Membranous septum and ventricular septal defect
A ventricular septal defect is a hole between the two ventricles, most often left when the thin membranous septum, the last part of the wall to close, forms incompletely.
Remember
The membranous part of the wall closes the last gap between the ventricles, using tissue from the endocardial cushions (swellings of tissue in the middle of the heart), so it is the part most often left incomplete.
Memory aid
the muscle comes up, the membrane comes down
Use it to remember the direction of growth: muscle rises from the floor of the ventricles, and the membranous part comes down from the endocardial cushions to close the last gap.
The wall between the two ventricles is the interventricular septum. It forms in two steps. First, a ridge of muscle called the muscular septum grows upwards from the floor of the ventricles. It does not reach the endocardial cushions, which are swellings of tissue in the middle of the heart. The gap that remains is the interventricular foramen, the opening that lets blood pass between the ventricles.
Tissue from the endocardial cushions then grows down to meet the muscular septum and mainly closes that gap. The closed gap is the membranous septum, a thin wall at the top of the ventricular septum.
If this closure is incomplete, the hole that remains is a ventricular septal defect (VSD), an opening between the two ventricles. Because the membranous region closes the final gap, it is the part most likely to be left incomplete. Defects here are called perimembranous (they involve the membranous septum and a little surrounding muscle). They account for about 70 to 80% of all VSDs, and they lie next to the tricuspid valve, usually just below the aortic valve.
Defects of the muscular part, which are surrounded entirely by muscle, are a less common cause and make up about 5 to 20% of cases.
Key facts
- Muscular septum
- grows up from the floor of the ventricles but stops short of the endocardial cushions.
- Interventricular foramen
- is the gap that the muscular septum leaves, and it lets blood pass between the ventricles.
- Membranous septum
- grows down from the endocardial cushions and mainly closes the interventricular foramen.
- If closure is incomplete
- a ventricular septal defect remains, which is a hole between the ventricles.
- Commonest type
- perimembranous, next to the tricuspid valve, about 70 to 80% of defects.
- Muscular defects
- about 5 to 20% of ventricular septal defects.
Exam traps
- Upward growth of the muscular septum is a possible source of a ventricular septal defect, but defects in that part make up about 5 to 20% of cases, whereas the membranous part is the commonest site.
- The septum primum is the first of the two partitions that divide the atria, so failure of it to fuse with the endocardial cushions leaves a gap between the atria and not a ventricular septal defect.
- A defect just below the pulmonary valve (a subpulmonary outlet defect) is not the commonest site, because these make up only about 5 to 7% of cases in the United States.
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