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Canine Patent Ductus Arteriosus
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The term PDA conjures up images
of handheld mini-computers for most people but in veterinary cardiology,
PDA stands for the condition patent ductus arteriosus, the most common congenital
heart defects in dogs.
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
circulation 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, blood diverts there, by-passing the
lungs, and circulating through the rest of the fetal body.
At birth, everything changes. We take our first breath. Our lungs begin
to work as they were meant to: to exchange gases. The fill with air and the
circulatory resistance drops. It then becomes easier for the blood to flow
to the lung rather than through the ductus. The ductus closes within the
first 3 days of life and is securely closed by day 7 to 10 of life.
Blood now flows the way it is supposed to: from the right side of the heart
to the lung to pick up oxygen then back to the left side of the heart to
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, blood
from the aorta will want to flow, not to the body but to the low resistance
ductus opening and into the pulmonary artery. This is called a left to right
shunt and creates an assortment of problems.
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.
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 heart disease by age 2 to
Read about management of heart failure
Diagnosis of PDA
A characteristic murmur can be heard in patients with a 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.
An increased index of suspicion exists for breeds of dogs with a known
predisposition to PDA. These breeds include German Shepherd Dog, Miniature
Poodle, Keeshond, Cocker Spaniel, Pomeranian, Collie, and Shetland Sheepdog.
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.
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.
Treatment: Coil Embolization
If the patient is fairly small, a second option exists: percutaneous transarterial
coil embolization. Here, coils made of steel and Dacron are placed in the
ductus from an external artery in the leg or throat. The coil essentially
causes small clots to form and plug the narrow end of the ductus. A 2.4%
mortality rate is associated with this procedure. It is appealing as it is
less invasive.
Without treatment 2/3 of affected puppies will die before reaching age
one year.
Copyright 2006 - 2008 by the Veterinary Information
Network, Inc. All rights reserved.
This work was originally published by Veterinary
Information Network, Inc. (VIN)
and is republished with VIN's permission.
*****************************
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Patent Ductus Arteriosus (PDA)
Vickie Halstead, RN, CCRN, CEN, CVNS, LNC
Significance
Cardiac disease in dogs is prevalent, accounting for about 11% of dogs
seen by veterinarians. At least 3 million dogs examined by veterinarians
in the US have acquired heart disease and may be in heart failure.
Acquired heart diseases, both inherited and obtained from influences outside
the body such as infections, will be addressed in successive articles.
Clinical Signs of Heart Failure
Cardiac diseases eventually lead to heart failure, AKA congestive heart
failure (CHF), which is defined as failure of the heart as a pump, meaning
the heart is unable to supply the body and organs with sufficient flow
of oxygen-rich blood. This inefficiency is due to weakened muscles in the
ventricles, the pumping chambers of the heart, causing backup of blood
into the cardiovascular system. Despite many compensatory mechanisms
in the heart and body that preserve the balance short term, in time the
stress on the heart takes its toll. Initially, clinical signs of CHF
are mild, vague, and easy to overlook. As time passes more blood backs
up into the lungs causing exercise intolerance, loss of appetite, rapid
respirations, and frequent coughing. Blood also backs up into the
venous system causing engorged neck veins, liver enlargement, and swelling
in the abdomen and legs. Without adequate blood flow, organs in the
body malfunction and eventually fail, thus causing more severe clinical
signs, such as kidney failure.
Heart murmurs detected during a veterinary exam are the hallmark of
most cardiac diseases and may be detected before any clinical signs are
present, therefore the importance of that yearly exam and the first puppy
exam before leaving the breeder. Other clinical signs of heart failure
that may be detected due to inadequate blood flow to the body include slow
capillary refill in the gums (it takes longer than 2 seconds for the gums
to regain the pink color after pressing with a finger), weak and rapid
pulses, cardiac rhythm irregularities (arrhythmias), fainting episodes
that may appear to be a seizure, and cyanosis (blue tint) in the white
part of the eyes or gums.
Dogs and humans can be born with several cardiac structural defects
including tetralogy of Fallot (4 distinct structural defects), holes between
the chambers in the heart called septal defects (ASD, VSD), narrowing of
the outflow tract of the left ventricle called subaortic stenosis (SAS),
and persistent fetal circulation (patent ductus arteriosis, or PDA).
Pathophysiology of Patent Ductus Arteriosus (PDA)
While in the uterus the fetus obtains oxygen from the mother’s blood
via the placenta and umbilical cord, instead of breathing it in through
the lungs. There is a communication between the 2 major blood vessels
that conduct blood out of the ventricles, the pulmonary artery (carries blood
to the lungs) and the aorta (carries blood to the body and organs), which
allows most of the blood to bypass the lungs that are not being used.
Shortly after birth, once the lungs are expanded, the connection between the
aorta and pulmonary artery, the ductus arteriosus, closes. If it remains
open (patent) blood is shunted through it from the high-pressure aorta to
the low-pressure pulmonary artery. The result is excessive blood flow to
the lungs, which places a burden on both ventricles that eventually fail.
Key: IVC = inferior vena cava, SVC = superior vena cava, RA =
right atrium, RV = right ventricle, PA = pulmonary artery, LA = left atrium,
LV = left ventricle, AO = aorta, and black arrow = shunting of blood through
the PDA
Diagnosis of PDA
A puppy born with PDA may appear normal for a few weeks, except that
it may be the first to tire while playing and the smallest puppy in the litter.
Upon auscultation, there will be a loud, continuous murmur. Murmurs
are graded on a scale of I to V, with grade I being very soft and grade
V being very loud, and can even be palpated on the chest wall. Eventually
the puppy will develop clinical signs of heart failure. The diagnosis
can be confirmed by ultrasound, x-ray to visualize the congested lungs
and enlarged heart, and electrocardiogram that reflects changes in the
ventricles.
Treatment of PDA
At the age of 2-3 months, before significant heart failure develops,
surgery is indicated to correct the defect, without which 60% will die within
one year. Surgery, which is very successful, consists of opening the
chest from the left side through the ribs (thoracotomy) and tying off the
patent ductus. Prognosis for surviving surgery is 90%, and is excellent
for a normal life if surgery is completed early.
Breeding Advice
PDA is believed to be polygenic, caused by more than one pair of genes,
so random in nature. The affected dog and its parents should not
be bred, plus littermates should only be used for breeding after careful
screening. According to Dr. Jerold Bell in his discussion of polygenic
diseases (see link below to his article), “If there are multiple generations
of normalcy in the breadth of the pedigree, then you can have some confidence
that there are less liability genes being carried.” He defines liability
genes as, “a number of genes must combine to cross a threshold and produce
an affected dog”.
Diseases of
the Heart by Charles K. Friedberg
“Matters of the Heart” by Mara Bovsun. AKC Gazette, October, 2005
“Facts on Canine Cardiac Health” by Kevin Schargen. AKC Gazette,
March, 2005
OFA web sites: http://www.upei.ca/~cidd/Diseases/cardiovascular%20diseases/PDA.htm
http://www.vetgo.com/cardio/concepts/concsect.php?conceptkey=20141#20141
http://www.offa.org/cardiacinfo.html
http://www.bichonhealth.org/HealthInfo/Startegies.asp
The above information is simply informational.
It's intent is not to replace the advice of a veterinarian nor to assist you
in making a diagnosis of your pet. Please consult with your own veterinarian
for confirmation of any diagnosis. Your pets life may depend on it.
