Atrial Septal Defect (ASD) — #2 Primum ASD

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.