Atrial Septal Defect (ASD)

An atrial septal defect (ASD) is an opening that allows blood to shunt between the left and right atria. It’s the third most common form of congenital heart disease (an estimated 56 per 100,000 live births by older figures, closer to 100 per 100,000 with improved echocardiographic detection of clinically silent defects), and — after patent foramen ovale — the most common cause of an atrial-level shunt overall.

A Nomenclature Point Worth Understanding

Clinically, four lesions get grouped together as “types of ASD,” but strictly speaking only one of them is a true defect of the atrial septum. Only a defect within the oval fossa is a true atrial septal defect. The atrial septum itself is a surprisingly small area — essentially just the floor of the oval fossa and its muscular rim — and as long as the fossa’s flap valve overlaps its rim (regardless of whether they’re actually fused) and left atrial pressure exceeds right, there’s no shunt. This isn’t a pedantic distinction; it’s made independently by two different textbook traditions — Anderson’s strictly morphological school and Van Praagh/Geva’s developmental school — which is a notable degree of agreement across otherwise quite different classification systems:

• A sinus venosus defect is not, anatomically, an atrial septal defect at all. It doesn’t allow direct communication between the atrial cavities through a hole in the septum — instead, a caval vein (almost always the SVC) has biatrial connections that override the rim of the oval fossa, producing an extraseptal interatrial communication. A sinus venosus defect should not be diagnosed unless it can be positively shown that the communication lies outside the confines of the oval fossa.
• Primum “ASD” is a form of partial atrioventricular canal (endocardial cushion) defect — an interatrial communication between the anteroinferior margin of the fossa ovalis and the AV valves, with two separate AV valve annuli and no AV-canal-type VSD. It belongs conceptually with Atrioventricular Septal Defect, not with true ASDs.
• A coronary sinus defect is a communication through the mouth of the coronary sinus itself, ranging from a simple fenestration to complete unroofing — again outside the true atrial septum.

None of this changes how these lesions are diagnosed or managed day to day, and this page covers all four under the umbrella term “ASD” as is standard clinical practice — but the distinction explains some real diagnostic pitfalls covered below, particularly around sinus venosus defects and neonatal “pseudo-shunts.”

Types and Epidemiology

Type	Location	Relative frequency	Key associations
Secundum	Central portion of the septum, within the oval fossa	Most common true ASD; size ranges from several mm to 2–3 cm	Mitral valve prolapse; a “high” secundum defect can reach the SVC-RA junction but, unlike a sinus venosus defect, does not involve the right upper pulmonary vein
Primum	Anteroinferior margin of the fossa, adjacent to the AV valves	Second most common	Cleft anterior mitral leaflet; part of the AV septal defect spectrum
Sinus venosus	Outside the oval fossa — ~87% between the right upper pulmonary vein and SVC (SVC-type); less commonly an inferior IVC-type	Roughly 4–11% of ASDs	Partial anomalous pulmonary venous return, almost always involving the right upper (and sometimes right lower/middle) pulmonary veins — see Pulmonary Venous Anomalies for the broader spectrum
Coronary sinus	Through the mouth of the coronary sinus	Rarest type	Unroofed coronary sinus; persistent left SVC draining to the roof of the left atrium

Most secundum ASDs are sporadic, but familial clusters exist with several described gene associations (NKX2.5, GATA4, MYH6) and syndromic associations (Holt-Oram, Noonan, Down, Budd-Chiari, Jarcho-Levine syndromes, among others).

Physiology and Shunt

The shunt is predominantly left-to-right, driven by the normally higher compliance of the right ventricle relative to the left. A brief flow reversal can occur during atrial contraction/early ventricular systole, but this doesn’t change the net shunt direction in an uncomplicated ASD. Chronic right heart volume overload from a large, long-standing shunt is what eventually drives right atrial and ventricular enlargement, and — in a minority of patients — progressive pulmonary vascular disease that can eventually reverse the shunt (Eisenmenger physiology).

