Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

146 Small Animal Radiolo g y and Ultrasono graphy
Fig. 2-121 A 6-year-old male
Persian cat had had a heart murmur
since birth with no clinical signs. The
murmur was currently graded 3/6
and was loudest on the left sternal
border. A two-dimensional echocardiogram
revealed a defect (arrow) in
the septum between the left atrium
(LA) and the dilated right atrium
(RA). A Doppler study confirmed a
left-to-right shunt through the
defect. Diagnosis: Atrial septal
defect.
Atrial Septal Defects. Defects limited to the atrial septum are uncommon and rarely cause
changes that are radiographically apparent. Large defects with significant shunting of
blood flow from the left to the right may cause right heart enlargement and dilation of the
pulmonary arteries and veins.
Echocardiography usually will demonstrate the defect in the septum (Fig. 2-121).
Care must be used in interpreting an apparent defect due to “dropout” of portions of the
septum as an artifact due to the thinness of the septum and the geometry of image
formation.
Doppler studies should be helpful. Color-flow studies should document the site of the
defect and the direction of flow, including bidirectional flow. Spectral studies should be
used to assess the velocity of the shunt flow.
Ventricular Septal Defect. VSD is seen in both dogs and cats. This, along with mixed septal
lesions, is the most common congenital anomaly of cats. 322 The most frequent site in
the septum is in the membranous portion (i.e., high, nearer the aortic root).
The size and location of the septal defect and the amount of blood and direction
of blood flow through the shunt determine the radiographic changes that are observed
(Fig. 2-122). In many septal defects, the cardiac silhouette and pulmonary vessels are
radiographically normal. When radiographic changes are present, enlargement of the
left ventricle is usually somewhat more obvious than right ventricular enlargement,
although both occur. Pulmonary arterial and venous enlargement may be
evident; however, the size and number of vessels depend on the volume of blood
shunting from left to right. The vessels appear normal or only slightly overperfused in
most cases.
The echocardiographic changes are likewise dependent upon the size of the defect
and the amount of blood that is shunted. Two-dimensional studies are best performed
with the right parasternal long-axis four-chamber view or the right parasternal shortaxis
view. Typically, septal defects are located close to the aortic root (high) (Fig. 2-123).
They vary in size from pinpoints to those involving the majority of the interventricular
septum. The majority of VSDs occur in the thin, membranous part of the septum. An
apparent defect may be seen in normal individuals in this area due to dropout of echoes
because the tissue is so thin. It is important to be able to define the septal defect in mul-