| Ann Kavanaugh-McHugh, MD |
Nov 3, 1999 |
A ventricular septal defect (VSD) is one of the most common congenital cardiac abnormalities in the newborn, but it is less common in the adult due to spontaneous closure of muscular VSDs during early growth. It can occur as an isolated finding or in combination with other congenital defects. VSD can also be an acquired disorder, occurring after acute myocardial infarction. (See "Mechanical complications of acute myocardial infarction").
The echocardiographic evaluation of VSD will be reviewed here. The pathophysiology and clinical features of this defect are discussed separately. (See "Pathophysiology and clinical features of ventricular septal defects").
ECHOCARDIOGRAPHY EVALUATION ！ Echocardiographic evaluation of ventricular septal defects includes:
• Identification of the location of defects on the septum
• Establishing the number of defects
• Delineation of associated anatomic features
• Assessment of the size and hemodynamic significance of the defects
The septum is a complex curved surface, and careful assessment using multiple echocardiographic planes is necessary to define the location and extension of defects. Although defects are traditionally described as membranous, muscular, subpulmonary, and inlet, they may not be confined to any one region of the septum but may extend into multiple adjacent regions. Multiple defects may also be present. (See "Pathophysiology and clinical features of ventricular septal defects").
Color flow Doppler is a very useful adjunctive method to search and screen for VSDs. It can be used to help localize and confirm their presence by characteristic flow patterns.
Membranous VSD ！ Defects in the membranous septum are also called paramembranous, perimembranous, or infracristal defects. They are located at the intersection of the trabecular, inlet, and outlet regions of the septum, lie just beneath the aortic valve and behind the septal leaflet of the tricuspid valve. In the parasternal long axis echocardiographic view, these defects are seen just below the aortic valve (show echocardiogram 1 ).
In the orthogonal short axis views of the left ventricular outflow tract, perimembranous defects can be seen beneath the septal leaflet of the tricuspid valve (show echocardiogram 1 ).
Membranous defects can also be seen beneath the aortic valve in apical imaging planes by angling the transducer anteriorly to view the aortic outflow tract. In smaller patients with good subcostal imaging windows, the boundaries of these defects are imaged well in the coronal and sagittal views of the membranous septum (show echocardiogram 4).
Membranous defects may close spontaneously, either partially or completely, due to apposition of the septal leaflet of the tricuspid valve. In some cases, septal apposition results in an "aneurysm of the ventricular septum," which is best seen in the parasternal long and short axis views (show echocardiogram 5).
Color flow Doppler mapping is useful for establishing the presence of residual shunt flow through the "aneurysm."
Membranous defects may also close by prolapse of an aortic cusp into the defect (see below).
Muscular defects ！ Defects in the muscular septum are often multiple, especially if they are a complication of a myocardial infarction, and may be associated with defects in other regions of the septum (show echocardiogram 6).
Color flow Doppler mapping is invaluable for detecting smaller defects that may be difficult to identify within the trabeculations of the right ventricle when only two dimensional imaging is used; defects in the apical septum are the most likely to be missed. The septum should be interrogated carefully by sweeping the transducer from the right ventricular inflow to the outflow in long axis and subcostal coronal views, obtaining serial sections of the ventricle from apex to base in parasternal short and subcostal sagittal views, and by sweeping from posterior to anterior in apical views.
Color flow Doppler mapping is very helpful for identifying muscular defects, even at suboptimal angles of interrogation. However, in patients with increased right ventricular pressure, the color flow patterns for the flow velocity across these and all other types of VSDs become less apparent, decreasing the sensitivity of Doppler for their detection [1,2].
Subpulmonary defects ！ Subpulmonary defects are located beneath the pulmonary valve and above the crista supraventricularis. Although they may be difficult to distinguish from membranous defects in the long axis view of the two dimensional echocardiogram, they are easily identified below the pulmonary valve in parasternal short axis images obtained at the level of the arterial roots (show echocardiogram 8).
In younger patients, the relationship between the defect and the pulmonic valve can also be seen in the subcostal sagittal view (show echocardiogram 9).
Prolapse of the unsupported right coronary cusp of the aortic valve into the defect may occur with subpulmonary VSDs and occasionally with perimembranous defects. Prolapse of the leaflet into the defect is best observed in parasternal and subcostal long axis and apical views (show echocardiogram 10).
Although it can be difficult to appreciate the prolapse by transthoracic echocardiography, it is readily seen with transesophageal imaging. Aortic insufficiency may occur with leaflet prolapse due to the progressive distortion of the leaflet; this distortion may be seen in parasternal short axis views of the aortic valve leaflets.
Inlet defects ！ Inlet defects occur at the crux of the heart, posterior and inferior to membranous and outlet defects, and at the junction of the atrioventricular valves. Similar to other septal defects, inlet defects may be seen in many echocardiographic imaging planes; however, these defects and their characteristic relationship to the atrioventricular valves are best demonstrated in apical and subcostal coronal views (show echocardiogram 11).
