3 year old Chihuahua

Discussion

The age of the dog and concentric heart enlargement point towards a congenital heart disease. Right heart enlargement can be caused by pressure overload, such as from a stenotic pulmonic valve or pulmonary hypertension. Patent ductus arteriosus is the most common congenital heart disease in dogs, and Chihuahuas are predisposed (along with Maltese, Pomeranian, Shetland sheepdog, Kerry blue terrier, Keeshond, Miniature and Toy Poodle, Bichon frise, Yorkshire terrier, English Springer Spaniel, Collie, Cocker Spaniel, German shepherd, and Irish Setter).
The lack of enlarged pulmonary arteries and an interstitial pattern suggest that there may be pulmonary hypertension contributing to the heart disease. The PDA usually causes left-to-right shunting as the pressure in the aorta is much greater than in the pulmonary artery. This increased blood flow to the lungs (or congential pulmonary hypertension) can cause fibrosis and increasedd pulmonary arterial pressure. If the pressure in the pulmonary arteries is greater than in the aorta, the shunt will flow from right to left. Deoxygenated blood then flows to the aorta causing hypoxia and a resulting increase in red blood cells (polycythemia).
The bubble study confirms a right-to-left shunt. If blood passed normally from the right heart through the lungs, the bubbles would be filtered by the lungs. Only if the blood bypasses the lungs are the bubbles seen in the descending aorta.