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Pulmonary Hypertension
Pulmonic Stenosis And Right-Side Heart Failure
Their Signs And Treatment In Your Dog 

 

Ron Hines DVM PhD

Cardiomyopathy In Your Dog

What Are  Pulmonary Hypertension  And Right Side Heart Failure ?

Pulmonary hypertension (PH/ PAH) is a term used to describe unusually high blood pressure in the blood vessel transporting blood from the right side of your dog’s heart to its lungs.  That large vessel is the pulmonary artery. (ref) All blood returning from your dog’s body makes that journey to replenish its oxygen content.

Ordinary hypertension (high blood pressure) describes the situation in which your dog’s blood pressure is abnormally high in the arteries and capillaries throughout its body - not just its pulmonary artery and the right side of its heart.

When a dog (or a cat for that matter) develops pulmonary hypertension, the right side of its heart is working harder than it should. Pulmonary hypertension is the result - not the cause - of a number of things that can go wrong when blood makes its circuit through the lungs to become re-oxygenated. (ref1, ref2, ref3)     

No matter what the cause, restricted blood flow to your dog’s lungs eventually causes destructive changes in the shape of the right side of its heart. The upper and lower right portions of the heart enlarge (hypertrophy/ dilatation), and the two valves that prevent back flow (the pulmonary valve and the tricuspid valve) become distorted, failing to close and/or open satisfactorily.

Your local veterinarian might have told you that your dog has developed right-side heart failure, or a veterinary cardiologist might have used a the term, cor pulmonale. They both describe the same problem. Read about that problem (as it affects people) here: (ref

What Are Some of The Causes Of Pulmonary Hypertension In Dogs ?

Your dog’s oxygen-depleted blood returns to its heart through a large vein called the vena cava. The posterior branch of this large vein drains the body from tail to heart (ref); while the anterior portion conveys blood to the heart from its head and neck. (ref) Any obstruction to this returning blood flow from the point it enters the heart to the point where it leaves re-oxygenated from the lungs increases the heart’s work load and has the potential to cause increased pressure (pulmonary hypertension) and right side heart failure.

 

Congenital Birth Defects :

The most common of these genetically-based obstruction defects is Pulmonic Stenosis . Pulmonic stenosis (PS) is a defect in the one-way semilunar valve that guides blood from the right ventricle of your dog's heart into the pulmonary artery that leads to its lungs. PS occurs more frequently in some dog breeds than others - particularly those with short snouts (brachycephalic breeds) like bulldogs, boxers and Boston terriers. But it is also more common in Jack Russell terriers, Labrador retrievers, Samoyed and Newfoundlands than in the general dog population. When these defects exist on the left or right, the valve - or the area just below it - does not allow adequate blood to pass. (ref)  Sometimes an abnormally narrow fibrous ring at the base of the valve (the subvalvular annulus) forms. In other cases the valve leaflets are thicker and less flexible than they should be or fused together. Less commonly, it is the area just past the valve (downstream) that is narrow (the supravalvular area). The cause is genetic – a failure of the embryonic heart to develop properly. Any of these restrictions in blood flow causes the heart to force blood through the restriction at higher pressure, dilating the pulmonary artery and increasing blood pressure within it. With time, the increase workload on the right side of the dog’s heart weakens its ability to pump blood to the lungs.

Since the defective genes involved regulate more than pulmonic valves development, it is not unusual for puppies with this problem to have a defective aortic valve as well. You can read about left side aortic valve stenosis here: (ref) The same or similar gene combination occasionally causes the wall (septum) dividing the left from the right side of the heart to have a hole in it as well (ventricular septal defects). (ref1, ref2, ref3)     

These birth defects come in all degrees of severity. Many result in heart murmurs your veterinarian might pick up on during the physical exams given at a puppy’s early vaccinations. When these defects are mild, many dogs show no signs of ill health. When heart defects are multiple or severe, collapse upon exercise, bluish gums and tongue and failure to thrive are common. Veterinary cardiologists and interventional radiologists can attempt to stretch some of these strictures (blood flow restrictions) using a technique called balloon valvuloplasty. (ref1, ref2) The procedure is delicate. the long-term benefits are unknown. Great expertise is required by the surgeon and even then, these procedures are not always successful. (ref)

