Page 49 - Feline Cardiology

Chapter 6: Radiography 43

cava (CVC) is visible as it courses cranially from the
diaphragm to the right of midline and into the right
caudal margin of the heart. Its diameter is normally
Ao Arch variable depending on the stage of the cardiac cycle. If
it is persistently large, right-sided heart disease should
PA be considered (Bahr 2007). In cats, an exact reference
RA 12
interval is lacking, but the canine equivalent may be
LA extrapolated: in dogs, the CVC measurement correlates Diagnostic Testing
moderately with the height of the vertebral body of T5
9 3 or T6. The mean CVC : V ratio was 0.75 + 0.13 in 100
clinically normal dogs (Buchanan and Bucheler 1995).
When congestive heart failure (CHF) occurs in the
RV 6 LV cat, heart enlargement is usually present because most
disorders that lead to CHF cause cardiac hypertrophy.
Rarely, in the early stages of hypertrophic cardiomyopa-
thy, left atrial enlargement may not yet have occurred at
the time of CHF, especially if a trigger for decompensa-
tion (parenteral fluids, repositol corticosteroids, other)
has recently occurred. However, pulmonary venous con-
gestion with engorgement and distension of the pulmo-
nary veins occurs in the earliest stages of congestive
heart failure and can be recognized radiographically
when the pulmonary vein diameters are greater than
those of their corresponding arteries (Figure 6.6).
Unfortunately, in cats, this finding can be inconsistent
Figure 6.5. Superimposing a clock face on the cardiac silhouette and it is possible for the pulmonary veins to appear
is useful for localizing important structures. The aortic arch (Ao unremarkable in a subset of cats with congestive heart
arch) extends from 11 o’clock to 1 o’clock while the main pulmo- failure, particularly if diuretic treatment has been
nary artery (PA) is located in the 1 to 2 o’clock position. The body
of the left atrium (LA) forms the 2–3 o’clock position of the cardiac recently initiated. As congestion worsens, interstitial
silhouette, the left ventricle (LV) forms the left heart margin from 3 pulmonary edema develops and is recognized as a diffuse
to 5 o’clock and the right ventricle (RV) is located between 7 and 9 increase in lung field radiopacity (Figure 6.7). Because
o’clock. The right atrium (RA) is located at 9 to 11 o’clock. of the perivascular nature of cardiogenic edema,
pulmonary vein and artery borders are indistinct. As
Assessment of the pulmonary vessels is critical in both congestion continues to worsen and fluid floods the pul-
projection(s). The branch (lobar) pulmonary arteries monary alveoli, a coalescing, fluffy alveolar infiltrate
originate from the main pulmonary artery and the right develops (Figure 6.8). Air bronchograms (black bronchi
branch courses transversely, superimposed over the in a white radiopaque background) and air alveolo-
cranial portion of the cardiac silhouette in the DV/VD grams (lung parenchyma with the radiopacity of liver
projection. The left pulmonary artery courses caudally, and absent vascular markings) are present. Because
superimposed over the caudal left ventricular portion of bronchi are not flooded with edema, they appear as
the cardiac silhouette. As in the lateral projections, the radiolucent structures in fluid-filled lungs radiographi-
DV/VD projections allow the pulmonary veins to be cally (air bronchograms). With acute pulmonary edema,
visualized best as they enter the left atrium along the groups of fluid-filled (radiopaque) acini mix with air-
caudal margin of the cardiac silhouette. They are more filled (radiolucent) acini resulting in a finely stippled or
axially oriented in the VD/DV projection than the arter- nodular pulmonary pattern (air alveolograms). This
ies. Feline pulmonary vessels are generally more difficult sequential progression is rarely so distinct, and in reality,
to identify and trace than are canine pulmonary vessels. the pulmonary infiltrate secondary to heart failure is
However, the sizes of the caudal lobar arteries and veins most frequently a mixed interstitial and alveolar pattern.
should be similar to each other and should not exceed In cats, cardiogenic alveolar consolidations often appear
the diameter of the ninth rib where they intersect (Bahr patchy and asymmetrically distributed. The air alveolo-
2007). The aortic arch is often not visible, but the gram pattern predominates, and, in contrast to the dog,
descending aorta is superimposed over the heart and a perihilar distribution is uncommon; the margins of
extends caudally, dorsally, and medially. The caudal vena the pulmonary vessels are often completely obscured.