The cardiac silhouette can appear larger in small-breed dogs because the heart occupies a large amount of thoracic space; conversely, the cardiac silhouette can appear smaller in large-breed dogs (eg, greyhounds) due to the relatively larger size of the thorax (Figure 3).2
In dogs and cats, the vertebral heart score (VHS) system measures the width of the cardiac silhouette (ie, distance from cranial to caudal [ie, short axis] along the estimation of where the atria and ventricles meet) and from the carina of the trachea to the apex of the heart at its most ventral point (lines [ie, long axis] should be perpendicular to each other). These measurements are transferred caudally starting at T4 to calculate the VHS (Figure 4).2 Although this measurement is a good general guide, it can be overinterpreted, as cardiac chambers can change size without changing the shape of the cardiac silhouette.3