Valvular Heart Disease

Chapter 149 Valvular Heart Disease



John E. Rush, John D. Bonagura



VALVE FUNCTION AND DYSFUNCTION



Cardiac Valve Structure


The aortic and pulmonic valves separate the left and right ventricles from their respective great vessels. The semilunar valves consist of three individual leaflets that are closed during ventricular diastole and open during ventricular systole. Atrioventricular (AV) valves positioned between the atria and ventricles of the left and right heart are respectively called mitral and tricuspid valves. The left and right AV valves consist of two major leaflets (“tricuspid” is a misnomer in dogs and cats). The AV leaflets are attached via the chordae tendineae to the papillary muscles, which attach to the ventricular wall. The base of each AV valve is supported by a valve annulus. The AV valves open during ventricular diastole and close during ventricular systole.



Normal Function


The heart valves serve as one-way passages for blood and prevent retrograde (regurgitant) blood flow. The cardiac valves open and close in a cyclical pattern, and valve motion is dictated by changes in pressure within the cardiac chambers and great vessels. Competent AV valves permit the development of high ventricular pressures during systole. In a similar fashion, normally functioning semilunar valves prevent retrograde diastolic flow into the ventricles allowing for low ventricular diastolic pressures. Vibration of the heart, blood, and major vascular structures attending closure of the AV valves results in the first heart sound (S1) at the beginning of systole, and oscillation of these structures at the time of closure of the semilunar valves results in the second heart sound (S2) at the onset of ventricular diastole.



Valve Dysfunction


Cardiac valves may be dysfunctional as a consequence of congenital or acquired heart disease. General patterns of disease include:


• Obstruction to flow, termed valvular stenosis.

• Incompetent closure allowing valvular regurgitation (also called valvular insufficiency).

Valvular stenosis is almost always a congenital abnormality in animals. Valvular regurgitation may develop from congenital malformation or acquired diseases. Common anatomic and functional causes of AV valvular regurgitation are listed in Table 149-1.



Table 149-1 CAUSES OF ATRIOVENTRICULAR VALVULAR REGURGITATION*


Congenital mitral valve dysplasia (MR)


Congenital tricuspid valve dysplasia (TR)


Chronic degenerative (myxomatous) valvular disease (endocardiosis causing MR, TR)


Bacterial endocarditis (MR)


Ruptured chordae tendineae (MR, TR)


Avulsion of the papillary muscle (MR, TR)


Transmural myocardial infarction (MR)


Causes of ventricular or atrial dilation leading to AV valve regurgitation


Patent ductus arteriosus (MR)


Ventricular septal defect/endocardial cushion defect (MR, TR)†


Atrial septal defect (TR)


Aortic regurgitation (MR)


Pulmonary regurgitation (must be severe to cause TR)


Myocarditis—for example, parvovirus, Chagas’ disease (MR, TR)


Dilated cardiomyopathy (MR, TR)


Intermediate/restrictive cardiomyopathy (MR, TR)


Atrial muscular dystrophy—“silent atrium” (MR, TR)


Right ventricular cardiomyopathy—RV dysplasia (TR)


Hyperdynamic circulation—AV fistula, anemia, hyperthyroidism (MR, TR)


Chronic bradyarrhythmia—for example, complete AV block (MR, TR)


Pulmonary hypertension, including heartworm disease and chronic left-sided congestive heart failure (TR)


Causes of left ventricular hypertrophy (causing MR)


Subaortic stenosis


Hypertrophic cardiomyopathy


Systemic hypertension—for example, chronic renal disease


Hyperthyroidism


Acromegaly


Causes of right ventricular hypertrophy (causing TR)


Pulmonic stenosis


Pulmonary hypertension


Arrhythmic causes (MR and TR)


Cardiac arrhythmias preventing synchronous closure of AV valves—for example, ventricular premature beats


MR, mitral regurgitation; TR, tricuspid regurgitation.



* While this list is extensive, note that the most common causes of atrioventricular (AV) regurgitation are AV dysplasia, myxomatous degeneration, cardiomyopathy (any form), endocarditis, and hyperthyroidism (cats).


† May include mitral cleft or other AV valve malformation.




Key Point


Because the AV valve consists of multiple anatomic components, disease of any portion of the apparatus or geometric changes in the ventricle can lead to AV valvular regurgitation.



Cardiac Murmurs


A hallmark clinical finding in valvular heart disease is a cardiac murmur. Murmurs are produced by high velocity and turbulent blood flow across the valve as the blood moves from a higher to a lower pressure chamber or vessel. With few exceptions, the absence of a cardiac murmur usually excludes the presence of clinically significant valvular disease (see Chapter 142).



