Aortic valve stenosis is a pathological constriction that can occur above, below, or at the aortic valve. It is characterized by restricted outflow from the left ventricle into the aorta. Partial obstruction to the flow of blood ranges from small nodules to a fibrous band, most commonly just below the aortic valve, causing subvalvular aortic stenosis. Due to the obstruction, the heart must work harder to pump out an adequate blood volume. In response to the obstruction to blood flow, the heart muscle becomes thicker over time resulting in left ventricular hypertrophy. As the condition progresses, the dog's heart becomes less able to compensate and signs associated with left-sided heart failure, such as tiring on exercise, difficulty in breathing, coughing, and poor growth will become apparent.
Changes in the heart muscle can also lead to cardiac arrhythmias (abnormal heart rhythms) and sudden death. Congenital aortic stenosis is probably the most common heart defect seen in large breed dogs with Newfoundlands having the highest risk for the defect. It is also important in the Golden Retriever, Rottweiler, and Boxer. There is a mildly increased risk of aortic valve stenosis in the German Shepherd Dog, German Shorthaired Pointer, Great Dane, Samoyed, and Bulldog.
Clinical signs and long-term outcome depend on the degree of narrowing. 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. Dogs with mild stenosis will generally show no clinical signs and have a normal life expectancy. With moderate to severe stenosis, signs will be variable. 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 your dog just seems to run out of steam or fainting due to inadequate blood supply to the brain. In young animals less than six months of age there may be no clinical signs. Thus the first indication that a dog may have a problem may come when a veterinarian hears a heart murmur during physical examination. Some low-grade murmurs are "innocent" and disappear by six months of age, but if the murmur is significant, a veterinarian will suggest a diagnostic workup to determine the cause. He or she will listen very carefully to the 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 ultrasonography if available. To determine the extent of the narrowing, the pressure gradient across the aortic valve can be measured using special procedures.
Re-printed with permission from Canine Inherited Disorders Database, a valuable resource on dog health.