Right Atrium Anatomy #6: ASD Anatomy and Surgical Strategy

Atrial septal defects are not a single anatomical entity, but a spectrum of interatrial communications involving different regions of the atrial septum and venous atrial junctions. They include secundum ASD, primum ASD, sinus venosus defect, coronary sinus defect, and patent foramen ovale. Although these lesions share the common physiology of atrial-level communication, each subtype has a distinct embryologic basis, anatomical neighborhood, associated lesions, and surgical strategy [1].

From a surgical standpoint, an ASD should not be understood simply as a “hole between the atria.” The operative question is broader: what structure is missing, what venous pathway is abnormal, and what critical tissue lies near the repair?

The surgeon must define:

1. The anatomical type of the defect
2. The relationship to the fossa ovalis, AV valves, coronary sinus, SVC, and IVC
3. The presence or absence of anomalous pulmonary venous return
4. The proximity of the conduction system
5. Whether closure requires direct suture, patch closure, venous rerouting, or AV valve repair

The dominant physiology of most significant ASDs is left-to-right shunting, producing right atrial and right ventricular volume loading, increased pulmonary blood flow, and progressive atrial and ventricular remodeling [1, 2]. Over time, this can lead to exercise intolerance, atrial arrhythmias, right-sided dilation, tricuspid regurgitation, pulmonary hypertension in selected late presentations, and paradoxical embolic risk when right-to-left shunting occurs intermittently or under specific hemodynamic conditions [2, 3].

1. General Physiology of Atrial Septal Defects

In most patients with a clinically significant ASD, left atrial pressure slightly exceeds right atrial pressure. Because the pressure gradient between the atria is low, the shunt is usually not a high-velocity jet. Instead, the physiological burden is primarily volume overload.

Hemodynamic sequence

1. Oxygenated pulmonary venous blood enters the left atrium.
2. A portion crosses the atrial septal communication into the right atrium.
3. The right atrium and right ventricle receive excess volume.
4. Pulmonary blood flow increases.
5. Chronic right-sided volume loading leads to chamber dilation.
6. Atrial stretch and remodeling increase the risk of atrial tachyarrhythmias.

This is a major conceptual difference from ventricular septal defect physiology. In VSD, the dominant issue may be high-pressure shunting and pulmonary vascular exposure. In ASD, the dominant issue is often chronic, silent, low-pressure volume loading.

Clinical consequences

Clinically important ASDs may produce:

• Right atrial enlargement
• Right ventricular dilation
• Pulmonary overcirculation
• Functional tricuspid regurgitation
• Atrial flutter or atrial fibrillation
• Exercise intolerance
• Paradoxical embolism
• Late pulmonary hypertension in selected untreated patients

Closure is generally considered when there is a significant left-to-right shunt with right-sided chamber enlargement, provided pulmonary vascular resistance is not prohibitive [1, 2].

2. Secundum ASD

Secundum ASD is the most common atrial septal defect and is located at the region of the fossa ovalis. It reflects deficiency of the septum primum, septum secundum, or both. Because it is usually central within the atrial septum, it is often suitable for transcatheter device closure when the surrounding rims are adequate [1, 4].

Anatomical landmarks

Important structures around a secundum ASD include:

• Fossa ovalis
• Aortic rim
• Superior rim
• Inferior rim
• Posterior rim
• Coronary sinus
• Septal leaflet of the tricuspid valve
• Triangle of Koch
• AV node region

The Triangle of Koch is especially important from the right atrial surgical view. It is bordered by the tendon of Todaro, the septal leaflet of the tricuspid valve, and the coronary sinus orifice. The AV node lies near the apex of this triangle. Therefore, even during a routine secundum ASD closure, the inferior-posterior suture line must be placed with awareness of the conduction system.

Management strategy

Secundum ASD closure may be performed by either transcatheter or surgical means.

Transcatheter closure

Device closure is often appropriate when:

• The defect is centrally located.
• Septal rims are adequate.
• The defect size is suitable for device anchoring.
• There is no associated lesion requiring surgery.

Recent reviews emphasize that secundum ASD management has increasingly shifted toward catheter-based closure in anatomically suitable patients, while surgery remains essential for large, complex, multiple, or rim-deficient defects [4].

Surgical closure

Surgical closure remains appropriate when:

• The defect is very large.
• Multiple fenestrations are present.
• Septal rims are deficient.
• There is associated PAPVR or another lesion requiring repair.
• Device closure is unsafe or unsuccessful.