• Semilunar valve stenosis or AV valve insufficiency leads to systolic murmurs.

• Semilunar valve insufficiency and AV valve stenosis leads to diastolic murmurs. In general, diastolic murmurs are softer than systolic murmurs of similar hemodynamic consequence.

• Cardiac murmurs may not be evident in the following situations:
• Trivial valve regurgitation or stenosis. (Note: trivial stenosis or insufficiency may be relevant in breeding soundness examinations.)

• Dynamic ventricular outflow tract obstruction in cats with hypertrophic cardiomyopathy or in dogs with subtle mitral valve malformation. Murmurs are often labile in these situations.

• Massive, “wide-open” AV valve regurgitation in which the atrium and ventricle act as a common chamber and there is minimal pressure difference across the valve.

• Animals with very low cardiac outputs.

• Heart sounds muffled by lung sounds, pleural or pericardial fluid, or pneumothorax.


VALVULAR DISEASES OF CLINICAL IMPORTANCE



Chronic Valvular Disease


Chronic valvular disease occurs primarily in dogs. This progressive, degenerative disease is also referred to as endocardiosis, chronic degenerative valvular heart disease, chronic “myxomatous” valvular heart disease, or chronic mitral and tricuspid valvular fibrosis.




Key Point


Chronic valvular disease, or endocardiosis, a degenerative disorder of the cardiac valves, is the most important cause of heart disease in dogs.



Congenital Aortic Stenosis


This lesion usually is a subvalvular fibrous obstruction, although the lesion may extend to the valve proper (see Chapter 154 for a discussion of congenital valvular disorders).



Congenital Pulmonic Stenosis


A variety of anatomic lesions may contribute to narrowing of the right ventricular outflow tract, pulmonic valve, or pulmonary artery (see Chapter 154).



Congenital Mitral and Tricuspid Valve Dysplasia


Anomalies of the chordae tendineae, papillary muscles, valve cusps, or valve annulus can lead to incompetency or, less commonly, to stenosis of the valves (see Chapter 154).



Infective (Bacterial) Endocarditis


This condition almost always involves the left-sided heart valves in dogs and cats. Destruction of valve tissue commonly results in valvular insufficiency, although large vegetation(s) or valvular fusion may obstruct blood flow and create stenosis.



AV Valvular Regurgitation Due to Cardiomegaly or Cardiomyopathy


Valvular regurgitation may develop secondary to ventricular or atrial dilation or ventricular hypertrophy related to a variety of causes (see Table 149-1). For example, acquired mitral regurgitation in cats is typically caused by a form of cardiomyopathy.



CHRONIC VALVULAR DISEASE (ENDOCARDIOSIS) IN DOGS


Chronic valvular heart disease is a degenerative disorder of unknown cause affecting the endocardial and subendocardial portions of the valve leaflets, primarily in middle-aged and older dogs. It is not a consequence of endocarditis, although there is good evidence for a genetic predisposition to develop the disease in a number of canine breeds. The AV valves are principally involved, with the mitral valve affected in nearly 100% of cases, the tricuspid in 34%, and the aortic valve in a smaller proportion of cases based on pathology studies.



Pathology



• The valve leaflets in advanced disease are thickened and distorted by nodular changes in the free edge and base of the valve. The valve surface glistens and the endocardium is grossly intact, distinguishing this degenerative change from endocarditis. In some dogs the valve surface may be increased with redundant connective tissue and billowing of a portion of the leaflets towards the atrium. Histological lesions are myxomatous in nature, characterized by deposition of acid-staining glucosaminoglycans within the valve leaflet and cusp. Collectively these lesions predispose to valvular regurgitation, which is typically slowly progressive in nature.

• The chordae tendineae are thickened near the valve attachment, and chordal stretch or rupture may be observed. Stretch predisposes to valve prolapse. Sudden rupture of a chord may lead to a flail leaflet and peracute congestive heart failure.

• Secondary dilation of the left atrium and ventricle develops in dogs with mitral valvular endocardiosis. Ventricular hypertrophy is of the eccentric type (with dilation of the ventricle) in isolated chronic valvular disease. The mitral valve annulus dilates as the ventricular dimensions increase, leading to further regurgitation and left atrial enlargement. In cases complicated by intercurrent systemic hypertension, there may be inappropriate thickening of the left ventricular wall.

• Focal subendocardial myocardial fibrosis may be evident, especially involving the papillary muscles. These fibrotic lesions may be related to narrowing of small, intramural coronary arteries.