Clinical Findings

• Auscultation — a pulmonary flow (ejection-type) murmur from increased flow across the pulmonary valve, and a fixed split S2 that doesn’t vary with respiration, reflecting delayed RV emptying that’s already maximized by the chronic volume load.
• ECG — incomplete right bundle branch block, right atrial enlargement (peaked P waves in II, III, aVF), and right axis deviation are typical of secundum-type defects. Primum defects instead typically show left axis deviation and can show first-degree AV block, reflecting the different conduction axis displacement in AV-canal-spectrum lesions.
• Chest X-ray — cardiomegaly (RA, RV, and main PA enlargement) with pulmonary plethora from increased pulmonary blood flow.
• Cardiac catheterization — an oxygen step-up at the right atrial level compared with the venae cavae confirms a left-to-right shunt at that level, and allows direct Qp:Qs quantification via oximetry (in addition to the echocardiographic Doppler method below).

Echocardiographic Findings

The atrial septum is imaged from several acoustic windows — subxiphoid, apical, left parasternal, and high right parasternal — and 2D and 3D imaging are used together with color and spectral Doppler to evaluate the fossa ovalis, sinus venosus region, and coronary sinus. A defect seen from one window has to be confirmed from other windows before it’s called real.

A View-by-View Approach

• Subxiphoid views are the workhorse. The normally oriented septum is relatively echo-reflective from here, which minimizes false dropout of the acoustic signal — the artifact that can lead to an erroneous ASD diagnosis. Image in the long-axis and short-axis planes, and use “in-between” transducer angles as well. A secundum ASD appears as a defect within the fossa ovalis, confirmed by transseptal flow on color Doppler. In the long-axis view, a secundum ASD is not contiguous with the posterior RA free wall or the right pulmonary veins — a defect that is suggests a right atrial-type sinus venosus defect instead. In the short-axis view, the superior limbic band of the fossa ovalis separates a secundum ASD from the SVC and right upper pulmonary vein. This view is also the best for judging RV volume load (RV enlargement with diastolic septal flattening) and RV hypertension (systolic septal flattening).
• Parasternal short-axis view. Helpful for imaging the atrial septum and for measuring the anteroposterior diameter of the defect, and for looking at RV volume and pressure load. A low left parasternal short-axis view can often image the septum adequately even when the subxiphoid window is poor.
• High right parasternal window. Ideal for the fossa ovalis and the superior sinus venosus area, because those structures sit perpendicular to the ultrasound beam. Placing the patient in the right lateral decubitus position makes this window easier to obtain.
• Apical four-chamber view. Not ideal for the fossa ovalis — the beam runs parallel to the septum, so there’s a real risk of false dropout. It’s still useful for measuring the tricuspid regurgitation jet velocity (to estimate RV systolic pressure) and for picking up contrast signals during a bubble study: bubbles in the LA and LV indicate right-to-left flow, while a negative jet in the RA indicates left-to-right flow.
• 3D imaging gives en face views of the defect from the right and left atrial perspectives, with better depth perception and a clearer relationship to neighboring structures, and shows how the defect’s size changes through the cardiac cycle. Restricted transthoracic windows can limit it in some patients, but TEE usually gives excellent 3D quality.

Key 2D Findings

• RA and RV enlargement from chronic volume overload
• Paradoxical (flattened) septal motion of the interventricular septum, from RV volume overload
• Pulmonary artery enlargement, from the increased pulmonary blood flow
• A cleft anterior mitral leaflet, when present, points toward a primum defect

M-Mode

M-mode over the interventricular septum shows the paradoxical septal motion pattern characteristic of RV volume overload — flattening or reversed motion of the septum during systole, distinct from the septal flattening pattern seen with RV pressure overload.

Preoperative Assessment: Key Elements

A complete study should cover: the size, location, and number of defects by imaging and color Doppler, with the diameter measured from all views; the septal rims and total atrial septal length — superior and inferior rims from subxiphoid and right sternal border bicaval views, anterior (retroaortic) and posterior rims from parasternal short-axis views, and total septal length from the subxiphoid in-between and bicaval views, the right sternal border bicaval view, and the apical four-chamber view; ASD flow by color Doppler from all views, with spectral Doppler (and the mean transseptal gradient if flow is high-velocity) where the angle allows; pulmonary venous return, including the high right parasternal window in long- and short-axis to rule out an accessory connection to the SVC and to exclude partial anomalous pulmonary venous connection to a systemic vein; RV pressure, estimated from tricuspid and pulmonary regurgitation jet velocities and from the systolic septal configuration; and RV size and function, with attention to dilation and volume overload. En face 3D views from the RA and LA perspectives are added when feasible.

