Case Studies In Small Animal

Cardiovascular Medicine

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Case 23

Tricuspid Valve Dysplasia Section from "Small Animal Cardiovascular Medicine" On-Line

Text from "Small Animal Cardiovascular Medicine

TRICUSPID VALVE DYSPLASIA

Tricuspid valve dysplasia (TVD) is defined as congenital malformation of the tricuspid valve leaflets, chordae tendineae, and/or papillary muscles. It is an uncommon congenital heart disease, seen in both dogs and cats. A specific type of congenital tricuspid valve malformation seen in humans in which the basal attachments of the tricuspid valve are displaced ventrally into the right ventricle is called Ebsteinís anomaly. This anomaly is rare in dogs and not reported in cats.

Pathology

Tricuspid valve dysplasia is not one type of lesion. Rather, numerous abnormalities can occur. These abnormalities almost always result in tricuspid valve regurgitation. Described lesions in humans include focal or diffuse thickening of the valve leaflets, underdevelopment of chordae tendineae and papillary muscles, incomplete separation of valve components from the ventricular wall, and focal agenesis of valvular tissue. In dogs and cats, chordae tendineae are commonly absent or very short. Consequently, the papillary muscles often attach directly to the valve leaflets. The leaflets are often irregularly thickened and may contain fenestrations. Leaflets, especially the septal leaflet, may be directly adhered to the ventricular wall.

Other congenital heart defects may be present. These may include mitral valve dysplasia, septal defects, subaortic stenosis, and pulmonic stenosis.

Prevalence

Tricuspid valve dysplasia has been reported in numerous dog breeds including old English sheepdogs, great Danes, German shepherds, and Irish setters. We see a preponderance of Labrador retrievers in our clinic with this malformation. We identified 62 dogs with tricuspid valve dysplasia between 8/1/86 and 8/1/96. Of these, 16 were Labrador retrievers. The majority of the dogs with this abnormality were purebred. Only eight mixed breed dogs were identified. The vast majority of the dogs were large-breed dogs (>20 kg). In the same period we diagnosed tricuspid valve dysplasia in 23 cats. The majority of these cats were mixed-breed.

Clinical findings

Tricuspid valve dysplasia in dogs and cats may initially be discovered because a heart murmur is identified. The heart murmur is systolic and generally loudest over the right apex. In many cases, however, the first clue that the patient has TVD is the discovery of ascites. At the time ascites is discovered, all dogs and most cats have a heart murmur. Some cats, however, apparently have a tricuspid orifice so large that regurgitant flow is laminar and no heart murmur is present. Most dogs and cats are young (< 2 years of age) at presentation. However, we have identified TVD in cats as old as 12 years of age and dogs as old as five years of age.

Pathophysiology

The malformed tricuspid valve in TVD allows blood to leak from the right ventricle into the right atrium during systole. The increase in systolic flow into the right atrium increases right atrial volume. The right atrium expands to accommodate this increased volume. The right ventricle grows larger (volume overload hypertrophy) to accommodate the increased venous return (normal venous return plus the return of the blood ejected into the right atrium in systole) in diastole. The increased right ventricular diastolic volume allows the right ventricle to eject a larger stroke volume, to compensate for the volume lost into the right atrium and maintain a normal forward (into the pulmonary artery) stroke volume. As the right heart grows larger, the tricuspid valve annulus increases in size. Because the tricuspid valve cannot grow to maintain coaptation, the orifice in the tricuspid valve becomes larger. The cycle of increasing right ventricular size leading to worsening tricuspid regurgitation leading to further increases in right ventricular size becomes a cycle of worsening hemodynamics that ultimately culminates in massive regurgitation that overwhelms the cardiovascular systems compensatory mechanisms. Congestive right heart failure is the end-result. Right heart failure results from massive regurgitation into the right atrium resulting in increased right atrial pressure and decreased forward flow into the pulmonary vasculature. The decrease in pulmonary flow results in a decrease in venous return to the left heart. As a consequence, the left heart becomes underloaded and probably atrophies in response. The potential net result is a decrease in systemic blood flow.

Animals with severe TVD develop right heart failure. The most common manifestation of right heart failure is ascites. Tricuspid valve dysplasia is the most common cause of pure right heart failure leading to ascites in cats. Along with ascites, hepatomegaly and/or jugular vein distension may be identified. Pleural effusion may be identified on occasion but is rarely severe.

