Ventricular Septal Defects: Embryology and Imaging Findings : Journal of Thoracic Imaging

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Ventricular Septal Defects

Embryology and Imaging Findings

Rojas, Carlos Andres MD*; Jaimes, Camilo MD; Abbara, Suhny MD

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doi: 10.1097/RTI.0b013e31824b5b95



VSDs are abnormal openings in the septum that allow shunting of blood between the ventricles.1 VSDs are the most common congenital abnormality diagnosed in children, with a reported incidence of 53 per 10,000 live births.2 VSDs occur in 50% of children with congenital heart disease and are an isolated finding in 20% of them.1,2 In adults, VSDs are the second most common congenital heart condition (after bicuspid aortic valve), with an estimated prevalence of 0.3 per 1000.3

Echocardiography is the main imaging modality for the diagnosis and follow-up of VSDs.1 Magnetic resonance imaging (MRI) and computed tomography (CT) are not routinely used in cases of VSDs. However, they are frequently used to characterize complicated cardiovascular malformations, many of which can have associated VSDs.4,5 Furthermore, with the recent surge in the use of MRI and CT in cardiothoracic imaging, the detection of these defects as incidental findings has increased substantially.5,6


The IVS is the anatomic structure that divides the right (RV) and left (LV) ventricles. The formation of the IVS starts around the fifth week of embryonic development and involves the sequential fusion of 3 independent septa: muscular, outlet, and inlet septa (Fig. 1).7–9 A disruption in this process leads to the development of VSDs (Fig. 2).

Diagrammatic representation of normal development of the IVS. The IVS is formed from 3 separate septa: muscular, outlet, and inlet septa. Early in embryologic development, the muscular septum (MS) grows upward from the floor of the ventricles toward the already fused endocardial cushions (EC). The gap between the edge of the muscular IVS and EC is called the interventricular foramen (IVF). Meanwhile, 2 spiral ridges of tissue, the conotruncal ridges or truncoconal swellings, appear on the sides of the truncus arteriosus (TA). The conotruncal ridges grow toward each other and fuse, forming a spiral-shaped septum termed the aortopulmonary septum (APS). The APS divides the TA into the pulmonary trunk and aorta. The conotruncal ridges also grow downward into the ventricles, meeting with the already fused endocardial cushions and the muscular portion of the IVS. By the seventh to eighth week of gestation, the membranous septum is formed when the APS, endocardial cushions, and muscular septum completely fuse, closing off the IVS.
Diagrammatic representation of normal common developmental anomalies of the IVS. Defects in the fusion of the muscular septum (MS) and the endocardial cushions (EC) result in membranous VSDs. Openings in the trabecular portion of the IVS lead to muscular VSDs. Incomplete fusion of the aortopulmonary septum (APS) with the EC-MS septum results in supracristal VSDs (SC-VSD).


During prenatal life, the vascular resistance in the systemic and pulmonary circulations is similar; thus, minimal flow occurs through VSDs regardless of their size.10 In the early neonatal period, pulmonary vascular resistance drops substantially, creating a pressure gradient. This pressure gradient is the main determinant of the degree and direction of the shunt across the VSD. The size of the defect also influences the amount of blood that crosses the VSD.11 Smaller defects, also called restrictive defects, provide intrinsic resistance to flow and limit the amount of shunted blood, maintaining a gradient between the 2 ventricles. In contrast, large defects allow for unrestricted flow through the defect and equalization of interventricular chamber pressures.11

In the early stages of disease, blood crosses from the LV into the RV. The increase in blood flow in the pulmonary circulation leads to pulmonary hypertension and to an increase in the preload in the left atrium and ventricle.12 At first, the vascular resistance in the pulmonary vessels is only mildly elevated. However, chronic shunting across the defect leads to structural and functional changes in the pulmonary vessels, which ultimately result in an increase in the pulmonary vascular resistance and worsening of pulmonary hypertension.12 If the pressure in the pulmonary circulation exceeds that in the systemic circulation, the direction of the shunt is reversed (right-to-left shunting). This condition, known as the Eisenmenger complex, is a late stage in the course of disease, has a worse prognosis, and leads to cyanosis, as venous blood crosses the septal defect and reaches the systemic circulation.13,14


Conventional radiographs demonstrate findings secondary to the increase in pulmonary blood flow. Cardiomegaly and prominent pulmonary vascularity are the most common features (Fig. 3). However, these findings are nonspecific and can be seen with other causes of left-to-right shunts, such as an atrial septal defect (ASD) or a patent ductus arteriosus. Additional imaging features such as an enlarged left atrium and a normal-sized or small aorta are characteristic of VSDs and can help narrow the differential considerations, given that ASDs typically present with a normal-sized left atrium, and a patent ductus arteriosus is commonly associated with an enlarged aorta.15 In VSDs, the degree of cardiomegaly is always proportional to the increase in pulmonary vascularity. Disproportionate findings should raise the concern for complex congenital heart disease (with additional malformations) or a different diagnosis.

