Patent Ductus Arteriosus (PDA) in Dogs

Patent ductus arteriosus, usually shortened to PDA, is a congenital heart defect. This means that the abnormality is present from birth, although a puppy may initially appear healthy.

PDA occurs when a blood vessel required before birth—the ductus arteriosus—fails to close normally after the puppy begins breathing.

What Is the Ductus Arteriosus?

Before birth, a puppy receives oxygen through the placenta. Because the lungs are not yet being used to breathe air, much of the blood does not need to pass through them.

The ductus arteriosus is a normal foetal blood vessel connecting the pulmonary artery, which leads towards the lungs, to the aorta, which carries blood to the body. This connection allows blood to bypass the developing lungs.

After birth, the puppy takes its first breaths and blood begins flowing through the lungs to collect oxygen. Changes in oxygen levels and pressure normally cause the ductus arteriosus to constrict and close.

When the vessel remains open, or patent, blood continues to flow abnormally between the aorta and pulmonary artery.

Left-to-Right PDA

The usual form is a left-to-right PDA. Pressure is higher in the aorta than in the pulmonary artery, so oxygenated blood passes from the aorta through the open ductus and back towards the lungs.

This causes an excessive volume of blood to circulate through the lungs and return to the left side of the heart. Over time, the left atrium and left ventricle may enlarge because they are continually handling more blood than normal.

A substantial PDA can eventually result in pulmonary oedema—fluid within the lungs—and left-sided congestive heart failure.

Signs of PDA

Many puppies have no obvious symptoms when the condition is first detected. A veterinary surgeon may discover it while listening to the heart during a routine puppy examination or vaccination appointment.

Possible signs include:

• A continuous heart murmur, often described as machinery-like.
• A noticeable vibration or “thrill” over the heart.
• A very strong or bounding pulse.
• Reduced stamina or reluctance to exercise.
• Breathing more rapidly or with increased effort.
• Coughing.
• Poor growth or difficulty gaining weight.
• Weakness or collapse.
• Signs of congestive heart failure.

The loudness of a murmur does not always show how severely the circulation is affected. A cardiology assessment is required to determine the size and direction of the shunt and its effects upon the heart.

How Is PDA Diagnosed?

A characteristic continuous murmur may create a strong suspicion of PDA, but further investigation is necessary before deciding upon treatment.

Tests may include:

• A full veterinary examination.
• Echocardiography—an ultrasound scan of the heart.
• Doppler assessment to show the speed and direction of blood flow.
• Chest X-rays to assess the heart and lungs.
• An electrocardiogram to examine the heart rhythm.
• Blood pressure and blood tests where appropriate.

Echocardiography helps confirm the diagnosis, assess enlargement of the heart, identify any additional congenital defects and distinguish an ordinary left-to-right PDA from a reverse PDA.

Reverse PDA

In a small proportion of affected dogs, pressure within the pulmonary arteries becomes sufficiently high for the direction of blood flow to reverse. This is known as a right-to-left or reverse PDA.

With a reverse PDA, some poorly oxygenated blood bypasses the lungs and enters the aorta. Because the ductus joins the descending aorta, the hindquarters may receive blood containing less oxygen than the front of the body.

Possible signs include:

• Exercise intolerance and severe tiredness.
• Weakness or collapse during activity.
• Weakness affecting the hind legs.
• Differential cyanosis, where the membranes or skin towards the back of the body appear blue while those towards the front may remain pink.
• An excessive number of red blood cells, known as secondary erythrocytosis or polycythaemia.

Treatment of Left-to-Right PDA

Closure is generally recommended for a significant left-to-right PDA. Waiting until a puppy is older or develops symptoms can allow irreversible heart damage or congestive heart failure to occur.

If congestive heart failure is already present, medication may be needed to stabilise the dog before an anaesthetic and corrective procedure can be considered.

The two principal methods of closing a PDA are:

The cardiologist or surgeon will recommend the most appropriate technique according to the dog’s size, age, heart function and the size and shape of the ductus.

Is Every PDA Closed?

Most clinically significant left-to-right PDAs are treated because of the risk of heart failure and premature death. Occasionally, a veterinary cardiologist may advise monitoring a very small PDA that is not causing measurable strain on the heart.

This decision must be based on specialist imaging. An owner should not assume that a symptom-free puppy has only a small defect.

Prognosis

The outlook following successful closure of a left-to-right PDA is usually excellent, especially when the procedure is performed before congestive heart failure or permanent heart damage develops.

Many successfully treated dogs can return to normal activity and have a normal or near-normal lifespan. Follow-up examinations and heart scans may be recommended to check that the abnormal flow has stopped and that any heart enlargement is improving.

The prognosis is less predictable when treatment is delayed until severe heart enlargement, heart failure or an abnormal rhythm has developed.

Without treatment, a significant PDA carries a serious risk. Published veterinary guidance reports that approximately two-thirds of affected puppies with a clinically important PDA may not survive beyond their first year. Individual outcomes vary according to the size and direction of the shunt.

Dogs with reverse PDA have a more guarded or poor long-term outlook because the underlying pulmonary vascular changes prevent safe closure of the ductus.

Breeds and Inheritance

PDA is one of the more frequently diagnosed congenital heart defects in dogs, although congenital heart disease itself is uncommon compared with many acquired conditions.

An increased occurrence has been reported in several breeds, including:

• Yorkshire Terriers.
• Pomeranians.
• Maltese.
• Toy and Miniature Poodles.
• Shetland Sheepdogs.
• German Shepherd Dogs.
• Collies.

Females are reported more frequently than males. However, a PDA can occur in any breed or crossbreed and in either sex.

A hereditary contribution is suspected in some breeds. An affected dog should not be used for breeding, and careful consideration should be given to its parents, siblings and close relatives.

If a case occurs, informing the breeder can help them review the pedigree and future mating plans. A congenital condition is present from birth, but this does not necessarily mean that either parent had a detectable heart problem.

Checking Puppies

Every puppy should have its heart listened to by a veterinary surgeon. However, some murmurs in young puppies are temporary and harmless, while others indicate an important congenital defect.

A persistent, loud, continuous or otherwise suspicious murmur should be investigated. Referral to a veterinary cardiologist should not be delayed simply because the puppy appears lively and is growing normally.

If a breeder has been advised that a puppy has a murmur, the prospective owner should be informed and provided with copies of the veterinary findings. A specialist diagnosis is important before decisions are made about treatment or rehoming.