Radiology plays an important role in veterinary cardiology, along with other methods, such as electrocardiography, echocardiography, and biomarkers, in the diagnosis of cardiac diseases. Particularly, thoracic radiography is often used to assess heart size and for confirmation of left-sided congestive heart failure by the presence of pulmonary edema. Many cardiac diseases in dogs are associated with a heart size increase, especially in more advanced stages, and cardiac size assessment has been of great interest for a long time for both clinicians and researchers. A substantial number of methods for heart size measurement has been described; however, two methods, the vertebral heart score and the vertebral left atrial size are currently widely used in dogs. Therefore, the literature provides an abundant number of research articles for using these measurements in both healthy dogs and dogs with cardiac and extra-cardiac pathologic conditions. This review article aims to cover the current use of two radiographic measurements of the heart in healthy and diseased dogs, based on the available literature.

Since the beginning of the 21st century, Stanley Kent described for the first time in the literature the presence of connections between the right atrium and the right ventricle (Kent, 1893). Following this study, these connections are called atrioventricular accessory pathways, later also described in dogs (Hill and Tilley, 1985; Thomas et al., 2006; Santilli et al., 2006; Wright et al., 2018). Accessory pathways are abnormal muscle bundles that connect the right atrium to the right ventricle, outside the usual conduction system through the atrioventricular node–His bundle–Purikinje network (Willis, 2018). Subaortic obstruction, also called subaortic stenosis, is characterized by the abnormal presence of tissue of endocardial origin at the subvalvular level, of the left semilunar valves, causing obstruction of the ejection tract of the left ventricle. This congenital anomaly occurs more frequently in dogs, males being more prone to this pathology than females (De Madron, 2016).