Aortic Stenosis (AS)


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Written: Sept 03
 THE HEART
The heart is a “pump” comprising of four chambers:
~ a left and right atrium, which receive blood;
~ a left and right ventricle, which pump out blood.
Diagrammatic representation of the heart
Left heart
~ The left atrium receives oxygenated blood via blood vessels (the pulmonary veins) from the lungs.
~ The left atrium pumps the blood through the left atrioventricular (mitral) valve into the left ventricle.
~ The left ventricle pumps the oxygenated blood, via the aortic valve, into the aorta.
~ The oxygenated blood is then circulated around the body.
Right heart
~ The right atrium receives deoxygenated blood via blood vessels (the superior vena cava and the inferior vena cava) from the body.
~ The right atrium pumps the blood through the right atrioventricular (tricuspid) valve into the right ventricle.
~ The right ventricle pumps the deoxygenated blood, via the pulmonic valve, into the pulmonary artery.
~ The deoxygenated blood then goes back to the lungs.
 DEFINITION OF AORTIC STENOSIS
Aortic stenosis (AS) is a „narrowing” just above or below the aortic valve, or (rarely) of the actual valve, causing a partial obstruction of blood flow from the left ventricle of the heart, through the aortic valve, and into the aorta.
~ Subvalvular aortic stenosis is the most common form of AS.
   The defect is located just below the aortic valve.
~ Supravalvular aortic stenosis is not so frequently seen.
   The defect is located just above the aortic valve.
~ Valvular aortic stenosis is rare.
   The defect is located within the aortic valve.
The stenosis (narrowing) is caused by the abnormal formation of nodules, or a fibrous ridge or ring of tissue. The heart therefore has to work harder to pump an adequate supply of oxygenated blood through the stenosed (narrowed) part.
 INHERITANCE
Aortic stenosis is known to be a hereditary heart defect, which affects some larger breeds of dog e.g. Newfoundland, Golden Retriever, Boxer and Rottweiler.
Canine geneticist Bruce Cattanach writes:
„The problem is that the mode of inheritance of Aortic Stenosis has not been clearly established even in the researched breed, the Newfoundland. In this breed some evidence was obtained that a single dominant gene was responsible but this became confused upon crossing to another breed. From my background in genetics I would expect that a single gene is primarily responsible for the condition but this is not to say that any other genes, such as those playing a role in heart development, blood vessel walls, elasticity of the wall, or the biochemical milieu behind the nodule and fibrous ring formation that leads to AS, etc., do not play a part.”
[Reference: www.boxerunderground.com/bu2000/jan2000/a_s_q_a.htm]
Unfortunately, it is not possible to predict the severity of the defect from one generation to the next; in other words, breeding from a dog with mild aortic stenosis could produce offspring with severe aortic stenosis. It is therefore usually recommended that affected dogs / bitches should not be used in a breeding programme.
 SCREENING & INVESTIGATIONS
Screening
Initial screening to identify aortic stenosis (and other heart problems) is „auscultation” i.e. listening to the dog’s heart, to find out whether or not there is a heart „murmur”. Murmurs, in the absence of any clinical signs, are very often incidentally picked up at routine annual check-up. This is one of the reasons why annual and pre-breeding checks are so important.
„Examination conditions must be appropriate for recognition of subtle cardiac malformations. Identification of soft cardiac murmurs is impeded by extraneous noise or by poorly restrained, anxious, or panting dogs.”
[Reference: www.offa.org/cardiacguide.html – Limitations 3.]
Investigations
There are several investigations, which may be undertaken by a veterinary cardiologist, to diagnose and evaluate aortic stenosis and other heart conditions.
Heart murmurs are graded from 0 to 6, according to the severity of the murmur:
Grade 0/6 denotes no murmur.
Grade 1/6 and 2/6 murmurs are generally considered to be „mild”.
Dogs with Grade 1/6 and 2/6 murmurs are very unlikely to develop any symptoms as a result of their aortic stenosis and should lead perfectly „normal” lives.
Grade 3/6 and 4/6 murmurs are generally considered to be „moderate”.
The majority of dogs with Grade 3/6 and 4/6 murmurs are also unlikely to develop any symptoms, but a small number will go on to become symptomatic.
Grade 5/6 and 6/6 murmurs are generally considered to be „severe”.
Dogs with Grade 5/6 and 6/6 murmurs are most at risk of developing symptoms – (see below).
X-ray looks at the shape and size of the heart and can identify abnormal enlargement of the heart chambers e.g. left ventricle in severe AS.
Electrocardiography measures the electrical impulses of the heart and can identify any abnormal heart rhythms e.g. VPCs (ventricular premature contractions) in severe AS.
Ultrasound with Doppler studies:
Ultrasound enables the vet to measure all the internal dimensions of the heart, in particular the thickness of the left ventricular wall with regard to aortic stenosis.
Doppler studies allow the vet to measure the speed of blood flow through the heart valves i.e. from one chamber to another or, in the case of aortic stenosis, specifically from the left ventricle to the aorta via the aortic valve; this is the aortic outflow velocity.
 DISEASE PROGRESSION
In the majority of cases, development of the defect does not usually progress beyond puppyhood i.e. the grading of the heart murmur would be unchanged from the age of 1 to 2 years throughout the rest of the dog’s life. However, in a small number of dogs (estimated to be about 10% of those affected) the defect appears to get progressively worse over the years. This certainly gives weight to the argument that annual auscultation should be considered in those breeds known to be affected by AS.
 SYMPTOMS
This hereditary heart defect may be mild and not affect the quality or longevity of the dog’s life, or it may be severe and result in symptoms, such as exercise intolerance and syncope (fainting). It is also one of the causes of sudden death.
Symptoms can occur [a] as a direct result of the defect or [b] secondary to left ventricular hypertrophy.
[a] A severe defect can significantly compromise the outflow of oxygenated blood from the heart to the brain and other vital organs.
[b] Over time, the walls of the left ventricle may become „hypertrophied” (thickened). This is the left ventricle „compensating” for its increased workload. If there is excessive hypertrophy, dogs can then develop abnormal heart rhythms; these VPCs (ventricular premature contractions) can compromise the delivery of oxygenated blood from the left ventricle through the aortic valve and into the aorta.
Syncope (fainting) occurs when there is a lack of oxygen to the brain.
If the stenosis (narrowing) is severe, this can result in significantly restricted outflow of oxygenated blood from the heart and, therefore, decreased oxygen supply to the brain.
VPCs are abnormal heart rhythms, which can be initiated by stress or exertion, whereby the left ventricle contracts too early i.e. before it has filled properly. This can also result in restricted outflow and decreased oxygen supply to the brain.
Sudden death can occur after a „run” of VPCs, which then cause the heart to go into a state of ventricular tachycardia. Instead of pumping normally, the left ventricle pumps too quickly and is therefore unable to effectively deliver oxygenated blood into the aorta, and to the brain and other vital organs, resulting in collapse and death.
 TREATMENT OPTIONS
General veterinary advice to the owners of dogs with AS
   ~ Avoid strenuous exercise for the dog, especially in hot weather.
   ~ Do not allow the dog to become overweight.
These are sensible precautions, which avoid putting excessive strain on the dog’s heart.
Medication
Beta-blockers may be prescribed for the following:
   ~ Dogs with severe aortic stenosis.
   ~ Dogs with evidence of left ventricular hypertrophy.
   ~ Dogs with evidence of VPCs.
Beta-blockers help to improve exercise tolerance, reduce cardiac (heart) workload and prevent abnormal heart rhythms.