Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Two The Thorax 137
Congestive left heart failure develops in stages. Pulmonary venous congestion may be
recognized radiographically when the size of the pulmonary vein reproducibly exceeds the
size of the adjacent pulmonary artery. This is followed by interstitial pulmonary edema,
which produces a blurring of vascular margins beginning in the perihilar area and radiating
peripherally in a nearly symmetric fashion. This, then, progresses to a more diffuse,
poorly defined alveolar pattern infiltrate. In some dogs with cardiomyopathy, especially
those with chronic left heart failure, a nodular pattern may be present. Pleural fluid may
follow the pulmonary edema. Although decreased cardiac output may reduce the size of
the aorta, this generally is not recognized radiographically. Congestive left heart failure
usually occurs after left heart enlargement; however, the left heart size may be normal with
acute left heart failure secondary to conditions such as ruptured chordae tendineae or
myocarditis.
Although echocardiography generally is superior to conventional radiography for evaluating
cardiac morphology, motion, myocardial function, and blood flow patterns, it cannot
evaluate the pulmonary parenchyma or pulmonic vessels. Therefore echocardiography
alone cannot confirm a diagnosis of congestive left heart failure. Radiographic evidence is
required.
Congestive right heart failure results in systemic venous hypertension and congestion.
Enlargement of the caudal vena cava usually occurs. This cannot always be identified
radiographically, because the normal caudal vena cava size varies with cardiac cycle
and respiration. 271 Hepatomegaly, splenomegaly, ascites, and pleural and pericardial
effusions may be observed radiographically in patients with congestive right heart
failure.
The sonographic diagnosis of congestive right heart failure requires evaluation of the
heart as well as the size of the caudal vena cava and hepatic veins. The size of the caudal
vena cava and hepatic veins is evaluated subjectively. The enlargement that occurs with
right heart failure is marked and recognized easily after a few normal livers have been
examined. There is also a loss of the normal respiration-related diameter changes in the
caudal vena cava. The presence of ascites or pleural effusion is helpful in confirming the
diagnosis of right heart failure. Right heart enlargement or pericardial fluid along with
dilation of the hepatic veins and caudal vena cava is indicative of congestive right heart
failure.
SPECIFIC CARDIAC DISEASES
When evaluating either the thoracic radiograph or echocardiogram, it is useful to categorize
cardiac disease as either acquired or congenital. The radiograph, only one part of the
cardiac evaluation, should not be used as the sole basis for a definite diagnosis. The
echocardiogram often allows a definite diagnosis. If there is a disparity between clinical,
electrocardiographic, echocardiographic, and radiographic findings, an angiocardiogram
is recommended to establish a definitive diagnosis. 272
CONGENITAL CARDIAC DISEASES
There are specific radiographic and echocardiographic features described for most congenital
cardiac diseases. 273-283 An algorithm for evaluating these diseases with echocardiography
has been published. 284 Because some variation exists, one or more of the expected
features in any specific case may be absent. Unless specified, the features described apply to
both dogs and cats.
PAT E N T D U C T U S A RT E R I O S U S
PDA is the failure of the fetal ductus arteriosus to close at birth. This situation usually
results in a left-to-right shunting of blood, which causes increased blood flow through
the lungs and the left side of the heart (Fig. 2-109). As a result, the left ventricle, left
atrium, pulmonary arteries, and pulmonary veins are all enlarged or increased in number
or both. 285-287 An aneurysmal dilation usually occurs in the descending aorta and in
the main pulmonary artery. The aortic aneurysm produces a bulge usually seen on the