ANNE FRENCH MVB DVC MRCVS RCVS
Specialist in Veterinary Cardiology
Hospital for Small Animals,
University of Edinburgh,
Easter Bush, Roslin.
Midlothian EH25 9RG.
What is mitral valve disease?
This disease is known by many different names. The names used apply either to the heart valve affected (mitral valve disease), to the changes that are seen on the valves (endocardiosis) or to the time course of the disease (chronic valvular disease).
Mitral valve disease is the most commonly diagnosed cause of a heart murmur and heart failure in all dogs in small animal veterinary practice and is the single most common heart disease of dogs. It is a degenerative disease of the heart valves affecting mainly the mitral valve on the left side of the heart but can also affect the tricuspid valve on the right side of the heart. It causes distortion of the valves which results in leakage of the affected valves. This disease usually occurs in older dogs, however it is seen in the Cavalier King Charles breed at a very young age and may result in premature death.
What happens to the heart in mitral valve disease?
Mitral valve leakage (insufficiency) leads to dilation of the left atrium arid the left ventricle, the two chambers on the left side of the heart. If the changes occur slowly then the left atrium distends and accommodates the increased volume, however eventually the pressure within the left atrium increases and the drainage of fluid from the lungs is compromised resulting in left sided congestive heart failure. If a chordae tendinae (chords which hold the heart valves in place) ruptures then left atrial pressures increase suddenly and acute severe pulmonary congestion results. Tricuspid valve leakage leads to right atrial and right ventricular dilation If right atrial pressure increases then drainage of the liver and abdomen is compromised resulting in right-sided congestive heart failure. If not enough blood is being pumped forward because of valve leakage then the kidneys sense this change and retain fluid making matters worse. The heart muscle will eventually fail due to the chronic presence of excess fluid.
Which breeds are affected?
Any breed may be affected however the following breeds are more commonly affected:
Cavalier King Charles Spaniel, Cocker Spaniels, Poodles, Terriers, Dachshunds, Irish Setters and there seems to be a higher predisposition in males.
What are the typical presenting signs of mitral valve disease?
Chronic coughing is a common presentation due to the large left atrium pressing on the airways. Exercise intolerance is frequently noted, many dogs will tire easily or breathe heavily during exercise. Some dogs will present at annual vaccination and the murmur is an incidental finding. Other dogs will present in acute congestive heart failure with evidence of severe difficulty breathing (dyspnoea) and fluid retention in the abdomen (ascites). Fainting (syncope) is seen in some dogs in the later stages of the disease, this is due to poor forward flow of blood.
How is mitral valve disease diagnosed?
A murmur is heard by auscultation using the stethoscope over the mitral valve area. The murmur starts off as a low-grade murmur at the apex of the heart on the left side of the chest. Murmurs are graded on a scale 0-6. As the disease progresses the murmur grade increases and the murmur duration lengthens. The mitral murmur radiates to the right side and it can be impossible to distinguish a murmur of mitral insufficiency radiating to the right from a separate tricuspid murmur. In the early stages of the disease, the gums will be pink and the pulse, which is felt inside the hindleg over the femoral artery, will be strong. As the disease progresses the gums may be pale and the pulse weak. If congestive failure is present then laboured breathing may be noted or fluid in the abdomen detected.
Thoracic radiographs are useful to assess chamber enlargement and lung congestion. Radiographs of the chest will show left atrial/left ventricular enlargement with significant mitral insufficiency or in the case of tricuspid insufficiency right atrial/right ventricular enlargement. Generalised cardiac enlargement (cardiomegaly) will occur with long standing disease. Thoracic radiography is the most useful method of determining pulmonary congestion. The pulmonary veins may be congested and fluid may be seen in the lung fields (oedema). Radiographic changes may occur before there are clinical signs of left sided congestive heart failure.
The ECG will again show if the heart chambers are enlarged, however it will also show if abnormalities of heart rhythm are present. Arrhythmias are seen due to dilation and stretch of the chamber walls and due to poor oxygen supply of the heart muscle. It is not unusual to get fast rhythm disturbances (tachycardia) with mitral valve disease which may require medication.
Echocardiography is very useful to show the thickened valves and also to see if chamber dilation is present. The ability of the heart to contract and pump blood forward can also be assessed. Doppler echocardiography is a specialised form of echocardiography which allows confirmation and quantification of valvular insufficiency.
What is the prognosis for dogs with mitral valve disease?
This is a progressive disease. The rate of progression of the disease can vary markedly in different breeds and in different lines within the same breed. The prognosis is guarded once signs of congestive heart failure develop ie difficulty breathing and/or fluid retention in the abdomen. Careful regular monitoring of dogs by a veterinary surgeon, once congestive heart failure develops, can prolong life considerably and the use of a wide range of medications (normally in combination with each other) does improve survival, control clinical signs and give good quality of life in many cases but eventually medical therapy ceases to work, and with deterioration in clinical condition euthanasia is often the only and kindest option.
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