Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

126 Small Animal Radiolo g y and Ultrasono graphy
that is sometimes observed between the left atrium and ventricle becomes flattened. The
caudal vena cava becomes dorsally displaced at the point where it contacts the heart.
Overlap, if any, between the heart and the diaphragm increases, although the reliability of
this change depends on consistently obtaining an inspiratory radiograph.
Dorsal enlargement of the cardiac silhouette will increase the apicobasilar cardiac
dimension and decrease the normal caudoventral angulation of the trachea. This always
must be interpreted with consideration of the animal’s thoracic conformation. Generally,
right heart enlargement elevates the trachea cranial to the tracheal bifurcation, left
ventricular enlargement causes elevation at the bifurcation, and left atrial enlargement
causes elevation caudal to the bifurcation. These changes rarely occur as isolated events and
a considerable amount of overlap is observed.
In the dorsoventral or ventrodorsal radiograph, cardiac enlargement may be cranial,
caudal, or to either side. The cranial cardiac margin blends with the cranial mediastinum,
and evaluation of this dimension is difficult. Cranial right enlargement, usually
associated with the right atrium, accentuates the curvature of this cardiac margin.
Cranial left enlargement, usually associated with the aortic arch and main pulmonary
artery, or very rarely the right atrial appendage, produces bulges or knobs on this part
of the cardiac margin. Caudal left enlargement usually is associated with the left ventricle
and results in a rounder silhouette. Caudal cardiac silhouette enlargement also
causes rounding of the cardiac apex, which results in a flattened or blunted tip. This is
documented best in a ventrodorsal rather than a dorsoventral radiograph. Enlargement
of the cardiac silhouette in a craniocaudal dimension, resulting in an increased apicobasilar
dimension and elongation of the cardiac silhouette, is associated mainly with
left ventricular enlargement.
Enlargement of the cardiac silhouette on the right side is mostly due to right ventricular
enlargement. The location of the cardiac apex relative to the midline is critical. An apex
shift to the left will decrease the appearance of right-sided cardiomegaly, while a shift
toward the right will increase the apparent enlargement. Enlargement of the right cardiac
margin accentuates its normal convexity. The cardiac apex shifts to the left and the distance
from the cardiac margin to the right thoracic wall decreases. Enlargement of the cardiac
margin on the left side is mostly due to left ventricular enlargement. The normal, relatively
straight, left cardiac margin becomes more convex and the distance to the left thoracic wall
decreases. Apex shift to the right is an uncommon occurrence. Left atrial appendage
enlargement produces a convex bulge at approximately the three to five o’clock position.
This occurs with moderate to marked left atrial enlargement and rarely is seen without left
ventricular enlargement.
Because the shape of the normal heart differs in the comparison of ventrodorsal and
dorsoventral radiographs, reevaluation of a patient should include use of the same view
each time. Consistent and accurate positioning is more important than the specific position
used.
Cardiomegaly in cats differs in appearance from that observed in dogs. On the lateral
radiograph, this produces a bulging cardiac contour, because enlargement is most apparent
in the craniodorsal and caudodorsal portion of the silhouette. The cardiac silhouette
may be elongated and tracheal elevation may be observed. On the dorsoventral radiograph,
cardiac enlargement usually produces changes in the right and left cranial margins. This
increases the dimension cranially while maintaining a relatively normal caudal dimension,
producing a somewhat triangular or valentine-shaped cardiac silhouette.
The nature of the cardiac enlargement depends on the specific cardiac disease that
is present, as well as the duration and severity of the disease. Once a diagnosis of cardiomegaly
is made, the cardiac silhouette should be evaluated critically for evidence of
a specific chamber enlargement or great vessel or pulmonary vessel changes. Sequential
radiographs are invaluable in detecting cardiomegaly and specific chamber
enlargement.
Usually a diagnosis of cardiomegaly is not made from an echocardiographic examination.
The information obtained is more cardiac chamber–specific than simply a general
assessment of overall cardiac size. M-mode and two-dimensional measurements usually
are made from leading edge (acoustic interface closest to the transducer) to leading edge.