Yellow Labrador puppy

Labrador retrievers are a breed with a higher risk of getting a PDA. Photo courtesy of Depositphotos.

Patent ductus arteriosus, often called PDA, is one of the most common congenital heart defects in dogs.

In adult mammals, the heart is one organ but has two separate sides that are not supposed to connect other than by vessels. All the parts in both sides are encased in one small sac called the pericardium. The right side pumps blood to the lungs through the pulmonary artery where it picks up oxygen, and then the oxygenated blood goes to the left side via the pulmonary veins from which it is pumped back to the body via the aorta.

While in the womb, though, it doesn’t work that way because the fetus doesn’t breathe in oxygen yet. A developing fetus depends on the umbilical circulation to pick up oxygen from the mother and provide it to the growing body. Because the lungs are still developing in the womb and are full of fluid, blood vessels growing there are relatively underdeveloped as there’s little reason to send a lot of blood to the lungs.

Thus, in all mammals the developing fetus has a short bypass vessel called the ductus arteriosus, which is similar to a shunt, and is a wide-open, low-resistance channel in which blood coming from the right side of the heart through the first part of the pulmonary artery is diverted from the lungs directly into the aorta and then out to the body. Of course, a small volume of blood goes to the lungs because they need nutrients and oxygen to grow, and that blood goes from the lungs into the left heart.

When the baby is born, that first breath of air fills the lungs with oxygen and inflates them. Blood can now circulate through inflated lungs to deliver oxygen to the rest of the body.  The pressure in the pulmonary circulation plummets, and blood now prefers to flow out to the lungs. The bypass system (the ductus arteriosus) is no longer needed or used.

Meanwhile, oxygen and other hormones cause the ductus arteriosus to close almost immediately; the ductus is lined with muscles that squeeze down and “lock” in that position. This squeezing ultimately fuses the ductus lining to create a ligamentum: in other words, the tube becomes a ligament! In most animals, the ductus closes within the first three days of life and is securely closed by day 7 to 10.

After it closes, the lungs work the way they should for the rest of the mammal’s life: 

• Blood flows from the right side of the heart through the pulmonary artery
• Then it goes to the lungs to pick up oxygen
• While in the lungs, it also gets rid of carbon dioxide
• It goes back to the left side of the heart through the pulmonary veins
• Then it goes through the aorta to deliver the oxygen to the body. 

At least, that’s what happens to a normal, healthy ductus arteriosus.

Sometimes, however, the ductus arteriosus fails to close and become a ligament. “Patent” means open, so a patent ductus arteriosus remains open after it should have closed.  In dogs, this closure usually fails because the muscles in it formed incorrectly and cannot squeeze it shut. Now that the ductus is open, and the lungs have blood flowing through them, blood shunts across the PDA from the aorta to the pulmonary artery (remember that in utero it went the other way, from the pulmonary artery to the aorta). This happens because the resistance (and pressure) in the pulmonary artery is much lower than in the aorta – and blood flows along the path of least resistance.  Of course, some blood still flows out to the body – it has to, so that the baby doesn’t die.  To compensate for this “perceived” decrease in blood going out to the body, the baby starts to retain fluid to increase the blood volume and compensate for that amount being “lost” through the shunt.  A continuous murmur can usually be heard on the left side because flow through the shunt is turbulent. This type of flow, called a left-to-right shunt, causes heart problems. The puppy or kitten with this condition now has a heart defect that can lead to heart failure.

Resulting Problems

If some of the blood destined to provide oxygen to muscles, brain, gut, etc. gets diverted and goes back through the lungs, the body gets upset and starts to retain fluid. This retention causes an increase in blood volume so that the left heart can still pump enough blood out to the body. However, this additional blood volume increases the size of the left heart because it has to pump both the blood destined to go out to the body and the blood that is destined to go through the shunt (which essentially gets wasted in a pointless loop).

The heart can only get so big, and, if the shunt is big enough, it eventually hasproblems dealing with the extra blood; the pulmonary vein pressure increases to the point that fluid starts to leak into the lungs, causing pulmonary edema, or “fluid in the lungs,” and is commonly referred to as left-sided congestive heart failure (CHF). The probability of developing CHF with a left-to-right shunt (there are others than just PDA) depends on the size of the defect and how much blood is going through the shunt back to the lungs.

A small shunt is easily dealt with. A big shunt can be rapidly lethal.

Affected pets may not have any signs or they may be short of breath, exercise intolerant, or experiencing a failure to thrive or exercise-induced collapse.

If your pet has heart failure, that will need to be controlled with medication before surgery is possible.

The Rare Cases of Right-to-Left Shunting 

A rare variant of the PDA is one in which the blood continues to flow after birth as it did during fetal development: from the pulmonary artery into the aorta and out to the body.  Only now it isn’t carrying oxygen from the placenta, but delivering oxygen-depleted blood back to the body without picking up oxygen in the lungs. This type of PDA is called a reverse ductus or a right-to-left shunting PDA.  

Generally, this happens in the first few weeks of life, where initial left-to-right flow changes direction after a few weeks.  The reason for this is that the PDA is quite big, allowing a lot of blood to pass from the aorta to the lungs for a week or two. However, the lungs don’t like all this extra blood and start to clamp down on the pulmonary vessels, increasing pulmonary arterial resistance (creating high blood pressure). But they can’t limit how much they clamp down the pulmonary arteries, and eventually, the resistance in what should be a low-resistance circuit increases above that in the aorta’s high-resistance circuit.  When that happens, it’s easier for blood to flow across the PDA from the pulmonary artery into the aorta than into the lungs! 

