Expiratory radiographs.

The bony structures are within normal limits.

The diaphragmatic outline is not clearly visible ventrally as it is silhouetting with the cardiac silhouette. Steep orientation of the diaphragm.

The liver extends beyond the costal arch and the caudo-ventral liver margin is not clearly delineated.

The cardiac silhouette is enlarged. It occupies 4 intercostal spaces and 90% of the thoracic height on the lateral radiograph and there is buldging of the cranial contour of the cardiac silhouette. Increased sternal contact is present.

On the dorsoventral (DV) radiograph the cardiac silhouette appears mainly broadened. At the level of the 7th rib it occupies almost 100% of the thoracic width. The tip of the cardiac silhouette is displaced to the left side. There is prominent buldging of the cardiac silhouette between 7 and 11 o’clock. At the 7 o’clock the cardiac silhouette becomes slightly smaller; however, a convex bulging is still visible up to the level of the tip of the cardiac silhouette at 5 o’clock. The trachea runs parallel to the spine (lateral radiograph).

No splitting of the main stem bronchi is visible on the lateral view and no widening of the space between the two main stem bronchi is evident on the DV view.

The caudal vena cava (CVC) has an increased diameter and measures approximately 1.3times the height of the vertebral bodies.

The course of the CVC is normal, extending from a cranioventral in a caudodorsal direction.
The lung shows a mildly increased opacity with fine reticular pattern.

The vessel size is unremarkable.

A sail-like, soft tissue opaque structure is visible on the DV radiograph paralleling the left side of the cardiac contour.

• Severe, right sided cardiomegaly with bulging of the cardiac silhouette between 7–11 o’clock and to a lesser degree between 5–7 o’clock
• Increased diameter of the CVC
• Suspicion of hepatomegaly
• Sail-like structure left hemithorax

The buldging of the cardiac silhouette between 7-11 o’clock is compatible with an enlarged right atrium and right ventricle.

However, the severe dilation cannot be traced to the tip of the cardiac silhouette but instead terminates abruptly at 7 o’clock.

This makes it more likely that the bulge represents the severely enlarged right atrium. The bulge between 5-7 o’clock is compatible with an enlarged right ventricle which is less pronounced than the enlargement of the right atrium.

There are no signs for an enlarged main pulmonary artery.

The elevation of the trachea points towards an enlargement of the left side of the heart.
However, no splitting or bowing of the main stem bronchi is present and tenting of the LA is absent. Thus the left atrium is not enlarged.

Additionally, the caudal contour of cardiac silhouette is not straight or abnormally upright and there is no bulging between 12 -2 o’clock.

The normal course of the CVC speaks against a significant enlargement of the left ventricle, because this would lead to an elevation of the CVC.

On the DV view no bulging is visible along the left contour of the cardiac silhouette. Therefore, an enlargement of the left atrium and ventricle is unlikely.

The increased diameter of the CVC points towards the presence of a right-sided backward failure. Hepatomegaly would tie in with this finding and would be caused by hepatic congestion. Overall findings are compatible with an enlarged right atrium with right heart failure with hepatic congestion and enlargement of the CVC.

The young age of the dog makes a congenital disease most likely.

The sail like structure at the left side of the cardiac silhouette represents the thymic remnant, which is likely displaced by the enlarged heart and therefore better visible than expected in an 18 months old dog.

Differential diagnoses for right sided enlargement include:

• Tricuspid valve dysplasia
• Persistent artrio-ventriclar (AV) canal
• Atrial septal defect
• Cor triatrium dexter
• Pulmonic stenosis

Tricuspide valve dysplasia is a possibility.
Due to the valvular insufficiency, regurgitant blood enters the right atrium during systole leading to atrial enlargement and possible right heart failure.

Persistent AV canal is possible, especially the complex form with a connection between all 4 chambers, which leads to an enlargement of right atrium and ventricle.

For a cor triatrium dexter the CVC is not big enough it cannot be eliminated from the list of differential diagnoses.

An atrial septal defect does usually not cause radiographic changes. Shunting of the blood usually occurs during diastole with blood entering the right ventricle, bypassing the right atrium. During systole the blood enters the lung. The resulting volume overload of the lung can lead to pulmonary hypertension with subsequent concentric hypertrophy of the R ventricular wall. A severe enlargement of the right atrium, as visible in this case, would be unlikely.

Pulmonic stenosis (PS) is unlikely due to the lack of an enlarged main pulmonary artery and normal sized lung vasculature (PS is associated with hypoperfusion).

Check the echogradiography for the final diagnosis.