6 Acquired Valvular Disease
Canine valvular disease most often takes the form of age-related degeneration, and, in fact, degenerative valvular disease is the most common acquired cardiac disease in the dog. This disorder is sometimes known as myxomatous valvular degeneration, and this term arguably most accurately describes its pathology. Some of the many synonyms include endocardiosis, chronic degenerative mitral valve disease, and, simply, chronic valve disease. Inflammatory valvular disease—infective endocarditis—is much less common, but is the only other important acquired valvular disease in the dog.
The mitral valve apparatus consists of the mitral valve leaflets and the supporting structures: the left ventricular papillary muscles, chordae tendineae, and the mitral valve annulus. The mitral valve annulus is the fibrous ring that forms the left atrioventricular junction and the site at which the basilar aspect of the mitral leaflets are attached. There are two mitral valve leaflets. The septal, or anterior, leaflet has fibrous continuity with the aortic valve and forms the caudal boundary of the left ventricular outflow tract; the caudal or mural leaflet is more often referred to as the posterior mitral valve leaflet.
In a healthy animal, the leaflets are thin sheets of connective tissue covered by cardiac endothelium. The free edges of the leaflets are tethered by the chordae tendineae to the two left ventricular papillary muscles. These chords serve to anchor the mitral leaflets; during systole, they prevent prolapse of the leaflets into the left atrium. The function of the mitral valve is straightforward; it prevents the regurgitation of blood into the left atrium during ventricular systole and thus ensures that the force of ventricular contraction results only in useful flow into the aorta.
Degenerative valvular disease is characterized by the accumulation of mucopolysaccharides within the spongiosa and fibrosa layers of the mitral leaflets; histologically, this lesion is known as myxomatous degeneration. Grossly, the leaflets are abnormally thick and have a nodular appearance. Lengthening of the chordae tendineae is also observed. Both atrioventricular valves can be affected; however, degenerative disease affecting the mitral valve is of greater clinical importance than degenerative tricuspid disease. Endocardiosis is a sterile degenerative process; inflammation or infection is not a feature.
Degenerative mitral valve disease (MVD) commonly affects chondrodystrophoid canine breeds that also have a high prevalence of collapsing trachea and intervertebral disc disease; consequently, it has been suggested that MVD is but one manifestation of a more generalized connective tissue disorder. The cause of MVD is unknown, although a genetic basis is likely. MVD is common in certain breeds of dog; in the Cavalier King Charles Spaniel, the age at which MVD becomes clinically apparent has a heritable basis.
Distortion of the mitral valve leaflets prevents normal coaptation of the mitral valve leaflets; the consequence is mitral valve regurgitation. Lengthening and rupture of the mitral chordae tendineae predisposes to mitral valve prolapse, which can also contribute to mitral valve incompetence. Inadequate leaflet apposition allows a portion of the left ventricular stroke volume to be ejected backward into the left atrium. This regurgitant volume is augmented by the pulmonary venous return and then returns to the left ventricle during diastole. Incompetence of the mitral valve therefore imposes a volume overload on the left atrium and the left ventricle. Volume loading of the left heart is a stimulus for eccentric hypertrophy (hypertrophy accompanied by dilation).
Small volumes of mitral valve regurgitation (MR) are usually well tolerated, but two factors can potentially contribute to worsening of MR. Although the speed at which it does so is highly individual, MVD is generally progressive; ongoing alterations in valvular structure worsen mitral valve incompetence. Additionally, atrial/ventricular dilation itself distorts valvular anatomy and this further limits leaflet coaptation worsening MR. Systolic myocardial function of the left ventricle is not obviously affected by MR unless the valvular lesion is severe and longstanding. When substantial MR is present, the isovolumic phase of ventricular contraction does not take place because the ventricle can unload into the low-pressure reservoir provided by the left atrium. Therefore, myocardial oxygen demand is relatively low in the setting of MR; as a result, MR tends to be a lesion that is well tolerated at least in the sense that contractility can be preserved until late in the natural history of the disease. Longstanding, severe MR can, however, result in myocardial cell death, replacement fibrosis, and myocardial dysfunction that is known as cardiomyopathy of overload.
MVD is a degenerative disease that develops primarily in older dogs. Valvular lesions may develop in dogs of virtually any breed; however, the clinical consequences of MVD are observed almost exclusively in older dogs of the toy and small breeds.
The prevalence of MVD is not known with certainty, but it can be as high as 33% in dogs older than 10 years. It is the most common cardiac disease in the dog. Miniature and Toy Poodles, Pomeranians, Bichons Frises, Miniature Schnauzers, Dachshunds, Cavalier King Charles Spaniels, and mixed-breed dogs are commonly affected.
Mitral valve disease in the Cavalier King Charles Spaniel is indistinguishable from MVD that affects other breeds. However, MVD in this breed is distinctive in that the disease becomes clinically evident at a relatively early age and in some individuals the disease is severe and rapidly progressive. In other breeds, clinical signs related to MVD are uncommon in dogs that are younger than about 8 years. In the Cavalier King Charles Spaniel, murmurs of mitral valve regurgitation are occasionally encountered in dogs as young as 2 and 3 years. Pedigree studies have demonstrated that the tendency to develop mitral valve lesions at an early age is heritable; the age of onset of MR in individual dogs can be approximately predicted by the age at which MR became evident in the parents.
It should be emphasized that murmurs resulting from MVD are more common than clinical signs related to the disease. Many animals have mild and only slowly progressive valvular lesions and ultimately succumb to extracardiac disease before the development of clinically consequential MR.
