Echocardiographic Assessment of Atrial Septal Defect (ASD)

 

Echocardiographic Assessment of Atrial Septal Defect (ASD): A Practical Guide

Atrial septal defect (ASD) is one of the most common congenital heart defects encountered in adults. Echocardiography is the cornerstone of diagnosis because it not only demonstrates the defect but also determines its anatomical type, size, direction of shunting, hemodynamic significance, effects on the right heart, pulmonary artery pressure, and suitability for device closure.

A good ASD echocardiographic examination should therefore answer more than simply: “Is an ASD present?”

It should answer:

What type of ASD is present?

How large is the defect?

What is the direction and magnitude of the shunt?

Is there right ventricular volume overload?

Is pulmonary hypertension present?

Are the pulmonary veins normally connected?

Is the anatomy suitable for transcatheter device closure?

These elements form the basis of a systematic echocardiographic assessment.

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Types of Atrial Septal Defect

An ASD should first be classified according to its anatomical location.

1. Ostium Secundum ASD

Secundum ASD is the most common type and results from deficiency of the septum primum in the region of the fossa ovalis.

It is located centrally within the interatrial septum.

This is the principal ASD type that can usually be treated by transcatheter device closure when the defect size and surrounding rims are appropriate.

2. Ostium Primum ASD

A primum ASD is located in the inferior portion of the atrial septum adjacent to the atrioventricular valves.

It is part of the spectrum of atrioventricular septal defects.

Associated findings may include:

Cleft left AV valve

Left AV valve regurgitation

Abnormal AV valve morphology

AV septal deficiency

Because of its anatomy, a primum ASD is not suitable for routine transcatheter device closure and generally requires surgical treatment.

3. Sinus Venosus ASD

Sinus venosus defects are usually located near the junction of the superior vena cava and right atrium.

Less commonly, an inferior sinus venosus defect occurs near the IVC-right atrial junction.

An important association is partial anomalous pulmonary venous return, particularly anomalous drainage of the right upper pulmonary vein.

These defects can be difficult to visualize with routine TTE. TEE, cardiac CT or CMR may therefore be required when a sinus venosus defect is suspected.

4. Coronary Sinus Defect

This is a rare interatrial communication related to partial or complete unroofing of the coronary sinus.

It may be associated with a persistent left superior vena cava.

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Step-by-Step Echocardiographic Evaluation

Step 1: Identify the Defect

Multiple echocardiographic windows should be used because relying on a single view can result in missed defects or incorrect measurements.

Important TTE views include:

Parasternal short-axis view

Apical four-chamber view

Subcostal four-chamber view

Subcostal bicaval or sagittal views

Color Doppler should be added to identify flow across the interatrial septum.

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Step 2: Subcostal View — One of the Most Useful TTE Views

The subcostal four-chamber view is particularly useful for evaluating the interatrial septum because the ultrasound beam is relatively perpendicular to the septum.

This reduces the problem of acoustic dropout.

This point is important because apparent discontinuity of the atrial septum in an apical four-chamber view can sometimes represent dropout rather than a true ASD.

The subcostal view can demonstrate:

Defect location

Defect diameter

Septal margins

Direction of shunting

Multiple fenestrations

Relationship to the IVC and SVC

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Color Doppler Assessment

Color Doppler confirms flow across the defect and helps determine its direction.

Most uncomplicated ASDs produce predominantly:

Left-to-right shunting

This occurs because right ventricular compliance is normally greater than left ventricular compliance and right-sided filling pressures are lower.

The shunt may vary during the cardiac cycle.

With severe pulmonary vascular disease, the shunt may become bidirectional or predominantly right-to-left.

Therefore, the report should specify:

Left-to-right

Right-to-left

or

Bidirectional shunting.

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Spectral Doppler Across the ASD

Pulsed-wave Doppler may be placed across the defect to assess flow characteristics.

Unlike a restrictive VSD, a typical ASD is a low-pressure communication. Therefore, the velocity across the defect is generally relatively low.

The interatrial pressure gradient is small.

