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Sub Aortic Stenosis
What is Aortic Stenosis

THE PET HEALTH LIBRARY

By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com
http://www.VeterinaryPartner.com

Subaortic Stenosis

 (SAS)

Subaortic stenosis, affectionately know as “SAS,” is the most common congenital heart disease of large breed dogs. This article attempts to present a review of this condition, including its diagnosis and treatment.

But First

In order to understand what subaortic stenosis is, it is necessary to understand some normal heart anatomy. The heart sits more or less centrally in the chest and is divided into a left side, which receives oxygen-rich blood from the lung and pumps it to the rest of the body, and a right side, which receives “used” blood from the body and pumps it to the lung to pick up fresh oxygen. Because the left side of the heart must supply blood to the whole body, its muscle is especially thick and strong. Blood is pumped from the left ventricle (pumping chamber) to a particularly large blood vessel called the aorta. (The aorta is the body’s largest artery.) The valve that separates the left ventricle from the aorta is called the aortic valve. The left ventricle narrows as it leads to the aorta and this area is called the aortic outflow tract.
subaorticstenosis1
Diagram of the (human) heart. The aorta and left ventricle are colored blue. 

In subaortic stenosis, the left ventricular outflow tract just below the aortic valve has a scar-like narrowing or “stenosis” (which is another word for narrowing.) This means that the left ventricle must pump extra hard to move the correct blood volume through the narrowed area. The blood squirts through in a turbulent fashion (like if you squeezed down on a garden hose) that creates a sound known as a heart murmur. While any cause of turbulent blood flow can be heard as a murmur, and a murmur does not always indicate disease is present, the murmur is usually the first sign that the puppy in question might have SAS.
subaorticstenosis2
 
This picture from the Mayo Clinic shows a normal left ventricle compared to one where the patient has aortic stenosis. Much less blood can pass the aortic valve. The extra hard work of the ventricle manifests as extra thick muscle. While this is a human heart graphic, the concept is the same.

The most commonly affected breeds for SAS include the Golden retriever, Rottweiler, Newfoundland, Great Dane, Boxer, German Shepherd, and German Short-haired pointer.

When a puppy with SAS is born, the stenosis is very small, barely a ridge near the valve, but over the first four to six months of life the stenosis grows and the murmur (hopefully) becomes more apparent.

The murmur is best heard on the left side of the chest at the level of the base of the heart. Generally, the louder the murmur, the worse the obstruction of the valve. The murmur is famous for radiating its sound up the carotid arteries of the neck. Over time, the muscle of the left ventricle thickens and grows due to the excess work it must perform. Eventually this interferes with the pumping chamber’s flexibility and ability to fill. (See how small the chamber in the above graphic has become compared to normal). Abnormal muscle in the heart makes for abnormal electrical conduction in the heart and soon the heart’s normal electrical rhythm may be disrupted. These pumping and electrical issues can lead to fainting spells or even sudden death. How long a dog with SAS lives is very much a function of how severely the outflow tract is narrowed.  How long a dog with SAS lives is very much a function of how severely the outflow tract is narrowed. Most dogs with severe SAS do not survive beyond age 3 years without treatment, though dogs with milder cases can have normal life spans. A dog with SAS is always predisposed to electrical arrhythmia and sudden death, heart failure, or infection of the abnormal aortic valve.

Recognizing the Disease
Obviously, the pup is not going to receive proper management unless the condition is recognized. The first step is hearing the murmur.

As mentioned above, a murmur is the sound made by turbulent blood flow. In other words, a murmur is a sound that might or might not indicate heart disease. Puppies under age six months sometimes demonstrate what are called “innocent murmurs,” which simply represent temporary turbulent blood flow. An innocent murmur is not very loud and should disappear by age six months; any murmur that persists beyond this time or is felt to be loud should be pursued as potentially abnormal. Prior to age six months, diagnostics may be difficult to interpret, depending on the patient’s size.

After the Murmur
Chest radiographs are helpful in assessing any evidence of actual heart failure, and may even show a dilation of the aorta near the valve (caused by the high pressure squirt of blood through the narrowing). This said, the real key to diagnosis is ultrasound (echocardiography) where the heart's chamber sizes and wall thicknesses are measured. Generally the cross-sectional area of the left ventricle outflow tract is compared to that of the aorta in a ratio to assess the severity of the stenosis. This information is generally adequate to confirm the diagnosis though a mild case might have values that overlap the normal range. Such a patient might have to be followed over time. A special kind of ultrasound called "Doppler" is particularly helpful in measuring the severity of the stenosis/narrowing.

