Atrial Septal Defect (ASD) — #2 Primum ASD

Date
November 25, 2025
URL
https://x.com/WithAScalpel/status/1993361658245529758?s=20
Categories
ASDCardiac Surgery

Atrial Septal Defect (ASD) — #2 Primum ASD

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Overview

Primum atrial septal defect (ASD) is a defect of the inferior atrial septum contiguous with the atrioventricular (AV) junction. It accounts for roughly 15–20% of all ASDs and is classically regarded as part of the atrioventricular septal defect (AVSD) spectrum, rather than a simple isolated ASD.[1,3] It is frequently associated with Down syndrome and almost always coexists with abnormalities of the AV valves, most notably a “cleft” in the anterior leaflet of the left AV (mitral) valve.[1,3]

Because the lesion combines an interatrial communication with AV-valvar incompetence, the hemodynamic burden is typically greater than in secundum ASD, and AV valve function becomes a key determinant of symptoms, timing of intervention, and late outcomes.[3,4]

Embryology, anatomy, and pathophysiology

During normal development, the septum primum grows toward the endocardial cushions and eventually fuses with them, closing the ostium primum and completing atrial septation.[2,10] In primum ASD, this fusion fails. The result is a defect in the lower atrial septum, just above the crest of the ventricular septum and immediately adjacent to the AV valve plane.[2,3]

Morphologically, primum ASD is better understood as an atrioventricular septal defect with shunting confined to the atrial level.[3,5] The heart typically exhibits:

  • a common AV junction with separate right and left AV valve orifices;
  • a three-leaflet left AV valve whose “cleft” is actually the zone of apposition between bridging leaflets rather than a simple split in a normal mitral leaflet; and
  • fusion of the bridging leaflets to the crest of the ventricular septum, which closes the potential ventricular component and restricts the shunt to the atrial level.[3,5]

Hemodynamically, blood flows from the left atrium to the right atrium and often also from the left ventricle to the right ventricle via the regurgitant left AV valve. This produces combined atrial and AV-valvar volume loading of the right ventricle and pulmonary circulation, leading to right-sided dilation, increased pulmonary blood flow, and, if unrepaired, progressive pulmonary vascular disease.[1,4] Symptoms range from asymptomatic murmur in older children to overt heart failure and failure to thrive in infants with large shunts and significant left AV valve regurgitation.[4,6]

Conduction system: displaced AV node and bundle course

In primum ASD/partial AVSD, the AV node and His bundle are displaced inferiorly and posteriorly compared with their position in the classic triangle of Koch.[5] The node lies near the junction of the atrial septum and AV valve plane, close to the posteroinferior margin of the defect. From there:

  • the penetrating bundle traverses the fibrous skeleton at a more inferior point, and
  • the right bundle branch runs along the crest of the ventricular septum, in direct proximity to the ventricular edge of the patch suture line.[5]

This distinctive arrangement has two practical consequences:

  1. Patch sutures placed too low on the septal crest or too deep into the fibrous AV junction risk direct injury to the AV node or bundle.
  2. Even with meticulous technique, traction or cautery near the septal crest frequently results in postoperative right bundle branch block (RBBB); this is common but typically well tolerated.[5,6]

Understanding this altered conduction anatomy is essential to avoid complete heart block while still achieving a secure reconstruction of the AV septal region.

Indications and timing for repair

Because of the combined atrial and AV-valvar shunt, elective repair is recommended earlier than for isolated secundum ASD. Classic series advocate operation between 2 and 4 years of age, before irreversible pulmonary vascular changes or long-standing AV valve remodeling occur, with earlier intervention in infants who develop heart failure or have severe left AV valve regurgitation.[4,6]

Key indications include:

  • evidence of significant left-to-right shunt with right-sided volume overload;
  • moderate or greater left AV valve regurgitation;
  • symptoms of heart failure or failure to thrive; and
  • absence of advanced, fixed pulmonary vascular disease.[1,4,6]

Adults with unrepaired primum ASD are at risk for pulmonary hypertension, atrial arrhythmias, and reduced functional capacity; surgical repair can still be beneficial but carries higher perioperative risk and a greater likelihood of residual pulmonary vascular disease.[4,6]

Surgical repair principles

The operative strategy mirrors partial AVSD repair, combining closure of the atrial component with reconstruction of the left AV valve.[4,5,7]

  1. Approach and inspection.

    2. Through a right atriotomy, the surgeon inspects the AV junction, defines leaflet morphology, and assesses the degree of left AV valve regurgitation. Particular attention is paid to the attachments of the bridging leaflets and the crest of the ventricular septum.[5,7]

