Atrial Septal Defect

Summary of Atrial Septal Defects

An atrial septal defect is a common congenital cardiac malformation characterized by a persistent opening in the interatrial septum that allows an initial (typically) left-to-right shunt between the atria. These communications are categorized into four primary anatomical types, with the ostium secundum defect being the most prevalent, accounting for up to ninety percent of cases. Located within the fossa ovalis, secundum defects result from a deficiency in the septum primum. In contrast, ostium primum defects are situated in the lower atrial septum above the atrioventricular valves and are part of the spectrum of endocardial cushion defects, frequently occurring in patients with trisomy 21. Sinus venosus defects occur at the junction of the superior or inferior vena cava with the right atrium and are almost invariably associated with partial anomalous pulmonary venous connections. The rarest variant is the coronary sinus septal defect, which involves an unroofing of the coronary sinus. 

The hemodynamic significance of an atrial septal defect is determined by the size of the communication and the relative compliance of the right and left ventricles. Because the right ventricle is typically more compliant than the left, blood is shunted from left to right, primarily during late systole and early diastole. This chronic volume overload leads to progressive dilatation of the right atrium, the right ventricle, and the pulmonary arteries. While infants and children remain predominantly asymptomatic due to the compliant nature of the right heart and normal pulmonary pressures, the increased flow can eventually lead to pulmonary hypertension or the development of an Eisenmenger reaction, usually manifesting in the third or fourth decades of life. Clinical signs in older children include a characteristic wide, fixed splitting of the second heart sound and a systolic ejection murmur at the upper left sternal border. Notably, this murmur does not arise from flow across the septum itself, which is low-velocity and silent, but rather from increased blood volume passing through a normal pulmonary valve, creating a relative "pulmonary stenosis" (or acceleration through the pulmonary valve which can create turbulence and an acoustic manifestation). Diagnosis is primarily established through echocardiography and color Doppler imaging, which delineate the defect’s size, location, and functional impact on the heart chambers. Electrocardiographic findings often reveal a rightward axis deviation and an incomplete right bundle branch block pattern, characteristically presenting as an rsR' pattern in the right precordial leads. In children with atrial septal defects, this pattern often represents a longer conduction pathway through a dilated ventricle rather than a true disruption of the bundle branch. Chest radiography may demonstrate cardiomegaly with right-sided dominance and increased pulmonary vascular markings in hemodynamically significant lesions. 

The natural history of secundum defects includes a high rate of spontaneous closure, particularly in those measuring less than eight millimeters during the first year of life. Consequently, elective intervention is often deferred until three to five years of age unless symptoms such as failure to thrive or recurrent respiratory infections necessitate earlier action. For anatomically suitable secundum defects with a sufficient septal rim, transcatheter device closure using nitinol-framed occluders has become the standard of care. Surgical repair remains mandatory for ostium primum, sinus venosus, and coronary sinus defects, as well as for secundum defects with deficient rims or excessive size. Long-term outcomes following timely repair are generally excellent, returning the individual's life expectancy to that of the general population.

• Superior Sinus Venosus Defect

  • Located between the base of the superior vena cava (SVC) and the right upper pulmonary vein (RUPV).

  • Frequently associated with abnormal right pulmonary venous (RPV) return to the SVC or RA.

  • In the subcostal sagittal view, the SVC appears to "override" the defect.

  • Superior sinus venosus defects often require surgical patch repair that redirects the anomalous pulmonary venous drainage toward the left atrium while preserving unobstructed SVC flow.

4. Unroofed Coronary Sinus

• A rare and unusual type of left-to-right shunt.

• Suspect this defect when a large coronary sinus is present without a left superior vena cava (LSVC), along with signs of right ventricular volume overload (RVVO).

• The defect results from incomplete formation of the wall covering the coronary sinus, allowing a shunt from the LA → coronary sinus (CS) → RA.

5. Patent Foramen Ovale (PFO) - Most frequent. 

• A persistent, flap-like opening between the atria due to incomplete fusion of the septum primum and septum secundum. 

• Though not a true ASD (as the septal tissue is present but not sealed), it can allow right-to-left shunting under certain conditions.

