One of the myriad heart problems which can affect your pet is a condition known as aortic stenosis or subvalvular aortic stenosis. With this condition, the blood flow is partially blocked as it leaves the left side of the heart (the left ventricle) into the aorta, which is the major blood vessel that transports blood to the rest of the dog's body. The obstruction can be a small nodule or a fibrous band of tissue, either of which is usually located just below the aortic valve, which is the dividing structure between the heart and the aorta.
Because of this obstruction, the dog's heart must work harder to pump an adequate supply of blood to the dog's body. Over time, this valve usually narrows even further, a condition known as stenosis.
Aortic stenosis is an inherited condition in many dogs, especially the larger breeds. Newfoundland dogs have the highest risk, but it also appears frequently in the Boxer, Golden Retriever and Rottweiler. Also at an elevated risk (although not as much as the previously-mentioned breeds) are Bulldogs, German Shepherds, German Short-Haired Pointers, Great Danes and Samoyeds.
When the stenosis is mild, the dog will show no ill effects and usually have a normal lifespan. When the condition is moderate to severe, however, there will be a number of symptoms. The dog often will not be able to exercise for long periods of time without becoming unusually fatigued. Some dogs may even faint, since the narrowing prevents them from pumping an adequate supply of blood to the brain when active. In the worst-case scenario, the dog will simply collapse and succumb to sudden death.
Over time, the obstructions to the dog's blood flow cause the heart muscle to become thicker, a condition known as left ventricular hypertrophy. As this process progresses, the dog's heart becomes less able to compensate, and other symptoms develop, including poor growth, difficulty breathing and a chronic cough. This thickening of the heart muscle also can cause the dog to develop abnormal heart rhythms (known as cardiac arrhythmias), which can cause a heart attack or sudden death.
Aortic stenosis is often diagnosed through a simple stethoscope when the animal is less than 6 months old, as the result of your veterinarian hearing an abnormal rhythm commonly known as a heart murmur. If this murmur is significant, then follow-up X-rays, blood work, an electrocardiogram (ECG or EKG) and/or ultrasound are often performed to confirm the diagnosis. In older dogs, the diagnosis is made through these same tests, plus an assessment of the symptoms mentioned above. Mild cases of aortic stenosis usually are left untreated, although in some cases the dog may be prescribed antibiotics, to prevent an infection in the abnormal valve tissue. It's also recommended that the animal not be used for breeding and its littermates be screened for the problem. (Mildly affected dogs can pass along a severe form of the defect to their offspring.)
When the stenosis, or narrowing, of the artery is more pronounced, the dog's exercise will be restricted, and beta-blocking drugs may be prescribed. Both of these actions help to slow down and minimize thickening of the heart muscle. Surgery can be performed to remove the blockage; however it is a risky and expensive procedure, and there is no real documented increase in the animal's longterm survival rate.