  2. Atrial septal patch.

    3. The primum defect is closed with an autologous pericardial or synthetic patch. The suture line begins along the ventricular septal crest, using tissue on the right-sided (tricuspid) aspect of the common junction to avoid the displaced AV node.[5] Sutures are kept slightly ventricular and annular-side, rather than deep into the septum, to stay away from the conduction axis that runs along the crest.[5]

  3. Continuation along the left AV valve annulus.

    4. From the septal crest, the patch is sewn along the left AV valve annulus, effectively separating the common AV junction into distinct right and left orifices while closing the atrial component. This annular-side orientation of the suture line is crucial to keeping a safe distance from the AV node and His bundle.[4,5,7]

  4. Left AV valve repair.

    5. The characteristic “cleft” in the left AV valve is usually closed with interrupted sutures, carefully preserving leaflet mobility and avoiding stenosis.[4,7,8] Additional techniques—such as commissuroplasty, annuloplasty, or chordal re-support—are tailored to leaflet and subvalvar morphology.[7,8]

  5. Assessment and fine-tuning.

    6. Intraoperative transesophageal echocardiography is used to confirm patch integrity, left AV valve competence, and unobstructed LV outflow tract. Any significant residual regurgitation or stenosis is addressed before weaning from bypass.[7–9]

Thus, repair of primum ASD is best viewed not as closure of a simple “hole,” but as a three-dimensional reconstruction of the AV septal region that simultaneously abolishes the shunt, restores a competent bifoliate left AV valve, and protects the conduction tissue.

Postoperative course and long-term outcomes

Early mortality after repair of primum ASD in contemporary series is low, typically <2–3%, with higher risk concentrated in infants presenting with heart failure or complex associated lesions.[4,6] RBBB on the postoperative ECG is common, whereas complete heart block is uncommon when the patch is kept away from the septal crest and AV junction.[5,6]

Long-term follow-up studies show:

  • Excellent survival. Freedom from death exceeds 90–95% at 20–30 years after repair in large series of partial AVSD/primum ASD patients.[6,7,9]
  • Meaningful reoperation rates. Approximately 20–30% of patients require reintervention over 20–30 years, most often for progressive left AV valve regurgitation or, less commonly, left ventricular outflow tract obstruction.[6–8]
  • Functional status. The majority of survivors enjoy good functional capacity with regression of right-sided dilation and normalization of pulmonary artery pressures when repair is performed before the onset of severe pulmonary vascular disease.[6,7,9]

Given this profile, the central goals of surgery are not only to close the primum defect but to achieve a durable left AV valve repair and preserve normal conduction, as these factors largely dictate the need for reoperation and long-term quality of life.

References

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

[2] Naqvi N, McCarthy KP, Ho SY. Anatomy of the atrial septum and interatrial communications. J Thorac Dis. 2018;10(Suppl 24):S2837-S2847. PMC

[3] Anderson RH, Mohun TJ, Brown NA. Clarifying the morphology of the ostium primum defect. J Anat. 2015;226(3):244-257. PMC

[4] Sadeghi AM, Laks H, Pearl JM. Primum atrial septal defect. Semin Thorac Cardiovasc Surg. 1997;9(1):2-7. PubMed

[5] Pillai R, Ho SY, Anderson RH, Lincoln C, Shore DF, Shinebourne EA. Ostium primum atrioventricular septal defect: an anatomic and surgical review. Ann Thorac Surg. 1986;41(5):458-461. PubMed

[6] Najm HK, Williams WG, Chuaratanaphong S, et al. Primum atrial septal defect in children: early results, risk factors, and freedom from reoperation. Ann Thorac Surg. 1998;66(3):829-835. PubMed

[7] Buratto E, Ye XT, Konstantinov IE, et al. Repair of partial atrioventricular septal defect: a 45-year experience. Eur J Cardiothorac Surg. 2015;47(5):796-803. PubMed

[8] Chowdhury UK, Airan B, Malhotra A, et al. Specific issues after surgical repair of partial atrioventricular septal defect. J Thorac Cardiovasc Surg. 2009;137(3):548-553. J Thorac Cardiovasc Surg

[9] Mery CM, De León LE, Molossi S, et al. Contemporary results after repair of partial and transitional atrioventricular septal defects. J Thorac Cardiovasc Surg. 2019;157(1):228-237.e1. PubMed