Imaging by Defect Type

• Secundum ASD — a defect within the fossa ovalis, best from subxiphoid and high right parasternal windows as described above.
• SVC-type sinus venosus defect — best seen from the subxiphoid short-axis and high right parasternal views, as a communication between the upper right corner of the LA (where the right upper pulmonary vein enters) and the SVC just above its RA junction. The right pulmonary artery appears as an oval vessel crossing just above the defect, behind the SVC. Color Doppler shows flow from the LA through the SVC into the RA and confirms pulsatile flow in the right pulmonary artery. A transverse view of the chest at the cardiac end of the SVC from a left or right parasternal window shows the unroofed sinus venosus septum — absent tissue between the right upper pulmonary vein and the SVC. The fossa ovalis is intact.
• Right atrial-type sinus venosus defect — best from the subxiphoid long-axis and parasternal short-axis views, as a posterior–inferior atrial defect with no posterior margin, near the IVC–RA junction; the fossa ovalis isn’t involved, and the right middle and lower pulmonary veins may drain to the RA.
• Coronary sinus defect — often first suspected in a patient with ASD physiology and an enlarged coronary sinus ostium, seen as an inferior interatrial communication just above and slightly anterior to the IVC–RA junction. It can be mistaken for a primum ASD, but the anterior mitral leaflet is intact and the other features of an AV canal defect are absent. A coronary sinus that can’t be seen at all means it’s completely unroofed; if part of the sinoatrial septal tissue is visible, it’s partially unroofed. Always establish whether a persistent left SVC is present (Raghib syndrome): its LA termination is anterior to the left upper pulmonary vein and posterior to the LA appendage. Injecting agitated saline in the left arm while imaging from the apical four-chamber view shows contrast in the left upper corner of the LA first, then in the RA.

How to Diagnose It: A Practical Sequence

1. Start from the subxiphoid window in both long-axis and short-axis planes, using in-between angles, and look for a defect within the fossa ovalis with color Doppler flow across it.
2. Confirm from a second window — low left parasternal or high right parasternal — before believing it, since parallel-beam dropout can create a false defect.
3. Locate it precisely: within the fossa ovalis (secundum), cranial to the superior limbic band and tied to the SVC (sinus venosus), posterior–inferior near the IVC (right atrial-type), inferior at an enlarged coronary sinus ostium, or low with a cleft mitral valve (primum).
4. Trace the pulmonary veins and look for anomalous connection, especially with a sinus venosus pattern.
5. Measure the defect and all rims, and count the defects — multiple defects matter for planning closure.
6. Judge the consequences: RA and RV size and function, diastolic septal flattening for volume load, systolic septal flattening and TR/PR jet velocities for pressure load, and Qp:Qs (below).
7. If a shunt is suspected but not seen, use agitated saline, ideally with a Valsalva release.
8. If transthoracic windows are inconclusive, move on to TEE, or to cardiac MRI in adults.

Doppler and Qp:Qs Quantification

Color flow Doppler identifies the shunt jet across the defect, and pulsed- and continuous-wave Doppler characterize its velocity and direction; turbulent, increased-velocity flow in the main pulmonary artery reflects the elevated volume passing through it. Right-sided regurgitant lesions (tricuspid and pulmonic regurgitation from annular stretching) and, separately, mitral regurgitation from an associated cleft mitral valve should both be assessed.

Qp:Qs is calculated, not estimated, from Doppler stroke volumes at two separate sites:

• Qp (pulmonary flow) = cross-sectional area of the PA × VTI of PA flow (assuming a circular cross-section)
• Qs (systemic flow) = cross-sectional area of the LVOT × VTI of LVOT flow

A ratio of 1:1 is normal; ≥1.5:1 is the conventional threshold for a hemodynamically significant shunt, and this figure feeds directly into the closure criteria discussed below.