When MR is substantial, it imposes a volume overload on the left atrium and left ventricle. Elevated left atrial pressures are reflected back on the pulmonary venous circulation, potentially resulting in the development of pulmonary edema. The presence of cardiogenic pulmonary edema defines the clinical syndrome of left-sided congestive heart failure (CHF).
Cough is the clinical sign that is most commonly associated with MVD. Respiratory distress is usually a feature of the history of those animals that have pulmonary edema. Other clinical signs such as syncope, ascites, weight loss, lethargy, and lack of appetite can also be observed.
The cause of cough in dogs with MVD is multifactorial. Cough is an explosive, reflex-mediated exhalation, and the receptors that initiate this reflex are located primarily in the larger airways. Pulmonary edema can stimulate cough receptors, but generally does so when edema fluid floods the small airways; when pulmonary edema is the cause of cough, dyspnea is usually evident.
Compression of the left main stem bronchus by an enlarged left atrium is another potential cause of cough in dogs with MVD. Cough from bronchial compression resulting from left atrial enlargement seems to be unique to the animal population that has MVD. This might be partly explained by the high prevalence of concurrent primary airway diseases such as collapsing trachea in the dogs that commonly have MVD. The stimulation of juxtapulmonary receptors might also play a role in the pathogenesis of cough in some dogs with MVD. Stimulation of juxtapulmonary or J receptors by pulmonary venous distention results in reflex-mediated bronchoconstriction and increases in mucus production. Stimulation of J receptors might therefore contribute to cough in animals that have MVD but do not have pulmonary edema.
Regardless, it is important to recognize that animals with MVD can have a cough in the absence of CHF; this cough can be explained by cardiac disease, although in some cases concurrent primary respiratory tract disease plays a role.
MVD becomes clinically apparent when it results in a cardiac murmur. Mitral valve regurgitation causes a systolic murmur that is usually heard best over the left cardiac apex. The correlation between murmur intensity and MR severity is imperfect; however, mild MR resulting from MVD generally results in a soft murmur, whereas severe MR usually results in a loud murmur. A soft murmur is typically evident early in the course of MVD; the murmur becomes louder as the animal ages and has progressively more severe MR. Mitral regurgitation continues as long as left ventricular pressure exceeds left atrial pressure; as a result, MR can persist beyond aortic valve closure and the murmur may therefore obscure the second heart sound. The murmur of moderate or severe MR has an intensity that typically changes little during the course of systole and is said to have a plateau-shaped configuration.
In small-breed dogs with MVD, heart disease or heart failure can explain cough; in some cases, however, the murmur of MR is incidental to the clinical presentation and the cough results from primary respiratory tract disease, such as collapsing trachea or chronic bronchitis. Although primary respiratory tract disease can certainly coexist, one of the two often dominates the clinical presentation.
A history of months or years of cough that occurs in the absence of dyspnea tends to support a diagnosis of airway disease. When MVD is sufficiently severe that it causes clinical signs, it is generally progressive. Therefore, untreated animals in which cardiac disease contributes importantly to cough tend to have a relatively short history; the clinical course often progresses to include dyspnea.
The body condition of the animal can provide useful clues. Animals that cough from heart disease or heart failure are often thin. Although exceptions certainly occur, obesity is more commonly associated with primary respiratory disease. The vital signs may also be useful. Healthy dogs often have a respiratory-associated arrhythmia that is evident on auscultation. In accordance with phasic variations in autonomic traffic, heart rate increases during inspiration and decreases during expiration. This respiratory sinus arrhythmia results primarily from fluctuations in vagal tone. When cardiac performance is impaired by severe MR, vagal discharge is inhibited and sympathetic tone becomes dominant. Thus in many animals with clinical signs related to cardiac disease, tachycardia develops and there is loss of physiologic, respiratory arrhythmia; the clinical finding of respiratory sinus arrhythmia is virtually incompatible with a diagnosis of heart failure. In contrast, many animals that cough mainly from primary respiratory disease have preserved and sometimes accentuated sinus arrhythmia.
In geriatric small-breed dogs, the absence of a cardiac murmur is generally an assurance that coughing results from primary respiratory tract disease. Soft murmurs resulting from MVD are seldom of clinical consequence. In contrast, animals that have clinical signs related to MVD almost always have loud cardiac murmurs.
It should be emphasized that these are guidelines only and exceptions do occur. Diagnostic studies, most particularly radiography, provide information that can help to solve the dilemma. As a general rule, however, respiratory disease is likely to be responsible for a chronic cough in an overweight dog with a soft murmur and preserved sinus arrhythmia. In contrast, heart disease, or even heart failure, is more likely responsible for cough in a thin animal with a loud murmur and tachycardia.
The appearance of the thoracic radiograph in animals with MVD is highly variable. Animals with mild MR may have a normal cardiac silhouette. Moderate or severe MR results in radiographic cardiomegaly that has a left-sided emphasis. Left ventricular enlargement typically accompanies left atrial enlargement, although the latter is more noticeable radiographically. Left atrial enlargement is evidenced by separation of the main stem bronchi and, in the lateral projection, loss of the “caudal waist” of the cardiac silhouette (Fig. 6-1). Engorgement of the pulmonary veins is sometimes observed in animals with elevated left atrial pressures, although this sign is inconsistent. The presence of pulmonary opacities in association with radiographic left atrial enlargement is diagnostic of CHF.