Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Two The Thorax 127
Diastolic dimensions are measured at the Q or R wave of the ECG. Systolic measurements
are obtained at the point of peak downward septal movement.
Right Atrial Enlargement. Right atrial enlargement is not identified radiographically
unless the atrium becomes severely enlarged. When present, it is usually associated with
evidence of generalized right heart disease (Fig. 2-101). The exception to this is when a
right atrial thrombus or neoplasm is present.
On the lateral radiograph, a severely enlarged right auricular appendage may produce
a convex bulge on the cranial dorsal margin of the cardiac silhouette, with loss of the cranial
cardiac waist. That portion of the trachea cranial to the bifurcation may be mildly displaced
dorsally over the heart base.
On the ventrodorsal or dorsoventral radiograph, the enlarged right atrium will produce
a bulge on the right cranial cardiac margin at approximately the nine to ten o’clock
position. Severe enlargement may protrude across the midline to the left cranial cardiac
margin.
An echocardiogram can identify right atrial enlargement prior to when it will be radiographically
evident. Although normal diameters have not been well defined, the right
atrium should not appear larger than the left atrium on the right parasternal long-axis view
or the left parasternal four-chamber view.
Right Ventricular Enlargement. Right ventricular enlargement causes accentuation of the
cranial convexity of the cardiac silhouette on the lateral radiograph; it eventually causes a
vertically oriented cardiac border (Fig. 2-102). Contact between the cardiac silhouette and
the sternum increases, the cardiac apex may become elevated or separated from the sternum,
and the trachea may become elevated cranial to or at the bifurcation as a result of
right ventricular enlargement. If a line is drawn from the tracheal bifurcation to the cardiac
apex, the amount of the cardiac silhouette cranial to this line will be greater than the normal
two thirds of the total cardiac area.
In the ventrodorsal or dorsoventral radiograph, right ventricular enlargement will
make the right cardiac margin become more convex and the distance to the right thoracic
wall will decrease. The cardiac apex will shift to the left. As the cardiac apex becomes elevated
from the sternum, the right ventricle may bulge caudally beyond the interventricular
septum, producing two bulges on the caudal cardiac margin (an apparent double apex). If
a line is drawn from the cardiac apex to the point where the cranial mediastinum merges
with the right cranial cardiac border, more than one half of the heart area will be to the
right of this line.
Radiographic evidence of right ventricular enlargement can be due to either dilation of
the right ventricular cavity or hypertrophy of the right ventricular myocardium. An
echocardiogram can determine if either or both of these lesions are present. The normal
diameter of the right ventricular cavity at end diastole is not well defined in the dog, but it
should not exceed 33% to 50% of the diameter of the left ventricle as measured on the right
parasternal long-axis view. The diameter in the cat at end diastole should not exceed
7 mm. 21 Likewise, the normal thickness of the right ventricular free wall is poorly defined
but should be approximately 33% to 50% of the thickness of the interventricular septum
or left ventricular free wall. 55
Main Pulmonary Artery Enlargement. On the lateral radiograph, a markedly enlarged
main pulmonary artery may bulge cranial to the cranial dorsal cardiac border, exhibiting
loss of the cranial cardiac waist (Fig. 2-103). In most cases the enlarged pulmonary artery
does not extend beyond the cardiac margin or is lost within the soft-tissue density of the
cranial mediastinum.
On the ventrodorsal or dorsoventral radiograph, the enlarged main pulmonary artery
bulges beyond the left cranial cardiac margin at the one to two o’clock position. The cranial
border of the bulge may blend with the shadow of the cranial mediastinum. In a
slightly overexposed radiograph, the caudal border of the bulge may be traced to the left
main pulmonary artery, which will facilitate differentiating it from other regional structures.