Diagnosing a PDA

Black Pomeranian

Pomeranians are another breed at higher risk for having a PDA. Photo by Dr. Carol Hershey

A left-to-right PDA can usually be diagnosed by your veterinarian just listening carefully. The PDA has a characteristic murmur that is often described as sounding like a washing machine because the murmur’s sound is continuous. It’s continuous because flow through the PDA goes on all the time, not just at certain parts of the cardiac cycle. However, a PDA can be missed if the veterinarian does not listen in the correct location. The murmur of a PDA is best heard with the stethoscope positioned in the patient’s left “arm pit,” way forward of the usual place where veterinarians usually listen to the heart. 

An increased index of suspicion exists for breeds of dogs with a known predisposition to PDA. PDA occurs more often in female purebred dogs. The Maltese, Pomeranian, Shetland sheepdog, English springer spaniel, American cocker spaniel, keeshonden, bichon frise, German shepherd dog, collie, Irish setter, Kerry blue terrier, Labrador retriever, Newfoundland, miniature and toy poodle, Chihuahua, and Yorkshire terrier are at increased risk.

The work-up to confirm a ductus might include chest radiographs to rule out fluid buildup in the lungs from heart failure and to look for characteristic enlargement of the aorta and the left side of the heart where the extra blood volume is contained. However, echocardiography clinches the diagnosis as all the chamber sizes are measured and the patent ductus can actually be seen. Furthermore, echocardiography enables additional congenital heart defects to be discovered as it is not unusual for a puppy or kitten to have more than one. And finally, echocardiography can help determine the best way to treat the PDA. 

Unlike the usual left-to-right shunting PDA, which has a loud continuous murmur, a right-to-left shunting PDA is silent. It has no murmur, and clinicians will not hear it. It is usually suspected because the patient starts to show signs of hind-end weakness with minimal exercise at a young age. This is because the muscles in the hind limbs are being fed with deoxygenated blood, so the animal develops cramps and weakness; the ductus is downstream of all the vessels going to the head. The front limbs are spared in most cases because of the anatomical location of the PDA. 

A right-to-left shunting PDA is generally diagnosed by specialized echocardiographic evaluations, including something called a bubble study (contrast echocardiogram).

Treatment

It’s best not to wait until the pet is an adult or seriously ill to begin treatment because without it, about 2/3 of affected puppies will die before they are a year old. Additionally, the older the animal at the time of correction, the more friable (crumbles easily) the PDA is and the riskier the procedure.

Left-to-Right PDA

There are multiple ways to treat the more common left-to-right shunting PDA. If the PDA is small, a cardiologist might elect to do nothing because it will not cause any problems over the life of a dog or cat (in humans, other long-term complications come into consideration when deciding whether to close a PDA). If the PDA is large enough, then several options exist to close it. 

Surgery 

Ligation is the traditional method of repair. The chest is opened and a piece of suture is used to tie off the patent ductus. This procedure is not something most general practitioners are likely to attempt or feel comfortable with, as this extremely delicate operation usually requires a specialist surgeon. Talk with your veterinarian about whether a referral to a cardiovascular surgeon is in your pet’s best interest. The complication rate is less than five percent with less than two percent requiring a second procedure due to re-opening of the ductus. However, it does require open-chest surgery. 

Surgery is the method generally chosen for PDA repair in cats because of their body size and the size of their vessels.

Endovascular Occlusion

In dogs (as in humans), the most common option today is to deploy a device via the aorta into the PDA to plug it up. Indeed, one of the devices is known as the Amplatzer Vascular Plug! (See a video about ihow it works.) Originally, small coils were used, and occasionally still are. These are positioned inside the PDA and cause a clot to form around them, plugging up the PDA.  More recently a specific device, the Amplatzer Canine Ductal Occluder (ACDO) has been developed for use in dogs. This device works like a two-sided umbrella that is opened up inside the PDA with a part of the device on each side of the PDA opening. It fits snuggly in the PDA and causes almost immediate clotting within the PDA, stopping the shunting of blood. 

Complications rates with occlusion devices are similar to or lower than with surgery and have much lower morbidity (sickness) because no surgery is required – only a catheter that is introduced through the femoral artery to deploy the device. However, in some cases, it (endovascular occlusion with a catheter) is not feasible, specifically if the PDA is very large or in toy breed dogs that are too small at less than five pounds. 

Reverse PDA 

Surgery is not an option for a right-to-left PDA. You can’t correct the defect in these cases.  The only treatments are directed at the secondary complications. With right-to-left shunting, the body has an oxygen debt. Its response is to produce more red blood cells, resulting in a condition called polycythemia.  With more red blood cells in the blood, the blood becomes thicker and more viscous, and has difficulty flowing. In dogs, this can lead to seizures.  Therefore, the treatment is aimed at decreasing the number of red blood cells, either by “blood-letting,” to use a medieval term – literally removing some blood and replacing it with saline – or by giving drugs that interfere with red cell production.  However, these are palliative measures as the dog will never be normal and will likely succumb to the disease at around 5 to 6 years of age. 

Outcomes

If surgery for a left-to-right shunting PDA is successful, the dog should be able to lead a normal life.  This is one congenital heart defect that is essentially curable, even if treated later in life, assuming the dog lives that long. The sooner it is treated, the better: in most cases, cardiologists will correct PDAs within the first 6to 12 months of life. Indeed, the longer you wait to correct a PDA, the more difficult it can be because they become more fragile and can tear more easily. If your pet has irreversible damage before the surgery, it may need medication for the rest of its life.

Dr. Wendy Brooks contributed to this article.