The Doppler pattern may vary with:

Respiration

Atrial contraction

Ventricular compliance

Pulmonary hypertension

Left ventricular diastolic pressure

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Size of the ASD

The defect should be measured in more than one imaging plane because ASDs are frequently oval rather than perfectly circular.

The measured diameter may therefore differ substantially between views.

The echo report should ideally document defect dimensions in two orthogonal axes.

Measurements should avoid including color Doppler blooming because the color jet may overestimate the true anatomical size.

Three-dimensional echocardiography is particularly useful for demonstrating the actual shape of the defect.

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3D Echocardiography

Three-dimensional echocardiography provides an en-face view of the interatrial septum.

It can demonstrate whether the defect is:

Circular

Oval

Irregular

Fenestrated

Multiple

3D TEE is particularly useful before and during device closure because it provides an anatomical view resembling the interventional or surgical perspective.

It can also help demonstrate the relationship between the defect and surrounding structures.

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The Most Important Hemodynamic Finding: RV Volume Overload

A crucial principle in ASD assessment is:

The physiological significance of an ASD is determined not simply by its diameter, but by its effect on the right heart.

A hemodynamically significant left-to-right ASD typically causes:

Right atrial enlargement

Right ventricular dilatation

Increased pulmonary blood flow

Possible tricuspid annular dilatation

Functional tricuspid regurgitation

The ESC guidelines emphasize RV volume overload as a key marker of the hemodynamic relevance of an ASD.

Therefore, whenever an ASD is detected, carefully assess RV size and function.

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Right Ventricular Volume Overload and Septal Flattening

Significant ASD produces RV volume overload.

The interventricular septum may flatten predominantly during diastole.

On parasternal short-axis imaging, this may produce a D-shaped left ventricle, particularly during diastole.

A useful conceptual distinction is:

RV volume overload → predominantly diastolic septal flattening

RV pressure overload → more prominent systolic flattening

Severe pulmonary hypertension can produce flattening throughout the cardiac cycle.

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Right Atrial Assessment

The right atrium commonly becomes enlarged because of chronic increased right-sided volume.

Assessment should include:

RA dimensions or area

RA volume when appropriate

Associated tricuspid annular dilatation

Presence and severity of TR

Long-standing ASD may eventually lead to marked RA enlargement and atrial arrhythmias.

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Estimation of Pulmonary Artery Pressure

Pulmonary artery pressure should be assessed in every significant ASD.

When an adequate tricuspid regurgitation signal is present:

RVSP = 4(TR Vmax)² + estimated RAP

In the absence of RV outflow obstruction or pulmonary stenosis:

PASP ≈ RVSP

Additional signs suggesting pulmonary hypertension include:

RV hypertrophy

Septal flattening

Dilated pulmonary artery

Short pulmonary acceleration time

Reduced RV function in advanced disease

Right-to-left or bidirectional atrial shunting

The 2020 ESC guideline recommends invasive assessment of pulmonary vascular resistance when there are non-invasive signs of elevated pulmonary artery pressure; it specifically notes an estimated systolic PAP >40 mmHg as a trigger for further evaluation.

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Qp:Qs — Quantifying the Shunt

The pulmonary-to-systemic flow ratio can be estimated by Doppler echocardiography.

The basic concept is:

Qp:Qs = Pulmonary blood flow / Systemic blood flow

For Doppler calculations:

Stroke volume = CSA × VTI

Therefore:

Qp = CSA of pulmonary annulus × pulmonary VTI

Qs = CSA of LVOT × LVOT VTI

and:

Qp:Qs =

(Pulmonary annulus CSA × RVOT/PV VTI) ÷

(LVOT CSA × LVOT VTI)

Because:

CSA = π(D/2)²

small errors in diameter measurement are squared and can produce substantial errors in calculated Qp:Qs.

For this reason, Qp:Qs should always be interpreted alongside right-heart chamber enlargement rather than in isolation.

A Qp:Qs ≥1.5:1 generally represents a physiologically important left-to-right shunt in the appropriate clinical context. Current 2025 ACC/AHA adult congenital heart disease guidance incorporates Qp:Qs ≥1.5 together with RV dilatation when determining indications for closure.

