The most common congenital heart disease of the dog is a patent ductus arteriosus or a PDA. In dogs, the condition is relatively common and some breeds are more likely to be affected than others. PDA does occur in cats but rarely.
What Is a Patent Ductus Arteriosus?
During fetal development, all animals have a ductus arteriosus. This vessel is responsible for shunting blood past the lungs, which are still filled with fluid in a fetus and not ready to function properly. By shunting blood past the lungs, the fetus' heart is able to pump the blood normally to other areas of the body.
At birth, when a newborn takes the first breath and the lungs fill up with air, the dynamics of the cardiovascular system change and blood starts flowing through the pulmonary artery to the lungs instead of being shunted past the lungs through the ductus arteriosus. As the blood flows to the lungs, the ductus arteriosus begins to close up. In a normal healthy puppy, it should be firmly closed by the time the puppy is 7 days old.
In the case of a patent ductus arteriosus, the ductus arteriosus does not close as it should. This results in blood flowing backward through the patent ductus arteriosus from the aorta instead of being pumped to the rest of the body as it should. This backward flows of blood results in an extra load on the heart.
If the shunt is large enough, the heart will compensate and the left ventricle will become enlarged. If the shunt is large enough, heart failure will occur. This scenario is referred to as left to right shunting because the blood is shunted from the left side of the heart back to the right side.
In some situations, when there is increased resistance in the lungs to blood flow, the shunting may instead become right to left. The increased resistance in the lungs is referred to as pulmonary hypertension. Pulmonary hypertension may occur if the left to right PDA continues to cause an overload to the heart and the circulation to the lungs indefinitely.
An untreated left to right PDA may become a right to left shunt if left untreated. This right to left shunt is sometimes called a reverse PDA.
The symptoms of a patent ductus arteriosus in a dog are those of heart disease and eventually heart failure if the shunt is severe enough.
Symptoms will vary depending on the severity of the shunt. If the shunt is small, there may be very few if any symptoms present. However, if the shunt is larger, heart failure will occur. Symptoms associated with heart failure include coughing, difficulty breathing and weakness.
Diagnosing Patent Ductus Arteriosus in Dogs
Diagnosis of a patent ductus arteriosus involves several things. A heart murmur will be present in most puppies with a patent ductus arteriosus.
Certain breeds are predisposed and if the puppy with a heart murmur is from one of those breeds, the suspicion of a patent ductus arteriosus may be higher. Breeds predisposed to patent ductus arteriosus include the German Shepherd, Miniature Poodle, Keeshond, Cocker Spaniel, Pomeranian, Collie, and Shetland Sheepdog.
Radiographs are usually indicated to evaluate heart size and determine whether there is fluid build-up in the lungs due to heart failure.
Definitive diagnosis of patent ductus arteriosus is usually done with an echocardiogram (an ultrasonographic study of the puppy's heart). In an echocardiogram, the blood flow through the patent ductus arteriosus can actually be visualized.
Treatment of Dogs with Patent Ductus Arteriosus
The preferred treatment of dogs with a patent ductus arteriosus is either surgical ligation of the vessel or implantation of a coil which effectively blocks off the vessel. The coil is placed through the use of a catheter inserted into one of the larger blood vessels and passed into the patent vessel.
In cases where heart failure is present, this must be treated before surgical ligation or implantation of a coil can be attempted.
Once the patent ductus arteriosus has become a right to left shunt or a reverse PDA, surgery is no longer possible. Ligation of a reverse PDA will lead to right-sided heart failure and death.