Subaortic Stenosis – When The Left Side Exit From Your Puppy’s Heart Is Too Narrow
Subaortic Stenosis (SAS) aka Subaortic Valvular Stenosis, Aortic Stenosis, AS
Ron Hines DVM PhD
|Right side heart blockage?|
Subaortic Stenosis (SAS) is an abnormal restriction of the flow of blood as it leaves your dog’s heart to enters its aorta and flow to all parts of its body – something a bit similar to your garden hose having a kink. The obstruction is due to an abnormal ring of fibrous tissue (stenotic ring). (ref1, ref2) And like a kinked hose, it makes a distinctive sound – one that your veterinarian will pick up through his or her stethoscope. You can listen to that sound through the link above. SAS is a heartbreaking discovery for every owner of a new puppy. And it is usually a veterinarian like me who has the sad job of break the news.
When any form of aortic stenosis (SAS or AS ) exists, your dog’s blood must pass through a channel that is too narrow for an unrestricted, unhindered flow. Some say that SAS and AS are the second most common inherited heart diseases of dogs. Among large breeds, some even believe that it might be the most frequent cause.
Reports that focus solely on SAS tend to place the frequency of the disease considerably higher than those that review the frequency of all developmental heart issues in dogs. Those, probably more objective, reports place subaortic stenosis as a component in about 20% of the young dogs presented with heart issues. (ref1, ref2, ref3, ref4)
Veterinarians see SAS most often in golden retrievers and boxers. But the problem plagues rottweilers, Newfoundlands, other larger breeds and a few smaller breeds as well. That is particularly true when the dog’s parents were too closely related. Restrictions in the flow of blood (stenosis) makes your dog’s heart work harder than it should. Eventually, that can lead to heart failure. X2X
Many things can go wrong when a young heart is developing in the womb. Some dogs with SAS have more than one heart blood flow defect.
Subaortic stenosis or SAS is a complex genetic problem. How it is inherited has yet to be worked out and there may well be several avenues that lead to it. We assume that puppies that suffer from SAS have inherited a gene-combination from one or both of their parents that caused their heart to develop abnormally. In the case of Newfoundland dogs, the presence of a defective single member of a two-gene pair (alleles) appears to favor SAS; while the presence of two defective genes in the allele pair is lethal to the puppy embryo. (ref) Others came to the conclusion that a number of defective gene pairs were required before SAS occurred in Newfoundlands (a polygenetic trait). (ref)
In Dogue de Bordeauxs (ref) a single non-defective gene of a two-gene pair was thought to be enough to prevent SAS. (ref) So perhaps there are multiple potential genetic ways for SAS to occur. Some of those gene combinations that appear to favor SAS also favor abnormal narrowing of the blood channels leading from the dog’s heart to its lungs (pulmonary artery stenosis [ref]). (ref)
Through 2019, veterinarians cannot accurately predict which matings might produce puppies with this condition and which will not – other than the more closely related the parents are, the more likely genetic disease is to occur. The Orthopedic Foundation For Animals (OFA), an organization founded to discover the genetic basis of hip dysplasia in an attempt to eradicate it, established a registry for “cardiac disease-free” dogs that includes SAS. To date, I do not know that that approach has reduced the incidence of SAS. (ref1, ref2) Until we better understand how SAS is inherited that is not unexpected. North Carolina State Veterinary School has a current project attempting to discover ways to predict the likelihood of SAS occurring in Newfoundland dogs. They solicit DNA samples (and a fee). (ref)
There are those who have associate SAS in dogs with heart infections (bacterial endocarditis). That is probably because in people, restricted blood flow to the aorta can occur subsequent to strep throat infections which have damaged the aortic valve. (ref) But I know of no cases where such an infection was ever confirmed to have been the cause of SAS in a dog.
When And How Is A Case OF SAS Usually Discovered?
The most common scenario regarding dog owners and SAS is a heart murmur picked up by your veterinarian when your puppy received its puppy shots. If the pet’s heart was not examined during those vaccinations, it is usually the unexpected death or near death of the puppy that makes this disease a suspect. This is where I go into to make my usual pitch that you visit your veterinarian for those critical shots (ref), not some traveling econo- vaccination bus.
