5.5.4 Further diagnostic aids
A definitive diagnosis of a VSD is best achieved using echocardiography, once a suspicious murmur is detected. Semi-membranous VSDs are usually identified in the long-axis view of the LV outflow-tract just below the right coronary cusp (RCC) of the aortic valve (Figure 5.3). It is important to rotate the transducer to examine this area in the short-axis view. This ensures that the lesion will not be missed and also allows measurement of the size of the defect in two image planes. Often the septal leaflet of the tricuspid valve will be seen to have high-frequency vibrations during systole. Abnormal looking chordae may insert on this valve leaflet. Occasionally, the RCC of the aortic valve will be seen pro-lapsing into the defect, or the whole of the aortic root may appear to be displaced into the defect.
Contrast echocardiography is a useful technique to document the presence and location of a left to right shunt (see section 4.2.8). If a VSD is present, a jet of negative contrast (normal blood) will be seen entering the contrast-filled RV just below the tricuspid valve, adjacent to the aortic root. It is useful to record an ECG on the echocardiogram and to review these studies on video tape because echo-free blood from the caudal vena cava tends to flow down the atrial septum adjacent to the aortic root and can be mistaken for a jet of blood flowing through the VSD. To distinguish the two, the VSD shunt occurs during systole and the caudal vena cava flow is seen during diastole. The venous flow also enters the right side of the heart on the atrial side of the tricuspid valve, although this can be difficult to identify in the contrast-filled chamber. Selective contrast echocardiography is also possible but is much more invasive and is seldom required.
DE is a useful and sensitive technique for detecting the abnormal shunt flow. Pulsed-wave DE is used to detect blood flow by placing the sample volume on the RV side of the suspected defect. Echocardiography and DE should also be used to conduct a thorough examination to rule out the presence of other congenital anomalies which might complicate the clinical picture.
Catheterisation has been used to demonstrate an increased pressure and oxygen tension in the RV as a result of blood shunting from the LV. The extent of these changes will depend on the size of the shunt. Small shunts may be missed.
Angiography may demonstrate intracardiac shunting of blood. Selective angiography is ideal although non-selective angiography can also be performed along similar lines to non-selective contrast echocardiography; however, this requires a pressure injector in order to inject sufficient contrast to produce a diagnostic image. Angiography requires expensive equipment and has largely been replaced by echocardiography.
Electrocardiography is of little or no value in assessing heart chamber changes as a result of a VSD, but is of use in identifying arrhythmias which may comthe condition. The most frequently occurring arrhythmia which is of clinical significance is atrial fibrillation. The presence of this arrhythmia in a foal with a VSD is suggestive of volume overload and indicates a poor prognosis.