Small defects may be closed primarily. Larger defects are commonly closed with an autologous pericardial patch or synthetic patch. Patch closure reduces tension on the septal tissue and helps avoid distortion of adjacent structures.

Surgical pearl

For secundum ASD, the technical goal is simple but precise:

Close the fossa ovalis defect securely while avoiding distortion of the coronary sinus, AV valve tissue, and conduction system.

3. Primum ASD

Primum ASD is located in the inferior atrial septum adjacent to the atrioventricular valves. It belongs to the atrioventricular septal defect spectrum and is related to abnormal development of the endocardial cushions [1].

This lesion should not be considered merely a “low ASD.” Its clinical and surgical significance comes from its relationship to the AV valves and conduction tissue.

Key anatomical features

Primum ASD is associated with:

• Inferior atrial septal deficiency
• Abnormal AV septal junction
• Cleft left AV valve
• Potential left AV valve regurgitation
• Displacement of the conduction system
• Close relationship to the ventricular septal crest

The left AV valve cleft is often central to the repair. If left untreated, residual or progressive left AV valve regurgitation may become a major postoperative issue.

Surgical strategy

Repair generally includes:

1. Patch closure of the primum ASD
2. Assessment of left AV valve morphology
3. Closure or partial closure of the left AV valve cleft when appropriate
4. Protection of the displaced AV node and His bundle
5. Avoidance of AV valve stenosis or distortion

The conduction system in primum ASD is positioned differently from that in normal atrial septal anatomy. AV block is uncommon with modern techniques, but the risk is anatomically meaningful. Williams and Perry noted that ostium primum defects have a higher association with spontaneous or postoperative AV block than secundum defects, although clinically significant conduction injury remains rare with contemporary repair [3].

Surgical pearl

The repair of primum ASD is best conceptualized as:

Atrial septal reconstruction plus AV valve management, not simply ASD closure.

4. Sinus Venosus ASD

Sinus venosus ASD is fundamentally different from a secundum ASD. It is not a deficiency of the fossa ovalis. Rather, it represents an abnormal communication at the systemic venous–atrial junction and is strongly associated with anomalous pulmonary venous connection [1, 5].

There are two major forms:

1. Superior sinus venosus defect
• Located near the SVC–right atrial junction
• Frequently associated with anomalous drainage of the right upper pulmonary vein into the SVC or high right atrium
2. Inferior sinus venosus defect
• Located near the IVC–right atrial junction
• Less common
• May be more difficult to distinguish from a low secundum ASD preoperatively

Physiologic principle

In sinus venosus ASD, the shunt is not only across the atrial communication. The associated anomalous pulmonary venous return also contributes oxygenated blood to the right atrium or systemic venous pathway, increasing right-sided volume load.

Therefore, the operation must achieve two goals:

• Close the interatrial communication.
• Redirect anomalous pulmonary venous blood to the left atrium.

This distinction is critical. A sinus venosus defect cannot be repaired adequately by “closing the hole” alone if pulmonary venous drainage is abnormal.

Surgical techniques

Common surgical strategies include:

1. Single-patch repair

A patch is used to baffle anomalous pulmonary venous blood across the defect into the left atrium. This approach can be effective when the anomalous pulmonary vein enters near the atrial junction and the SVC pathway is not compromised.

2. Two-patch repair

One patch redirects pulmonary venous return to the left atrium. A second patch enlarges the SVC or right atrial pathway to prevent systemic venous narrowing.

This strategy is particularly relevant when the baffle may narrow the SVC lumen.

3. Warden procedure

In the Warden procedure, the SVC is divided above the anomalous pulmonary venous connection. The cardiac end of the SVC is closed, and the cephalic SVC is anastomosed to the right atrial appendage. An intracardiac patch then directs anomalous pulmonary venous flow into the left atrium.

This technique is especially useful when the anomalous pulmonary veins enter high into the SVC.

Outcomes and postoperative concerns

Long-term surgical outcomes for sinus venosus ASD are generally excellent. In a Mayo Clinic series of 115 patients, anomalous pulmonary venous connection was present in 97%, early mortality was low, and most patients reported clinical improvement after repair [5]. A more recent systematic review and meta-analysis including 40 studies and 1,292 patients reported very low in-hospital mortality and low pooled incidences of major complications, supporting the overall safety and effectiveness of surgical repair [6].