Diagnostic Tests

Thoracic radiography

Thoracic radiographs of dogs and cats with severe TVD reveal severe cardiomegaly primarily due to massive right atrial enlargement. The enlarged right atrium enlarges the entire right side of the cardiac silhouette, often pushing the left heart further to the left. The net result can be an impression of generalized cardiomegaly. More commonly, one can distinguish the markedly enlarged right atrium as a separate structure. Massive right atrial enlargement in a young animal with a right apical systolic heart murmur is pathognomonic for severe tricuspid regurgitation, usually due to TVD. Even in older animals, massive right atrial enlargement should suggest TVD, as acquired tricuspid regurgitation rarely produces such pronounced right atrial enlargement. Animals that present in right heart failure commonly have a massively enlarged caudal vena cava that can be identified on both views.

Electrocardiography

The electrocardiogram is commonly abnormal in dogs and cats with severe TVD. The most common finding is right axis deviation due to deep S waves in leads I, II, III, and aVF. Tall P waves, indicative of right atrial enlargement, may also be present. This finding is not a sensitive means of detecting right atrial enlargement, however, and absence of this finding does not rule-out right atrial enlargement in this or any other disease. Massive right atrial enlargement may lead to supraventricular tachyarrhythmias, most commonly atrial fibrillation.

Echocardiography

The most striking finding on the echocardiogram is the markedly enlarged right atrium. The right atrial size may be larger than the rest of the heart. A right ventricular volume overload is also present but never to the same degree as the right atrial volume overload. The left heart is often diminutive, with both the end-diastolic and end-systolic diameters smaller than normal. The left heart can be so small that it takes some time to identify it during an examination. The valve leaflets are commonly abnormal. The septal leaflet may appear to be adhered to the interventricular septum and may have little movement. The mural leaflet may appear to be very large. The papillary muscles may be adherent to this leaflet or the papillary muscles may be malpositioned. A large turbulent jet is always present during systole in the right atrium on color flow Doppler in dogs. In some cats, laminar regurgitant flow may be identified. Laminar regurgitant flow occurs when the tricuspid valve orifice is very large in systole, probably similar in size to the pulmonic valve orifice, resulting in very little resistance to blood flow. In dogs and cats with uncomplicated TVD, the velocity of the regurgitant jet, measured with continuous wave Doppler, is in the 1.5-3 m/s range.

Contrast echocardiography using microbubbles can be performed in dogs and cats with tricuspid valve dysplasia. When the right heart is examined following injection of the saline, the bubbles remain in the right heart for a prolonged period. In severe tricuspid regurgitation, blood is pushed all the way back into the hepatic circulation in systole. Consequently, following a saline injection into a cephalic or jugular vein, bubbles can often be visualized in the hepatic veins. Retrograde hepatic vein flow can also be documented with spectral Doppler echocardiography.

Treatment

Medical Therapy. Medical therapy is palliative and aimed at improving quality of life by reducing the amount of ascites. Standard therapy consists of administering furosemide and an angiotensin converting enzyme inhibitor. If the animal is uncomfortable at the first examination, the ascitic fluid is removed. Medical therapy is commonly effective at prolonging the time until the abdomen is severely distended again but is commonly ineffective at completely stopping fluid accumulation. Consequently, periodic abdominocentesis and fluid removal is often required. Periodic removal of ascitic fluid results in removal of large quantities of protein each time the procedure is repeated. However, in our experience this rarely results in clinically significant sequela. Serum albumin concentration may decrease in these cases but rarely to less than 2.0 gm/dL. Consequently, we never discourage an owner from returning for repeated procedures. Some owners can even learn to do the procedure themselves. As opposed to dogs with left heart failure, dogs with right heart failure commonly feel good and so have a good quality of life between procedures. Ultimately, however, the interval becomes too short for most owners to tolerate and euthanasia becomes a viable option.

Surgical Therapy. Surgical therapy is usually a poor option in our experience. We have observed the clinical courses of three dogs following the replacement of the tricuspid valve with a bioprosthetic valve. In each case, the prosthetic valve has been constructed from a stent and the dogís own pericardium. In each case, extensive thrombosis and fibrosis have occurred shortly after surgery. One dog has survived with moderate tricuspid stenosis. One dog died within 24 hours of surgery. The last dog died one week following surgery with severe tricuspid stenosis and possible pulmonary thromboembolism.

©Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.