• PFO is a physiological remnant of normal fetal circulation where the septal tissue is present but not sealed, usually remaining functionally closed unless right atrial pressure increases (e.g., Valsalva maneuver). Ostium Secundum ASD, on the other hand, is a structural defect where there is actual missing atrial septal tissue, leading to continuous left-to-right shunting and potential volume overload in the right atrium and right ventricle. Small atrial septal defects of secundum type or PFO type tend to close in infancy but up to 25% of the population remains with an inter-atrial shunt of various significance. 

A few other elements to consider:

• A "true" ASD involves a deficiency in the actual interatrial septum (fossa ovalis), whereas sinus venosus and coronary sinus defects are technically communications outside the confines of the true septum.

• Vestibular ASD: This is a specific interatrial communication located in the muscular antero-inferior rim of the fossa ovalis, distinct from secundum or primum defects (see more information here).

PFO and Secundum inter-atrial shunt in preterm infants

We locally follow infants with an inter-atrial shunt ≥4 mm size. Otherwise, we consider the shunt a patent foramen ovale (depending also on its anatomy and presence of flap). See some examples of the patent foramen ovale in the subcostal section.

Key note to remember: Shunting at the atrial level is dependent on ventricular compliance and ventricular end-diastolic pressures. Inter-atrial shunting is predominantly a diastolic phenomenon when the atrio-ventricular valve(s) is(are) open.

As per Heidari et al: In preterm neonates (< 36 weeks GA), atrial septal defects ≥ 3 mm diagnosed in the first month of life show lower spontaneous resolution rates and higher intervention rates than in term infants, regardless of initial size. At ~15 months follow-up, overall ASD resolution was 78% in preterms, with resolution observed in 79% of small (3–5 mm), 76% of moderate (5.1–8 mm), and only 33% of large (> 8 mm) defects. Unlike term infants, defect size did not reliably predict closure in preterms, and 7% required intervention.

• In their study, they mention: "Most studies to date have examined the natural history of secundum ASDs among term-born infants. These studies have largely corroborated the findings of Radzik et al. in 1993 suggesting all ASDs < 3 mm identified in infancy by echocardiography are likely to close spontaneously [3, 6, 13, 16]. This has guided clinical practice for years with follow-up provided for infants with an ASD of ≥ 3 mm."

• Heidari N, Kumaran K, Pagano JJ, Hornberger LK. Natural History of Secundum ASD in Preterm and Term Neonates: A Comparative Study. Pediatr Cardiol. 2024 Apr;45(4):710-721. doi: 10.1007/s00246-023-03403-7. Epub 2024 Feb 16. PMID: 38366300.

• See also:

  • Radzik D, Davignon A, van Doesburg N, Fournier A, Marchand T, Ducharme G (1993) Predictive factors for spontaneous closure of atrial septal defects diagnosed in the first 3 months of life. J Am Coll Cardiol 22(3):851–853. https://doi.org/10.1016/0735-1097(93)90202-c 

  • Geva T, Martins JD, Wald RM (2014) Atrial septal defects. The Lancet 383(9932):1921–1932. https://doi.org/10.1016/s0140-6736(13)62145-5

  • Riggs T, Sharp SE, Batton D, Hussey ME, Weinhouse E (2000) Spontaneous closure of atrial septal defects in premature vs full-term neonates. Pediatr Cardiol 21(2):129–134. https://doi.org/10.1007/s002469910020

  • Ozcelik N, Atalay S, Tutar E, Ekici F, Atasay B (2006) The prevalence of interatrial septal openings in newborns and predictive factors for spontaneous closure. Int J Cardiol 108(2):207–211. https://doi.org/10.1016/j.ijcard.2005.05.023

  • Bostan OM, Cil E, Ercan I (2007) The prospective follow-up of the natural course of interatrial communications diagnosed in 847 newborns. Eur Heart J 28(16):2001–2005. https://doi.org/10.1093/eurheartj/ehm268 

Hemodynamic Consequences of an Atrial Septal Defect (ASD)