Treatment: Drugs
The goal in treating SAS is to create normal ability to exercise and normal life span. The most popular class of drug for SAS is the “beta blockers.” Beta receptors are the neurologic areas on the heart that respond to adrenaline (we call it “epinephrine” now) and cause the heart to rate to speed up during exercise. In SAS, this kind of racing pulse is what leads to the abnormal electrical rhythm (and ultimately fainting). It is hoped that the beta blockers will keep the heart from racing. Atenolol, a beta blocker, is currently being investigated. Of all the treatment options available for SAS, atenolol is certainly the least invasive and least expensive, but studies are on-going to determine if it is also the most effective.

Treatment: Surgery
Open heart surgery is uncommonly performed in dogs but it is possible to surgically excise the collar of scarring that is narrowing the outflow tract. One would think this would solve the whole problem, but in fact resulting survival times are similar to those for dogs simply taking beta blockers.

Treatment: Balloon Valvuloplasty
With balloon valvuloplasty, the patient is anesthetized and a type of catheter is threaded into the heart so that it spans the stenosis. The catheter has a tough balloon at the end that is then inflated, breaking down the scarring and dilating the stenosis. (Think of using a shoe stretcher in too-tight shoes.) Again, one would think that this would solve the problem, but survival times are similar to those for dogs simply taking beta blockers.

At this time, invasive procedures cannot be recommended over medication. More complete studies in the future may change this, so we’ll update here as we learn more information.

Copyright 2006 - 2007 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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What is aortic stenosis?

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related terms: Type A aortic stenosis, Type B aortic stenosis, subvalvular aortic stenosis

In aortic stenosis, there is a partial obstruction to the flow of blood as it leaves the left side of the heart (the left ventricle) through the main blood vessel (the aorta) that carries blood to the rest of the body. The obstruction ranges from small nodules to a fibrous band, most commonly just below the aortic valve ("subvalvular aortic stenosis"). Due to the obstruction, the heart must work harder to pump out an adequate blood volume. Clinical signs and long-term outcome depend on the degree of narrowing, or stenosis.

How is aortic stenosis inherited?
In Newfoundlands, this defect has been shown to have an autosomal dominant mode of inheritance, with variable expression.

In the mildest form, the condition is undetectable and will not cause any problems for the dog. However the defect may still be passed on to offspring. The challenge for breeders and veterinarians is to identify affected dogs with very mild or no clinical signs of the disorder.

What breeds are affected by aortic stenosis?
Congenital aortic stenosis is probably the most common heart defect seen in large breed dogs. Newfoundland dogs have the highest risk for this disorder. It is also important in the golden retriever, Rottweiler, and boxer.

There is a mildly increased risk of aortic stenosis in the German shepherd, German short-haired pointer, Great Dane,samoyed and bulldog.

For many breeds and many disorders, the studies to determine the mode of inheritance or the frequency in the breed have not been carried out, or are inconclusive. We have listed breeds for which there is a general consensus among those investigating in this field and among veterinary practitioners, that the condition is significant in this breed.

What does aortic stenosis mean to your dog and you?
Dogs with mild stenosis will generally show no clinical effects and have a normal life expectancy. With moderate to severe stenosis, signs will be variable. Because of the narrowing in the aorta as the blood leaves the left ventricle, your dog's heart must work harder to pump an adequate volume of blood to the rest of the body. Depending on the degree of obstruction, your dog's heart may be able to compensate at rest but not keep up with the body's demands during exercise. Thus you may see reduced exercise tolerance  - your dog just seems to run out of steam - or fainting due to inadequate blood supply to the brain.

In response to the obstruction to blood flow, the heart muscle becomes thicker over time (left ventricular hypertrophy). As the condition progresses, your dog's heart becomes less able to compensate and you and your veterinarian may see signs associated with left-sided heart failure such as tiring on exercise, difficulty in breathing, coughing, and/or poor growth. Changes in the heart muscle can also lead  to abnormal heart rhythms (cardiac arrythmias) and sudden death.