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Pulmonary Veins Must Be Assessed

Pulmonary venous connections should be evaluated whenever an ASD is diagnosed.

This is particularly important in suspected sinus venosus ASD.

A superior sinus venosus ASD is strongly associated with partial anomalous pulmonary venous return, often involving right-sided pulmonary veins.

If pulmonary venous anatomy cannot be confidently established by TTE, further evaluation with:

TEE

Cardiac CT

or

CMR

should be performed. Current ACC/AHA guidance recommends advanced imaging such as CMR, TEE or cardiac CT to define defect morphology, rims and pulmonary venous connections in adults with unrepaired ASD.

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ASD Device Closure Assessment

Before transcatheter closure of a secundum ASD, echocardiography must answer several additional questions.

Number of defects

Determine whether there is:

Single ASD

Multiple ASDs

Fenestrated septum

Aneurysmal interatrial septum

Maximum ASD diameter

Measure the defect in multiple planes.

Total interatrial septal length

This helps determine whether an appropriate closure device can be accommodated.

Surrounding rims

Assessment of the surrounding tissue rims is fundamental.

The important rims include:

Aortic rim

SVC rim

IVC rim

Posterior rim

AV valve rim

RUPV rim

A rim of approximately ≥5 mm is generally considered favorable for transcatheter closure, although a deficient aortic rim is relatively common and is not necessarily an absolute contraindication. Other deficient rims may have greater implications for device stability and suitability.

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TEE Assessment of ASD

TEE becomes particularly important when:

TTE images are inadequate

Sinus venosus ASD is suspected

Multiple defects are suspected

Pulmonary venous anatomy is uncertain

Device closure is planned

Precise rim assessment is required

TEE provides high-resolution visualization of the interatrial septum and adjacent structures.

Important TEE views include:

Midesophageal four-chamber view

Midesophageal aortic short-axis view

Bicaval view

Modified bicaval views

3D en-face views

The bicaval view is especially valuable for evaluating the superior and inferior margins of a secundum ASD.

TEE should assess the defect dynamically because the dimensions may change during the cardiac cycle.

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ASD Rims — Why They Matter

The surrounding septal rims anchor the closure device.

The aortic rim lies adjacent to the aortic root.

The AV valve rim separates the defect from the atrioventricular valves.

The SVC rim separates it from the superior vena cava.

The IVC rim separates it from the inferior vena cava.

The posterior rim separates the defect from the posterior atrial wall.

The RUPV rim lies toward the right upper pulmonary vein.

Inadequate rims, particularly important non-aortic rims, can make device closure difficult or unsafe.

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Bubble Study in ASD

Agitated saline contrast is particularly useful when right-to-left interatrial passage needs to be demonstrated.

After injection into a peripheral vein, microbubbles opacify the right atrium.

Normally they do not enter the left atrium.

Appearance of bubbles in the left atrium indicates right-to-left passage across an intracardiac communication when timing and imaging are consistent with an intracardiac shunt.

Provocative maneuvers such as Valsalva can transiently increase right atrial pressure and make right-to-left passage more apparent.

However, color Doppler is usually more useful for demonstrating the predominant left-to-right shunt of a typical ASD.

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ASD Versus PFO on Echocardiography

ASD and PFO should not be used interchangeably.

An ASD represents a true deficiency of atrial septal tissue.

A PFO represents persistence of a potential flap-like communication between septum primum and septum secundum.

Therefore:

ASD → true tissue defect

PFO → flap-like communication without true deficiency of the septal tissue

This distinction has important implications for anatomy, hemodynamics and management.

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When Should ASD Closure Be Considered?

Current adult congenital heart disease guidance emphasizes the combination of significant left-to-right shunting and RV volume overload.

The 2025 ACC/AHA guideline recommends ASD closure in adults with a significant left-to-right shunt, Qp:Qs ≥1.5, and RV dilatation when significant pulmonary arterial hypertension or LV disease is absent. For isolated secundum ASD suitable for intervention, transcatheter closure is generally preferred over surgery.

