PATENT DUCTUS ARTERIOSUS
The term “PDA” conjures up images of handheld mini-computers for most people but in veterinary cardiology, “PDA” stands for patent ductus arterioles, the most common congenital heart defect in the dog.
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 we really need the blood to bypass the non-functional lung and that 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.
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.
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 all the way to being 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.
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: 7/23/2019