Abnormal connections between different chambers of the heart, or between heart vessels, are called cardiac or arterial shunts. The most common of these in dogs is known as patent ductus arteriosus, or "PDA". A PDA is an abnormal persistent arterial connection between the aorta and the pulmonary artery after birth. During prenatal life, the ductus arteriosus is a normal anatomical feature that allows most fetal blood to bypass the nonfunctioning fetal lungs. However, this fetal channel should close shortly before or within a day or two after birth - once the puppy takes its first few breaths and its lungs become inflated. When the ductus arteriosus remains open -or "patent" - it puts increased burdens on the heart's left chambers, which causes them to enlarge or dilate. This, in turn, reduces the flow of blood out through the aorta and thereby decreases oxygen delivery to all body tissues. PDAs occur in all breeds and ages of dogs, but they most commonly are identified in young animals. Left untreated, a PDA will contribute to congestive heart failure in dogs and will almost always be fatal.
Causes of Patent Ductus Arteriosus in Dogs
Arterial shunts are normal in fetal animals (including people), but they should close within a short time of birth. A patent ductus arteriosus is one of the most common congenital heart defects in dogs (congenital defects are those which are present at birth). Unfortunately, little is known about why PDA develops so frequently in domestic dogs. It may be influenced by a number of factors or combinations of factors, including environmental, infectious, nutritional, pharmaceutical and/or toxicological factors, among others. Usually, the exact causes of PDA are never determined. However, there are strong genetic factors which predispose certain dogs to retain a patent ductus arteriosus, especially in some smaller dog breeds. This is considered to be an inherited disorder by most veterinary cardiologists.
Affected dogs should not be bred and should be spayed or neutered once they are stable enough for elective surgery. There is no other known way to prevent patent ductus arteriosus in companion dogs.
Signs of patent ductus arteriosus (PDA) in domestic dogs can take years to develop, but some dogs show mild signs from this disorder as early as one year of age. It is critical to detect PDA as early in a dog's life as possible, because this congenital defect is almost always surgically correctable. Surgery has a much better chance of success if the PDA is detected before the onset of congestive heart failure or other irreversible heart damage. Also, reputable breeders will want to take affected animals out of their breeding programs.
Symptoms of Patent Ductus Arteriosus
Owners of affected dogs may not notice any symptoms of PDA. Most cases are identified by a veterinarian during the initial puppy wellness examination and vaccination visit, based upon hearing a loud, continuous heart murmur when ausculting the puppy's chest (listening to the heart and lungs through a stethoscope). When symptoms do become noticeable, they are typically signs of congestive heart failure and can include one or more of the following:
Palpable heart murmur; feels like a washing machine churning behind the dog's left elbow.
Exercise intolerance; tiring with exercise
Failure to thrive
Respiratory distress – difficulty breathing (dyspnea); rapid breathing (tachypnea); shallow breathing
Collapse; episodes of fainting
Death from congestive heart failure if not treated
Dogs At Increased Risk
For some unknown reason, female dogs are more likely than males to retain a patent ductus arteriosus. This is a heritable condition. Predisposed breeds include the English Springer Spaniel, Maltese, Pomeranian, Yorkshire Terrier, Shetland Sheepdog, Keeshond, Bichon Frise, Collie, Chihuahua, Cocker Spaniel, German Shepherd, Cavalier King Charles Spaniel, Labrador Retriever and Standard, Toy and Miniature Poodle.
Patent ductus arteriosus (PDA) is not extremely difficult to diagnose, but it can require a keen ear and insight on the part of the diagnosing veterinarian. Most cases of PDA are diagnosed during the initial puppy wellness examination and puppy vaccination series.
How Patent Ductus Arteriosus is Diagnosed
Patent ductus arteriosus is usually diagnosed during a routine physical examination of a young dog showing no overtly abnormal clinical signs. PDA shunts cause a continuous heart murmur that is louder and then softer at identifiable phases of the heart cycle. A skilled veterinarian can detect this murmur when listening to the heart through a stethoscope. This murmur often can be felt by placing the palm of a hand behind the dog's left elbow; the palpable sensation has been described as feeling like a churning washing machine, giving it the name a "machine murmur." Dogs with PDAs also can have bounding pulses. Normally, the veterinarian will want to take radiographs (X-rays) of the chest to assess heart, lung and vessel size. An electrocardiogram (ECG) should be conducted to identify any dilation or hypertrophy of the heart chambers and to assess any irregular heart rhythms. Most veterinarians will also recommend evaluating a blood sample to determine whether the dog is anemic.
More advanced testing will involve an ultrasound examination of the heart, which is called an echocardiogram, or simply an "echo." Advanced tests to confirm a diagnosis of PDA usually are performed by a specialized veterinary cardiologist. An echo can disclose the size of the heart chambers, the thickness of the chamber walls and the degree of contractility of the heart muscle. Special types of echocardiograms can evaluate the speed of blood flow through the patent ductus arteriosus and determine the pressures in the various heart chambers.
It is important to remember that the presence of a heart murmur in a puppy does not necessarily mean that the puppy has heart defects or disease. Many puppies have a low-grade heart murmur early in life that disappears by about 6 months of age. These puppy murmurs are completely normal.
Fortunately, patent ductus arteriosus ("PDA") is one of the congenital canine conditions that is highly treatable – in fact, surgically correctible – in most affected dogs. The therapeutic goal is to occlude or tie off the shunt defect and thereby restore normal blood flow between the right and left heart chambers. This treatment it is extremely effective in young animals that have not yet deteriorated to the point of congestive heart failure. Dogs that have developed congestive heart failure due to a PDA will face increased surgical risks. However, pre-operative steps are available to help manage these risks, including the use of diuretics, vasodilators and enforced cage rest.
Several surgical procedures are available to correct a PDA. Depending on her background and expertise, a veterinarian may perform the surgery in a local general clinic. Otherwise, she can refer the dog's owner to a board certified veterinary cardiologist who specializes in diagnosing and treating heart problems in companion animals. The current preferred surgical technique is called a ductus ligation, which basically involves entering the thoracic (chest) cavity and tying off (ligating) the abnormal vessel. This procedure is not especially long or complicated, but of course any surgery involving general anesthesia carries with it certain risks. If a dog has progressed to congestive heart failure before surgery, diuretics and vasodilators may be prescribed pre-operatively, together with enforced cage rest, to help manage the condition and increase the dog's chances of successful surgery. Diuretics help to reduce the fluid retention that commonly accompanies congestive heart failure, and vasodilators can relieve the effects of high blood pressure. After surgery, followed by a few weeks of post-operative physical restriction, dogs usually return to their normal pre-surgical activity level. Depending upon the dog's condition, its veterinarian may recommend Furosemide (Lasix), a sodium-restricted diet and exercise restriction indefinitely. Affected dogs should get a great deal of rest and should avoid stressful situations.
Outlook for Dogs with Patent Ductus Arteriosus
The outlook for most dogs after PDA repair surgery is very good to excellent. Once the shunt is closed off and blood flow returns to normal, the heart and heart vessels seem to bounce back to good function and health. Indeed, many dogs are able to enjoy physical activities that they could not before the surgery. While heart surgery understandably is intimidating for many pet owners, it really is the best option for dogs with a patent ductus arteriosus. Most animals have a normal lifespan following surgical treatment. Without treatment, the prognosis is grave. Untreated PDA almost always leads to congestive heart failure and, ultimately, to death.