Radiographic findings of VSDs. A, Frontal radiograph of the chest of a 1-year-old boy with a membranous VSD demonstrates an enlarged cardiothymic silhouette with increase in pulmonary vascularity. The VSD was surgically closed at this time. B, Frontal radiograph of the chest of the same boy 3 years later shows improved cardiomegaly and normal pulmonary vascularity. Sternal wires are seen as well.


Advanced cardiac imaging with CT and MRI continues to evolve and its use continues to increase. Currently, CT serves to assess morphology and detect associated anomalies when echocardiography is limited. Imaging of VSDs by CT requires an appropriate imaging protocol to obtain ideal intracardiac opacification. At our institution, contrast material is administered using a biphasic protocol consisting of an initial contrast bolus at a rate of 4 to 7 mL/s, followed by 40 to 50 mL of saline to flush out the contrast from the right heart.16 This method provides an ideal opacification of the left heart for the detection of left-to-right shunts. Right-to-left shunts result in low attenuation jets entering the opacified LV.

Images can be acquired with prospective triggering or with retrospective gating. Prospectively triggered acquisition yields images during a specific point in time, significantly reducing radiation exposure. However, ventricular function cannot be assessed. Retrospectively gated acquisition uses helical scanning, exposing the patient to radiation during the entire cardiac cycle.16,17 This method is preferred for morphologic assessment, as it allows visualization of the VSD during the entire cardiac cycle and also allows calculation of the biventricular volumes and function. It is important to note that ventricular stroke volumes calculated using the Simpson method do not allow quantification of shunting in the setting of VSDs because of maintained stroke volumes in both ventricles.18

MRI aids in the morphologic and functional evaluation of VSDs. Multiple sequences including black blood images, balanced steady-state free precession (BSSFP) images, velocity-encoded phase-contrast images, magnetic resonance angiography images, and three dimensional (3D) whole-heart BSSFP images are usually acquired.19 The appearance of VSDs varies depending on their location and hemodynamic characteristics. Although large defects may be readily apparent on morphologic or BSSFP imaging, a subtle jet of dephasing across the IVS may be the only evidence of a small VSD.

BSSFP images in the short-axis plane covering the ventricles can be used to determine biventricular volumes and function using the Simpson method. As mentioned above, this method is inaccurate to calculate the degree of shunting in the presence of VSDs because of the maintained stroke volumes in both ventricles.18 An alternative way to estimate the degree of shunting is to use phase-contrast MRI. Velocity-encoded phase-contrast images acquired of regions above the level of the semilunar valves can be used to calculate the volume of blood exiting the pulmonary artery and aorta.18 This data can be used to calculate the flow in the pulmonary (Qp) and systemic (Qs) circulations.19 The rate of shunting (Qp:Qs) and the shunt fraction ([Qp−Qs]/Qp) can be calculated on the basis of these estimates (Fig. 4).20 In general, shunts that result in a pulmonary blood flow that is 1.5 times greater than the systemic blood flow (Qp:Qs ratio>1.5) are considered significant.

Quantification of the shunt using MRI. A, Short-axis BSSFP MRI of the heart of an 8-year-old girl shows a small restrictive perimembranous VSD (arrow) with a small jet into the right ventricular outflow tract from left-to-right shunting. B, Magnitude and (C) velocity images obtained above the semilunar valves were used to quantify the flow in the main pulmonary artery (PA) and the aorta (Ao) using a flow analysis software. The Qp:Qs shunt was estimated to be 1.5.

The estimates obtained using MRI are comparable to those obtained with catheter-based approaches (invasive oximetry), although a small nonsignificant overestimation in the Qp:Qs ratio and shunt fraction has been described with MRI.20,21 A caveat to phase-contrast imaging are patients with chronic shunting that has resulted in equalization of left and right ventricular pressures, which reduces, stops, or even reverses the shunt flow. Furthermore, phase-contrast imaging perpendicular to the shunt jet can also assess the volume and peak gradient across the defect.22


Congenital VSDs can be classified according to their location in the IVS as perimembranous, muscular, subarterial, or inflow (Fig. 5).23

Schematic diagram depicting the various types of VSDs. Perimembranous VSD (yellow), subarterial VSD (red), muscular VSD (blue), inlet VSD (green). IVC indicates inferior vena cava; SVC, superior vena cava.