The hemodynamics of an atrial septal defect (ASD) are primarily governed by the size of the communication and the relative compliance (distensibility) of the two ventricles . Because the right ventricle is normally more compliant than the left, blood is shunted from the left to the right atrium. This creates a chronic volume overload on the right heart, leading to progressive dilatation of the right atrium, the right ventricle, and the pulmonary arteries. An ASD, with or without partial anomalous pulmonary venous return (PAPVR), leads to a left-to-right shunt at the atrial level under normal circumstances. The shunt may change depending on the relationship between LA and RA filling pressures. This may depend on RV-end diastolic and LV end-diastolic pressure, AV valves integrity, as well as degree of pulmonary/systemic venous return. The shunting of blood across an ASD is not constant but occurs in distinct phases throughout the cardiac and respiratory cycles: 

• Late Ventricular Systole: The maximal left-to-right shunt occurs during this phase, coinciding with the peak of the left atrial v wave (atrio-ventricular valves are closed), when the pressure difference between the atria is greatest. 

• Early Diastole: Flow continues from left to right as the atrioventricular valves open and the right ventricle, being more compliant, accepts a larger volume of blood than the left. 

• Transient Reversals: Brief periods of right-to-left shunting may occur at the beginning of ventricular diastole (associated with rapid systemic venous return) or at the onset of ventricular contraction. 

• Respiratory Influence: During inspiration, negative intrathoracic pressure increases systemic venous return to the right atrium while decreasing pulmonary venous return to the left atrium, which momentarily reduces the left-to-right shunt. 

• In large, unrestrictive ASDs, the mean and phasic pressures in the left and right atria tend to equalize. The right atrial pressure tracing often shows a characteristic change where the a and v waves become equal in height, accompanied by deep x and y descents.

An ASD is considered a dependent shunt because its magnitude and direction are determined by the ratio of pulmonary to systemic vascular resistances and the relative filling properties of the ventricles. In neonates, the right ventricle is still thick-walled and relatively non-compliant, which often restricts the shunt . As right ventricular pressure falls and its compliance improves during the first few years of life, the left-to-right shunt increases. Conversely, conditions that decrease left ventricular compliance—such as systemic hypertension or coronary artery disease—will increase the left-to-right shunt over time by raising left atrial filling pressures. A left to right inter-atrial shunt eventually results (typically after months/years) in right ventricular volume overload (RVVO). This does not usually happen in the neonatal period since there is still a transitional aspect to the physiology. The RVVO causes:

• • Right ventricular (RV) dilation and hyperdynamic function.

  • Flattening or paradoxical motion of the interventricular septum (IVS).

  • Main pulmonary artery (MPA) enlargement due to increased pulmonary blood flow.

  • Eventually the excess pulmonary blood flow leads to chronic remodelling of the pulmonary vasculature (usually in the 30s - 40s). Eisenmenger syndrome is the late-stage consequence of an untreated atrial septal defect (ASD) or other congenital heart defects that cause a left-to-right shunt. Over time, chronic volume overload and increased pulmonary blood flow lead to pulmonary vascular remodeling and progressive pulmonary hypertension. As pulmonary vascular resistance rises beyond systemic levels, the shunt direction reverses (bidirectional and eventually right-to-left), resulting initially in attenuation of symptoms (paradoxical balance in Qp:Qs) and eventually in cyanosis and systemic hypoxemia. This leads to right ventricular failiure and increased mortality. This typically develops in adulthood, often in the third or fourth decade of life, though the timeline varies based on defect size and pulmonary vascular response. Once Eisenmenger physiology is established, closure of the ASD is contraindicated due to the risk of worsening right ventricular failure.

• ASD is a common intracardiac shunt (considered a pre-triscupid shunt). The shunt direction is typically left-to-right due to lower RV compliance compared to LV compliance.

• The primary determinant of shunt direction and volume in an ASD is the difference in compliance between the ventricles, rather than just the absolute atrial pressures (unless there is an inflow obstacle that will inherently increase atrial pressure on the right or left atrial side). In general, because the RV is more compliant than the LV, blood preferentially flows from the left atrium to the right atrium. Even with similar atrial pressures (e.g., 4-5 mmHg), a significant shunt can exist due to compliance differences. Indeed, if an ASD is large, by definition the two atriums are connected and pressure will equalized. Flow or volume shunt will depend on the underlying ventricular compliance.

