What is aortic stenosis?
In aortic stenosis, there is a
partial obstruction to the flow of blood as it leaves the left
side of the heart (the left ventricle) through the main blood
vessel (the aorta) that carries blood to the rest of the body.
The obstruction ranges from small nodules to a fibrous band,
most commonly just below the aortic valve ("subvalvular
aortic stenosis"). Due to the obstruction, the heart must
work harder to pump out an adequate blood volume. Clinical signs
and long-term outcome depend on the degree of narrowing, or
stenosis.
How is aortic stenosis
inherited?
In Newfoundlands, this defect has
been shown to have an autosomal
dominant mode of inheritance, with variable expression.
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 of the
disorder.
What breeds are affected by
aortic stenosis?
Congenital aortic stenosis is
probably the most common heart defect seen in large breed dogs.
Newfoundland dogs have the highest risk for this disorder. It is
also important in the golden retriever, Rottweiler, and boxer.
There is a mildly increased risk
of aortic stenosis in the German shepherd, German short-haired
pointer, Great Dane, Samoyed and Bulldog.
For many breeds and many
disorders, the studies to determine the mode of inheritance or
the frequency in the breed have not been carried out, or are
inconclusive. We have listed breeds for which there is a general
consensus among those investigating in this field and among
veterinary practitioners, that the condition is significant in
this breed.
What does aortic stenosis mean
to you and your dog?
Dogs with mild stenosis will
generally show no clinical effects and have a normal life
expectancy. With moderate to severe stenosis, signs will be
variable. Because of the narrowing in the aorta as the blood
leaves the left ventricle, your dog's heart must work harder to
pump an adequate volume of blood to the rest of the body.
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 reduced exercise
tolerance - your dog just seems to run out of steam - or
fainting due to inadequate blood supply to the brain.
In response to the obstruction to
blood flow, the heart muscle becomes thicker over time (left
ventricular hypertrophy). As the condition progresses, your
dog's heart becomes less able to compensate and you and your
veterinarian may see signs associated with left-sided heart
failure such as tiring on exercise, difficulty in breathing,
coughing, and/or poor growth. Changes in the heart muscle can
also lead to abnormal heart rhythms (cardiac arrythmias)
and sudden death.
Your veterinarian can do various
tests (see below) to determine the severity of the defect. Based
on the results, he or she will discuss with you the long-term
prognosis, and ways to manage this condition in your dog.
How is aortic stenosis
diagnosed?
In young animals (less than 6
months of age) there may be no clinical signs. Thus the first
indication that your dog may have a problem may come when your
veterinarian hears a heart murmur during physical examination.
Some low-grade murmurs are "innocent" and disappear by
6 months of age, but if the murmur is significant, your
veterinarian will suggest a diagnostic workup to determine the
cause. He or she will listen very carefully to your 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/or ultrasonography if available. To determine the extent of
the narrowing, the pressure gradient across the aortic
valve (between the left ventricle and the aorta) can be measured
using special procedures for which your veterinarian can refer
your dog to a specialist.
In an older animal or when the
obstruction is pronounced there may be clinical signs associated
with left-sided heart failure.
Based on the results of these
various tests, your veterinarian will discuss with you the
prognosis and long term management of your pet. S/he will also
be able to tell you if any (and how much) change has occurred in
the heart already as a result of the stenosis.
FOR VETERINARIAN:
- MURMUR: systolic, left
hemithorax, radiates into thoracic inlet and up the neck,
PMI left heart base (3rd to 4th intercostal space), may be
equally loud at right heart base.
- ECG: may be normal, over time
shows left ventricular enlargement, left axis shift, may
show ST depression, may show ventricular arrhythmias
- RADIOGRAPHS: may see
left ventricular enlargement, cranial aortic enlargement,
left axis shift. Pulmonary vasculature is normal.
- ECHOCARDIOGRAPHY: left
ventricular hypertrophy, subvalvular fibrous ring, post-stenotic
dilation of aorta
- The arterial pulse may be of
reduced intensity and slow to rise.
Echocardiography (with Doppler)
or cardiac catheterization is usually required to determine the
severity of the defect.
How is aortic stenosis
treated?
In dogs with mild aortic
stenosis,there is no special treatment required. The dog should
not be used for breeding and littermates should be carefully
screened. Your veterinarian may suggest antibiotics in certain
circumstances as a precaution against infection of the abnormal
valve tissue.
With moderate to severe stenosis,
the dog's exercise should be restricted. Beta-blocking drugs may
be prescribed by your veterinarian to try to minimize the
effects of left ventricular hypertrophy. Your veterinarian will
recommend other therapy if required to manage congestive heart
failure. Medical management for congestive heart failure is
similar no matter what the cause, and consists of medications to
support the heart muscle and decrease the work load of the
heart, together with dietary recommendations.
Various surgeries have been
attempted to alleviate the obstruction with limited success. The
surgery itself carries a high risk, and there is little, if any,
increase in survival rates compared with dogs whose condition is
managed medically.
Breeding Advice
Affected individuals should not
be used for breeding, and littermates should be carefully
screened.
FOR MORE INFORMATION
ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
Resources
Bussardori, C. 1998. Breed
related echocardiographic prognostic indicators in pulmonic and
subaortic stenosis. ACVIM-Proceedings of the 16th Annual
Veterinary Medical Forum: 140-142.
Bonagura, J.D. and Darke, P.G.G.
1995. Congenital heart disease. In S.J. Ettinger
and E.C. Feldman (eds.)Textbook of Veterinary Internal Medicine,
p. 892-943. W.B. Saunders, Toronto.
Patterson, D.F. 1996. The
genetics of canine congenital heart disease. ACVIM-Proceedings
of the 14th Annual Veterinary Medical Forum: 225-226.
This reference has good information for breeders and
veterinarians regarding screening and genetic counselling for
congenital heart defects.