Not all puppies with the SAS condition have heart murmurs when they are first examined. Some don’t developed audible heart murmur before they received their last puppy vaccinations. Sometimes these murmurs come and go in their early stages. And sometimes these juvenile heart murmurs are not signs of SAS or any other heart problem at all. Heart murmurs that have no apparent cause are called innocent heart murmurs. They can leave as mysteriously as they appeared. Significant anemia also causes heart murmurs. Anemia can be due to anything from heavy flea or hookworm infestation, to nutritional deprivation. (ref) Nevertheless, in cases of SAS, the loudness of a heart murmur is often an indication of the severity of the blood flow problem. (rptref)
Heat murmurs due to SAS are often loudest over the left lower chest during the first heart sound (systole) . (rptref) Others find SAS murmurs also present higher on the right side of the chest. I suppose that depends on the dog’s chest conformation as well as other co-existing heart defects that may be present.
When SAS is discovered later in life, the dog was usually brought to the vet because of general fatigue and/or fainting.
What Are The Signs Of SAS That I Might See In My Puppy or Dog?
The progression and severity of SAS vary greatly between dogs. So the symptoms the disease causes – if any – vary greatly as well. Some dogs never become incapacitated or visibly ill. The blood “outlet channel” from your dog’s left ventricle consists of the area on each side of the aortic heart valve as well as the valve itself (subvalvular-valve-supravalvular). The amount of tissue distortion in those three areas (they are also called the left ventricular outflow tract or LVOT) vary dog-by-dog.
As I mentioned, most often, you will see no signs of ill health at all when your dog is a puppy. That is because SAS is a problem of the growing heart. And most of that growth occurs in the puppy after birth. But as time goes by, your puppy’s heart muscle walls might thicken (hypertrophy) due to the back pressures they must pump against. With that thickening, come problems in the heart’s electrical system that can cause fainting (syncope), weakness, lethargy and even unexpected sudden death. The fainting is thought to be due to electrical disturbances in the heart’s nerve network (caused by scaring). A healthy cardiac nerve system is necessary to keep the organ beating properly (ie prevent ventricular arrhythmias ). Although it has not been worked out in dogs, it is assumed to be similar to what occurs in humans with aortic valve stenosis. (ref) Blessedly, I do not believe that SAS is a painful condition.
If the dog survives those electrical heart disturbances the situation can progress to congestive heart failure (CHF). When it does, the symptoms include a weak pulse, respiratory difficulties, coughing and perhaps rear leg weakness. Irregular (left ventricular) heartbeats have a forward and a backward pathology. They provide inadequate forward oxygen to the body while the blood “waiting in line” to pass through the SAS restriction pools and backs up into the dog’s lungs. (ref)
What Diagnostic Tests Can My Veterinarian Perform?
To decide how serious your pet’s heart murmur really is, an echocardiogram (doppler ultrasound) needs to be performed by a veterinary cardiologist. Veterinary cardiologists are also better trained in evaluating the distinct sounds a heart murmur makes. To an experienced ear, the loudness, interval and tone of a heart murmur can be an indication of the severity of the heart abnormality and the point on the chest where the murmur is the loudest can give cardiologists clues as to what portion of the dog’s heart might be responsible for it. Read more about Cardiac Doppler Ultrasound below.
X-rays are not particularly helpful in diagnosing the early stages of SAS. But they can help rule out other chest and heart issues.
Electrocardiograms (EKGs) in puppies with SAS are often normal. But once heart arrhythmias have begun, EKGs detect them. Your veterinary cardiologist might suggest a Holter monitor (ref) to detect these arrhythmias as early as possible and decide if medications are warranted.
Cardiac Doppler Ultrasound – Echocardiography
This sophisticated apparatus allows veterinarians to visualize the four chambers of your dog’s heart, its valves and its major arteries – all in real time. The machine’s doppler capability allows the veterinarian to see indications of the speed at which blood passes through the organ (flow velocity) as well and the area(s) where blood flow is restricted (pressure gradient(s)). Blood flow speed is also an indication of the pressure your dog’s left ventricle must exert to force blood through the narrowed SAS restriction. So that gives us an indication of the amount of extra exertion your SAS dog’s heart undergoes. During this examination, dogs needs to be as relaxed as possible to give the most accurate results. (ref)
As dogs mature, the degree of increased thickness of the walls of the left ventricle as well as the doppler pressure gradient are good predictor of the severity of SAS. (ref) In younger dogs, blood back flow from the aorta to the left ventricle is often the first sign of clinically significant SAS.