• Factors increasing left-to-right shunt through an ASD (shifting LV compliance curve upwards on Frank-Starling Curve). Displacing the curve upwards means that the LV becomes less compliant:

  • Left ventricular hypertrophy (LVH): Conditions like systemic hypertension, cardiomyopathy (infant of diabetic mother) or coarctation of the aorta stiffen the LV, reducing its compliance and increasing its diastolic pressure, thus promoting more left-to-right shunting through the ASD.

  • Aging: A normal physiological process where the LV becomes less compliant with age, which can lead to late presentation or increased significance of a previously well-tolerated ASD ("aged ASD"). Closing such an ASD without addressing underlying LV compliance issues (e.g., hypertension) can lead to left heart failure post-closure (pulmonary edema post-closure due to LA hypertension).

• Factors decreasing left-to-right shunt through an ASD  by improving LV compliance (LV compliance curve now shifts downwards):

  • Addressing a left ventricular obstacle (aortic stenosis, coarctation) will improve LV compliance and decreased left to right inter-atrial shunting.

  • Improvement in LV compliance: Treating systemic hypertension or left-sided obstructions can improve LV compliance, reducing left-to-right shunting through the ASD.

  • Improvement in systemic hypertension will decrease LV stiffeness and improve compliance. 

  • Treatment of an ischemic cardiomyopathy of the left ventricle (revascularization of coronaries), will improve relaxation and improve compliance. 

  • Treatment with beta-blockers which improves relaxation (lusitropy), will reduce the LV compliance and decrease the left to right shunting at the atrial level. 

• Factors decreasing left-to-right shunt through an ASD or even reversing the shunt by decreasing the RV compliance (RV compliance curve now shifts upwards):

  • Pulmonary Hypertension (PH) or RV outflow obstruction: Conditions that increase RV afterload or reduce its compliance (e.g., severe PH, pulmonary stenosis, post-surgical stiff RV) can lead to a right-to-left shunt or decrease the left-to-right shunt.

  • RV hypertrophy for other reasons (cardiomyopathy, Noonan's syndrome, etc.)

  • Progression of a pulmonary valvular stenosis. 

• Factors increasing left to right shunt through an ASD by shifting the RV compliance curve and improving the RV compliance (the RV becomes more compliant). Examples:

  • Postnatal decrease in PVR: The normal drop in pulmonary resistance after birth causes the RV to become less muscular and more compliant, leading to an increase in left-to-right shunting progressively. This explains why a large ASD might not be symptomatic or clinically obvious in a neonate but becomes more prominent later.

  • Treatment of pulmonary hypertension, as well as treatment of a RVOT obstruction.

  • Beta-blockers (lusitropic) given to a patient with RV hypertrophy may improve its compliance and increase the lef to right shunting at the inter-atrial level 

• In contrast to VSD, the structures that become dilated with time are the right ventricle and the pulmonary artery, while the left ventricle typically remains undilated as it only handles systemic flow. Filling Pressures: Due to the right ventricle's high compliance, the filling pressures are typically less modified in ASD compared to VSD. Patients with normally compliant right ventricles usually do not exhibit signs of right heart failure.

Key Message: 

• Shunting at the atrial level is determined by the compliance of the underlying ventricle - Atrial Level Shunting (ASD) is determined by the relative compliance of the two ventricles. The less compliant (stiffer) ventricle will receive less flow during filling, diverting blood across the ASD

Echocardiographic Goals for Atrial Septal Defects (ASD)

1. Assess Defect Characteristics

   • Measure size, location, and number of defects using imaging and color Doppler.

2. Evaluate ASD Flow

   • Assess ASD flow using color Doppler in all views.

   • In views with an acceptable interrogation angle:

     • Use Pulsed-Wave (PW) Doppler (preferred) and Continuous-Wave (CW) Doppler to measure the mean pressure gradient.

3. Identify Pulmonary Venous Connections

   • Pay special attention to:

     • Right upper pulmonary vein (RUPV) in superior sinus venosus defects.

     • Right lower pulmonary vein (RLPV) in inferior sinus venosus defects. Best views are the subcostal short (sagittal / bicaval) axis view. 