Lung Disease

Dogs with long-term lung disease that obstructs blood flow are also at risk of ultimately developing pulmonary hypertension and right side heart failure.  The lungs are a vast network of small blood capillaries that expose left side heart blood to oxygen before returning the blood to the heart for recirculation. Anything that hinders blood flow within those lung capillaries increases backpressure (pulmonary hypertension) in the pulmonary artery and right side of the dog’s heart. A number of inflammatory diseases (ref) increase the wall thickness of the lung’s  alveoli preventing normal blood flow and taxing the right side of the dog’s heart. Dogs with this problem have serious respiratory distress. They breath rapidly and uncomfortably in an attempt to better oxygenate their bodies. X-rays of dogs with chronic lung disease (pulmonary veno-occlusive disease) show a patchy or diffuse thickening of their lung tissue.  (ref1, ref2, ref3, ref4)    

Dogs with weakened immune systems are at increased risk of pneumonia. When pneumonia is a long term issue, it too impedes lung circulation which can lead to right side heart failure. (ref1, ref2

Tricuspid Valve Issues

The tricuspid heart valve (ref) divides the upper and lower right chambers of your dog’s heart. Like the mitral valve on the left side of its heart, the tricuspid valve is at risk of improperly closing (leaking/regurgitating blood) when the right ventricle forcefully contracts. In some right side heart issues, it opens insufficiently. In other cases it closes insufficiently. In some cases both. When the mitral valve on the left fails, it is usually due to the wear and tear of time. When the tricuspid valve fails, the causes are often genetic. (ref) At other times blockages elsewhere in the flow of blood to the lungs cause ventricular pressure to be so high that the valve fails. (ref)

Heartworms

The preferred location of mature dog heartworms is in the pulmonary artery and right upper chamber (atrium) of your dog’s heart. These worms kill their host through a combination of physical obstruction of blood flow and the lung inflammation they cause. Read about heartworm disease and treatment here and here. The inflammation these parasites cause in the dog's lungs is called pulmonary endarteritis. It restricts lung blood flow and increases pressure within the lungs and pulmonary artery leading to pulmonary hypertension. (ref)  When many adult worms are present, that eventually leads to right side and then total heart failure. The problem is sometimes referred to as core pulmonale. (ref)

Cats also develop heartworm disease. Because their pulmonary artery is smaller, it only takes a few mature heartworms to be lethal to them. But what the parasites do to right side blood circulation and pressure is the same as it is in dogs. (ref)

Tricuspid valve issues also occur in dogs with heavy heartworm infections. The large masses of worms involved often drape through the tricuspid valve, preventing its proper closure and restricting blood flow through the right side of the heart. Heart function in those canine heartworm cases is further compromised by blockage of the pulmonary artery branches with large numbers of worms, as well as by the inflammation, fibrosis, and embolic disease within the capillaries, arterioles, and arteries of the lungs. Impeded blood flow through these diseased vessels leads to increased pulmonary artery pressure, further compromising right-sided cardiac output. (ref)

Left Side Heart Failure Leading To Right Side Heart Failure

One side of your dog’s heart cannot change in shape or function without affecting the shape and of the other side of the heart (=ventricular interdependence phenomenon). (ref) Other than when due to heartworms, heart failure begins on the left side of a dog’s hearts considerably more frequently than beginning on the right side. You can read about left side heart failure here: (ref) Which ever the case, eventually, both sides no longer work satisfactorily. Being a closed system, blood backing up (pooling) on one side eventually causes blood to back up on both sides. (ref)

What Are The  Signs And Symptoms Of Pulmonary Hypertension
And Right Side Heart Failure ?

When right side heart problems are substantial, the most common symptoms dog owner report are respiratory distress and an tiring after even moderate exercise. In more advanced cases, coughing and even fainting occur. In one Colorado heart study, 45% of dog owners had brought their dogs in because of exercise intolerance, 30% because of a persistent cough, 28% because of difficult respiration and 23% because of fainting. (ref) It is quite easy for these symptoms to be mistaken for the much more common generalized congestive heart failure, pneumonia or lung cancers since the symptoms of the three are so similar. It might only be when diuretics, ACE inhibitors or antibiotics fail to improve the dog’s situation that right side heart problems come to mind.  (rptref1, rptref2)

How Will My Veterinarian Be Sure That  Pulmonary Hypertension Is My Dog’s Problem ?