Your veterinarian can do various tests (see below) to determine the severity of the defect. Based on the results, he or she will discuss with you the long-term prognosis, and ways to manage this condition in your dog.

How is aortic stenosis diagnosed?
In young animals (less than 6 months of age) there may be no clinical signs. Thus the first indication that your dog may have a problem may come when your veterinarian hears a heart murmur during physical examination. Some low-grade murmurs are "innocent" and disappear by 6 months of age, but if the murmur is significant, your veterinarian will suggest a diagnostic workup to determine the cause. He or she will listen very carefully to your dog's heart to determine the point of maximal intensity of the murmur and when the murmur occurs during the cardiac cycle. Other diagnostic aids include chest x-rays, an electrocardiogram (ECG) and/or ultrasonography if available. To determine the extent of the narrowing, the pressure gradient  across the aortic valve (between the left ventricle and the aorta) can be measured using special procedures for which your veterinarian can refer your dog to a specialist.   

In an older animal or when the obstruction is pronounced there may be clinical signs associated with left-sided heart failure.

Based on the results of these various tests, your veterinarian will discuss with you the prognosis and long term management of your pet. S/he will also be able to tell you if any (and how much) change has occurred in the heart already as a result of the stenosis.

FOR VETERINARIAN:
MURMUR: systolic, left hemithorax, radiates into thoracic inlet and up the neck, PMI left heart base (3rd to 4th intercostal space), may be equally loud at right heart base.

ECG: may be normal, over time shows left ventricular enlargement, left axis shift, may show ST depression, may show ventricular arrhythmias

RADIOGRAPHS:  may see left ventricular enlargement, cranial aortic enlargement, left axis shift. Pulmonary vasculature is normal.

ECHOCARDIOGRAPHY: left ventricular hypertrophy, subvalvular fibrous ring, post-stenotic dilation of aorta

The arterial pulse may be of reduced intensity and slow to rise.

Echocardiography (with Doppler) or cardiac catheterization is usually required to determine the severity of the defect.

How is aortic stenosis treated?
In dogs with mild aortic stenosis,there is no special treatment required. The dog should not be used for breeding and littermates should be carefully screened. Your veterinarian may suggest antibiotics in certain circumstances as a precaution against infection of the abnormal valve tissue.

With moderate to severe stenosis, the dog's exercise should be restricted. Beta-blocking drugs may be prescribed by your veterinarian to try to minimize the effects of left ventricular hypertrophy. Your veterinarian will recommend other therapy if required to manage congestive heart failure. Medical management for congestive heart failure is similar no matter what the cause, and consists of medications to support the heart muscle and decrease the work load of the heart, together with dietary recommendations.

Various surgeries have been attempted to alleviate the obstruction with limited success. The surgery itself carries a high risk, and there is little, if any, increase in survival rates compared with dogs whose condition is managed medically.

Breeding advice
Affected individuals should not be used for breeding, and littermates should be carefully screened.

FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.

Resources
Bussardori, C. 1998. Breed related echocardiographic prognostic indicators in pulmonic and subaortic stenosis. ACVIM-Proceedings of the 16th Annual Veterinary Medical Forum: 140-142.
Bonagura, J.D. and Darke, P.G.G. 1995.  Congenital heart disease. In S.J. Ettinger and E.C. Feldman (eds.)Textbook of Veterinary Internal Medicine, p. 892-943. W.B. Saunders, Toronto.
Patterson, D.F.  1996. The genetics of canine congenital heart disease.  ACVIM-Proceedings of the 14th Annual Veterinary Medical Forum: 225-226.   This reference has good information for breeders and veterinarians regarding screening and genetic counselling for congenital heart defects.
Copyright © 1998 Canine Inherited Disorders Database. All rights reserved.Revised: August 18, 2003.
This database is funded jointly by the Animal Welfare Unit at the Atlantic Veterinary College, University of Prince Edward Island, and the Canadian Veterinary Medical Association.
reprinted with kind permission from:-  Alice Crook, DVM,Coordinator, Sir James Dunn Animal Welfare Centre,
 Atlantic Veterinary College,University of Prince Edward Island, 550 University Ave.Charlottetown, PEI C1A 4P3
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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.