Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

148 Small Animal Radiolo g y and Ultrasono graphy
Fig. 2-123 A 6-month-old male Rottweiler with a grade 4/6 systolic
ejection-type murmur that was best heard at the heart base.
The right parasternal long-axis aortic (ao) outflow view reveals a
focal defect (arrow) high upon the interventricular septum (vs).
Doppler studies confirmed the defect. Diagnosis: Ventricular
septal defect.
sonography (bubble studies) can be used to help identify some VSDs. In those that have
right-to-left shunting of blood, the bubbles may be seen in the left ventricle before they
appear in the left atrium. In those that shunt from left to right, there may be an area of
apparent diminished bubble numbers in the right ventricle due the shunting of
non–contrast laden blood across the septum into the area.
Doppler studies are very important in clearly identifying and grossly quantitating
blood flows through a VSD. Color-flow studies should be used to identify specifically the
site of the lesion and indicate the direction of blood flow. Aortic regurgitation also may be
noted in some cases.
Spectral Doppler studies should be directed across the lesion in a manner as parallel as
possible to the flow through the defect. Because high velocities may be encountered, continuous
wave (or high pulse repetition frequency) studies are preferable.
The echocardiography findings may be helpful in making a prognosis. Factors that
favor patient longevity include a defect in the septum of less than 40% of the area, a
maximal shunt velocity of greater than 4.5 m/s, and no evidence of significant aortic
regurgitation. 323
Atrioventricular Septal Defects. Atrioventricular septal defects, or endocardial cushion
defects, are cases in which all or part of the atrioventricular septum is missing. This malformation
is relatively common in cats with congenital cardiac disease. 324,325 Complete failure
of the endocardial cushion to form has been termed a common atrioventricular canal.
T E T R A L O G Y O F FA L L O T
Tetralogy of Fallot is the combination of pulmonic stenosis, VSD, overriding (rightward
displaced) aorta, and right ventricular hypertrophy (Fig. 2-124). Tetralogy has been
reported in both cats and dogs. It is a heritable disease in the Keeshond. 326-328 The cardiac
silhouette size is usually normal or only slightly enlarged. The right ventricular margin may
be slightly more round and the left ventricular margin straighter than normal. Both
changes result in a somewhat rectangle-shaped heart. The poststenotic pulmonary artery
dilation is usually small and not radiographically visible. A striking radiographic feature is
the decreased pulmonary vascular size and hyperlucent lung. The aorta may be small, but
radiographic recognition is difficult.
The echocardiographic findings are consistent with the intrinsic pathology. The interventricular
septal defect is usually high in the membranous portion of the septum, resulting
in a “malalignment VSD.” The aortic root is seen straddling the septal defect (Fig.
2-125). The main pulmonary artery usually shows a mild aneurysmal dilation. The right
ventricular free wall is thickened. The left atrium may be small. The shunting of blood may
be documented with contrast (bubble) or Doppler studies.
E I S E N M E N G E R’ S P H Y S I O L O G Y
Eisenmenger’s physiology is the shunting of blood from the right side of the circulatory
system to the left due to increased pulmonary vascular resistance. This is a very uncommon
finding and usually results in a right-to-left shunt. These reversals usually occur early in
life. Lesions associated with Eisenmenger’s physiology include atrial septal defect, VSD,
aorticopulmonary defect, PDA, and combinations of the above. 329-331