Approximately 75% to 80% of VSDs occur in the membranous septum. These defects are located below the crista supraventricularis and anterior to the septal leaflet of the tricuspid valve. Membranous VSDs are also known as infracristal, subaortic, perimembranous, or paramembranous. These defects can be associated with misalignment of the aortopulmonary septum, as in the setting of Tetralogy of Fallot (anterior misalignment) or interrupted aortic arch (posterior misalignment). Approximately a third of isolated perimembranous VSDs close spontaneously, usually by apposition of the septal leaflet of the tricuspid valve or prolapse of an aortic cusp (right or noncoronary) into the defect.24 These mechanisms of spontaneous defect closure can result in the formation of an aneurysm in the IVS with or without residual shunting (Fig. 6).

Interventricular septal aneurysms in 2 different patients. A, Short-axis view from a cardiac CT in a 40-year-old man demonstrates the presence of an interventricular septal aneurysm (black arrows). No associated VSD was noted. B, A 5-chamber view of a cardiac CT in a 67-year-old man with a history of perimembranous VSD. Note a ventricular septal aneurysm (white arrow) with high attenuation jets entering the RV (black arrows) consistent with a partially closed membranous VSD. RVOT indicates right ventricular outflow tract.

On transthoracic echocardiography, perimembranous VSDs are better visualized on parasternal short-axis views at the level of the aortic valve. Typically, a flow jet arising at the 11 o’clock position can be readily identified.

The characteristic features of perimembranous VSDs are well demonstrated in this 2D echocardiographic cine clip from an asymptomatic 8-year-old boy. The parasternal short-axis view with color flow mapping at the level of the aortic valve shows the characteristic location of the defect and the typical jet at the 11 o'clock position. (Supplemental Digital Content 1, Video, The videoclip of a 2D echocardiogram on a subcostal frontal view shows that the VSD is located in the anterosuperior portion of the IVS between the tricuspid and aortic valves. The septal leaflet of the tricuspid valve is seen in close proximity to the defect (Supplemental Digital Content 2, Video, The videoclip from the subcostal left anterior-oblique view with color Doppler demonstrates left-to-right shunting through the VSD and aneurysmal tricuspid valve tissue partially occluding the defect. There is no obstruction of the left ventricular outflow tract. A PFO with left-to-right shunting is also evident (Supplemental Digital Content 3, Video,

On electrocardiogram-gated multidetector computed tomography images with contrast and a saline chaser, perimembranous VSDs are seen as a high attenuation jet crossing through the septal defect and entering the washed-out infracristal RV. A potential pitfall on CT imaging is the incomplete washout of contrast on the ventricular side of the tricuspid septal leaflet accompanied by a thin membranous septum, which can simulate a membranous VSD (Fig. 7). Large defects can result in right ventricular enlargement because of long-standing increased volume in the low-pressure chamber. Morphologically, perimembranous VSDs are bound by both membranous and muscular tissue. Diagnostic imaging can also identify associated conditions, such as a subaortic ridge, aortic valve prolapse, and aortic regurgitation.24

A 5-chamber view from a cardiac CT demonstrates the presence of high attenuation contrast (arrows) on the ventricular side of the septal leaflet of the tricuspid valve that could be confused with a small membranous VSD. The transthoracic echocardiogram of the same patient excluded the presence of a defect in this location. Ao indicates aorta; LA, left atrium.

A Gerbode-type defect is a communication between the LV and right atrium (RA) through the membranous IVS.23 Gerbode-type defects can be further classified as infravalvular or supravalvular in relation to the tricuspid valve annulus. Supravalvular Gerbode defects involve the atrioventricular portion of the membranous septum and result in a direct communication between the LV and RA. Infravalvular Gerbode defects include a defect in the interventricular portion of the membranous septum and fenestrations of the tricuspid septal leaflet, resulting in LV-to-RA shunting. Imaging features that can aid in the identification of Gerbode defects include atypical direction of the flow jet, from the LV to the RA, and persistent shunting during diastole.

Acquired causes of Gerbode defects include surgery, endocarditis, trauma, and ischemia.25 Echocardiographic images in this 6-year-old boy with persistent shunting after surgical repair of an atrioventricular canal demonstrate the characteristic imaging features in a postsurgical setting. 2D echocardiographic images with color flow mapping on a 4-chamber apical view show a flow jet extending from the LV into the RA, consistent with a Gerbode-type VSD. The RA appears slightly enlarged. Color Doppler images also demonstrate smaller jets across the left and right AV valves, which represent mild residual insufficiency. The RV and LV appear normal in size (Supplemental Digital Content 4, Video,

