The aorta is the great artery that leaves directly from the left ventricular chamber of the heart. It is the main trunk of the circulatory system and supplyies blood to virtually all bodily tissues. "Stenosis" is defined as the narrowing or contraction of a body opening or passageway, from whatever cause. Therefore, "aortic stenosis" is an obstruction to the outflow of blood from the left ventricle into the aorta due to some narrowing or constriction. It is a common condition in domestic dogs. In fact, it is reported to be the second most common congenital canine heart defect, second only to another condition called patent ductus arteriosis. Some authorities place aortic stenosis first in this category.
Causes of Aortic Stenosis
Aortic stenosis can be caused by a number of things. It can result from some defect or malformation of a heart valve (valvular aortic stenosis), by an obstruction in that part of the aorta known as the ascending aorta (supravalvular aortic stenosis), or by an obstruction in the outflow path of blood leaving the left ventricle through the descending aorta (subvalvular aortic stenosis). Supravalvular aortic stenosis is the most common form of this condition in domestic cats; subvalvular aortic stenosis is the most common type in domestic dogs. What actually causes these various physical defects in the heart can vary. Many times, aortic stenosis is congenital, which means that it is present at birth but is not necessarily genetic in origin. However, most authorities believe that subvalvular aortic stenosis – also called subaortic stenosis or "SAS", does have a strong hereditary component in almost all cases. Genetic SAS has been soundly documented in the Newfoundland breed. There is no known genetic sex linkage to this disorder, which means that, without other evidence, it cannot be specifically attributed to the father or the mother of affected puppies. SAS is occasionally caused by a bacterial infection of the aorta and/or the left atrioventricular (AV) valve. This condition is called endocarditis.
Congenital SAS in dogs typically involves an obstruction in the outflow of blood from the left ventricle into the descending aorta as a result of abnormal fibrous tissue or some other malformation in or near the area of the left atrioventricular valve. The left AV valve is the doorway between the left ventricular heart chamber and the aorta. It is also referred to as the bicuspid valve or the mitral valve. According to some reports, the name "mitral" valve was coined in reference to the hats worn by Catholic bishops, called "miters." Apparently, someone thought that the left AV valve resembled those hats in shape and appearance, leading to the designation "mitral valve."
When the outflow of blood from the left ventricle into the aorta is compromised, that chamber has a volume overload of blood. The heart must contract with more force than normal in order to maintain acceptable forward blood flow and blood pressure. This extra work causes the walls of the left ventricle to thicken (hypertrophy) over time. It also causes a number of other detrimental physiological changes.
Prevention of Aortic Stenosis
Because SAS is thought to be a genetic condition, the sire and dam of a litter that has one or more affected animals probably should not be used as part of a responsible breeding program. Certainly, dogs with clinical SAS should not be bred. SAS due to bacterial endocarditis may be prevented by careful use of antibiotics before, during and/or after procedures such as dental cleanings and surgeries. Otherwise, there is no known way to prevent a dog from developing subaortic stenosis.
SAS is almost always considered to be a hereditary disease, or at least to have a strong genetic component.
How Aortic Stenosis Affects Dogs
Subaortic stenosis (SAS) typically develops after a puppy is born, during the first weeks to months of its life. However, clinical signs of the disorder often do not appear until later. The onset of noticeable signs of SAS can happen at any age and depends largely upon the severity of the blockage or obstruction of blood outflow from the left ventricle into the aorta. In most cases, the disorder can be identified by the time the puppy reaches about one year of age. It is hard to say whether affected dogs are consciously hindered by the condition before they develop symptoms of disease. However, at some point, they most likely will become extremely ill and may die suddenly from the condition.
Symptoms of Aortic Stenosis
The signs of SAS range from none (subclinical disease) to the signs of full-blown congestive heart failure. In most cases, the condition is diagnosed when a veterinarian detects a heart murmur in a young, large breed dog on a routine physical examination, or when the dog is in the veterinary clinic for some unrelated reason. When symptoms are present, may include one or more of the following:
Difficulty breathing (dyspnea)
Rapid breathing (tachypnea)
Fainting episodes (syncope)
Heart murmur (most prominent after 2 months of age)
Irregular heart beats (arrhythmias)
Weak femoral pulses (the femoral artery is the large artery that is palpable [can be felt] on the inside of a dog's upper thigh)
Unfortunately, if SAS is not properly diagnosed in an affected young dog, its owner may only learn of the existence of the disorder when the dog literally "drops dead," which usually occurs during a period of exercise or exertion.