The symptoms I just mentioned, along with possible heart murmurs, a weak, rapid pulse and increased lung sounds will alert your vet to a chest problem – probably involving the heart and/or lungs. In advanced cases, the pet’s tongue might be abnormally bluish (cyanotic) and fluid (ascites) might be noted in the dog’s abdomen.

Some blood tests (WBC count and a “diff”, serum chemistry profile & urinalysis) might be suggested to get a general picture of your dog’s general health – probably with the addition of a BNP test. Perhaps even a c-Troponin assay if there are some doubts about the significance of proBNP results. (ref) A heartworm antigen test might be in order as well. No blood or urine value tests are specific for right side heart failure, but they could rule out or rule in other diseases that might either make pulmonary hypertension more severe or influence decision in preparing your dog’s treatment plan.

X-rays often reveal an abnormal heart silhouette, a ballooned right atrium (ref), an enlarge pulmonary artery and abnormally radiopaque (ref) lungs (due to increased cellularity of the lungs = pulmonary infiltrates).

Electrocardiograms (ECGs) results are not specific for pulmonary hypertension. Early in the disease they might be normal or perhaps show some right axis deviation or a right-sided heart enlargement pattern.

This is the point where most veterinarians defer to a board-certified veterinary
cardiologist for a more refined examination and diagnosis
:


In 2019, less than 10% of the USA's 113,394 licensed veterinarians were board-certified in any specialty. And thorough 2017, the AVMA only knew of 292 who were board-certified in veterinary cardiology. So a refined diagnosis for your dog by a veterinary cardiac specialist is relatively expensive. Besides reviewing EKGs performed by your local veterinarian, and your dog's current treatment plan, veterinary cardiologists rely heavily on an apparatus called an echocardiograph. What follows below includes an echocardiographic exam:

On 2/22/2019, the University of California, Davis quoted me $4,400-5,000 for this work up. The cost of the cardiology work up at North Carolina State University was quoted as $500-$1000. But I was told that a bare bones echocardiographic study with interpretation performed by one of their 4 board certified veterinary cardiologists might cost as little as $270. At the Animal Medical Center in Manhattan, a doppler echocardiogram plus consultation and suggested treatment plan from one of their 3 board-certified veterinary cardiologists runs $589-799. At Angell Memorial SPCA Hospital in Boston, the cost of the consultation and echocardiographic study is $545. These are receptionist quotes, confirm them.

Echocardiography

But to be more certain of the diagnosis, its severity and the best treatment options available, your vet often recommends that a veterinary cardiologist examine your pet. Veterinary cardiologists own a marvelous machine I covet but cannot afford. It is called a spectral doppler echocardiograph. Two common versions are pictured in the photo above. Less sophisticated ultrasound machines in common use by veterinarians accurately show the heart's chambers. But they do not readily show blood flow or pressures within the heart. (ref)

This machine allows a veterinarian to see your dog’s heart, lungs and blood vessels in real-time (in action). It allows the veterinarian to visualize blood flow and estimate arterial blood pressure (through valvular regurgitation jet estimates) (ref). When blood pressure on the right side of the heart and the pulmonary artery leading to the lungs are too high, your primary vet’s suspicions are confirmed. An extensive study of the use of this machine in establishing the diagnosis and treatment plan for dogs was done at the Veterinary College in Blacksburg Virginia in 2004. You can read it here: (ref) In the intervening 15 years, veterinary cardiologists have refined their methods and ability to make sophisticated observations that help them decide the degrees of damage to the many structures that play a part in right side heart failure, pulmonary artery stenosis and pulmonary hypertension.

Because a great deal of subjectivity is required to interpret these echocardiography images, diagnostic accuracy probably remains at about 85%. You can read some of those highly technical articles if you are inclined to here: (ref1, rptref2, ref3, ref4)

Echocardiography is painless, the procedure is rapid and rarely requires anesthetics or other drugs. Based on the estimated increase in blood pressure to your dog’s lungs, the cardiologist can classify your pet’s pulmonary hypertension as mild, moderate or severe.

What Medications Might Help My Dog ?