The location of these defects in the posterior septum can be well seen in the parasternal short axis view (show echocardiogram 12).
The relationship of the atrioventricular valves to the VSD must be carefully assessed. Tricuspid valve chordal attachments commonly insert on the crest or right ventricular surface of the septum. Anomalous chordal attachments of the mitral valve may also insert on the septum. In addition, either atrioventricular valve may straddle the defect, with chordal attachments crossing the defect and attaching anomalously on the septum or free wall of the opposite ventricle, complicating approaches to the defect. It is important to assure that two atrioventricular valves are present, since these inlet VSDs may be part of a larger endocardial cushion defect.
Malalignment defects ！ Assessment of the relationship between the components of the ventricular septum and the atrial and ventricular septa must be included in the evaluation of a ventricular septal defect. Malalignment defects occur when there is an abnormal relationship between the atrial and ventricular septa or between the individual components of the ventricular septa ( and show echocardiogram 14) [2].
Overriding and straddling atrioventricular valves are seen when there is malalignment of the atrial and ventricular septa. Malalignment of the conal septum, which partitions the great arteries and the trabecular septum dividing the ventricular chambers, may result in aortic override with or without subpulmonary stenosis when there is anterior deviation of the conal septum; subaortic obstruction occurs when there is posterior deviation of the conal septum (show echocardiogram 14).
HEMODYNAMIC ASSESSMENT ！ In addition to evaluating the anatomy of the VSD and its relationship to other cardiac structures, echocardiography also provides information about the hemodynamic abnormalities associated with the VSD, primarily the right and left ventricular pressures and the degree of shunting of blood across the defect.
Shunt determination ！ Two dimensional imaging can give important qualitative information about the degree of shunting associated with the VSD. Both left atrial and left ventricular dilation are present in VSDs that produce significant left-to-right shunting. Flow-related increases in transpulmonary and transmitral velocities, assessed by Doppler, may be documented in such lesions, while pulsed Doppler and Doppler color flow mapping can delineate the timing and direction of shunting during the cardiac cycle [3].
In most individuals, left-to-right shunting predominates in mid and late diastole and throughout systole, while right-to-left shunting, which may not be clinically significant, is commonly seen in early diastole. However, with evolving pulmonary hypertension and pulmonary vascular disease, right-to-left shunting may occur in early and mid diastole, and even in late systole [4]. (See "Pathophysiology and clinical features of ventricular septal defects").
The magnitude of left-to-right shunting at a ventricular septal defect can be estimated using any of several methods.
• Traditionally, the ratio of pulmonary to systemic blood flow (Qp/Qs) has been estimated using volumetric flow data, relying on measurements of pulmonary and aortic velocity or velocity time integrals, and corresponding great artery luminal diameters or cross-sectional areas [5,6,7,8]. The accuracy of this method is lower in adults.
• Estimates of Qp/Qs using color Doppler and flow convergence [9,10] and of volumetric flow at the ventricular septal defect [11] have also been shown to correlate with the measured Qp/Qs in the cardiac catheterization laboratory.
The degree of shunting through a defect is influenced by the size of the defect and by the balance of resistances in the pulmonary and systemic circulations. The pressure and volume loads imposed by large VSDs result in elevations in pulmonary arterial pressure and resistance. As a result, calculations of Qp/Qs have less clinical utility than measurements of right ventricular and pulmonary arterial pressure, which reflect the effect of the VSD on the pulmonary vascular bed.
Assessment of right ventricular and pulmonary artery pressures ！ A number of methods can be used to estimate right ventricular and pulmonary artery pressures [12,13,14,15].
• Using the modified Bernoulli equation (gradient [mmHg] = 4 x [peak velocity]2), the maximum velocity of flow across the VSD, measured by Doppler at an optimal angle of interrogation, can be translated into the pressure gradient between the left and right ventricles (show figure 2).
This value can then be subtracted from the patient's cuff pressure to estimate the systolic right ventricular and pulmonary arterial pressure [12]. In the absence of coexistent right ventricular outflow tract obstruction, large gradients are seen in patients with smaller defects and low right ventricular pressures, and small gradients are seen in patients with elevated right ventricular and pulmonary arterial pressures.
• The common findings of tricuspid and pulmonary insufficiency often allow estimation of right ventricular and pulmonary arterial pressures. Right ventricular systolic pressure can be estimated using the sum of the estimated right atrial pressure and the gradient between the right atrium and right ventricle as derived from the modified Bernoulli equation applied to the peak velocity of tricuspid regurgitation (show figure 3) [13].
• Mean and end diastolic pulmonary arterial pressures can be derived from analysis of the peak and end diastolic velocities of the pulmonary insufficiency jet, respectively (show figure 4) [14,15].
In the assessment of ventricular septal defects, these measurements of right ventricular and pulmonary artery pressures give crucial information regarding patient hemodynamics.