Patent Ductus Arteriosus

TL;DR. Patent Ductus Arteriosus (PDA) is a common congenital heart defect where a fetal blood vessel fails to close after birth, but timely treatment with surgery or minimally invasive procedures offers an excellent long-term cure.

A patent ductus arteriosus allows blood to bypass the lungs by shunting directly from the aorta to the pulmonary artery.

What is it?

To understand Patent Ductus Arteriosus (PDA), it helps to look at how a puppy or kitten develops in the womb. Before birth, a fetus does not use its lungs to breathe; instead, it receives oxygen-rich blood directly from the mother. Because the lungs are collapsed and do not need a full blood supply, the fetal cardiovascular system uses a natural bypass vessel called the ductus arteriosus. This vessel connects the pulmonary artery (which carries blood to the lungs) directly to the aorta (the main artery that carries blood to the rest of the body), routing blood away from the non-functional lungs.

When a puppy or kitten is born and takes its first breath, its lungs expand, and the pressure dynamics within the cardiovascular system change dramatically. In a normal newborn, these changes signal the muscular walls of the ductus arteriosus to contract, closing the vessel within the first few days of life. Once closed, it eventually degenerates into a harmless band of fibrous tissue.

If this vessel fails to close, the pet is left with a "patent" (meaning open) ductus arteriosus. Because the pressure in the aorta is much higher than the pressure in the pulmonary artery after birth, blood begins to flow backward through the open vessel. This abnormal flow is called a left-to-right shunt. Instead of traveling out to the body, a significant portion of oxygenated blood is pumped back into the lungs. As a standard veterinary surgical reference explains:

"When pulmonary vascular resistance is less than that of the systemic circulation, blood shunts from aorta to pulmonary artery. The resultant increase in pulmonary venous return imposes a volume load on the left atrium and ventricle."

This constant recycling of extra blood floods the lungs (pulmonary overcirculation) and forces the left side of the heart to work much harder to pump the excess volume. Over time, this chronic volume overload causes the left atrium and left ventricle to stretch and enlarge, ultimately leading to congestive heart failure.

Causes & risk factors

PDA is a congenital defect, meaning it is present at birth. The primary cause is genetic, resulting from an abnormal development of the smooth muscle layer within the wall of the ductus arteriosus, which prevents the vessel from contracting and closing properly.

Genetics play a major role in who develops this condition. In some breeds, such as the Miniature Poodle, the defect is known to be inherited through a polygenic trait, meaning multiple genes contribute to whether an individual puppy will inherit the disease.

There is also a strong sex predisposition associated with this condition: female dogs are significantly more likely to be born with a PDA than male dogs. While the condition is relatively common in dogs, it is considered uncommon in cats. Additionally, research indicates that PDA is extremely rare in brachycephalic (flat-faced) dog breeds.

Signs to watch for

In its early stages, a patent ductus arteriosus can be highly deceptive. Many puppies and kittens with a PDA are completely asymptomatic (common), showing no outward signs of illness. They may play, eat, and grow normally, which is why routine veterinary puppy and kitten exams are so critical.

When symptoms do appear, they are typically related to the progressive strain on the heart and the accumulation of fluid in the lungs. Watch for the following signs:

• Continuous Murmur (Cardinal): This is the definitive physical sign of a PDA. It is a unique, constant heart murmur that persists through both the contraction and relaxation phases of the heartbeat. It is often described by veterinarians as sounding like a continuous hum or a washing machine.
• Hyperkinetic Bounding Arterial Pulse (Common): Because blood is rapidly leaking out of the aorta into the pulmonary artery during the heart's relaxation phase, the diastolic blood pressure drops significantly. This creates a very wide difference between systolic and diastolic pressure, resulting in a pulse that feels unusually strong and sharp (often called a "water-hammer" or bounding pulse).
• Congestive Heart Failure (Common): As the heart fails to cope with the volume overload, fluid begins to back up. Signs include coughing, rapid or labored breathing, exercise intolerance, and general lethargy.
• Mitral Regurgitation (Occasional): As the left ventricle stretches to accommodate the extra blood, the mitral valve ring can stretch as well, preventing the valve leaflets from closing properly and causing a secondary leak.
• Pulmonary Edema (Occasional): This is the accumulation of fluid directly within the air sacs of the lungs, leading to severe respiratory distress.
• Third Heart Sound (Occasional): An extra heart sound heard via stethoscope, indicating rapid ventricular filling and abnormal heart muscle vibrations.
• Weakness of the Hind Legs (Rare): This can occur in advanced, atypical cases where the pressure in the lungs becomes so high that the shunt reverses (right-to-left), sending oxygen-poor blood to the lower half of the body.
• Cyanosis of the Genital Membranes (Rare): Also associated with a reversed shunt, where the tissues of the rear half of the body do not receive enough oxygen, causing the mucous membranes of the vulva or prepuce to appear blue or purple.

A continuous 'washing machine' murmur heard through a stethoscope is the classic sign of a PDA.

How vets diagnose it

Diagnosis typically begins during a routine physical examination. Because of the distinct nature of the murmur, an experienced veterinarian can often identify a suspected PDA simply by listening to your pet's chest. As a leading veterinary surgery reference notes:

"A left-to-right PDA results in a continuous murmur; the murmur begins during systole, peaks in intensity at the time of the second heart sound, and persists through at least..."