Pulmonary hypertension and right side heart issues are often progressive problems. However, with medications and good veterinary care, many dogs live happily with these problems for quite some time.

Most dogs with mild forms of pulmonary stenosis and right heart issue do not need medication and no medications are known to slow or prevent the progress of these heart diseases. Dogs with no visible signs of ill health related to their heart or lungs form this group.

Sildenafil (Viagra®)

If and when symptoms of ill health related to right side heart issues begins, the most common medication veterinarians dispense to lower right side heart and pulmonary artery pressure is sildenafil. Sildenafil increases the diameter of blood vessels (vasodilation) by relaxing smooth muscle within the vessel walls. Its most beneficial point of action in these types of heart disease is in the small blood vessels of the dog’s lungs where it allows blood to pass more easily through the lungs and pulmonary artery.  It was first utilized as a heart drug in people suffering from pulmonary hypertension. In dogs sildenafil is believed to improve the dog’s general energy level and overall quality of life. A 2006 study in 13 dogs with pulmonary hypertension appeared to show that the drug was beneficial in lowering pulmonary hypertension (pressure). (ref) A 2007 study in 16 dogs seemed to show that the dogs receiving the medication had a reduction in their symptoms. But neither study found that the dogs' arterial pressure or tricuspid valve function improved while taking sildenafil. (ref) Those were both small studies

I would go with the fact that larger studies in humans did confirmed that sildenafil lowered pulmonary artery pressure, eased the blood's passage through the lungs and aided tricuspid valve problems and leave it at that. I do have to add that no work in humans or dogs confirmed that taking the drug increased patient lifespan.

Several online veterinary pharmacies will prepare sildenafil for your dog after they receive a prescription from your veterinarian. Certain formulations can also be given by suppository. That can be beneficial when dogs have progressed to respiratory distress. In those cases, holding the dog's mouths closed until it swallow a pill or capsule is not without danger. (ref)

Sildenafil is a PDE5 inhibitor. (ref) There are other drugs in its class, (Tadalafil/ Cialis®), and vardenafil/ Levitra®). The large market for all three is human erectile dysfunction. All PDE5s have the ability to increase blood flow through the lungs by the same mechanism. Some veterinarians mention these other two medications as alternatives to sildenafil – but if sildenafil is ineffective in helping your dog, I am unsure why the other two would offer advantages.

Endothelin Antagonists  ERAs

Endothelins are peptide compounds the body produces to constricts blood vessels and raises blood pressure. That makes them undesirable in dogs that already have elevated right side heart pressure. Several compounds have been designed to block endothelin activity (ambrisentan (ref), bosentan (ref) and sitaxsentan). Ambrisentan has been given experimentally to dogs for other lung issues (ref) and bosentan for left side heart issues. (ref)

Those ERAs, with the addition of another, macitentan, (ref) have shown some promise in treating humans with pulmonary hypertension. They all have the potential to cause liver damage and I believe their use is more a current subject of discussion than actual use. (ref) How they might be beneficial to dogs with pulmonary hypertension and right side heart failure remains unknown. (ref)

Some veterinary cardiologists suggest pimobendan (Vetmedin®) to increase the strength of contraction of failing hearts with right side issues. In the only study I know of, adding Vetmedin to sildenafil-based therapy did not appear to extend the dog’s life. (ref) But when a dog has right and left side heart issues, adding pimobendan might gain your pet a bit more time. The same goes for diuretics like furosemide (Lasix®) to decrease fluids pooling in the chest and abdomen  (pulmonary edema & ascites). Read more about the drugs used in generalized heart failure here: (ref)

Supportive Care

Oxygen therapy is helpful when dogs struggle to breath in their final stages of heart failure. Some find that bronchodilators like theophylline allow these dogs to breath more freely as well.

The immune systems of these dogs has been weakened by the stress of heart failure. That, plus any fluid that might have accumulated in their lungs make them prime candidates for pneumonia. So antibiotics can have a place in their treatment plan as well.

It is quite possible for veterinarians and dog owners to keep these dogs alive when, in my opinion, it is no longer a humane or kind thing to do. My suggestion to you is to not let anyone  - regardless of distinguished degree, title or position – talk you into extending your dogs life longer than what you know in your heart is the loving thing to do.