• Endocardial sclerosis develops in the left atrium secondary to mechanical irritation from high velocity, regurgitant streams of blood. These “jet lesions” consist mainly of fibrous tissue. Fresh or healed linear left atrial tears commonly are found at necropsy, but rupture of the left atrium with pericardial tamponade rarely is encountered.

• The right ventricle and right atrium can become dilated because of concomitant tricuspid regurgitation, often complicated by pulmonary hypertension from left ventricular failure or intercurrent lung disease.


Pathophysiology


Endocardiosis represents a progressive process and does not cause detectable signs during the earliest period of structural changes. Progressive valvular distortion leads to detectable valvular insufficiency with accompanying cardiac enlargement. Mitral regurgitation (MR) causes left ventricular volume overload, left heart failure, pulmonary hypertension, and predisposes to cardiac arrhythmias (Fig. 149-1). However, heart failure does not develop in all dogs. The entire process usually requires many years, although some breeds (e.g., Cavalier King Charles spaniel) are affected relatively early in life and may have a rapid progression to heart failure.


image

Figure 149-1 Diagrammatic representation of mitral regurgitation. Consult the text for an explanation of abbreviations and symbols.




Key Point


Mitral valve disease is the most important cause of heart murmurs and left-sided congestive heart failure (CHF) in mature dogs.



Mitral Regurgitant Volume


Total left ventricular (LV) stroke volume often is greatly increased, but a large proportion of LV output can be regurgitated into the left atrium. Forward stroke volume, the blood pumped into the aorta, is often maintained at near normal levels until CHF is present or advanced.



Left-Sided Cardiomegaly


Chronic mitral insufficiency increases LV and left atrial (LA) volume. Increases in LA pressure and volume cause progressive LA dilatation. Eccentric hypertrophy with dilatation of the left ventricular chamber results from the volume overload.



Increased Pulmonary Venous Pressure


LA and pulmonary venous (PV) pressures increase for a number of reasons:



• Progressive MR increases LA pressure, initially during systole. If the regurgitant volume increases gradually, LA compliance increases, and the condition is well tolerated. This explains why many dogs have severe cardiomegaly but minimal clinical signs. If the regurgitant volume is very large, or if regurgitation develops suddenly (e.g., chordal rupture), the limits of atrial distensibility are exceeded and mean LA and PV pressures increase, leading to pulmonary edema.

• The left ventricle may develop fibrosis or with progressive myocardial failure demonstrate reduced diastolic compliance and increased resistance to filling. The resulting higher LV diastolic pressure is transmitted to the left atrium and pulmonary veins.


Pulmonary Edema


Elevation of PV pressures increases pulmonary capillary pressure and may lead to pulmonary edema. Reduced forward output may promote retention of sodium and water by the kidneys and contribute to fluid overload. Clinical consequences include tachypnea, hypoxemia, coughing, and exercise intolerance.



Bronchial Compression


The left main-stem bronchus can become trapped between the pulsating aorta and the enlarging atrium. During systole, the bronchus is compressed by combined expansion of the aorta and the left atrium, triggering pressure receptors in the bronchus. As a result, exertional coughing and wheezing may occur even in the absence of lung edema.



Pulmonary Dysfunction


Chronic passive congestion of the lung causes ventilation perfusion inequalities (leading to hypoxemia) and, with chronicity, also might induce pulmonary fibrosis, which increases lung stiffness and the work of breathing.



Myocardial Failure


Chronic volume work by the left ventricle, combined with recurrent neurohormonal activation (renin-angiotensin-aldosterone; sympathetic nervous system; pro-inflammatory cytokines) can cause structural remodeling of the myocardium. Eventually reduced LV contractility and cardiac muscle failure occur (“cardiomyopathy of overload”). LV dysfunction is especially likely in larger dogs with primary mitral valve disease. However, in most cases, CHF often is manifest before substantial myocardial failure can be appreciated by clinical testing.



Low Cardiac Output


If forward stroke volume is inadequate, signs of low output (weakness, exertional or cough-related syncope, and prerenal azotemia) may develop. Low output signs are often associated with the development of pulmonary hypertension and right-sided CHF, although syncope can be the result of arrhythmias or of altered baroreceptor function leading to vasodilation and bradycardia.



Right Ventricular (RV) Failure


Left heart failure with secondary pulmonary hypertension, often combined with tricuspid regurgitation due to endocardiosis, causes progressive RV dilation. RV failure may ensue, as evidenced by hepatomegaly and ascites.



