The heart is a powerful muscle that gives our animals life. Made up of four chambers, separated by valves, it is responsible for pumping oxygen and blood around the body. If the structure of the muscle or valves is abnormal, this may compromise its ability to do this important job. One condition that can affect the heart of our canine companions is aortic stenosis.
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The structure of the heart
When mammals breathe oxygen into their lungs, it is passed into the blood which then carries this fresh oxygen into the left side of the heart. It passes through the upper chamber (atria), through a valve (mitral valve) to a lower chamber (ventricle). From there, it is pumped through the aortic valve and out the aorta, a large blood vessel which takes this highly oxygenated blood out to the body.
Low-oxygen blood returns from its journey around the body by entering the right side of the heart. It passes through an upper atria, through a valve (tricuspid valve) to a lower ventricle and out the pulmonary artery to go to the lungs to pick up more oxygen. This cycle repeats itself over and over as the heart pumps throughout life.
The efficiency of the heart’s action is influenced by any abnormality that compromises this normal movement through the four chambers. This may occur in the pericardium (the membrane around the heart), the heart muscle, the valves, or the electrical impulses that control the beating of the heart. Abnormalities can be present from birth (congenital) or develop with age (acquired).
Aortic Stenosis
One congenital problem that affects some breeds of dogs, is aortic stenosis. This condition involves a stricture or narrowing of the exit point from the left ventricle, known as the left ventricular outflow tract. It is typically caused by a ring of fibrosis (scarring), but can sometimes involve nodules.
There are three locations where stenosis can occur and therefore there are three types of aortic stenosis. With supravalvular aortic stenosis, the narrowing is above the aortic valve. Valvular aortic stenosis refers to narrowing at the valve itself, and subaortic stenosis occurs below the valve. This is the most common form in dogs.
How does aortic stenosis affect my dog?
This condition typically affects large breed dogs and is a congenital condition. Some breeds are particularly predisposed, including German Shepherds, Golden Retrievers, Boxers, Rottweilers, and Newfoundlands. Initially, there may be no symptoms and your vet may just detect a heart murmur at your dog’s puppy exam.
A heart murmur is an extra sound audible between the heart beats. It is created by turbulent or backwards flow of blood within the heart. In the case of aortic stenosis, the murmur results from the reduced flow through the stenosis. The more restricted the outflow of blood, the louder the murmur tends to be. If the condition is mild, there may be no additional concerns with your dog.
As your dog grows, sometimes the stenosis worsens with age
Over time, the restricted outflow of blood causes the muscular wall of the left ventricle to thicken as it works harder to eject blood out of the chamber and into the aorta. This can lead to exercise intolerance as the heart struggles to cope with the increased demand for oxygen with activity. If it is severe enough, your dog may be lethargic even at rest.
The most concerning consequences of aortic stenosis is syncope, where the heart stops beating adequately to circulate oxygen to the brain. This causes your dog to pass out and can be a cause of sudden death. In some individuals, this may be the first and only symptom of underlying heart disease and can be distressing for all involved.
How is aortic stenosis diagnosed?
Many puppies have so-called ‘innocent’ murmurs detected at an initial puppy exam so your vet may recommend monitoring very mild murmurs. If your vet detects a more severe or concerning murmur, or your dog has symptoms of reduced heart efficiency, they will likely recommend some testing. These are designed to find both the cause and effects of the problem.
An x-ray, or radiograph, can allow assessment of the heart size and shape. Enlargement of the heart chamber as a result of muscle thickening can produce a visible bulge on an x-ray. An ECG, or electrocardiogram, is used to measure the electrical signals moving through the heart. Physical changes in the heart resulting from the stenosis can lead to an abnormal pulse signal on this test. The most accurate test, however, is an ultrasound. This can localise and visualize the stenosis, measure the thickness of the heart muscle to check for thickening, and measure the efficiency of the blood outflow as it leaves the heart. This can then be monitored and re-measured over time to assess if the condition is progressing.
What can be done to treat my dog?
If your dog has no symptoms and only a mild murmur or stenosis, they may need no treatment at all. Your vet will advise you if follow-up tests or monitoring are recommended. Surgery to correct the stenosis or the use of a balloon to stretch open the stenosis (balloon valvuloplasty) are recognised treatment options for this condition. However, they generally involve referral to a specialist veterinary hospital and come with risks both during and after the procedure. There is also evidence that these procedures do not offer huge advantages over medical management.
Medical treatment is therefore usually the main form of management. The goal of treatment is to minimise the stress on the affected ventricular wall and help with oxygen demand. These drugs are known as beta blockers and are typically a tablet that is given by mouth. Your vet can discuss with you if your pet would benefit from these, and what side effects could occur.
It is not advised to breed from an animal with aortic stenosis due to the potential to pass the condition on to offspring. Pregnancy also puts extra pressure on the heart which could worsen the symptoms of the condition. Aside from physically preventing the opportunity to mate, your vet can discuss with you if it is safe to surgically desex your animal under general anaesthetic. If it is not, hormonal contraceptives in the form of an implant may be an option.
Conclusion
Aortic stenosis primarily affects large breed dogs and can compromise the efficiency of the heart to pump oxygen around the body. Although mild cases may cause no symptoms and require no treatment, when severe, it can have an impact on your dog’s energy and ability to exercise. Medical and surgical options are available, and your vet can keep you informed on how best to diagnose and treat this condition in your dog.
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