalva are requisite to increase the RA pressure enough to force right to left flow of the saline contrast across the septum.
Clinical Presentation of Atrial Level Defects
Unrepaired atrial level defects presenting in adulthood can be found incidentally, as many patients are asymptomatic. If symptomatic, the most common complaints include dyspnea, fatigue, palpitations, and chest pain. Clinical findings suggestive of an unrepaired atrial level defect include a pulmonary flow murmur and fixed split S2. On electrocardiogram (ECG) patients often have right bundle branch block.
Atrial Septal Defects
Defects in the atrial septum are the most common type of adult congenital heart disease, but there are actually only two types of defects of the true atrial septum: secundum and primum ( Fig. 43.1 ). Secundum ASDs occur as a result of a deficiency in septum primum and occur near the center of the atrial septum. In contrast, primum ASDs result from incomplete endocardial cushion formation. For this reason, primum ASDs are often associated with other lesions that result from incomplete endocardial cushion formation such as inlet ventricular septal defects and cleft atrioventricular (AV) valves.
Venous Septation Defects
While venous septation defects are physiologically similar to ASDs, they are anatomically distinct. Understanding of this anatomic distinction is critical to appropriate image acquisition and interpretation. Defects can occur at the insertion of the superior vena cava (SVC) or the IVC and the edge of the atrial septum. Typically, there is also incomplete septation of the associated right pulmonary vein. Coronary sinus defects occur as the coronary sinus passes behind the LA a defect (“unroofing”) in this wall can occur, causing a communication between the LA and coronary sinus and flow from right to left at the atrial level.
Imaging Approach
The approach to imaging of atrial level shunting focuses on anatomic characterization and evaluation of physiologic effects ( Fig. 43.2 ). Additionally, complete imaging of the rims of the secundum ASD is particularly important, as it is currently the only type of ASD that is a candidate for percutaneous closure.
- 1.
Anatomy of the defect: The initial approach is to describe the location and dimensions of the defect. Detailed description of secundum defects are particularly important due to the potential for percutaneous closure. TEE is needed for complete evaluation of the secundum ASD rims, and three-dimensional (3D) imaging can be very useful to reconstruct the entire defect.
- a.
Secundum: Visualization of each “rim” of tissue in reference to its associated structure. Typically, the rims are identified as the aortic, tricuspid, superior, posterior, and inferior ( Fig. 43.3 ). For adequate device position, each rim should measure at least 0.5 cm. Often the anterior, or retro-aortic rim, is the smallest as shown in Fig. 43.4 .
FIG. 43.3
Transesophageal approach to the evaluation of secundum atrial septal defects. ASD , Atrial septal defect; IVC , inferior vena cava; LA , left atrium; LV , left ventricle; PFO , patent foramen ovale; RA, right atrium; RUPV, right upper pulmonary vein; RV, right ventricle; SVC , superior vena cava.
Courtesy of Bernard E. Bulwer, MD, FASE; Data from Silvestry FE, Cohen MS, Armsby LB, et al. Guidelines for the echocardiographic assessment of atrial septal defect and patent foramen ovale: from the American Society of Echocardiography and Society for Cardiac Angiography and Interventions. J Am Soc Echocardiogr. 2015;28[8]:910–958.
FIG. 43.4
Transesophageal imaging of a secundum atrial septal defect (ASD). (A) Midesophageal transesophageal echocardiography (TEE) image at ∼45 degrees demonstrates a deficient retroaortic rim (red arrow) . (B) 3D TEE view demonstrating all rims as viewed from the left atrial aspect. Note that use of 3D TEE can avoid underestimation of ASD diameters due to 2D imaging alone, although balloon-sizing is also used to measure the size of the balloon-stretched defect. Ao, Aortic valve; AV, aortic valve; RA, right atrium.
B, Modified from Solomon D, Wu J, Gillam L. Echocardiography. In: Mann DL, Zipes DP, Libby P, et al., eds. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine. 10th ed. Philadelphia: Elsevier, 2015:179–260.
- a.
- 2.
Evaluation of flow: Similar to PFOs, ASDs and venosus defects are typically first diagnosed with color Doppler. Flow is best visualized with low Nyquist and high frame rates. Additionally, the direction of flow should be evaluated typically with color flow Doppler, but pulsed wave Doppler is also used. Direction of flow can change typically due to either:
- a.
Change in ventricular compliance: In most cases, flow is typically left to right owing to the very compliant nature of the RA and RV. Compliance can change due to ventricular scar formation (e.g., due to volume loading), and shunt direction can change.
- b.
Change in pulmonary vascular resistance (pulmonary hypertension): if the patient develops pulmonary hypertension due to increasing pulmonary vascular resistance, atrial level shunting can shift from right to left (Eisenmenger syndrome).
- c.
In rare situations, right-to-left shunting can occur due to anatomic factors (e.g., a prominent eustachian valve can direct IVC flow preferentially across the ASD) or mechanical distortion of the atrial septum (e.g., after pneumonectomy, ascending aortic aneurysm) or in the setting of pericardial effusion or constriction. In these settings, pulmonary pressures are not necessarily elevated. Changes can even occur transiently with body position, causing a “platypnea-orthodeoxia” syndrome in which a patient desaturates when going from recumbent to upright position.
- a.
- 3.
Cardiac effects of the defect: As shunts are typically left to right, the RA and RV typically dilate. Characterization of right atrial dilation as well as right ventricular dilation and function are useful in determination of the physiologic effects of the defect. When there is persistent flow and development of pulmonary hypertension or right sided chamber dilation, tricuspid regurgitation can develop.
- 4.
Evaluation for associated defects: Most commonly, secundum ASDs are associated with pulmonary stenosis, ventricular septal defects, partial anomalous pulmonary veins, and Ebstein anomaly. Primum ASDs are often seen together with other endocardial cushion defects such as inlet ventricular septal defect, cleft mitral valve, and cleft tricuspid valve. In addition, evaluation for additional ASDs is done by interrogating the entire septum. Other septation defects can be present, including venous septation defects ( Table 43.1 ).