   • In sinus venosus defect, one key echo task is not only seeing the defect, but also proving pulmonary venous drainage, especially RUPV for superior sinus venosus defects. 

4. Measure Septal Rims and Total Atrial Septal Length (if secundum ASD)

   • Total septal length from:

     • Subcostal in-between and short-axis (bicaval) views.

     • Right sternal border bicaval view.

     • Apical 4-chamber view - although the angle of insonnation typically is parallel and may create acoustic shadows - as such it is not a good view to measure the actual defect.

   • Specific septal rims:

     • Retroaortic rim – Assessed in parasternal short-axis (PSAX) view.

     • Posterior rim – Assessed in PSAX view.

     • Inferior septum – Assessed in apical 4-chamber (A4C) and subcostal sagittal views.

     • Superior rim – Assessed in subcostal sagittal and right sternal border bicaval views.

5. Assess Septal Motion and Right Ventricular Volume Overload (RVVO)

   • Use M-mode to evaluate interventricular septal (IVS) motion and identify paradoxical motion due to RV volume overload.

6. Estimate Right Ventricular (RV) Pressure

   • Assess tricuspid regurgitation (TR) jet and pulmonary regurgitation (PR) jet for RV pressure estimation.

7. Evaluate Right Ventricular (RV) Size and Function

   • Obtain high-quality views of the RV to assess chamber size and contractility.

8. Post-ASD Device Closure Assessment

   • Focus on:

     • Residual atrial-level shunting.

     • Atrioventricular valves, assessing for any new regurgitation or dysfunction.

     • Pulmonary and systemic venous drainage, ensuring no interference from the device.

     • Aortic valve function, checking for any impact from device placement.

Criteria for Percutaneous Closure of an Atrial Septal Defect 

• Secundum atrial septal defect (ostium secundum ASD) 

• Body weight generally ≥10 kg (although closure may be performed in smaller infants in selected cases at experienced centers). Many centers routinely close ASDs in children weighing <10 kg when clinically indicated (e.g., failure to thrive, chronic lung disease, pulmonary hypertension), although the procedure is technically more challenging.

• Adequate inferior vena cava (IVC) rim to provide device stability. This is one of the most important anatomical criteria. Deficiency of the IVC rim substantially increases the risk of device instability or embolization and is generally considered a contraindication to percutaneous closure.

• The maximal ASD diameter + approximately 12–14 mm should be less than the total septal length to ensure that the left atrial disc can be safely accommodated without impinging on adjacent structures. The "ASD diameter + 14 mm < septal length" rule is a useful practical approximation because most occluders require adequate room for the left atrial disc. However, it is not a formal guideline criterion. Operators ultimately assess device size, left atrial dimensions, and the relationship to surrounding structures using transesophageal or intracardiac echocardiography.

• Adequate septal rims (generally ≥5 mm) surrounding the defect, except for the retro-aortic (anterosuperior) rim, which may be deficient. While an absent retro-aortic rim does not always preclude closure, it is a potential risk factor for late cardiac erosion .

• A sufficiently robust interatrial septum to provide secure device anchoring. A very thin, aneurysmal, or highly fenestrated septum may complicate device deployment and stability.

• Deficiency of multiple rims (particularly posterior or IVC rims) generally favors surgical repair. 

• Balloon Sizing Techniques during catheterization: 

  • The "pull-through" technique (using a spherical balloon) 

  • The "static" method (inflating a compliant balloon until a "waist" is seen). 

  • Stop-Flow Technique: Using the point at which color Doppler flow is abolished during gentle balloon inflation.

Indications for ASD Closure 

Closure of an atrial septal defect is recommended in patients with: 

• Symptomatic atrial septal defect. 

• Hemodynamically significant left-to-right shunt, typically defined as a pulmonary-to-systemic blood flow ratio (Qp:Qs) ≥1.5:1 associated with right ventricular volume overload (right ventricular dilation). 

• Evidence of right heart volume overload (right atrial and/or right ventricular enlargement), even in asymptomatic patients, is generally considered an indication for closure when pulmonary vascular disease is not advanced.

Ostium secundum ASD, left to right.

ASD osteum primum (in the context of an AVSD)

ASD Superior Sinus Venosus Defect