PATENT DUCTUS ARTERIOSUS

The PDA (patent ductus arteriosus) is the most common congenital heart defect in the dog. If it is not corrected, most puppies affected dgos will die at a young age.

SO WHAT IS A DUCTUS ARTERIOSUS AND WHY SHOULDN’T IT BE PATENT?

Everybody had a ductus arteriosus once but it was a long time ago, back in one’s fetal days. As a developing fetus, one depends on one’s umbilical cord to supply oxygen. After all, not only are the lungs not developed yet but there is no air to breathe inside mom’s uterus.

But we still have a heart and it still pumps blood even in the fetal stage. The problem is that the lungs are "under construction" and not yet working so we want the blood to bypass that whole area. This is where the ductus arteriosus comes in.

The ductus arteriosus is a small channel connecting the pulmonary artery (which will one day carry blood to the lungs) and the aorta (which already carries blood to the rest of the body). Because the lung is developing, full of fluid, and generally not ready to do anything, blood vessels growing there have high resistance. In other words, it would take a lot of force from the heart to circulate blood there. Since there is a low resistance channel wide open (i.e. the ductus arteriosus), blood diverts through it instead, by-passing the lungs, and circulating through the rest of the fetal body. The lungs can continue their development and fetal blood circulation continues using oxygen from the umbilical cord.

The ductus arteriosus is shown in pink and serves as a bridge or shunt to divert blood from
the main pulmonary artery (the blue side of the bridge) to the aorta (the red side of the bridge).
The ductus arteriosus prevents blood from attempting to circulate through
the lung vasculature which is still "under construction" and closed off.
(Photocredit: Public Domain Graphic via Wikimedia Commons)

At birth, everything changes. We take our first breath. Our lungs begin to work as they were meant to: moving gases in and out of the body. The lungs fill with air causing the circulatory resistance of fetal development to drop. This means that the blood flow pathway to the lung is easier than the blood flow pathway through the ductus. The ductus begins to close within the first 3 days of life and is securely closed by day 7-10 of life. With the ductus closed, blood now flows the way it is supposed to: from the right side of the heart, to the lung where it picks up oxygen, back to the left side of the heart, and to the rest of the body to deliver the oxygen. Or at least that is what is supposed to happen. Sometimes the ductus does not close. It remains open or “patent.” When the ductus stays open, some of the blood moving out of the left side of the heart will not go forward to supply oxygen to the rest of the body. Instead, this blood slips into the ductus and back to the flow pathway to the lung. Oxygen is not properly delivered to the body. This is called a Left to Right shunt and creates an assortment of problems.

Now that the lung vasculature is open for business, blood flow through the ductus reverses and now goes from the aorta to the main pulmonary artery instead of the other way around as it did during fetal development. This means less blood going through the aorta to the body where it is needed. (Photocredit: BruceBlaus via Wikimedia commons)

WHY LEFT TO RIGHT SHUNTING IS BAD

The body has its oxygen requirement and demands to be serviced by the heart. The problem is that a great deal of blood (how much depends on the size of the ductus) is shunting back to the pulmonary circulation. In order to meet the body’s oxygen demand, the heart is going to have to pump all the more blood to cover what circulates in the shunt as well as what the body needs. This is a lot of extra work for the heart and failure can result, leading to coughing, weakness, and difficulty breathing. In fact, more than 25% of pups have some degree of heart failure at the time their patent ductus is discovered.

Treatment for PDA involves surgical tying off of the ductus with a piece of suture or coil.
If this can be accomplished, all the heart problems are reversible.
If heart failure is present, it will need to be controlled with medicine before surgery is possible.

Click here to read about management of heart failure

If the volume overload to the lung’s circulation is allowed to go on indefinitely (and the patient has not died from heart failure during puppyhood), resistance increases in the lung and the shunt may diminish or even reverse to a Right to Left shunt. Increased resistance in the lung circulation is called “pulmonary hypertension.” A Right to Left shunt is no longer something that can be fixed and the patient will be very sick from their heart disease by age 2-3. There is not a lot that can be done if the shunt progresses from Left to Right to Right to Left. The window of opportunity for normal life has closed.

DIAGNOSIS OF PDA

A characteristic murmur can be heard in patients with a Left to Right patent ductus arteriosus. The murmur is described as sounding like a “washing machine” and is often called a “continuous” or “machinery” murmur. Discreet heart “thumps” cannot be made out; only the continuous whooshing of the murmur is heard. The murmur is best heard with the stethoscope positioned in the patients left “arm pit.” If the condition progresses to Right to Left shunting, no murmur will be heard. (At that point, the condition is probably too far gone for treatment).

An increased index of suspicion exists for breeds of dogs with a known predisposition to PDA. These breeds include:

The work-up to confirm the presence of the ductus will include chest radiographs to rule out fluid build up 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. Echocardiography clinches the diagnosis as all the chamber sizes are measured and the patent ductus can actually be seen. Further, echocardiography enables additional congenital heart defects to be discovered as it is not unusual for a puppy to have more than one.

TREATMENT: SURGICAL LIGATION This is the traditional method of repair. The chest is opened and a piece of suture is used to tie off the patent ductus. Generally a specialist is required to perform this procedure but complication rate is less than 5%, with less than 2% requiring a second procedure due to re-opening of the ductus. Because of the kitten's small size, this method of repair has been more successful in this species and is the treatment of choice for kittens with PDA.

Kittens can also be born with a patent ductus arteriosus. Surgical ligation is treatment of choice. (Photocredit: Morguefile.com)

THE RIGHT TO LEFT SHUNT

As mentioned, the right to left shunt represents the most advanced progression of the patent ductus arteriosus: blood is basically by-passing the lungs just like it did during fetal development only this time there is no umbilical cord to provide oxygen. This creates a gigantic oxygen deficiency and the body tries to respond by making more red blood cells. The result is a poorly oxygenated patient with very thick blood that does not circulate well. Treatment involves actually removing extra blood periodically and/or using medications to interfere with red blood cell production. This may work for a time but normal life span is no longer possible and surgery does not work at this stage. Most patients have died by age six.

This is a progressive disease so it is important not to wait until the patient is experiencing difficulty or for the patient to reach a mature size. The best outcomes and best chance for normal life require early intervention.

Without treatment 2/3 of affected puppies will die before reaching age one year.

Page last updated: 7/23/2023