Page 45 - Feline Cardiology

Chapter 6: Radiography 39

patient has responded to urgent care. Another option for tion among cats. In particular, the angle of the cardiac
obtaining thoracic radiographs in severely dyspneic cats long axis orientation is more variable. In older cats the
is a horizontal beam projection. Many models of radio- angle decreases (a more “horizontal” alignment) with
graph units can be rotated to direct the radiographic increased sternal contact (Moon et al. 1993; Nelson et
beam horizontally. Doing so allows a cat to remain in al. 2002). This finding has been termed by some a “lazy”
sternal recumbency while the radiographic plate is heart conformation. A tortuous, redundant aorta is
placed perpendicular to the tabletop, against the cat’s another common variation in older cats (Figure 6.2).
lateral thorax. Thus, a lateral projection can be obtained However, this change has also been associated with sys- Diagnostic Testing
without placing the cat in lateral recumbency. temic hypertension in humans and an association is
Thoracic radiographs are the diagnostic test of choice suspected in cats (Nelson et al. 2002). See Chapter 21 for
for evaluating the chest in most severely dyspneic cats, more information regarding systemic hypertension.
but an important exception can be the patient suspected The cardiac chambers can be defined by drawing a
of having large-volume pleural effusion. In these cats, a second line (short axis) perpendicular to the heart long
brief thoracic ultrasound exam with the cat in sternal axis at the level of the ventral aspect of the caudal vena
recumbency may effectively rule in or rule out pleural cava. The dorsal cardiac segment includes both atria, the
effusion and the need for immediate centesis. pulmonary arteries and veins, the cranial and caudal
Radiographic interpretation involves adherence to cavae and the aortic arch. The cranioventral segment
several steps. Any identified abnormalities supportive of is formed by both the right ventricle and right auricle.
disease should be confirmed on multiple radiographic The ascending aorta is superimposed over the right
views: atrium; the aortic arch extends caudodorsally to form the
ascending aorta. The main pulmonary artery cannot be
1. Evaluate films for technical quality and respiratory visualized on the lateral projection, but the left pulmo-
phase; if they are substandard, they should be repeated.
2. Review the entire thoracic cavity, notably the pleural nary artery can sometimes be seen extending dorsally and
caudally to the tracheal bifurcation while the right pul-
space, for abnormal presence of air or fluid.
3. Review the portion of the cranial abdomen included monary artery may be seen on “end” as it leaves the main
pulmonary artery immediately ventral to the carina.
in the projection.
A method developed by Dr. Jim Buchanan, the verte-
4. Evaluate the position, course, and diameter of the bral scale system, allows objective measurement of the
trachea and mainstem bronchi.
5. Evaluate the position of the cardiac apex and caudal heart size scaled against the length of specific thoracic
vertebrae (Figure 6.3) (Litster and Buchanan 2000a).
mediastium.
6. Evaluate the cardiac margin (including all parts as This form of internal control reduces error associated
with subjective interpretations and interpatient variabil-
described in the clock face analogy, below [see Figure ity. On the lateral radiograph, the long axis of the heart
6.5]) for enlargement. (L) is measured with a caliper extending from the ventral
7. Evaluate the size, shape, and course of the main pul- aspect of the left mainstem bronchus to the left ventricu-
monary artery and peripheral pulmonary vessels. lar apex. The caliper is repositioned along the vertebral
8. Evaluate the caudal vena cava for elevation and/or column beginning at the cranial edge of the fourth tho-
enlargement. racic vertebra, and the number of vertebrae (V) within
9. Evaluate the lung fields for hyper- or hypoinflation the distance L is recorded to the nearest 0.1 V. The
and for distribution and pattern of increased or
decreased opacity including bronchiolar prominence/ maximal perpendicular short axis (S) is measured in the
same manner and likewise is quantified beginning at the
thickness.
fourth thoracic vertebra. Normal cats measure less than
The lateral projection allows assessment of the right side 8 V; in a series of 100 normal cats, the mean vertebral
of the heart (the cranial margin of the silhouette) and heart size (VHS) was 7.5 V (±0.3) (Litster and Buchanan
the left side of the heart (the caudal margin). In the 2000a). Another study obtained very similar results in
normal cat, the cardiac silhouette is ovoid or egg-shaped, 50 clinically normal, adult stray cats, with a greater
with the apex pointed in shape compared to the wider degree of interindividual variation: the VHS was
cardiac base. The heart axis is defined by drawing an 7.3 ± 0.49 V in the right lateral, 7.3 ± 0.55 in the left
imaginary line from the tracheal bifurcation (carina) to lateral, 7.5 + 0.68 in the DV and 7.5 + 0.53 in the VD
the apex, with a resulting angle that usually is approxi- views (Ghadiri et al. 2008). Thoracic radiographs and
mately 45 degrees to the sternebrae (see Figure 1.3). Cats VHS have also been compared to echocardiographic
have far less breed-related cardiac variation than is seen findings in a group of cats presenting to an emergency
in dogs, but there tends to be more interindividual varia- service for dyspnea. None of the cats with a VHS ≤8 V