Cardiac Arrhythmias


Arrhythmias are common and can further decrease cardiac output (see Chapter 145).



• Atrial arrhythmias are triggered by dilation of the atria. Atrial premature complexes (APCs) frequently are recorded on the electrocardiogram (ECG). Atrial tachycardia and atrial fibrillation are evident in some cases.

• Ventricular premature complexes (VPCs) develop in some dogs, especially those with myocardial failure, but sustained ventricular tachycardia appears to be less common than in dogs with dilated cardiomyopathy.

• Iatrogenic arrhythmias (e.g., sinus bradycardia, AV block, APCs, and VPCs) may be precipitated by digitalis intoxication or diuretic-induced hypokalemia.


Ruptured Chordae Tendineae


This condition commonly is observed during echocardiographic examination of dogs with advanced MR. Catastrophic CHF may develop acutely; however, depending on the location of the chordal rupture, the added hemodynamic burden may be reasonably well tolerated.



Left Atrial Rupture


Rupture of the left atrium, although uncommon, can cause acute cardiac tamponade, cardiogenic shock, and sudden death. Rarely, splitting of the atrial septum produces an acquired left-to-right shunting atrial septal defect.




Key Point


Rupture of the mitral valve chordae tendineae, the development of sustained atrial tachyarrhythmias, and LA rupture are important diagnostic considerations when rapid hemodynamic deterioration occurs in dogs with chronic MR. Appropriate therapeutic intervention can stabilize many of these patients, allowing sufficient time for further compensation.



Clinical Signs and Diagnosis



Signalment


Chronic valvular disease is most common in toy and small breeds of dogs (e.g., poodle, Yorkshire terrier, Cavalier King Charles spaniel, schnauzer, cocker spaniel, and dachshund). Some breeds, particularly the spaniels, German shepherds, and Afghan hounds, are prone to both valvular degeneration and dilated cardiomyopathy. Endocardiosis is an incidental finding in many aged dogs. Male dogs are predisposed to the development of CHF with chronic valvular disease.




Key Point


Occasionally, CHF from primary mitral valve disease develops in large-breed dogs, but dilated cardiomyopathy is a more important cause of CHF in these larger dogs.



History


The presenting complaints typically are those attributable to bronchial compression, pulmonary congestion, or low cardiac output and include cough, tachypnea, tiring on exercise, and syncope. Weight loss is a frequent but subtle finding until biventricular or right-sided heart failure develops. Unless there are other metabolic problems, such as hyperadrenocorticism, few dogs with overt CHF are obese. The opposite is often true in dogs with chronic respiratory disease. In general, the clinical signs gradually become more severe, reflecting the progressive nature of the disease. Syncope may be related to one or more of the following problems:



• Cough-syncope—fainting after vigorous coughing caused by decreased venous return, transient bradyarrhythmias, or inappropriate baroreceptor reflex-mediated activation.

• Paroxysmal atrial or ventricular tachyarrhythmias.

• Orthostatic hypotension—sudden weakness after rising may be related to abnormal baroreceptor activity, particularly in volume-depleted dogs.

• Insufficient forward flow from severe MR or from pulmonary hypertension, typically observed at the onset of exercise or with excitement.

• Reflex-mediated (neurocardiogenic) syncope. This may be the most common cause of excitement-induced syncope.

• Iatrogenic issues—hypotension secondary to diuretics, vasodilators, or angiotensin-converting enzyme inhibitors (ACEIs).


Physical Examination



Auscultation


As the disease progresses, the left apical systolic murmur of mitral insufficiency generally increases in intensity, and the duration increases from early systole to midsystole until it extends throughout systole (holosystolic). The murmur of MR radiates in the direction of the regurgitant jet, typically dorsal to the mitral valve on the left hemithorax, but very often to the right hemithorax as well.




Key Point


A systolic murmur heard best over the (palpable) left apex almost always is caused by MR. The murmur may radiate widely.



• A systolic click or short systolic murmur usually indicates early disease. The murmur of MR usually evolves from a soft decrescendo to a loud holosystolic murmur over a period of months to years. In some dogs with peracute CHF or in dogs with systemic hypotension from drug therapy or due to ruptured left atrium, the murmur can become softer and shorter.

• Left apical midsystolic clicks often are detected in asymptomatic dogs and may be confused with a ventricular gallop. These clicks probably indicate chordal laxity and valve prolapse.

• A ventricular gallop (third heart sound) may be heard at the left apex in some dogs with CHF. This sound often resolves after successful therapy.

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Valvular Heart Disease

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