SUBAORTIC STENOSIS

Unless through some unusual set of circumstances, you are familiar with the anatomy of the heart, its outflow tracts, and the aorta itself, some basic explanation is going to be needed in order to understand subaortic stenosis and why it is bad. We will cover what subaortic stenosis is then cover how it is diagnosed and finally treatment. Subaortic stenosis (also simply called "SAS") is a scar-like narrowing (a scar-like narrowing is a "stenosis") under the aortic valve. In order to understand where this is and what happens when such a thing exists, let's back up and cover how things are supposed to work in the heart.

LET'S START WITH WHAT THE NORMAL SITUATION IS

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 “left ventricular outflow tract.”

Diagram of the normal heart. The left ventricle is shown in pink.
(original graphic by marvistavet.com)

In subaortic stenosis, the left ventricular outflow tract just below the aortic valve has a scar-like narrowing or “stenosis“ (which is basically just a medical word for “narrowing"). This means that the left ventricle must pump extra hard to get the correct blood volume through the narrowed area. The blood squirts through in a turbulent high pressure fashion which creates a sound known as a “heart murmur" and makes a bulge in the aorta where the high pressure blood squirted against it. While any cause of turbulent blood flow can be heard as a murmur and a murmur does not always indicate disease is present, a murmur is usually the first sign that the puppy in question might have SAS, especially if the murmur is loudest over the aortic valve.

	Normal heart on the left and a heart with SAS is depicted on the right. The left ventricle is shown in pink in each graphic. In SAS the left side of the heart must pump extra strongly to send blood through the narrowing (the stenosis). All this exercise makes for a very thick and strong heart muscle but doesn't leave much space for blood and makes for a stiffer muscle. Further, the blood that is expressed through the stenosis is coming through at a very high pressure. It splats against the wall of the aorta on the other side and creates a bulge that is visible on a radiograph. (original graphics by marvistavet.com)

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

Golden Retriever (Photocredit: Morguefile.com)

Rottweiler (Photocredit: Morguefile.com)

Dogue de Bordeaux (Photocredit: Ewa Ziemska via Wikimedia Commons)

Newfoundland (Photocredit: Morguefile.com)

When a puppy with SAS is born, the stenosis is very small, barely a ridge near the valve, but over the first 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, right over the aortic valve. 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, making for a murmur that can be heard (with a stethoscope) unusually far forward.)

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 is disrupted. Since pumping and filling are highly coordinated electrically, when this coordination is lost, fainting spells or even sudden death during exercise can result. Most dogs with 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, heart failure, and infection of the abnormal aortic valve.

RECOGNIZING THE DISEASE

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

AFTER THE MURMUR 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 chamber sizes and heart wall thickness 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. One of the most important aspects of the echocardiography evaluation is the Doppler study. This involves measuring the pressure difference across the stenosis which essentially tells us how hard the heart is working to eject blood through the narrowing. This information is correlated with survival time and need for treatment. A pressure difference of 40 mm of Hg (pressure is measured in "millimeters of mercury, abbreviated "mm of Hg") is mild where as a pressure difference of 80 mm of Hg is severe. Mildly affected dogs may not require treatment at all.

Chest radiograph of a puppy with SAS. The bulge of the aortic valve is conspicuous (graphic used with permission by ASEC Cardiology)

TREATMENT: DRUGS

The goal in treating SAS is to create normal exercise tolerance and normal life span. The most popular class of drug for SAS are 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 rate to speed up during exercise. In SAS, this kind of racing pulse is what leads to the abnormal electrical rhythm (and ultimately fainting). The beta blockers keep the heart from racing. In one study, dogs with SAS treated with a beta blocker called “atenolol” had a median survival of 56 months vs. 19 months for dogs receiving no treatment.

TREATMENT: EXERCISE RESTRICTION

Fatal heart rhythm episodes in SAS patients are associated with excitement and demanding exercise (though sudden death can certainly occur without either situation.) It is probably best to avoid strenuous exercise if possible.

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 atenolol.

TREATMENT: BALLOON VALVULOPLASTY

With balloon valvuloplasty, the patient is anesthetized and a special catheter is threaded into the heart so that it spans the stenosis. The catheter has a tough balloon at the end which is then inflated, breaking down the scarring and dilating the stenosis. Again, one would think that this would solve the problem but survival times are similar to those for dogs simply taking atenolol.

At this time, invasive procedures cannot be recommended over medication. More complete studies in the future may change this so watch this space.

Subaortic stenosis is a genetic disease.
Because inheritance is not simple, dogs with mild disease may produce puppies with severe disease.
No dog with subaortic stenosis of any degree should be bred.

Page last updated: 9/2/2021