Contrast Echocardiography

Agitated saline contrast is used when a shunt is suspected but not clearly seen on color Doppler — most often for smaller or borderline defects. Two distinct signs can be seen, and they’re worth distinguishing rather than conflating:

• Negative contrast — an unopacified stream of blood (from the left-to-right shunt) washing into the contrast-filled right atrium. In a predominantly left-to-right shunt, this is often the more reliably seen sign.
• Direct right-to-left bubble passage — bubbles appearing in the left atrium, seen more consistently when RA pressure is transiently raised above LA pressure (coughing or a Valsalva release), which is why provocative maneuvers are performed routinely, not just when a shunt is initially suspected.

A properly performed study should: use a view where right heart contrast won’t shadow the left atrium; demonstrate contrast in the left heart within three beats of its appearance in the right heart to support a genuine right-to-left shunt; record a longer clip to confirm timing accurately; and include at least two injections — one at rest and one with a provocative maneuver.

Transesophageal Echocardiography

TEE is superior for smaller defects, difficult transthoracic acoustic windows, and is specifically preferred for identifying sinus venosus defects and associated anomalous pulmonary venous return, both of which transthoracic imaging can miss. The relevant views are all midesophageal, not transgastric — transgastric views are oriented for LV short-axis and inflow assessment, not the atrial septum or venae cavae:

• ME four-chamber view (~0°) — shows the atrioventricular and posterosuperior rims of the defect
• ME aortic valve short-axis view (~60°) — shows the aortic and posteroinferior rims
• ME bicaval view (~90–120°) — shows the SVC and IVC rims, and is where sinus venosus defects and anomalous pulmonary venous connections are most reliably confirmed

Echocardiographic Guidance of Closure

Because fluoroscopy images the atrial septum poorly, echo guidance is essential for transcatheter closure of a secundum ASD or PFO. TEE has been used for this since the 1980s, usually under general anesthesia with an expert echocardiographer; intracardiac echo (ICE) is an alternative that the catheterization team can perform, though it requires an expensive disposable catheter, so the choice depends on institutional experience and resources.

• Before deployment — define the location and size of the defect(s) and their relationship to the AV valves, venae cavae, and right pulmonary veins; identify multiple defects; measure the rims and total septal length; many centers also measure the stretched diameter using a balloon across the defect. Evaluate the shunt with color and spectral Doppler, image the pulmonary veins to exclude partial anomalous connection, and grade AV valve regurgitation as a baseline for comparison afterward.
• After surgical or device closure — image the septum from multiple planes to check patch position; rule out residual ASD by color Doppler from multiple windows; check device position, with particular attention to seeing both the left and right atrial disks; rule out impingement on the systemic and pulmonary veins, AV valves, and aortic root; rule out thrombus or vegetation on the patch or device; and re-estimate RV pressure and RV size and function.

Adults and the Prenatal Setting

In adults, the main challenge is technical — getting clear images and Doppler signals through subxiphoid or transthoracic windows. TTE is even poorer at diagnosing sinus venosus defects in adults: only about one in four cases were correctly diagnosed in one report. Right heart chamber enlargement with diastolic flattening of the interventricular septum (RV volume overload) should prompt suspicion of an atrial-level shunt, and when TTE is inconclusive, TEE gives clear imaging of the fossa ovalis, sinus venosus septum, pulmonary vein ends, and coronary sinus. Cardiac MRI is a noninvasive alternative that avoids sedation, shows the extracardiac veins, and quantifies RV size and function and the pulmonary-to-systemic flow ratio; TEE’s advantage is spatial resolution, which clearly shows small secundum defects and whether a PFO is present.

Prenatally, a primum ASD can usually be diagnosed reliably, but secundum ASD is much less reliable — agreement between fetal and postnatal echo was only modest — because small-to-moderate secundum defects are hard to distinguish from the normal flow through the foramen ovale.

