Patent Ductus Arteriosus (PDA)

Patent Ductus Arteriosus (PDA)

There are a number of anatomical structures that arise as a puppy fetus develops, but then disappear or change before or shortly after birth. This makes sense, as a fetal puppy (just like a human fetus) is living in a liquid environment, with many of the bodily functions provided by the mother, but then transitions immediately to a self-contained, breathing newborn puppy. Patent Ductus Arteriosus is in the category of conditions where one of those necessary fetal structures remains after birth, when it should have naturally gone away or ceased its function.

In the normal dog, the pulmonary artery delivers blood to the lungs, where it is oxygenated and returned to the heart. The oxygen-rich blood is then pumped into the aorta and delivered to the body. However, a fetal puppy is not breathing... in fact, the lungs are basically collapsed... and its blood is getting the necessary oxygen from the mother. Therefore the body has a mechanism to divert that blood flow so that it bypasses the lungs and goes straight to the body. That mechanism is the Ductus Arteriosus, a "shunt" (bypass) from the pulmonary artery to the descending aorta.

When a puppy is born and begins breathing on his own, the Ductus Arteriosus should close. This will typically happen within a few days of birth. If it fails to close completely, still allowing blood to flow through, the puppy has a Patent Ductus Arteriosus, which is the most common congenital heart defect.

Breeds Affected by Patent Ductus Arteriosus
Most affected are the Kerry Blue Terrier, Maltese, Pomeranian, Miniature Poodle, Shetland Sheepdog, Cocker Spaniel, Chihuahua, German Shepherd, Bichon Frise, English Springer Spaniel, and Keeshond.

Types of Patent Ductus Arteriosus
There are two types of Patent Ductus Arteriosus: left-to-right and right-to-left.

Left-to right Patent Ductus Arteriosus is the more common. This occurs when the pressure in the aorta (which carries oxygenated blood to the body) is higher than the pressure in the pulmonary artery (which carries blood to the lungs), so blood travels through the shunt from the aorta to the pulmonary artery then to the lungs. The blood is actually moving through the shunt in the opposite direction it was prior to birth. This results in an increased blood flow to the lungs and a change in the relative volumes being pumped by the heart chambers.

Right-to-left Patent Ductus Arteriosus is less common, and it occurs when blood continues to flow through the shunt in the same direction it did before birth, when its purpose was to bypass the lungs. The result is a decreased blood flow to the lungs, and therefore the body is receiving poorly oxygenated blood.

Symptoms of Patent Ductus Arteriosus
Symptoms differ by the type of PDA present. Symptoms of left-to-right include coughing, fatigue, a low tolerance of exercise, collapse, and if severe and untreated, eventual death. Symptoms of right-to-left include shortness of breath and limb weakness.

Diagnosis
Often, diagnosis is made before any clinical symptoms appear, as heart auscultation (listening to the heart with a stethoscope) is a routine part of a puppy exam. Left-to-right PDA produces increased flow to the pulmonary artery and typically a heart murmur that can be heard on examination, so that is often detected in an early puppy exam. Further examination would typically be done via electrocardiogram or electrocardiography, which would reveal the changes in flow and heart chamber enlargement which are typical of the condition.

With right-to-left Patent Ductus Arteriosus, there is often little or no heart murmur, so the condition may be missed on a routine exam, which means noticeable physical signs may be the first indication of the condition. Typically, tissue toward the front of the puppy's body may be healthy and pink, with tissue toward the hind end appearing bluish. This is due to the poorly oxygenated blood circulating. Combined with signs of pulmonary edema and kidney problems, these symptoms may be what leads the vet to perform the more involved diagnostic procedures mentioned above.

Treatment
For left-to-right Patent Ductus Arteriosus, surgery is recommended, as early as possible, before significant changes to the heart have occurred. Depending on the amount of damage or change to the heart tissue, there may be a period of stabilization required prior to surgery, to help minimize risk. Surgery involves ligation (closing the shunt with sutures), and prognosis after surgery is typically very good, as blood flow returns to normal virtually instantly, and the effects on the heart typically normalize afterward. Catching the condition and operating at an early age is key, to minimize the effects the Patent Ductus Arteriosus has had on the heart and lungs.

Typically, right-to-left Patent Ductus Arteriosus is treated medically, with rest and low activity level, plus monitoring of other critical bodily functions. Dogs with right-to-left PDA typically have a markedly shortened life expectancy.

Mode of inheritance
The mode of inheritance for PDA is not well understood, although it is interesting to note that it affects females much more frequently than males. At any rate, dogs with Patent Ductus Arteriosus should never be bred, and most experts will recommend not breeding the parents of any puppy that has been diagnosed with PDA.