Fetal dogs have a short, broad vessel called the ductus arteriosus, which sends blood from the right ventricle of the heart to the aorta, thus bypassing the lungs. Shortly after birth the vessel should close naturally. If it does not close, normal circulation is impaired. This congenital heart defect is called patent ductus arteriosus (PDA), patent meaning "open". Cardiac surgery may be required to correct the problem. 
Picture courtesy of Inova Heart and Vascular Institue (www.inova.org)
Findings suggest that apparently the PDA is caused by a genetic defect in the structure of the ductus arteriosus that is similar or identical to that in the Poodle. The relatives of dogs with PDA, particularly parents, offspring, and siblings, should be screened for evidence of PDA. Dogs with PDA should not be used for breeding, regardless of breed.
There are two types of PDA: Left to right is the typical form of PDA in which the aortic pressure is higher than lung artery pressure, and blood shunts
continuously from the aorta to the lung artery. This results in a continuous heart murmur, and increased size of the heart. Approximately 64% of the dogs diagnosed with left-to-right shunting PDA will die from complications within a year of diagnosis without surgical correction. Some dogs with modest shunts will survive to maturity, and a few may live 10 years or more.
Right to left Patent Ductus Arteriosus (or reversed PDA) occurs when there is an increase in lung vessels pressure and is an uncommon condition. Many owners do not recognize obvious clinical signs in their pet during the first 6-12 months of life. Animals with reversed PDA often live 3-5 years; a few survive beyond 7 years.
The diagnosis of PDA is usually made by ausculting (listening) of the heart and detecting continuous heart murmur, which can be confirmed by examining a chest X Ray and evaluating the heart echocardiogram.
Clinical signs of PDA may include coughing, labored breathing, exercise intolerance,
collapse, decreased appetite. If the ductus is small, there may be no signs at all. The severity of these signs is related to the degree of heart failure. Dogs with heart failure are initially treated with a diuretic (medication that promotes urination) to remove excess fluid.
If the PDA is left untreated clinical signs usually appear within a year. Surgery is more successful if done in the young dogs, before permanent heart damage has occurred, and is often repaired without complications. Affected breeds include Miniature poodles, Collies, Pomeranians, Cocker spaniels, Chihuahua, Maltese, German shepherds, Irish setters, Keeshounds, and Shetland Sheep dogs.
PDA references
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