Perimembranous Ventricular Septal Defect 

Perimembranous ventricular septal defects (VSDs) are located in the left ventricle outflow tract beneath the aortic valve. They are the most common VSD subtype in the United States, occurring in 75-80% of cases. Defects may extend into adjacent portions of the ventricular septum. When tissue forms on the right ventricular septal surface (often thought to be atrioventricular valvular in origin), it is termed an aneurysm of the membranous septum. Such tissue serves as a mechanism of spontaneous closure. The defect may be partially or completely occluded by the septal leaflet of the tricuspid valve. (See Epidemiology, Prognosis, and Treatment.)

Normal closure of the ventricular septum occurs through multiple concurrent embryologic mechanisms that help to close the septum’s membranous portion: (1) downward growth of the conotruncal ridges forming the outlet septum, (2) growth of the endocardial cushions forming the inlet septum, and (3) growth of the muscular septum forming the apical and midmuscular portions of the septum.

Ventricular septal defects (VSDs) occur when any portion of the ventricular septum does not correctly form or if any of components do not appropriately grow together. The ventricular septum is complete by 6 weeks' gestation. VSDs are typically classified according to the location of the defect in 1 of the 4 ventricular components: the inlet septum, trabecular septum, outlet/infundibular septum, or membranous septum. This article specifically addresses defects in the trabecular muscular septum. (See Etiology.)

Small VSDs (defined as VSD dimension less than half the size of the aortic annulus diameter) are usually isolated defects with otherwise normal cardiac anatomy and function. Large VSDs (defined as defect size equal to the diameter of the aortic annulus) typically have left atrial and left ventricular dilation with normal left ventricular systolic function. (See Workup.)

Perimembranous VSD is caused by failure of the endocardial cushions, the conotruncal ridges, and the muscular septum to fuse at a single point in space.

Hemodynamic effects of VSD

Independent of the type of ventricular septal defect (VSD), the hemodynamic significance of the VSD is determined by 2 factors: the size of the defect and the resistance to flow out of the right ventricle, including the pulmonary vascular resistance (PVR) and anatomic right ventricular outflow obstruction.

In small to moderate VSDs, left-to-right shunting is primarily limited by the size of the defect. Conversely, in large VSDs without right ventricular outflow obstruction, the left-to-right shunting is determined by the relative degree of PVR and systemic vascular resistance.

Because PVR is high at birth and does not reach its nadir until age 6-8 weeks, the development of significant left-to-right shunting and pulmonary overcirculation, often termed congestive heart failure (CHF), can be delayed until the second or third month of life. Additional cardiac lesions that increase left-to-right shunting (eg, atrial septal defect, patent ductus arteriosus) may predispose patients to earlier development of CHF. Noncardiac abnormalities, including prematurity, infection, anemia, and other congenital anomalies, also may predispose infants to significant symptoms of heart failure.

Additional congenital heart lesions (eg, muscular right ventricular outflow tract obstruction, pulmonary valve stenosis, pulmonary venous obstruction, persistent elevation of PVR, mitral stenosis) can restrict shunting, possibly leading to right-to-left trans-VSD flow, depending on the ultimate resistance balance between the systemic and the total right-sided resistances.

Complications

Complications may include the following (see Prognosis):

• CHF
• Bacterial endocarditis
• Eisenmenger syndrome
• Aortic insufficiency
• Subaortic stenosis
• Double-chambered right ventricle

Patient education

Advise the patient and/or his or her parents regarding the risks of bacterial endocarditis indications and the importance of oral hygiene. Educate them concerning signs and symptoms of CHF.

For patient education information, see the Heart Health Center, as well as Congestive Heart Failure and Ventricular Septal Defect.

Perimembranous VSDs have a multifactorial etiology and are predominantly the result of spontaneous abnormalities in development. The precise etiology of muscular septal defect formation is unknown. However, the proposed mechanisms are many. Muscular defects may occur because of a lack of merging in the walls of the trabecular septum or because of excessive resorption of muscular tissue during ventricular growth and remodeling.

No significant correlation between the cause of VSDs and the age of the mother or the birth order of the child is observed.

VSDs are the most common congenital heart lesion associated with chromosomal anomalies and syndromes. VSDs are especially common in patients with trisomy 13, trisomy 18, and trisomy 21. However, nearly 95% of VSDs are not associated with chromosomal abnormalities. Noncardiac conditions associated with VSD include prematurity.

Regular maternal cannabis use slightly increases the incidence of VSD.^[1] The use of selective serotonin reuptake inhibitors (SSRIs) during early pregnancy also slightly increases the incidence of VSD.^[2]

Occurrence in the United States

Without regard to type, VSD is the most common congenital heart defect in the first 3 decades of life, with an incidence between 1.5-4.2 cases for every 1000 live-term infants. VSD is more common in premature infants with an incidence of 4.5-7 cases for every 1000 liveborn infants.

Clinically significant VSD that requires medical or surgical management accounts for only 15% of such defects (0.35-0.50 cases for every 1000 live births). When viewing congenital heart disease in total, solitary VSD cases account for 20-40% of congenital heart disease. Perimembranous VSD is the most common type, accounting for as many as 50% of VSD cases identified in most surgical or autopsy series.

Race-, sex-, and age-related demographics

Inheritance patterns of different VSDs vary widely by race. Perimembranous VSD has no known race predilection. Defects located in a subpulmonary position, such as supracristal defects, are more common in the Asian population. VSDs are slightly more common in females than in males.

Most perimembranous VSDs present clinically in the neonatal period secondary to a murmur. These defects, especially the smaller defects, are not typically suspected at birth and may not be identified by auscultation until PVR begins to fall in the first few days to weeks of life. Large perimembranous VSDs may not present until patients are aged 6-8 weeks, when decreased PVR allows significant left-to-right shunting and clinical signs and symptoms of CHF. VSDs may present soon after birth if associated with significant additional congenital heart lesions or if they occur with an associated chromosomal anomaly or syndrome.

Children with small-to-moderate sized ventricular septal defects (VSDs) have an excellent prognosis; infants and children with large VSDs have a good prognosis. Optimal medical management, with appropriate timing of surgical intervention, has the best outcome.

Morbidity and mortality

Morbidity and mortality are influenced by the number and size of VSDs, the degree of left-to-right shunting, the presence of associated congenital heart defects, the presence of associated noncardiac defects and syndromes, and age at repair of VSD.

Perimembranous VSDs may spontaneously decrease in size and eventually close. (This often occurs with a small defect.) Closure rates as high as 50% have been reported in some series, but continued follow-up care is warranted until documented VSD closure occurs.

Although patients with a small VSD have an excellent prognosis, small perimembranous VSDs may lead to the development of aortic insufficiency.

For patients with moderate-sized VSD, defects may allow the development of voluminous left-to-right shunting in the first few months of life as PVR falls. Failure of medical management, with persistent evidence of CHF, is the primary indication for surgical closure of moderate-sized defects. Fewer than 25% of moderate-sized defects require surgical closure.

For patients with large muscular VSDs, surgical repair is indicated at any time during the first year of life if the infant fails to grow appropriately despite optimal medical management. Surgical risk and mortality for patients with large VSDs is higher in the first 2 months of life (10-20%) than after age 6 months (1-2%), although these figures have been decreasing. Elective surgical closure of large VSDs should be planned within the first year of life to prevent development of irreversible pulmonary vascular obstructive disease (ie, Eisenmenger syndrome).