CDVD is the most common cause of cardiac disability in dogs. The disease process is best described as myxomatous degeneration of the heart valves wherein the integrity of the valves is compromised often resulting in valvular insufficiencies. In a necropsy study performed by JW Buchanan the mitral valve alone was affected in 62% of cases, the mitral and tricuspid valve in 33%, mitral and aortic valves in 3%, mitral, tricuspid and aortic in 1% and tricuspid valve alone in 1%. Small breed dogs such as toy and miniature Poodles, miniature Schnauzers, Pomeranians, Chihuahuas, cocker spaniels, Pekingese, fox terriers, and Boston terriers appear predisposed to the development of CDVD while males are more frequently affected than females (1.5:1.0).

Pathophysiology

During the initial stages of the disease there is no valvular insufficiency so there is no hemodynamic change or murmur ausculted, and the patients are entirely asymptomatic. A systolic click, possibly from mitral valve prolapse, is sometimes ausculted in these patients. As the lesions progress and the valves become incompetent, a systolic murmur results at the affected valve site and atrial pressure begins to rise. In the compensated patient, left atrial enlargement and eccentric hypertrophy of the left ventricle maintain normal cardiac output for an indefinite period of time, often months to years. Eventually, left atrial and pulmonary venous pressures rise resulting in pulmonary venous congestion and pulmonary edema.

Most of the early signs of mitral regurgitation result from pulmonary congestion and owners seek treatment for their dog after noticing coughing or respiratory distress. Coughing is a common but nonspecific sign of developing heart failure in dogs. Many older, small breed dogs cough because of chronic bronchitis or from a collapsing trachea. When due to heart failure, coughing is usually accompanied by an elevated respiratory rate (tachypnea) and increased respiratory effort (dyspnea). Some dogs with congestive heart failure become restless at night, are unable to sleep and may experience orthopnea, that is, they have more difficulty breathing lying down than sitting or standing.

Some dogs with CDVD develop signs of right heart failure due to degeneration of the tricuspid valve, as a consequence of pulmonary hypertension, or a combination of these disorders. Regardless of the cause, the development of abdominal distension and ascites is usually accompanied by signs of low output heart failure and a decline in the severity of pulmonary congestion. Generalized muscle weakness and progressive exercise intolerance become evident when forward output is impaired by severe valvular regurgitation, pulmonary hypertension, and/or declining myocardial contractility. Syncope tends to occur primarily when cardiac output declines precipitously.

Physical examination

Cardiac auscultation by a skilled examiner is the most practical and economical diagnostic method for detecting mitral regurgitation. The classic murmur of mitral regurgitation is holosystolic, constant in intensity (plateau-shaped), of mixed frequency, and loudest at the left apex. A soft, grade 1/6, systolic murmur over the mitral area is the first clinical sign of CDVD. A mid or late systolic click or a click-murmur can sometimes be auscultated prior to the development of a holosystolic murmur. There is a strong relationship between murmur intensity, heart size, the severity of regurgitation, and NYHA class of heart failure. Dogs with a murmur of grade 3/6 or less and a clearly audible second heart sound are likely to have mild disease. With the development of more severe disease, the murmur becomes louder, the second heart sound becomes softer or inaudible, and a gallop heart sound may be auscultated. The murmur of mitral regurgitation is usually best heard at the left fifth intercostal space, but dorsal, cranial, caudal, or rightward radiation of the murmur is common. It is particularly difficult to determine whether murmurs heard over the tricuspid valve area originate from that valve or if they are referred from an incompetent mitral valve. Cardiac arrhythmias can be readily identified as they interrupt the predominating cadence of the heart, create abnormal pauses in the rhythm, and alter the intensity of both murmurs and transient heart sounds.