Patent Foramen Ovale

A PFO — the persistent, unfused space between the septum primum and septum secundum — is a normal feature of fetal circulation that closes anatomically in roughly 70–75% of adults; the remainder retain a probe-patent foramen. Two prevalence figures are both worth knowing, since they answer different questions: echocardiographic detection at rest with contrast finds a PFO in only about 5% of the general population, but that rate rises to roughly 25% when a provocative maneuver transiently raises RA pressure during the injection — a figure that lines up well with autopsy-based prevalence data. A PFO becomes clinically relevant as a route for paradoxical embolism and stroke, and as a potential (though now less commonly needed, given TEE and color Doppler sensitivity) site for right-to-left shunting when right-sided pressures are elevated.

Atrial Septal Aneurysm

An interatrial septal aneurysm is defined specifically as more than 15 mm of total excursion of the fossa ovalis region from the septal plane, in the absence of chronically elevated LA or RA pressure. That last qualifier matters: a volume-overloaded, dilated right atrium can stretch the rims of the oval fossa and produce a bulging, shunt-permitting septum that resolves once the underlying volume overload is corrected — this is a real, described mechanism behind some “spontaneous closures” of apparent ASDs reported in the neonatal period, and it isn’t a true septal aneurysm. A genuine septal aneurysm carries a high likelihood (up to 90%) of an associated fenestration or PFO, and is itself a recognized potential source of both shunting and embolism.

Treatment Considerations

Indications for closure follow a structured, multi-parameter framework rather than a single cutoff:

	Class I (appropriate)	Class IIB	Class III (inappropriate)
Symptoms	Symptomatic	—	—
Right heart size	Enlarged	—	—
Qp:Qs	>1.5:1 (net left-to-right)	—	Under 1.0 (net right-to-left)
PA pressure	Under 50% systemic	>50% systemic	>2/3 systemic
PVR	Under one-third systemic	>1/3 systemic	>2/3 systemic

The primary contraindication is irreversible, severe pulmonary arterial hypertension with no remaining left-to-right shunt. During a closure attempt, an occlusion balloon test — transiently closing the defect while monitoring hemodynamics — can confirm the patient will tolerate permanent closure; the procedure is aborted if hemodynamic instability or acute pulmonary edema develops.

• Surgical closure — by direct suture or patch (pericardial or synthetic material), performed by surgeons with congenital heart disease expertise. Surgery remains the only recommended approach for primum, sinus venosus, and coronary sinus defects — secundum ASD is the only type where percutaneous closure with current devices is indicated, with surgery available as an alternative.
• Transcatheter (percutaneous) closure — for secundum ASDs with adequate anatomic rims. Device selection is based on direct 2D/3D TEE measurement of the defect (increasingly preferred over the older balloon “stop-flow” sizing technique), with 3D TEE particularly well suited to characterizing ASD size, shape, and rim anatomy from an en face view. There are six anatomic rims to assess (clockwise: SVC, aortic/anterior, atrioventricular, IVC, posteroinferior, posterosuperior) — a minimum rim width is needed to anchor the device securely, absence of the IVC rim is considered a contraindication to device closure, and absence of the aortic rim is a major risk factor for device erosion into surrounding structures.

Post-Closure Follow-Up

• Paradoxical or flat septal motion can persist after successful closure — a normal finding, not evidence of a residual problem on its own.
• Small residual color Doppler flow through a percutaneous device is normal immediately after closure and typically resolves as the device endothelializes over subsequent weeks; a peridevice leak at the edges between the device and the native rims is the abnormal finding to distinguish this from.
• Follow-up imaging should also confirm device position and screen for complications (thrombus, erosion), and patients are typically placed on antiplatelet therapy for several weeks after percutaneous closure.

Clinical Importance

Untreated or unrecognized ASD can lead to right-sided heart failure, atrial arrhythmias (particularly atrial fibrillation as patients age), progressive pulmonary hypertension with eventual Eisenmenger physiology, and paradoxical embolism with stroke. See Right Atrium Evaluation and Right Ventricle Evaluation for how the resulting chamber remodeling is assessed in more detail, Pediatric Transthoracic Echocardiography for the segmental approach used to characterize atrial septal anatomy systematically alongside other congenital findings, and Ventricular Septal Defect for the parallel shunt-lesion framework on the ventricular side. ASD frequently coexists with other shunt lesions — see Patent Ductus Arteriosus for another commonly-associated left-to-right shunt.