However, the repair is not risk-free. Important postoperative concerns include:

• Sinus node dysfunction
• Atrial arrhythmias
• SVC obstruction
• Pulmonary venous obstruction
• Residual atrial shunt
• Need for pacemaker in selected cases

In children, comparative data have suggested lower arrhythmia rates with the Warden procedure than with intracaval baffle repair in selected superior sinus venosus defects with PAPVR [7]. This supports the surgical principle that the repair should not only close the communication but also preserve sinus node function and unobstructed systemic venous return.

Surgical pearl

For sinus venosus ASD:

The central operation is pulmonary venous rerouting, not merely atrial septal closure.

5. Coronary Sinus ASD

Coronary sinus ASD, also known as unroofed coronary sinus, is rare. It results from partial or complete absence of the roof of the coronary sinus, allowing communication between the coronary sinus and the left atrium [1].

Anatomical concept

Normally, the coronary sinus drains systemic venous blood from the cardiac veins into the right atrium. In an unroofed coronary sinus, the coronary sinus communicates with the left atrium. Depending on associated venous anatomy and pressure relationships, this can create left-to-right shunting, right-to-left shunting, or systemic desaturation.

A persistent left superior vena cava is commonly associated with this lesion. If an LSVC drains into an unroofed coronary sinus, systemic venous blood may enter the left atrium, creating a right-to-left shunt and systemic desaturation.

Diagnostic importance

Coronary sinus ASD may be missed if the imaging focus remains only on the fossa ovalis. It should be suspected when there is:

• Unexplained right-sided dilation
• Dilated coronary sinus
• Persistent LSVC
• Unusual atrial-level shunting
• Systemic desaturation without another clear cause

Surgical strategy

The surgical plan depends on:

• Extent of coronary sinus unroofing
• Presence or absence of persistent LSVC
• Size of the bridging innominate vein
• Drainage pattern of the LSVC
• Relationship between the coronary sinus, left atrium, and right atrium

Possible repairs include:

1. Direct closure of the unroofed segment
2. Patch closure from within the atrium
3. Baffling of LSVC flow to the right atrium
4. Reimplantation or rerouting of systemic venous return in complex cases

Surgical pearl

For coronary sinus ASD:

The surgeon must define the systemic venous pathway before closing the communication. Otherwise, repair may inadvertently redirect systemic venous blood into the left atrium.

6. Patent Foramen Ovale

A patent foramen ovale is not a true deficiency of the atrial septal tissue. It represents persistence of the fetal communication between the septum primum and septum secundum [1].

During fetal life, the foramen ovale allows oxygenated blood from the IVC to cross from the right atrium to the left atrium. After birth, increased pulmonary venous return raises left atrial pressure, functionally closing the flap valve. Anatomical fusion may remain incomplete, leaving a PFO.

Clinical significance

Most PFOs are clinically silent and do not produce right-sided volume overload. They become clinically relevant in selected situations, such as:

• Cryptogenic stroke
• Paradoxical embolism
• Platypnea–orthodeoxia syndrome
• Elevated right-sided pressure
• Perioperative or catheter-related embolic risk

Unlike a true secundum ASD, a PFO generally does not require closure for right ventricular volume loading because it usually does not create a sustained left-to-right shunt.

7. Arrhythmia and Conduction Considerations

Atrial arrhythmias and conduction disturbances are important long-term issues in ASD patients. The mechanism is primarily chronic right atrial and right ventricular volume loading, which produces atrial stretch, electrical remodeling, and vulnerability to atrial tachyarrhythmias [3].

Factors associated with arrhythmia risk

Arrhythmia risk increases with:

• Older age at repair
• Larger shunt size
• Right atrial enlargement
• Pulmonary hypertension
• Sinus venosus anatomy
• Prior atrial surgery
• Residual shunt or venous obstruction

ASD closure reduces arrhythmia burden in many patients, but it does not fully normalize long-term risk, particularly in patients repaired later in life [2, 3].

Conduction-system risk by subtype

• Secundum ASD: AV block is rare with surgical or device closure.
• Primum ASD: AV node and His bundle displacement increases the importance of careful inferior suture placement.
• Sinus venosus ASD: Sinus node dysfunction is a particular concern, especially with older techniques or repairs near the SVC–right atrial junction.
• Coronary sinus ASD: Surgical risk depends on the anatomy of the coronary sinus, LSVC, and atrial septal reconstruction.

8. Surgical Decision-Making Framework

The best repair is determined by anatomy, not by the label “ASD.” A practical surgical framework is as follows.