To confirm the diagnosis and assess the severity of the condition, your vet will recommend several diagnostic tests:

• Echocardiography (Gold Standard): This is an ultrasound of the heart and is the definitive tool for diagnosing a PDA. It allows the veterinary cardiologist to directly visualize the open ductus arteriosus, measure the direction and velocity of the blood flow using color Doppler technology, and assess the degree of left-sided heart enlargement and muscle dysfunction.
• Thoracic Radiography (Chest X-rays): X-rays are essential for evaluating the overall size and shape of the heart and checking for fluid in the lungs (pulmonary edema). A classic PDA often shows a distinct enlargement of the left atrium and ventricle, along with a characteristic bulging of the aorta and pulmonary artery.
• Electrocardiography (ECG): An ECG records the electrical activity of the heart. In pets with a PDA, it often reveals electrical patterns consistent with left ventricular enlargement or cardiac arrhythmias.
• Cardiac Catheterization with Selective Angiocardiography: While less common for routine diagnosis, this advanced procedure involves injecting a contrast dye directly into the blood vessels under fluoroscopy (real-time X-ray). It is highly useful for mapping out complex vascular anatomy and is typically performed in conjunction with minimally invasive repair procedures.

Treatment options

Left untreated, a left-to-right PDA carries a very poor prognosis. Fortunately, it is one of the few congenital heart defects that can be completely cured if caught early. Treatment is divided into medical stabilization and definitive closure.

Medical Stabilization

If a pet is already showing signs of congestive heart failure, they must be stabilized medically before undergoing any corrective procedures. This typically involves a combination of medications:

• Furosemide (Loop Diuretic): This medication helps the body eliminate excess fluid that has accumulated in the lungs, making it much easier for the pet to breathe.
• Pimobendan (Inodilator): This drug works by helping the heart muscle contract more efficiently while simultaneously dilating the blood vessels, which reduces the workload on the failing heart.
• Digoxin (Cardiac Glycoside): Used occasionally to help support heart muscle contraction and control rapid heart rates associated with severe heart failure.

Definitive Correction

Once the pet is stable, or immediately upon diagnosis if they are asymptomatic, the abnormal vessel must be closed. There are two primary methods for achieving this:

1. Surgical Ligation: This is a traditional open-chest surgery (thoracotomy). The surgeon carefully navigates to the heart, isolates the patent ductus arteriosus, and ties it off securely with heavy suture material. While invasive, it is a highly effective and time-tested method.
2. Transcatheter PDA Occlusion: This is a modern, minimally invasive alternative. Instead of opening the chest, a veterinary cardiologist inserts a catheter into a major artery (usually in the hind leg) and guides it up to the heart. A specialized device is then deployed inside the vessel to block the flow of blood. As a leading veterinary internal medicine reference describes:

"Several methods of transcatheter PDA occlusion are available and involve placement of a vascular occluding device such as the Amplatz canine ductal occluder or wire coils (with attached thrombogenic tufts) within the ductus. Vascular access is usually..."

This minimally invasive procedure offers a faster recovery time and less post-operative pain compared to open surgery.

Echocardiography is the gold standard for visualizing the abnormal blood flow through the patent vessel.

Prognosis

The prognosis for a pet with a PDA depends heavily on when the condition is diagnosed and treated.

If the vessel is successfully closed before the heart suffers irreversible damage or before congestive heart failure develops, the prognosis is excellent. Most puppies and kittens go on to live completely normal, active lives with a normal life expectancy.

Conversely, if the condition is left untreated, the prognosis is poor. More than 50% of dogs diagnosed with a PDA will die within their first year of life from progressive congestive heart failure. If the shunt is allowed to persist for too long, the blood vessels in the lungs can become permanently damaged, leading to a dangerous reversal of blood flow (right-to-left shunt) that can no longer be surgically corrected.

Prevention

Because Patent Ductus Arteriosus is a congenital genetic defect, there are no lifestyle changes, diets, or vaccines that can prevent it from occurring.

Prevention relies entirely on responsible breeding practices. Dogs and cats that have been diagnosed with a PDA—even those who have been successfully cured through surgery—should never be bred. Because the inheritance pattern is polygenic in certain high-risk breeds, the parents and siblings of an affected pet should also be carefully evaluated by a veterinarian before being considered for breeding programs.

When to call your vet

If you have recently adopted a puppy or kitten, schedule a wellness exam as soon as possible so your vet can listen for a heart murmur.

If your pet has already been diagnosed with a PDA or is recovering from surgery, you should monitor them closely. Contact your veterinarian if you notice any of the following signs:

• A soft, persistent cough
• Breathing faster than normal while resting or sleeping (more than 30 breaths per minute)
• Unusual lethargy or reluctance to play
• A sudden decrease in appetite

Seek emergency veterinary care immediately if your pet experiences any of the following red-flag symptoms:

• Severe difficulty breathing or open-mouth breathing
• Sudden collapse or fainting spells
• Blue, purple, or pale gums and mucous membranes
• Sudden, severe weakness in the hind legs

For specific breeds

While any dog or cat can be born with a PDA, certain breeds carry a much higher risk. Miniature Poodles have a well-documented genetic predisposition with a polygenic inheritance pattern.

Other breeds with a suspected genetic predisposition include:

• Bichon Frise
• Cardigan Welsh Corgi
• Chihuahua
• Collie
• English Cocker Spaniel
• German Shepherd Dog
• Keeshond
• Labrador Retriever
• Maltese
• Newfoundland
• Pomeranian
• Shetland Sheepdog
• Toy Poodle
• Yorkshire Terrier
• English Springer Spaniel
• Kerry Blue Terrier

If you own one of these breeds, ensure your veterinarian pays close attention to cardiac auscultation during their puppy or kitten exams. Early detection is the single most important factor in securing a long, healthy life for your pet.

Sources

• Clinical Echocardiography of the Dog and Cat, p. 288
• Current Techniques in Small Animal Surgery, 5th Edition, p. 671
• Internal Medicine, 5th Edition, p. 130, 133