Tricuspid valve dysplasia
The atrioventricular (AV) valves in the heart ensure that the blood flows in the correct direction inside the heart, from the atria to the ventricles, when the heart beats. When the AV valve in the right side of the heart -the tricuspid valve- is malformed at birth (called dysplasia of the valve), blood flow through the heart is less efficient: with each heartbeat, a portion of the blood that is meant to travel in the normal direction instead spills backward to where it just came from. This process, called tricuspid valve insufficiency or tricuspid valve regurgitation, requires the heart to work harder to overcome this inefficiency.
Tricuspid valve dysplasia is the most common birth defect of the heart in Labrador retrievers (yellow, black, and chocolate). It is inherited as an autosomal dominant trait with variable penetrance; it affects male and female puppies equally, it can be transmitted to pups from the sire or the dam (or both), and the degree of severity is unpredictable: some pups inherit it as a severe, life-shortening disorder whereas others have a mild form that never causes symptoms (or they may be completely normal in every detectable way but carry the genetic defect that leads to tricuspid dysplasia in later generations). The genetic defect for tricuspid valve dysplasia is found on chromosome 9 in dogs.
The main determinant of the impact of tricuspid dysplasia is the degree of valve malformation. Dogs with a mildly or even moderately malformed tricuspid valve routinely live normal lifespans. However, dogs with severe tricuspid valve malformations, even as pups, may develop symptoms of congestive heart failure, especially a bloated, pot-bellied appearance due to fluid pooling in the abdomen (ascites), difficulty breathing due to fluid retention in the chest cavity (pleural effusion), or both. Such severely affected dogs require medications to reduce the impact of the problem and maintain an acceptable quality of life.
The veterinarian may detect a heart murmur long before an affected dog is showing any outwardly visible signs associated with tricuspid valve dysplasia. If a veterinarian detects a heart murmur and the murmur persists for more than 2-3 weeks, further investigation is always warranted. Tests can pinpoint tricuspid valve dysplasia as the problem and determine its degree of severity. Such tests generally include thoracic radiographs (X-rays of the chest) and an echocardiogram, also called sonogram of the heart, or cardiac ultrasound. Both are noninvasive procedures that can be performed awake or under mild sedation in virtually all dogs. The underlying problem (malformation of the tricuspid valve) as well as its impact (degree of distortion of surrounding heart chambers, for example) can be identified if present. This information helps determine whether treatment is necessary and whether the outlook is good, fair, or poor.
Mild and even moderate cases of tricuspid dysplasia usually do not require any treatment at all. Mild exercise restriction may be wise, to reduce the strain on the tricuspid valve (which is at its worst during bursts of intense physical activity). Surgical replacement of the tricuspid valve is not feasible in dogs as it is in people; therefore, pre-emptive/early-stage treatment is not appropriate in the dog. Rather, dogs with tricuspid valve dysplasia should be observed at home for signs of abdominal enlargement or difficulty breathing. If such symptoms occur, then a recheck with the veterinarian is warranted, both to confirm that the symptoms are due to the heart (there are many noncardiac disorders that can mimic these symptoms) and to begin medication immediately if confirmed.
- MURMUR: soft to loud holosystolic murmur over the tricuspid valve and right apex area (fourth intercostal space at costochondral junction). Failure to auscult carefully over the right side of the chest is a common reason for underdiagnosis of tricuspid dysplasia, and a meticulous auscultation in this region is warranted, especially in Labradors.
- ELECTROCARDIOGRAM: right atrial and ventricular enlargement patterns are possible (tall P waves and right axis deviation, respectively). Atrial arrhythmias, especially atrial fibrillation, are common. Ventricular conduction disturbances may be seen, notably splintering of the QRS complex.
- RADIOGRAPHS: evidence of right atrial and ventricular enlargement is almost always present in moderate cases and these changes may be dramatic in severe cases. Pulmonary undercirculation is possible in severe cases. Pleural effusion is possible with right-sided congestive heart failure.
- ECHOCARDIOGRAPHY: abnormal location, shape, movement, or attachment of the valve apparatus are the hallmarks of this disorder. Doppler assessment will show abnormal flow in most cases - a regurgitant jet, evidence of valvular stenosis, or both.
- OTHER: With right-sided congestive heart failure, jugular distension with or without jugular pulsation, cool extremities, dyspnea, hepatomegaly, and/or ascites may be apparent on physical examination.
Affected individuals and their parents should not be used for breeding. Siblings should only be used after careful screening, and their offspring should be evaluated thoroughly (echocardiography).
One obstacle to controlling tricuspid valve disease in the dog population in general and in specific breeds in particular is that overt symptoms are generally not evident until after a dog has reached breeding age. However, a heart murmur can often be detected long before the onset of symptoms. Breeders are encouraged to select mature rather than young dogs for breeding, and to use them only once they have been certified free of murmurs, preferably by a veterinary cardiologist (see www.acvim.org or www.ecvim-ca.org for directories of veterinary cardiologists in North America and Europe, respectively).
There is widespread agreement regarding echocardiography (cardiac ultrasound) for dogs that have murmurs: only by having an echocardiogram is it possible to tell whether the murmur comes from tricuspid dysplasia or any of dozens of other defects, many of which are harmless. However, controversy exists regarding whether all Labrador dogs, with or without heart murmurs, should have an echocardiogram at some point in their lives prior to being used for breeding. The advantage is the opportunity to identify "silent" (no murmur) tricuspid dysplasia and reduce its transmission through the gene pool; the drawback is the time and cost needed to have an ecocardiogram performed.
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
Wright KN. Tricuspid valve dysplasia. In Cote E, ed. Clinical Veterinary Advisor: Dogs and Cats, 2nd ed. (St. Louis, MO: Mosby Elsevier, 2011) pp. 1117-1119.
Adin DB. Tricuspid valve dysplasia. In Bonagura JD, Twedt DC, eds. Kirk's Current Veterinary Therapy XIV (St. Louis, MO: Saunders Elsevier, 2008) pp. 762-765.
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