The atrioventricular (AV) septum or septal atrioventricular junction forms a central part of the heart, where the interatrial and interventricular septum crosses the atrioventricular annular plane and join with the septal tricuspid and anterior mitral leaflet attachments.

Terminology

On a four-chamber view the conjunction of both septa, the tricuspid and mitral leaflets display a cruciate appearance and have been named 'echocardiographic crux cordis' ⁸.

Gross anatomy

A characteristic anatomic feature of the atrioventricular junction is the atrioventricular septum a septal component separating the right atrium from the left ventricle. It forms an offset of the atrioventricular annular anatomy with a length of up to 8 mm/m² BSA between the more apically located septal tricuspid leaflet and the hinge of the anterior mitral leaflet ^1-3. It consists of an anterior membranous and inferior muscular part evolving from the right atrial septal wall and an extension or the crest of the interventricular septum on the left with fibrous and adipose tissue from the inferior atrioventricular groove sandwiched in between the two muscular structures ^1-5.

Relations and/or Boundaries

The atrioventricular septum separates the right atrium from the left ventricle and is located inferior to the aortic root and immediately above the inferior pyramidal space. Towards the cardiac apex, it connects to the muscular interventricular septum and the septal tricuspid leaflet whereas towards the atria it fuses with the vestibular spine of the interatrial septum and the hinge of the anterior mitral leaflet ^1-6.

Arterial supply

The atrioventricular septum is supplied by septal branches and the atrioventricular nodal artery ¹.

Innervation

The septal atrioventricular junction is the location of the atrioventricular conduction axis and its membranous part is beyond the apex of the triangle of Koch ^4-6.

Radiographic features

The atrioventricular septum can be easily depicted on a four-chamber view with echocardiography, cardiac CT and cardiac MRI. The visualisation of the detailed anatomic ultrastructure is usually beyond the scope of all three imaging techniques but most likely feasible with cardiac MRI, which can demonstrate the three layers in the case of abundant adipose tissue ¹.

Ultrasound

Echocardiography

The septal atrioventricular junction can be delineated on a four-chamber view and its cruciate appearance has been first described on echocardiography ⁷.

CT

The atrioventricular septum can be best visualised on ECG-gated cardiac CT with a triphasic bolus injection technique suitable for additional opacification of the right-sided cardiac chambers ^1,2.

MRI

The three-layered architecture of the atrioventricular septum can be best differentiated by cardiac MRI on a four-chamber view immediately below the aortic root ^1-3.

History and etymology

The cruciate appearance of the atrioventricular septal area was already described by Norman H. Silverman and Nelson B. Schiller in 1978 ⁷.

Clinical importance

The atrioventricular node is located near the central fibrous body of the membranous atrioventricular septum just beneath the apex of Koch's triangle and is a significant landmark for electrophysiologists in planning radiofrequency ablation.

Related pathology

Pathological conditions related to the atrioventricular septum include ^1-3:

• supravalvular Gerbode defect: isolated defect of the membranous atrioventricular septum

• congenitally corrected transposition of the great arteries

  • preserved offset and relationship of the tricuspid septal leaflet attachment

  • the flipped axis of the valves together with the ventricular switch

• atrioventricular canal defect

  • no offset between the two valves and the tricuspid and mitral septal attachments

• Ebstein anomaly

  • apical migration of the septal tricuspid leaflet with increased offset (>8 mm/m² BSA)

See also

• innervation of the heart

• cardiac valves