A normal doppler ultrasound exam is reported back to you, the dog owner, as a “phenotypically normal dog”, meaning that no changes denoting the presence of SAS were seen. However, it does not mean that a dog is not a carrier of defective genes that might cause SAS in its offspring. That goes for OFA SAS-free certificates as well.
What Are My Dog’s Treatment Options?
The most common drugs used to treat SAS are in a class called beta-blockers (ß-blockers). The two most commonly used ones in dogs are probably propranolol (Inderal ®) and atenolol (Tenormin ®). Beta-blockers are given in the hope of reducing your dog’s heart rate, help control abnormal heart rhythms and reduce blood pressure. Although they are unproven in their ability to extend the lives of dogs with SAS they are often dispensed – particularly when the dog has a history of fainting or abnormal EKGs. (ref)
Some veterinarians suggest giving antibiotics to SAS dogs – particularly before and after dental procedures, surgery, skin or other infections. That is based on recommendations for humans with similar heart blood flow restrictions.
Open Heart Surgery
Open-heart surgery to correct SAS problems in dogs has not been very successful. Dogs in which this has been attempted lived about as long as dogs that just received medications. (ref)
A newer, more sophisticated and much less traumatic therapy for your dog is balloon valvuloplasty. This method inserts a small stretching balloon catheter through a small incision in your dog’s neck made over and into its right carotid artery. The device is advanced through the artery until it is positioned within the SAS-narrowed area. The technique and device were first adapted from a common procedure used to dilate (angioplasties to increase the diameter of) blocked human coronary arteries. Those are the ones most commonly implicated in human heart attacks. (ref) However, the benefits, if any, of the stretching procedure in dogs with SAS were found to be temporary at best. (ref)
More recently, the technique has been modified to make small slices partially through the walls of the portions of the blood outflow tract that are narrowed in SAS. Once they are incised (cut), the accompanying balloon is inflated in an attempt to stretch and widen the passageway. You can read how that is done here: (ref1, ref2)
I know of no studies that confirm that dogs that have had this procedure performed live any longer than dogs that do not have it performed. That is probably because a well-designed study to determine the value cutting balloon valvuloplasty would be extremely difficult to design and costly to perform.
Besides beta-blocking medications, many veterinarians would advise you to limit your pet’s strenuous exercise once it has been confirmed that it has SAS. They might suggest limiting your dog to short strolls on a leash. Boarding kennels and trips to the groomer might best be replaced by dog sitters and mobile services.
Hill’s and other major dog food manufacturers market “heart healthy” diets. They are nutritionally adequate for dogs; but there is no evidence that these diets have any effect on the progression of SAS. I would avoid those that replace traditional ingredients with more trendy, politically correct ingredients until we understand more about the long-term effects on dog heart health of doing so. (ref)
How Long Might This Puppy Live?
That is very difficult for me to predict for you. I mentioned that mild cases do occur. Those dogs should live long lives and I hope that your dog is one of them. Those more severely affected by SAS will have shorter lifespans. They often die suddenly in their young or middle age due to heart arrhythmias. The younger your puppy is when the problem is first noticed, and the louder the heart murmur, the bleaker its outlook is likely to be. Most dogs with the typical signs of SAS do not live past 3 years – even with medication.
Periodically having your veterinary cardiologist measure the pressure gradient across your dog’s SAS restriction is probably the best way to keep track of how the disease is progressing (pressure gradients of 50-65 mmHg are a positive sign). (ref) Higher numbers are not.
If your dog reaches the point where its heart can no longer maintain adequate circulation, all the medications used to treat congestive heart failure are likely to prolong its life. (ref) It is not unusual for dogs with SAS to have other heart abnormalities. The presence or absence of other coexisting heart issues (such as pulmonary stenosis ) also influence your pet’s likely longevity.