PATHOPHYSIOLOGY OF VSDs
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.
CARDIAC CT AND MRI
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.
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
TYPES OF VSDs AND IMAGING APPEARANCES
Congenital VSDs can be classified according to their location in the IVS as perimembranous, muscular, subarterial, or inflow (Fig. 5).23
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).
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, http://links.lww.com/JTI/A19). 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, http://links.lww.com/JTI/A20). 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, http://links.lww.com/JTI/A21).
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 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, http://links.lww.com/JTI/A22).
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.
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.
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.
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ventricular septal defects; interventricular septum; congenital heart disease
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