by Jennifer Zablotny, DVM
Subaortic stenosis is a congenital, genetic heart defect. It can affect any breed of dog, but is most commonly seen in Newfoundlands, Golden Retrievers, and Rottweilers. It is typically diagnosed in puppies 16 weeks of age or older, although more severe cases will present with a murmur at younger ages.
SAS is a narrowing (stenosis) of the left ventricular outflow tract just past the aortic valve (subvalvular). In a normal dogheart, blood lacking oxygen flows from the body to the right atrium and into the right ventricle where it is pumped through
the pulmonary artery and into the lungs. After picking up oxygen from the lungs, the blood returns to the heart through the left atrium and then into the left ventricle where it is pumped out into the body via the aorta. Pumping blood into the lungs doesn’t require much effort, so the right ventricle has a thinner wall than the left ventricle. Pumping blood into the body, however, requires a lot of force, so the wall of the left ventricle is much thicker. Muscles get larger when they are used.
When a dog is affected by SAS, there is a fibrous band of tissue just past the aortic valve. This band can range from a mild narrowing of the outflow tract to a severely narrowed tract. The narrower the tract, the harder the left ventricle has to work to push blood into the aorta and the body. Think of putting your hand over the end of a garden hose and narrowing the opening. Since the left ventricular muscle is working harder, it gets thicker than it should be. This can cause two scenarios. In the first, the muscle is so thick that it doesn’t get enough oxygen and part of it dies. This causes scar tissue to form, and scar tissue doesn’t conduct electrical impulses as well as normal muscle. Abnormal electrical impulses cause arrhythmias in the ventricle, and death occurs suddenly. In the second scenario, the left ventricular wall is so thick that it is making the chamber smaller and not enough blood is able to flow from the left atrium into the left ventricle. This can overload the lungs and the right ventricle causing congestive heart failure. This is not typical of SAS, but it can occur.
A related disease is pulmonic stenosis (PS). This is a narrowing of the right ventricular outflow tract. Since the right ventricle isn’t very thick to begin with, it has a lot more room to enlarge before there are problems. PS is also genetic and affected individuals shouldn’t be bred, but they aren’t in as much danger from sudden death as individuals affected with SAS. Dogs with a loud PS murmur can live a normal life span.
Diagnosis of SAS is anything but straightforward. The narrowed artery produces turbulent flow, which causes a murmur best heard on the left side of the chest at the heart base. In general, the louder the murmur and the earlier that it is heard, the more severe the disease. Many breeders have their puppies screened for cardiac diseases by a veterinary cardiologist prior to leaving for their new homes at eight to 10 weeks of age. This helps breeders avoid placing puppies with potentially serious heart defects. The problem with this is that not all murmurs that are heard at eight weeks are pathologic, and puppies with a mild narrowing may not have a murmur when they are checked.
Any heart disease that causes turbulent or abnormal blood flow will create a murmur. Pulmonic stenosis and patent ductus (PDA) are two other congenital heart diseases that are also seen in Newfoundlands. A PDA murmur is very characteristic and wouldn’t be confused with a murmur caused by SAS. Innocent murmurs in puppies are usually gone by 16 weeks of age. These murmurs aren’t caused by any underlying disease process and are fairly common across breeds. Puppies without a SAS murmur at eight weeks may have one by 12 or 16 weeks. That doesn’t mean they didn’t have SAS at eight weeks, it just means that the narrowed outflow tract was big enough for the eight-week-old puppy and didn’t grow. The puppy did and now the opening is abnormally small for the larger dog. Waiting as long as possible to check puppies will help avoid this problem.
Adult Newfoundlands should be ausculted by a veterinary cardiologist after a year of age. They have reached most of their growth, so any stenotic areas should be causing a murmur. While auscultation is a very cost effective and accurate method to screen adult dogs for SAS, any dog, especially breeding animals or puppies over the age of 16 weeks, that has a murmur should have an echocardiogram or ultrasound of the heart. Innocent murmurs are not uncommon in large and giant breed, athletic dogs. Exercise alone is enough to cause a murmur in some individuals. This is not pathologic, and an echo will confirm a normal heart.
In dogs affected by SAS, an echo can grade the severity. Colorflow Doppler measures the velocity of the blood across the aortic valve. Severe narrowing causes increased speed. (Remember the garden hose.) Gray areas occur when a dog may or may not have a murmur, there is no visible sign of a band of tissue below the aortic valve, left ventricular wall thickness is normal, and velocities are at the upper end of normal. Does this dog have SAS or is it an athletic heart in a giant breed
dog? Unfortunately, there are no established normal ranges for
blood flow velocity specific to large or giant breed dogs. (OFA
Cardiac database statement)
Definitive and accurate diagnosis of SAS is only possible with a necropsy. There are some individuals that will not have a murmur, will have a normal echo, and the SAS lesion will be found at necropsy. The use of this diagnostic is undesirable in breeding stock and most pets, so for now, we are left with auscultation and echocardiogram for diagnosis of SAS in living dogs. It is wise to perform a necropsy in breeding animals that die suddenly, regardless of previous auscultation or echo results.
Treatment of SAS varies with the severity of the disease. Mildly affected dogs can have normal lifespans without treatment. Medical therapy can increase the lifespan of moderately affected dogs and also improve quality of life if they are suffering from congestive heart failure. Severely affected dogs will require surgical intervention and medical therapy. The NCA Charitable Trust has funded a study examining the efficacy of cutting balloon valvuloplasty. The narrowed band of tissue is expanded and cut at the same time with a balloon on a catheter. This appears to be a promising technique as the narrowed section of artery is enlarged reducing the stress on the left ventricle and the cuts don’t allow it to narrow again. Prevention of this disease is the better course of action. Unfortunately, the genetics of the condition are muddled at best.
There are several genes involved in SAS with different modes of inheritance. Screening puppies prior to placement prevents unsuspecting owners from acquiring a heartache dog, and breeders can avoid investing time and money in a promising puppy. What to do with puppies that have murmurs is controversial. Some breeders euthanize all puppies with murmurs. That may be fine if the puppy has SAS, but these breeders may be euthanizing normal puppies with innocent murmurs or puppies with less serious pulmonic stenosis unless they are echoing all puppies with murmurs.
A veterinary cardiologist should listen to all Newfoundlands a year of age or older, regardless of the their breeding status. Cardiac clinics are commonplace in many areas at dog showsand there is always a cardiology clinic at the National. Results should be reported back to the breeder, at the very least. There is considerable controversy over whether all breeding animals should have an echocardiogram performed. Many cardiologists feel that auscultation is sufficient and detects the majority of SAS affected individuals. Echo is a more sensitive test, but it is also more expensive and may not detect enough affected individuals to make it cost effective for the population. If a dog is producing a high percentage of SAS puppies, it may be beneficial to echo that dog and make sure that individual is not affected.
Phenotypic testing obviously has its limits. Testing for the gene that actually causes SAS would be ideal. Dr. Linda Meurs at the North Caroline State University Cardiology lab is working on that. With funding from the NCA Charitable Trust, she is recruiting samples of normal and affected Newfoundlands. The dogs must have had an echo to confirm SAS or normal and copies of that report along with a three generation pedigree need to be sent with two to three cc’s of blood in a lavender top tube. For more information, please visit: http://www.cvm. ncsu.edu/vhc/csds/vcgl/newfoundlandcardiacresearch.html SAS can be a frustrating and heartbreaking disease to deal with. Educating breeders and potential puppy purchasers has raised awareness of this disease and the steps needed to reduce the incidence in Newfoundlands. Ultimately, identification of the abnormal gene will make it possible to eliminate SAS from our dogs.