Atrioventricular Septum: Fibrous Connective Tissue Forms Electrical Barrier, Bundle of His Penetrates

The atrioventricular (AV) septum (membranous septum) is a small fibrous portion of the interventricular septum that separates the right atrium from the left ventricle.

Structure and Function

• Tissue: Dense fibrous connective tissue (collagen-rich).
• Electrical Insulation: Electrically isolates atria from ventricles — prevents direct myocardial conduction.
• Penetration: The Bundle of His (AV bundle) is the only normal electrical connection traversing the AV septum, conducting the impulse from AV node to bundle branches.
• Anatomic Relations: Aortic valve (right/non-coronary cusps) overlies the membranous septum; tricuspid valve (septal leaflet) attaches to its right ventricular side.

Clinical Relevance

• AV Block: Disease in the AV septum (ischemia, fibrosis, surgery) can disrupt the His bundle → high-grade AV block.
• VSD (Ventricular Septal Defect): Perimembranous VSDs occur in the AV septum; risk of aortic valve prolapse, complete heart block after repair.
• Accessory Pathways (WPW): Bypass the AV septum via Kent bundles → pre-excitation.