VSDs located in the trabecular portion of the IVS are called muscular VSDs (Fig. 8). Approximately 5% to 20% of all VSDs occur in this region. These defects are entirely surrounded by muscle and vary in their number and size.26 The presence of multiple defects is known as the “swiss cheese septum.” Two thirds of muscular VSDs are located in the apical region. This type of VSD exhibits a marked tendency to regress spontaneously (68% to 75% close before 2 y of age); thus, they are more commonly seen in children.24 As with other types of VSDs, a high attenuation jet crossing through the defect can be seen on electrocardiogram-gated multidetector computed tomography images following administration of contrast and a saline chaser (Fig. 9). Associated dilation of the RV can occasionally be seen, especially with large defects (Fig. 10). Small defects usually close during ventricular systole limiting flow during this phase of the cardiac cycle. Potential pitfalls include deep crypts, Thebesian sinuses, and coarse trabeculations. The absence of high attenuation contrast material entering the RV can help differentiate these entities from true muscular VSDs.

Muscular VSD in a 36-year-old man. A short-axis BSSFP image incidentally demonstrates the presence of a small anterior muscular VSD (white arrow). No significant right ventricular enlargement is noted.
Muscular VSD in a 46-year-old woman. A, The 4-chamber view from a cardiac CT demonstrates the presence of a small defect in the apical muscular septum consistent with a muscular VSD (white arrow). B, The short-axis view clearly demonstrates contrast coursing through the muscular VSD (white arrow) and spilling into the unopacified RV. No right chamber enlargement is noted. LA indicates left atrium.
Muscular VSD in a 37-year-old woman. A, Four-chamber and (B) short-axis BSSFP images in a patient with a dual-chamber RV demonstrate a large apical muscular VSD (white arrows). The dense trabeculations of the RV (black arrowheads) serve as a barrier to the functional RV; therefore, no chamber enlargement is noted despite the large VSD.

VSDs located below the semilunar valve and above the crista supraventricularis are known as subarterial, outlet, supracristal, subpulmonary, infundibular, doubly committed, or conoseptal VSDs (Fig. 11).26 Overall, subarterial VSDs account for 5% to 7% of isolated VSDs; however, a substantially higher prevalence has been reported in Asian populations.27 Defects in this location are frequently associated with aortic valve prolapse or regurgitation, which results from the loss of support of the right cusp of the aortic valve.27 These defects can be bound by the fibrous annulus of the semilunar valves and/or muscular tissue. On transthoracic echocardiography, the shunt is best seen on a parasternal short-axis view at the level of the aortic valve at the 1 o'clock position. The shunt is well depicted on MRI and CT images as well, at the level of the aortic valve (Fig. 12). As with other VSDs, right ventricular chamber enlargement can be seen, especially in large defects.

Subarterial VSD in a 29-year-old man. The short-axis view from a cardiac CT demonstrates the presence of a subarterial VSD (black arrow). The small muscular ridge below the defect represents the crista supraventricularis (white arrow). RVOT indicates right ventricular outflow tract.
Subarterial VSD in a 31-year-old man with a history of subaortic membrane. A, Three-chamber and (B) short-axis BSSFP images demonstrate a small defect (white arrows) in the supracristal region. Note that the defect is above the crista supraventricularis (black arrow) in (B). C, A fat-saturated 3D SSFP image on the same patient in the short-axis plane demonstrates the small defect (white arrow) above the crista supraventricularis (black arrow). D, A phase-contrast image at the level of the aortic annulus demonstrates a systolic jet in the 1 o’clock position (white arrow) consistent with the subarterial VSD.

Inflow VSDs occur almost exclusively in association with endocardial cushion defects. Thus, inflow VSDs are uncommon in the general population but have a remarkably high prevalence in genetic conditions such as trisomy 18 and trisomy 21.28 These defects are bound by the tricuspid valve annulus and extend to the muscular septum and variably to the membranous septum. These defects can be associated with ASDs (primum type) and atrioventricular valve clefts.


Many factors influence treatment decision in patients with VSDs, such as the presence of symptoms, coexisting anomalies, age of the patient, right ventricular enlargement, and size, number, and location of the defects. By fully characterizing the anatomy, diagnostic imaging provides valuable information to guide therapy.

The main alternatives in the treatment of VSDs are open surgery with patch repair (Fig. 13) and endovascular closure with devices. Currently, endovascular closure is performed for perimembranous or muscular VSDs in patients at high risk for standard surgical closure.

Corrected Tetralogy of Fallot in a 27-year-old man. Volume-rendered image depicts patch closure of membranous VSD (white arrows) and a bioprosthetic valve in the pulmonic position. Incidentally noted is prominence of the pulmonary artery and right ventricular chamber. LVOT indicates left ventricular outflow tract; PA, pulmonary artery; RVOT, right ventricular outflow tract.


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ventricular septal defects; interventricular septum; congenital heart disease

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