The cardiac silhouette consists of pericardium, pericardial fluid, myocardium (including epicardium and endocardium), the origins of the major vessels and blood. Its size may change with the cardiac cycle, and it may be slightly larger during expiration than inspiration. Its appearance is slightly different between right and left lateral recumbency and between sternal and dorsal recumbency, and so a consistent technique should be adopted. Radiographic signs of heart disease include change in size or shape of the heart and evidence of right- or left-sided heart failure. Alteration in size or shape of the cardiac silhouette may be due to enlargement of any of its components and can often be distinguished only by angiography or ultrasonography; plain radiographs may be misleading. Conformation is the single most important cause for apparent cardiomegaly in barrel-chested dogs such as the Bulldog, Yorkshire Terrier and Dachshund, which have a relatively large heart with elevated trachea on lateral radiographs. The Golden Retriever also has an apparently large and square-shaped heart on the lateral radiograph. Generalized cardiomegaly may be evaluated in dogs by means of the vertebral heart scale measurement (Fig. 7.1); on the lateral recumbent radiograph, the distance between the ventral aspect of the carina and the cardiac apex is taken as a length value, and the maximum width of the heart perpendicular to the length line is taken as the width of the heart. Starting at the cranial aspect of the fourth thoracic vertebra, the number of vertebral lengths is determined for each measurement. In 100 clinically normal adult dogs, the mean (± SD) vertebral heart size was 9.7 ± 0.5. Cardiomegaly is usually considered present when the combined measurement exceeds 10.6 thoracic vertebrae, although in some breeds (e.g. Labrador, Golden Retriever, Boxer, Cavalier King Charles Spaniel, Greyhound, Whippet, Lurcher) this value is commonly exceeded in normal dogs. Reference ranges for some popular breeds have been published. In cats, the same technique can be used with a mean of 7.5 ± 0.3 and 8.1 being the cut-off point above which the heart is considered enlarged. Localized cardiac enlargement may be described according to the clock face analogy (Fig. 7.2).

It should be noted that radiography is an insensitive and relatively inaccurate method for diagnosis of heart diseases (especially congenital), and echocardiography is a much more efficient modality. Nevertheless, radiography is extremely important for assessment of signs of heart failure, especially in the lungs.

A normal cardiac silhouette may be present in spite of severe cardiac disease. Echocardiography and an electrocardiogram are essential diagnostic components of the cardiac examination for complete cardiac evaluation.

The heart silhouette is abnormally small and pointed, the ventricles appear narrower and the apex loses contact with the sternum. Thoracic blood vessels may appear smaller and the lungs hyperlucent (see hypovascular pattern, 6.23.5). The CdVC is also reduced in size (Fig. 7.4).

Some of the following diseases may only cause mild cardiomegaly or cardiomegaly only in advanced stages of the condition. Chamber lumen dilation and heart wall hypertrophy cannot be distinguished radiographically, and myocardial pathology is much more readily diagnosed by means of two-dimensional and M-mode echocardiography.

Because it is hard to differentiate the left and right sides of the heart radiographically, disease that is confined to one side may nevertheless appear radiographically as generalized heart enlargement (see 7.8, 7.9, 7.11 and 7.12); the list below includes only diseases that genuinely affect both right and left sides. Assessment of pulmonary vasculature is also important in radiographic assessment of cardiac disease (see 6.23).

Pericardial disease may be difficult to distinguish from generalized cardiomegaly radiographically. The main difference is that most cases of cardiomegaly have left atrial enlargement, whereas pericardial effusion produces an enlarged, globular cardiac silhouette lacking specific chamber enlargement (Fig. 7.5). Its margins may be sharp due to reduced movement blur. Pericardial effusion may also often be differentiated from generalized cardiomegaly by the type of failure that results, which is right-sided, whereas left-sided or generalized failure is seen with the most common cause of generalized cardiomegaly, cardiomyopathy. Ultrasonography is the imaging modality of choice to evaluate pericardial pathology (see 7.7).

The lateral view shows bulging of the cardiac silhouette at 12–2 o’clock, with elevation and compression of the left main stem bronchus. The caudal border of the heart is abnormally straight and upright or even slopes caudodorsally, and the caudal cardiac waist is lost (Fig. 7.7A). On the DV view, atrial enlargement may push the main stem bronchi further apart (to > 60°) and the enlarged left auricular appendage creates a bulge at 2–3 o’clock (Fig. 7.7B). The increased opacity of the dilated left atrium may be mistaken as lymphadenopathy or a lung mass on either view. Secondary pulmonary changes in the form of vascular congestion or pulmonary oedema may be present (see 6.14 and 6.24).

On the lateral view, cardiac enlargement is seen at 2–5/6 o’clock, with increased height of the heart and elevation of the trachea (Fig. 7.7A). Left atrial enlargement is usually also present. On the DV view, the heart may appear elongated and enlargement is seen at 3–5 o’clock; in severe cases this displaces the cardiac apex to the right (Fig. 7.7B) (right heart enlargement due to, for example, pulmonic stenosis may displace the cardiac apex further to the left on the DV radiograph, mimicking left ventricular enlargement).

On the lateral view, bulging of the cardiac silhouette occurs at 10–11 o’clock, with increased craniocaudal dimension of the heart and elevation of the terminal trachea and/or loss of the cranial cardiac waist in severe cases. There may be a widened CdVC as result of venous congestion (Fig. 7.10A). On the DV view, bulging is seen at 9–11 o’clock (Fig. 7.10B). In obese cats, the right atrial area outline may have a square or angular appearance on VD radiographs, due to pericardial fat rather than genuine chamber enlargement.