Doppler Evaluation of Atrial Septal Defects (ASD) and Ventricular Septal Defects (VSD):
©Walter Rasmussen, R.D.C.S.
Screening for atrial septal and ventricular septal defects should be part of the routine echocardiographic examination.
For ventricular septal defects, color Doppler mapping of the inter-ventricular septum in the parasternal long axis, apical four-chamber and subcostal four-chamber views will fulfill this objective. For atrial septal defects, color Doppler mapping of the atrial septum in the parasternal short axis view at the base of the heart, the apical four-chamber view and the subcostal four-chamber view will provide adequate screening.
The best views for finding atrial-septal defects.
It is relatively rare to encounter a previously unknown atrial septal defect on a routine adult echocardiogram however; if there is echo dropout suggestive of thinning or absence of a portion of the atrial septum, it should be routinely evaluated with color Doppler and the reading physician may want a shunt study. The presence of a highly-mobile inter-atrial septal aneurysm should raise the sonographer’s interest for further investigation with color Doppler and possibly obtaining a shunt study order.
Due to the typically low velocity of the ASD jet, color Doppler scale should be lowered to 45-50 cm/sec or lower, depending upon how much turbulence is present within the right atrium. If there is a lot of turbulence within the right atrium, further lowering of the scale will only add undesirable visual clutter.
Maintaining high frame rates is also very important for finding small, brief jets. This can be achieved by narrowing the two-dimensional sector width and making the color box only as large as absolutely needed to cover the atrial septum and portions of the right and left atria.
Large ASD’s can be of very low velocity and so will demonstrate a slowly moving dull red/blue area on color Doppler during most of the cardiac cycle. Small ASD’s will be seen as a jet of only a few millimeters in width and length, with greater length if the pressure gradient between the left atrium and right atrium is relatively high.
CW and PW Doppler should be used for follow-up to Color Doppler in order to the characterize blood flow velocity and pattern across septal defects. The shunt velocity profile can help to establish the severity of the lesion and the pressure relationship between the right and left sides of the heart. A slow chart speed at 25 to 50 mm/sec helps to demonstrate physiologic variations and can aid in assessing intra-cardiac pressures.
The most common location for ventricular septal defects is at the membranous ventricular septum, near the aortic root and is known as a perimembranous VSD. Color Doppler is the primary tool for locating this type of VSD because they are usually so small that they will not be visualized with certainty on two-dimensional echo alone. Shunting associated with a perimembranous VSD typically lasts throughout systole without interruption and is usually of a high velocity, reflecting the great difference in pressure between the left and right ventricles and the diameter of the VSD. During diastole, the shunting rapidly decreases to the baseline at a point based upon the pressure differences between the right and left ventricles and the size of the VSD.
The color Doppler scale should be set at the standard 50-60 cm/sec for assessment of ventricular septal defects, unless a large VSD is being investigated, in which case it may be necessary to lower the scale in order to see the Doppler shift information.
Color Doppler evaluation of ventricular-septal defects should be followed by both CW and PW Doppler modes, which allow for calculation of the pressure gradient between the chambers and more accurate assessment of the blood flow characteristics. Typical CW Doppler of a perimembranous VSD
Muscular VSD’s are investigated by searching the entire inter-ventricular septum with color Doppler and may be found from any acoustic window or view, dependent on the axis of the jet. Since the left ventricle normally achieves a higher systolic pressure than the right ventricle, the jet direction will be continuous throughout systole from left to right and extending into the right ventricle or the right ventricular outflow tract.
Restrictive muscular VSD’s may be quite small and shunting may diminish or completely cease during systole due to obstruction of the defect during myocardial thickening.
Since the apical view is relatively parallel to the heart’s septum, VSD’s and ASD’s generally do not produce as large of a Doppler shift as views that are perpendicular to the septum such as the parasternal and subcostal views. Off-axis apical views can be used to increase the Doppler shift caused by shunts, thus making visualization easier.