Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

134 Small Animal Radiolo g y and Ultrasono graphy
ascending aortic arch, as in aortic stenosis, the bulge, if noticeable at all, will be observed
cranial to the cardiac silhouette.
The proximal ascending aorta is visualized easily by echocardiography. As seen on
the right parasternal long-axis left ventricular outflow tract view, the aorta begins with
the aortic valves, then is followed by a slight dilation (sinus of Valsalva) and then continues
cranial and then dorsal. In some patients, part of the aortic arch can be visualized.
Dilation of this segment is recognized readily, and the diameter can be
compared with published normals (1.9 cm: 12 kg; 2.4 cm: 24 kg). 55 Dilation of the aorta
distal to this region usually is not exhibited on the echocardiogram, because the airfilled
lung blocks transmission of sound to these regions (e.g., at the level of ductus
arteriosus).
M I C R O C A R D I A
Microcardia is a nonspecific term that is used to denote a smaller-than-normal cardiac silhouette
(Fig. 2-108). It generally reflects a decrease in circulating blood volume. This may
occur from hypovolemic shock, dehydration, or emaciation and has been reported with
Addison’s disease. 270 The hypovolemia and hypoperfusion also cause the caudal vena cava
and pulmonary arteries to appear small. Microcardia may be diagnosed incorrectly when
the heart appears small in comparison to the lungs as a result of overinflation of the lungs
or tension pneumothorax.
On the lateral radiograph, an abnormally small heart may not contact the sternum. It
will measure 21/2 intercostal spaces or less in width, and its height will be less than 50% of
the lateral thoracic dimension. The apex often appears more sharply pointed than normal.
The heart will be almost perpendicular to the sternum.
On the ventrodorsal or dorsoventral radiograph, the cardiac silhouette will occupy
less than 50% of the thoracic diameter and will be separated from the diaphragm by
lucent lung. The heart may be oval in shape due to its upright position within the
thorax.
Echocardiographically, hypovolemia is recognized by a decrease in the diameter of all
chambers and great vessels. Cardiac wall thicknesses will remain normal or be slightly
increased, and fractional shortening may be normal or increased.
Fig. 2-106 A 4-year-old male Great
Dane had a cough and an enlarged
cardiac silhouette on radiographs.
The M-mode echocardiogram
revealed marked dilation of the left
ventricle. The septal (SL) and parietal
(PL) leaflets of the mitral valve
are observed. The E-point septal separation
(EPSS—i.e., that distance
between the maximal open position
of the septal leaflet and the interventricular
septum) is enlarged
markedly (16 mm). Diagnosis:
Dilated cardiomyopathy.