Step 1: Define the defect

Identify whether the lesion is:

• Secundum ASD
• Primum ASD
• Superior sinus venosus ASD
• Inferior sinus venosus ASD
• Coronary sinus ASD
• PFO
• Multiple fenestrated atrial communications

Step 2: Define associated lesions

Look specifically for:

• Partial anomalous pulmonary venous return
• Persistent left superior vena cava
• Cleft left AV valve
• AV valve regurgitation
• Pulmonary hypertension
• Deficient septal rims
• Multiple atrial fenestrations

Step 3: Define structures at risk

The critical structures include:

• AV node
• His bundle
• Sinus node
• Coronary sinus
• SVC pathway
• IVC pathway
• Pulmonary venous pathway
• AV valves

Step 4: Choose the repair

The operative strategy may include:

• Direct closure
• Patch closure
• Device closure
• AV valve repair
• Pulmonary venous baffle
• Two-patch repair
• Warden procedure
• Coronary sinus reconstruction
• LSVC rerouting

The best operation is not necessarily the smallest or fastest repair. The best operation is the one that achieves durable closure while preserving:

• Unobstructed systemic venous return
• Unobstructed pulmonary venous return
• Competent AV valve function
• Normal conduction
• No residual shunt
• No avoidable atrial pathway distortion

9. Practical Operative Pearls

Secundum ASD

• Confirm the size, number, and rims of the defect.
• Avoid excessive tension on thin septal tissue.
• Use patch closure for large, fragile, or multiple defects.
• Protect the coronary sinus and AV node region.
• Confirm absence of residual shunt after repair.

Primum ASD

• Treat the lesion as part of the AVSD spectrum.
• Evaluate the left AV valve cleft carefully.
• Avoid excessive narrowing of the left AV valve inflow.
• Place inferior sutures with conduction-system awareness.
• Confirm AV valve competence after repair.

Sinus venosus ASD

• Always define pulmonary venous drainage preoperatively.
• Avoid SVC obstruction.
• Avoid pulmonary venous obstruction.
• Consider Warden repair for high anomalous pulmonary venous insertion.
• Monitor for sinus node dysfunction and atrial arrhythmias.

Coronary sinus ASD

• Look for persistent LSVC.
• Define the roof of the coronary sinus.
• Avoid redirecting systemic venous blood into the left atrium.
• Confirm that the final repair does not create venous obstruction or systemic desaturation.

10. Key Message

Atrial septal defects are unified by atrial-level shunting but separated by anatomy. Their surgical management requires a precise understanding of septal morphology, venous connections, AV valve anatomy, and conduction-system proximity.

A secundum ASD is primarily a fossa ovalis defect.

A primum ASD is an AV septal and AV valve lesion.

A sinus venosus ASD is a venous rerouting problem.

A coronary sinus ASD is an abnormal coronary sinus–left atrial communication.

A PFO is a persistent fetal flap-like communication rather than a true septal deficiency.

The operative principle is:

Close the communication, preserve venous pathways, protect the conduction system, and restore physiologic atrial flow.

References

[1] Geva T, Martins JD, Wald RM. Atrial septal defects. Lancet. 2014;383(9932):1921-1932.

[2] Webb G, Gatzoulis MA. Atrial septal defects in the adult: recent progress and overview. Circulation. 2006;114(15):1645-1653.

[3] Williams MR, Perry JC. Arrhythmias and conduction disorders associated with atrial septal defects. J Thorac Dis. 2018;10(Suppl 24):S2940-S2944.

[4] Shaban Q, Hijazi ZM. Secundum atrial septal defects in adults: all you need to know with an emphasis on outcome. Expert Rev Cardiovasc Ther. 2025. doi:10.1080/14779072.2025.2495235.

[5] Attenhofer Jost CH, Connolly HM, Danielson GK, Bailey KR, Schaff HV, Shen WK, et al. Sinus venosus atrial septal defect: long-term postoperative outcome for 115 patients. Circulation. 2005;112(13):1953-1958.

[6] El-Andari R, Moolla M, John K, Slingerland A, et al. Outcomes following surgical repair of sinus venosus atrial septal defects: a systematic review and meta-analysis. J Am Heart Assoc. 2024;13(12):e033686.

[7] Sojak V, Sagat M, Balazova E, Siman J. Outcomes after surgical repair of sinus venosus atrial septal defect in children. Bratisl Lek Listy. 2008;109(5):215-219.

[8] Rao PS, Harris AD. Recent advances in managing septal defects: atrial septal defects. F1000Res. 2017;6:2042.