Septal Atrioventricular Junction Region: Comprehensive Imaging in Adults

Abstract

The septal atrioventricular junction is a centrally located region of the heart where the septal components of the atria and ventricles meet the aortic, mitral, and tricuspid valves. Important structures in this region include the membranous septum, the central fibrous body, the Koch triangle, the inferior pyramidal space, and the base of the interventricular septum. This small area is the home of the atrioventricular node and the atrioventricular conduction axis and has enormous importance to electrophysiologists owing to its prime role in the conduction system of the heart. The atrioventricular node lies within the triangle of Koch; and the atrioventricular bundle, or bundle of His, exits the atrioventricular node and penetrates the right fibrous trigone and runs underneath the membranous septum. The septal atrioventricular junction is a common location for intracardiac shunts such as membranous and perimembranous septal defects. Imaging classification of these defects can have important implications before surgical closure, because the atrioventricular conduction axis passes along the posteroinferior margin of most perimembranous defects. Extracardiac inflammatory and malignant pathologic conditions can extend from the mediastinum toward the inferior pyramidal space in this region through the epicardial fat planes. Although the anatomic structures are complicated, the components can be shown in exquisite detail with computed tomography (CT). In this review, the anatomic boundaries and important anatomic landmarks are examined with CT and magnetic resonance imaging. Also described are the anatomic variants of the membranous septum pertinent to percutaneous aortic valve implantation, the vascular anatomic variants, and commonly encountered pathologic conditions related to the septal atrioventricular junction. RSNA, 2016.

Fibrous Skeleton of the Heart: Anatomic Overview and Evaluation of Pathologic Conditions with CT and MR Imaging

“…Destruction of the aortic-mitral valvar continuity after aortic endocarditis can also destroy the fibrous trigones, and reconstructive surgery may be necessary to repair them (37). The membranous septum is located between the crest of the muscular ventricular septum and the interleaflet triangle interposed between the noncoronary and right coronary aortic valvar sinuses (7,38) (Fig 12). It is divided by the attachment on its right side of the septal leaflet of the tricuspid valve into AV and interventricular components.…”

“…The dimensions of the membranous septum and the relative sizes of its two components are variable. In rare cases, the membranous septum can be absent (38,39).…”

“…Deficiency of the ventricular myocardium in the environs of the membranous septum is the essence of perimembranous ventricular septal defects (38). A congenital or acquired isolated defect of the AV component of the membranous septum with a left-to-right shunt is known as a Gerbode defect (38). Perforation of the AV portion of the membranous septum during direct compact AV node ablation could induce a Gerbode-type shunt (40).…”

“…The left bundle of the cardiac conduction system enters the LVOT posterior to the membranous septum, with the His bundle located at the posteroinferior aspect of the membranous septum and right fibrous trigone (38) (Fig 12c). In cases of an absent or small membranous septum, the His bundle moves closer to the aortic root.…”

Variations in rotation of the aortic root and membranous septum with implications for transcatheter valve implantation

“…These differences are in marked contrast to the uniform muscular support provided for the leaflets of the pulmonary root 5 6. The membranous septum is also known to vary in size,7 8 variation recognisable using CT analysis 9. This analysis has also served to identify variation in the rotation of the aortic root relative to the underlying ventricular mass 10.…”