What are the typical echo findings of an atrial septal defect (ASD)?

Echocardiographic Findings of Atrial Septal Defect

Transthoracic echocardiography (TTE) is the primary diagnostic imaging modality for ASD, requiring 2-dimensional imaging of the atrial septum from parasternal, apical, and subcostal views with color Doppler demonstration of shunting across the defect. 1

Essential 2D Echocardiographic Features

Direct Visualization of the Defect

• Echo dropout at the mid-portion of the interatrial septum represents the actual defect, with sharp demarcation of the remaining septal edges 2
• The entire atrial septum from the superior vena cava orifice to the inferior vena cava orifice must be visualized to detect sinus venosus defects or extension of large secundum defects 1
• Subcostal views with deep inspiration and high right parasternal views are particularly helpful for imaging ASD in adults 1

Color Doppler Findings

• Color jet across the interatrial septum demonstrates active shunting, typically in the fossa ovalis area for secundum ASDs 1
• The maximal dimension of the transseptal color jet correlates well with the true orifice size (r = 0.745 with surgical measurements) 3
• Predominantly left-to-right flow with peaks in late systole and atrial systole is the typical pattern in patients with sinus rhythm and no pulmonary hypertension 4, 5
• Transient low-velocity right-to-left flow may occur in early systole, mid-diastole, and late diastole 4

Secondary Cardiac Changes

• Right ventricular volume overload with RV enlargement is the hallmark finding indicating hemodynamically significant shunting 1
• Paradoxical interventricular septal motion due to RV volume overload 2
• Right atrial enlargement 1
• The RV dimension and Qp/Qs ratio increase proportionally with defect size 2

ASD Type-Specific Features

Secundum ASD

• Located centrally in the fossa ovalis region 1, 6
• Single or multiple defects visible on en-face views 1

Primum ASD

• Defect in close proximity to atrioventricular valves with no offset between the valves 1
• Associated with varying degrees of left atrioventricular valve malformation 6

Sinus Venosus Defects

• Superior type (5% of ASDs): defect near the superior vena cava with associated anomalous drainage of right upper pulmonary veins 1
• Inferior type (<1%): defect at the mouth of the inferior vena cava involving the posterior-inferior atrium 1
• Often missed by TTE due to superior location; requires TEE for definitive diagnosis 1

Role of Transesophageal Echocardiography

TEE becomes necessary when: 1

• Transthoracic images are poor quality in adults
• Exact localization and sizing of the ASD is required for intervention planning
• Measurement of septal rims is needed for device closure assessment
• Identification of all pulmonary vein connections is required
• Unexplained RV volume overload exists despite negative TTE

TEE-Specific Assessment for Device Closure

• Rim characterization is paramount: rim width <5 mm (except at the aortic side) generally precludes device closure 1
• Rim segments are named after adjacent structures: superior vena cava, aorta, coronary sinus, inferior vena cava 1
• Assessment includes: largest defect diameter, tissue strands crossing the defect, extensive Eustachian valve or Chiari network, and concomitant pulmonary venous anomalies 1

3D Echocardiographic Advantages

3D echocardiography provides en-face views of the interatrial septum from both right and left atrial perspectives, offering superior spatial orientation for complex defects. 1, 7

• 3D TEE provides more reliable sizing for complex defects compared to 2D TEE, which may lead to selecting smaller devices 1
• 3D-derived ASD area or circumference reliably predicts device size through validated equations 1
• Allows visualization of defect shape, orientation, number and position of orifices in a single live view 7

Common Diagnostic Pitfalls

False-Positive Diagnoses

• Apparent septal dropout on 2D images may mimic ASD; use contrast echocardiography or TEE to confirm 1
• Misinterpretation of vena caval inflow as shunt flow on color Doppler 1

False-Negative Diagnoses

• Sinus venosus ASDs are commonly missed due to superior location 1
• Patients with unexplained RV volume overload by TTE should undergo TEE or alternative imaging to evaluate the atrial septum and pulmonary veins fully 1
• Partial anomalous pulmonary venous drainage without ASD causes RV volume overload and may be erroneously presumed to be ASD 1

Quantitative Measurements

• Defect size measured by color Doppler jet width shows good correlation with surgical measurements (r = 0.745 for TTE, r = 0.919 for TEE) 3
• Doppler-determined left-to-right shunt flow velocity correlates fairly well with Qp/Qs ratio by oximetry (r = 0.71) 5
• Echocardiographic measurements typically show slightly smaller defects than surgical measurements 2