Expansion
A large muscular part and a small membranous part
Expansion
- Muscular part - the large lower portion, thick and contractile, supplied mainly by septal branches of the left anterior descending artery
- Membranous part - a small fibrous area at the upper posterior margin, continuous with the fibrous skeleton
The membranous septum is where the bundle of His passes through, so a defect or a surgical repair here risks heart block.
Roughly 70 per cent of congenital ventricular septal defects are perimembranous. The resulting left to right shunt produces a loud pansystolic murmur, and the smaller the defect, the louder the murmur, since a small hole generates more turbulence.