Cardiac diseases can be classified as congenital or acquired. Evaluating cardiac size and shape on radiographs can be challenging and often requires echocardiography as an additional diagnostic step. When assessing cardiac pathology, it is essential to evaluate the pulmonary vasculature as well, in order to achieve a more accurate picture of the cardiovascular dysfunction.

Peripheral pulmonary arteries and veins should have roughly the same diameter. In case of change in size of pulmonary arteries, veins or both arteries and veins, a cardiovascular dysfunction should be suspected.

An enlarged cardiac silhouette on a thoracic radiograph does not necessarily mean true cardiomegaly. The cardiac silhouette visible on the radiographs is in fact composed by the pericardium and the heart, thus any fluid or tissue in the pericardial space will also contribute to an overall enlargement of the cardiac silhouette. Additionally, in obese patients, accumulation of fat around the pericardium can be misleading and, due to border effacement with the heart, can mimic cardiomegaly.

Atrial enlargement is usually the result of volume overload leading to increased diameter of its lumen.

Ventricular enlargement can be secondary to concentric or eccentric hypertrophy of the cardiac walls. In the first case this occurs at the expense of a decreasing ventricular volume, while in the second case leads to an increasing ventricular lumen. In case of ventricular concentric hypertrophy, radiographic signs of cardiomegaly can be absent. Cardiac radiography should be used as a screening method to evaluate cardiac changes in size and shape, to assess radiographic signs of cardiac failure and response to therapy.

When interpreting cardiac radiology, the first question we need to answer is if the cardiac silhouette is enlarged or not. If there is a cardiomegaly, it is necessary to further characterize if the cardiomyopathy is left sided, right sided or generalized. How do we assess if there is cardiomegaly? Expert radiologists can judge the cardiac size with a subjective evaluation, based on their experience. Students, residents and non-expert radiologists often need to measure the heart size to conclude if it is pathologic or not. The best way is to calculate the vertebral heart score (VHS) by Buchanan, correlating the cardiac long and short axes on a latero-lateral projection with the size of the thoracic vertebrae. When seeing the cardiac silhouette on a DV or VD view, it can be useful to consider the heart as a clock and use the heart-clock analogy to identify roughly the cardiac chambers and large vessels (aorta and pulmonary vessels).

Left Cardiomegaly

On the lateral view, dilation of the left atrium/ventricle causes an increased height of the caudodorsal border of the heart that appears either straight or bulging in a caudodorsal direction. The enlarged left atrium can also cause dorsal displacement of the thoracic trachea and compression of the mainstem bronchi.

On the DV or VD view, left atrial dilation can be seen as increase in soft tissue opacity superimposed to the mid-portion of the cardiac silhouette. A severely enlarged left atrium will cause lateral displacement of the mainstem bronchi that will diverge, assuming the shape of "cowboy legs." Dilation of the left atrial appendage (auricle) is visible on DV or VD views as focal bulge along the left cardiac border at the 2 to 3 o'clock position according to the "heart clock analogy."

Right Cardiomegaly

The main radiographic findings of a right cardiomegaly on a latero-lateral view are an increased sternal contact of the cardiac silhouette and, in case of severe enlargement, dorsal displacement of the cardiac apex from the sternum.

On the DV or VD views, bulging of the right cardiac contour from 6 to 9 o'clock is easily detectable. In severe cases, the cardiac silhouette can assume a reversed "letter D shape."

Generalized Cardiomegaly

The cardiac silhouette tends to be more rounded. In this situation it is important to differentiate a true cardiomegaly (secondary to dilated cardiomyopathy or atrioventricular valve degeneration) from pericardial disease (such as pericardial effusion or peritoneal-pericardial diaphragmatic herniation).

Enlargement of the Main Vessels Associated with Cardiac Disease

Aorta

Segmental enlargement of the aorta can involve the entire aortic arch (often associated with aortic stenosis) or its descending portion (often associated with patent ductus arteriosus). In case of aortic stenosis, a bulging in the contour of the cardiac silhouette is visible on the DV or VD views at the level of 12–1 o'clock, while in case of patent ductus arteriosus, the descending portion of the aortic arch is enlarged and has a sigmoid appearance.

In aged cats, the aortic arch can be elongated and redundant, assuming a sigmoid shape. On the DV or VD view, the projection of the redundant aortic arch, partially superimposed to the left cranial lung lobe, shouldn't be confused with a pulmonary lesion.

Main Pulmonary Artery

Enlarged main pulmonary artery can be associated with pulmonic stenosis, heartworm disease or patent ductus arteriosus and is visible on the DV or VD view as a bulging in the left cranial contour of the cardiac silhouette at the level of 1–2 o'clock.

Caudal Vena Cava

The size of the caudal vena cava is difficult to assess since it is easily subjected to variation secondary to changes in the respiratory phase (inspiration versus expiration) or pressure. Congestion of the caudal vena cava is mostly associated with right heart failure or with an obstructive lesion at the level of the right heart or caudal vena cava itself.

Lobar Pulmonary Arteries and Veins

The lobar pulmonary arteries and veins should have roughly the same diameter. In cases of pulmonary arterial or venous congestion, it is important to correlate these findings with the cardiac and pulmonary changes possibly visible on the radiographic study.

Radiographic Signs Associated with Cardiac Failure

Left-Sided Heart Failure

Left-sided heart failure leads to the development of pulmonary edema.

Depending on the severity of the lung edema, the lung changes visible on the radiographs can vary from an interstitial, peribronchial or alveolar pattern. The distribution of these lung changes differs in dogs and cats.

In dogs, pulmonary infiltrate secondary to left heart failure is mainly localized in the perihilar and caudodorsal region of the lungs. In deep-chested dogs, the pulmonary edema distribution can be multifocal and resemble more than one visible in cats.

In cats, lung edema is less predictable and more variable in location, having mostly multiple patchy distributions. Pleural effusion can also be seen in association with cardiogenic lung edema.

Lung edema is usually associated with congestion of the peripheral pulmonary veins.

Right-Sided Heart Failure

Right heart failure is associated with right-sided cardiomegaly, pleural and peritoneal effusion, congested caudal vena cava, hepatomegaly and ascites.

The combination of thoracic x-rays and echocardiography is often needed in order to have an overview of the cardiac disease and to establish a therapy for the patient.