Dogs at Increased Risk
SAS is among the most common of the congenital heart defects seen in domestic dogs. It certainly is the most common congenital heart defect of large and giant breeds. SAS is commonly seen in Golden Retrievers, German Shepherd Dogs, Newfoundlands, Boxers, Bull Terriers, Rottweilers, German Shorthaired Pointers, Bernese Mountain Dogs and the Bouvier des Flandres. Other breeds with a heightened risk of developing SAS include Great Danes, Samoyeds, Bull Terriers and English Bulldogs.
How Aortic Stenosis is Diagnosed
Any dog suspected of having heart problems or other systemic disease will be given a thorough physical examination by the attending veterinarian, who will also take a history from the dog's owner. Dogs with subaortic stenosis (SAS) usually have normal results on a urinalysis and on routine blood work, including a complete blood count (CBC) and a serum biochemistry profile. The next step in gathering the initial data base usually is thoracic radiographs (chest X-rays), which may or may not show an enlarged cardiac silhouette. In severe cases, the left side of the heart may be noticeably enlarged on the chest films. An electrocardiogram (ECG) can be performed and may or may not disclose signs of SAS based on thickening of the left ventricular wall.
More advanced testing includes echocardiography, which basically is an ultrasound of the heart. Also sometimes simply called an "echo," this is a recording of the position and motion of the heart walls and internal structures by the echo obtained from beams of ultrasonic waves directed through the chest wall. An echocardiogram is usually diagnostic of SAS, especially when what is called a "Doppler study" is performed. Doppler echocardiography effectively measures the speed (velocity) and direction of blood flow through the various heart chambers.
Contrast radiography can be used to identify the thickening of the left ventricular wall, the narrowing of the aorta as it leaves the left ventricle through the left atrioventricular (AV) valve and the dilation (widening) of the aorta from high blood pressure further down the line. This process is known as angiocardiography. Cardiac catheterization can also be performed to determine the pressure differences across the left AV valve and compare them to normal. However, these last two techniques are almost never used diagnostically in veterinary medicine. They are invasive, require that the dog be put under general anesthesia and do not reliably provide more beneficial diagnostic information than echocardiography.
Echocardiography is currently considered to be the gold standard for diagnosing SAS.
The goals and types of treatment for dogs diagnosed with subaortic stenosis (SAS) are somewhat controversial, even among veterinarians. There is no universally agreed-upon treatment protocol. Certainly in severe cases, the therapeutic goal is to try and reduce the risk of fainting episodes (syncope) and sudden death. If the dog has developed congestive heart failure as a result of SAS, treatment for that condition should be implemented; this typically involves administration of a combination of several different medications. If the dog has SAS but has not yet developed many or any recognizable symptoms of the condition, recommendations for treatment can vary. Activity restriction is usually suggested in most cases, as exertion can bring on bouts of fainting and can exacerbate the risk of sudden death. Low sodium diets can be beneficial for dogs with heart failure and with SAS. Affected animals should not be bred, and their siblings should be assessed and monitored closely for any signs of heart disorders.
The only truly definitive treatment for aortic stenosis is open-heart bypass surgery to remove the obstructive lesion. However, this is rarely performed in veterinary clinics – even in those with specialized veterinary cardiologists on their staff. Some experts have used balloons to dilate the aorta during cardiac catheterization. Unfortunately, the results of these invasive surgical procedures have not shown any prolonged survival times over conservative medical (drug) management.
Medical management of dogs with SAS is designed to relieve discomfort. It is not designed to "cure" the condition. Certain medications in a class of drugs called "beta blockers," together with other medications, have been used and advocated by some veterinarians to manage SAS, although little research has been done to establish the efficacy of drug therapy for dogs with SAS. Owners should consult with their veterinarian, and probably seek referral to a veterinary cardiology specialist, to decide upon the best course of treatment for their dog.
The outlook for dogs with subaortic stenosis is quite variable. Dogs with very mild disease may live a full and normal life, never showing any noticeable signs of their heart condition. On the other hand, dogs with moderate to severe SAS often die without warning within the first several years of life. Alternatively, they may develop congestive heart failure and will be euthenized or die from that condition.