Blood flows from the body to the right heart, the lungs, the left heart then out to the body (see ‘Normal Cardiac Circulation’). Blood flows from the left ventricle, out of the aorta to deliver oxygenated blood to the tissues. The aorta is the largest artery in the body. Between the left ventricle and the aorta is the aortic valve, which opens to allow blood to pass through to the body.
What is Subaortic Stenosis?
Subaortic stenosis is a narrowing in the left ventricle, just below the aortic valve. This narrowing is usually caused by a fibrous ridge of tissue. Depending on the severity of the narrowing, obstruction of blood flow out to the aorta results. In order to force the same amount of blood past this narrowing, the left ventricle has to work harder. The left ventricle becomes thickened to cope with this increased workload. The blood squirts through the narrowing in a turbulent fashion and comes out faster on the other side. This creates the heart murmur and, often, results in damage to the aortic valve.
How Common is Subaortic Stenosis?
Subaortic stenosis is a common disease in certain large breed dogs including the Newfoundland, Boxer, Golden Retriever, German Shepherd and German Shorthaired Pointer. It is considered to be an inherited disease so breeding from an affected dog is not recommended. Subaortic stenosis is rare but occasionally seen in small breed dogs and cats.
How Do I Know my Puppy has Subaortic Stenosis?
At birth, the stenosis will be minimal with only a slight ridge below the aortic valve. Often, no heart murmur will be heard at birth. However, over the next few weeks - months, the ridge grows resulting in obstruction to blood flow. The heart murmur then starts to develop. The disease generally continues to progress in the first 12 months of life then remains stagnant after this time. The first sign of subaortic stenosis is usually a heart murmur heard at the puppy checks. Your regular veterinarian will then recommend referral to a veterinary cardiologist for an echocardiogram (heart ultrasound).
An echocardiogram is the quickest and easiest way to diagnosis subaortic stenosis as it allows visualisation of all the cardiac chambers, evaluation of speed and direction of blood flow and visualisation of the fibrous ridge causing obstruction. Occasionally an electrocardiogram (ECG) and chest xrays may also be recommended to get a more thorough evaluation of the heart and lungs.
What are the Consequences of this Disease?
If the stenosis is mild, there may be no consequences and your puppy will likely live a normal live. If the stenosis is severe, the left ventricle has to work harder to pump the same amount of blood past the narrow region. As a result, it thickens. Over time, this thickening affects the ability of the left ventricle to relax due to increased stiffness. Heart muscle cells in the thickened region can also start to die. The end result is irregular heart beats (arrhythmia), back up of fluid into the lungs (congestive heart failure), fainting spells and even sudden death.
The turbulent blood flowing past the fibrous ring hits the aortic valve at high speed and can start to damage it. Dogs with subaortic stenosis (mild, moderate or severe) can be at risk of infection to this damaged aortic valve (bacterial endocarditis). While rare, this is a serious consequence of the disease. Hence, it is recommended that any dog with a diagnosis of subaortic stenosis be treated with antibiotics during any traumatic or surgical procedures.
Is There Treatment for Subaortic Stenosis?
Unfortunately, there is no real surgical option for subaortic stenosis in dogs. Balloon valvuloplasty (inserting a catheter with a balloon into the heart to dilate the stenosed area) has been trialled but currently does not appear to improve survival times significantly in dogs. Dogs with mild or moderate disease usually live a normal life and do not need any treatment. Severe cases will be prescribed a beta-blocker called Atenolol. The aim of beta-blockers is to reduce the workload on the left ventricle and reduce any potential arrhythmia. The aim is to maintain a good quality of life for your dog and hopefully prolong life.