Degenerative Valve Disease
(Endocardiosis, Chronic valvular disease, Chronic valvular fibrosis) |  |
| This acquired disease is characterized by nodular thickening of the cardiac valve leaflets, most severely at their free margins. The most commonly affected valves are the mitral or tricuspid valve leaflets. The etiology is unknown; however, in Cavalier King Charles Spaniels (which are prone to this disease), it is believed to be an inherited trait possibly resulting in a collagen abnormality. Degenerative valve disease is the most common cardiac disease in dogs and accounts for
~75% of cardiovascular disease in this species. Approximately 60% of affected dogs have myxomatous degeneration of the mitral valve, 30% have lesions in both the tricuspid and mitral valves, and 10% have only tricuspid valve disease. In dogs, the disease is age- and breed-related, with older, small-breed dogs demonstrating a higher incidence. There is also a slight predisposition among male dogs. Large animals are also affected by this disease (most commonly affecting the mitral
valve leaflets), however it is uncommon in cats. In horses, degenerative valve disease often affects the aortic valve and consists of valvular nodules or fibrous bands at the free borders of the valve. This condition is most common in middle-aged and older horses. In many horses, unlike dogs, clinical signs are uncommon because significant left ventricular volume overload and dilation do not occur. |
| Insufficiency of the AV valve results in turbulent, systolic (ie, during ventricular contraction) flow at the affected valve. This regurgitation of blood into the atrium results in an increase in pressure within the atrium. If the mitral valve is affected, the elevated left atrial pressures eventually result in elevated pulmonary capillary pressures and, ultimately, pulmonary edema. If the tricuspid valve is affected, elevated systemic venous pressures arise and ascites may
develop. The constant, high-velocity, regurgitant jet of blood at the affected mitral valve physically damages the endocardium of the left atrium, resulting grossly in jet lesions and, in severe cases, may result in left atrial rupture. The decrease in the amount of blood ejected by the left ventricle (cardiac output) results in an activation of compensatory mechanisms such as the RAAS (
Compensatory Mechanisms). The body responds to decreases in cardiac output by increasing
sympathetic tone and activating angiotensin-converting enzyme (ACE). On a chronic basis, these compensatory mechanisms become deleterious rather than beneficial. Chronically increased sympathetic tone causes sustained tachycardia, which increases the oxygen demand of the heart and predisposes to arrhythmias. ACE activation results in the formation of angiotensin II, which causes sustained arteriolar constriction and increased aldosterone. Vasoconstriction increases cardiac afterload,
hampering ventricular ejection of blood. Aldosterone release results in sodium and water retention and predisposes to pulmonary edema. |
| In dogs there are no clinical signs in the early stages of the disease, although a systolic murmur of low intensity (grade I-II/VI) can be heard with maximum intensity at the left apex. As the disease progresses, exercise intolerance, increased respiratory rate and effort, and cough develop. Syncope may also occur secondary to either compromised cardiac output or, more likely, a transient cardiac arrhythmia. Sudden death is rare, but may occur secondary to left atrial rupture
caused by severe and chronic mitral regurgitation. With continued progression, the murmur intensity generally increases (up to a grade VI/VI systolic murmur); however, the intensity does not always coincide with disease severity. Physical examination findings in patients that have developed left-sided CHF include respiratory crackles and wheezes and dyspnea. If tricuspid valve degeneration is significant, signs of right-sided CHF may be noted (eg, ascites, jugular pulses). |
| A CBC, serum chemistry profile, and urinalysis are usually within normal limits. Left atrial enlargement is the characteristic finding on thoracic radiographs of a patient with myxomatous degeneration of the mitral valve. Other changes include enlargement of the left ventricle and pulmonary veins. As heart failure develops, increased interstitial density to the pulmonary parenchyma occurs and ultimately air bronchograms (indicative of an alveolar pattern and severe pulmonary edema)
appear. Echocardiography demonstrates a thickened, enlarged, and irregular valvular leaflet of normal echogenicity. Chordae tendineae may be ruptured, and the AV leaflets may prolapse into the atrium during ventricular contraction. Ventricular enlargement is common. In most cases, contractility is normal initially, but left ventricular fractional shortening is increased due to off-loading of some of the left ventricular ejection into the low resistance left atrium rather than the
high resistance aorta. A decrease in contractility suggest the presence of myocardial failure. Electrocardiographically, asymptomatic patients with early degenerative valve disease demonstrate a normal sinus arrhythmia or normal sinus rhythm. As the disease progresses, and especially when CHF develops, left atrial enlargement promotes the occurrence of atrial arrhythmias such as atrial premature complexes and atrial fibrillation. Cardiac hypoxia may result in ventricular arrhythmias.
There may be evidence of left atrial enlargement (P-mitrale or widened P waves) and left ventricular enlargement (tall and widened R waves) when the mitral valve is involved. |
| A recent, large study in asymptomatic Cavalier King Charles Spaniels with degenerative valve disease demonstrated no reduction in time to onset of CHF with use of ACE inhibitors. Thus, treatment in small breed dogs should be reserved for symptomatic animals or those demonstrating cardiogenic pulmonary edema on thoracic radiographs. Treatment for early signs of CHF includes ACE inhibitors to reduce adverse neurohormonal effects caused by activation of the RAAS, and to reduce mitral
regurgitation and signs of pulmonary edema. Control of pulmonary edema is also accomplished by use of diuretics (furosemide is the drug of choice). Abnormal arrhythmias such as atrial fibrillation or other severe supraventricular arrhythmias, if present, should either be resolved or rate controlled with use of digitalis glycosides and, if needed, calcium channel blockers or β-blockers to improve cardiac output and reduce signs of CHF. Another indication for digitalis glycosides is
the presence of myocardial failure. Optimal therapy should be planned for each stage of disease. In acute and severe CHF, oxygen and nitroglycerin ointment, along with aggressive parenteral furosemide and other medication previously mentioned, would be warranted. Affected dogs can live for years with appropriate therapy. |
