The interventricular septum consists of two parts: the muscular portion and the membranous portion.
■ The muscular interventricular septum forms as an upward expansion of the base of the primitive ventricle. It extends toward the AV septum but does not reach it; the resulting gap is the interventricular foramen.
■ The membranous interventricular septum is created by the fusion of the aorticopulmonary septum with the muscular intraventricular septum. It grows downward from the AV cushions and fuses with the muscular interventricular septum, obliterating the interventricular foramen.
Ventricular septal defect (VSD), an abnormal opening in the interventricular septum, is the most common congenital heart malformation. The most common location is in the membranous interventricular septum, resulting from incomplete fusion of the AV cushions with aorticopulmonary septum. Clinical manifestations of a VSD vary depending on the size of the defect. Fifty percent of small VSDs undergo complete or sufficient partial closure by age 2 and do not require intervention. Larger VSDs result in left-to-right shunting of blood, and, as a result, may present with late cyanosis.
■ A classic symptom is easy fatigability.
■ Cardiac auscultation reveals a harsh holosystolic murmur heard best at the left lower sternal border.