Pulmonary vascular resistance becomes particularly important when pulmonary hypertension is present.

An ASD should therefore never be closed merely because a hole is visible on echocardiography.

The entire hemodynamic picture must be considered.

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ASD With Severe Pulmonary Hypertension

This is one of the most important situations in ASD assessment.

If severe pulmonary vascular disease develops, the ASD may function as a pressure-relief pathway for the right heart.

The shunt can become bidirectional or right-to-left.

Closing the defect in irreversible pulmonary vascular disease may be dangerous.

Therefore, significant pulmonary hypertension requires careful assessment of pulmonary vascular resistance, usually with right-heart catheterization.

The decision may range from closure to fenestrated closure or no closure depending on pulmonary vascular resistance, reversibility, LV filling pressures and the overall clinical situation.

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ASD in Older Patients and LV Diastolic Dysfunction

An important problem occurs when an older patient has:

ASD

RV volume overload

Hypertension

LV hypertrophy

LV diastolic dysfunction

The ASD may partially decompress the left atrium.

Sudden complete closure can increase left atrial and LV filling pressures and potentially precipitate pulmonary edema.

In selected patients, temporary balloon occlusion with hemodynamic assessment may therefore be useful before permanent closure. Both ESC and current ACC/AHA guidance recognize the importance of evaluating this issue in appropriate patients.

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Post-Device Closure Echocardiography

After ASD device closure, echocardiography should assess:

Device position

Device stability

Residual shunt

Relationship with the aortic root

Relationship with AV valves

SVC and IVC flow

Pulmonary venous flow

Pericardial effusion

New valve regurgitation

Thrombus when clinically suspected

RV reverse remodeling during follow-up

A small residual color Doppler jet may occasionally be seen early after closure and should be documented and followed appropriately.

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What Should Be Included in an ASD Echo Report?

A structured report should include:

ASD type and location

Number of defects

Maximum dimensions in two axes

Direction of shunting

Color and spectral Doppler findings

RA size

RV size and systolic function

Evidence of RV volume overload

TR severity

Estimated PASP

Qp:Qs when technically reliable

Pulmonary venous connections

Associated congenital abnormalities

For device closure assessment:

Defect dimensions

Total septal length

Aortic rim

SVC rim

IVC rim

Posterior rim

AV valve rim

RUPV rim

Presence of multiple/fenestrated defects

For post-closure studies:

Device position

Residual shunt

Interaction with adjacent structures

Pericardial effusion

The emerging ASE adult echocardiographic reporting framework similarly emphasizes ASD number/location, dimensions in two axes, shunt direction, right-heart size, Qp:Qs when possible, pulmonary hypertension, rims/total septal length for intervention, and device position or residual shunt after repair.

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Practical Echo Checklist

When you see an ASD, think:

1. WHERE?

Secundum, primum, sinus venosus or coronary sinus defect?

2. HOW BIG?

Measure in at least two planes.

3. WHICH WAY?

Left-to-right, bidirectional or right-to-left shunting?

4. WHAT HAS IT DONE TO THE RV?

Look for RA/RV dilatation and RV volume overload.

5. HOW MUCH SHUNT?

Estimate Qp:Qs when technically reliable.

6. WHAT IS THE PULMONARY PRESSURE?

Assess TR velocity and other signs of pulmonary hypertension.

7. WHERE ARE THE PULMONARY VEINS?

Especially important in sinus venosus defects.

8. CAN IT BE CLOSED WITH A DEVICE?

Assess defect morphology, dimensions and surrounding rims.

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Key Take-Home Message

The most important echocardiographic question in ASD is not simply the size of the hole.

It is:

Is this ASD producing significant right-heart volume overload, and is the anatomy and pulmonary vascular physiology suitable for closure?

A complete ASD echocardiogram therefore combines anatomical assessment of the interatrial septum with Doppler evaluation of the shunt, assessment of RA/RV remodeling, pulmonary artery pressure, pulmonary venous anatomy, and—when device closure is contemplated—detailed evaluation of the defect dimensions and surrounding rims.

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