Reviewed July 22, 2004
R.
M. Mills, MD
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The availability of two-dimensional Doppler echocardiography and the development of reliable valve prostheses, both bioprosthetic and mechanical, have quietly revolutionized the clinical management of valvular heart disease. The cardiac valves have two functions. By opening, they control the direction in which blood flows, and by closing they allow pressure differentials to exist in a closed system. Abnormal valve function produces either pressure overloading due to restricted opening or volume overloading due to inadequate closure. Valvular heart disease can be approached either on the basis of the pathologic lesion, ie, aortic stenosis or aortic regurgitation, or pathophysiologically as pressure overloading versus volume overloading. In this chapter, we will summarize our current approach to aortic valve disease, namely aortic stenosis and regurgitation, with particular emphasis on the indications for valve surgery. In general, we adhere to the recommendations given in the ACC/AHA guidelines published in 1998. |
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AORTIC STENOSIS AORTIC REGURGITATION
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Aortic stenosis, for the purposes of this chapter, refers to obstruction of flow at the level of the aortic valve, and will not include the sub- and supravalvular forms of this disease. Aortic stenosis is usually defined by restricted systolic opening of the valve leaflets, with a mean transvalvular pressure gradient of at least 5-10 mmHg. The etiology of the stenosis can be further defined based on the anatomy and disease process affecting the valve. |
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Rheumatic valve disease has declined dramatically in the United States during the past 50 years, and isolated rheumatic aortic valve is unusual in any event. Aortic stenosis due to bicuspid valves affects males much more often than females, but late-life calcific disease of a trileaflet valve involves both sexes equally. With our aging population, calcific aortic stenosis accounts for the vast majority of aortic valve disease. In the elderly, mild thickening and/or calcification of a trileaflet aortic valve without restricted leaflet motion (ie, aortic sclerosis) affects about 25% of the population > 65 years of age.1 Calcific aortic stenosis, however, affects approximately 2% to 3% of those > 75 years. Thus not all patients with aortic sclerosis will go on to develop obstructive aortic valve disease. For aortic stenosis due to a bicuspid valve, the approximate overall incidence of an anatomic bicuspid aortic valve is 1% to 2% of the population. Of these individuals, most will go on to present with aortic stenosis, while a minority will develop a regurgitant lesion. |
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The most common forms of aortic valve disease include: 1) congenital aortic stenosis, usually resulting from failure of the valve commissures to develop fully, often resulting in unicuspid aortic valves, 2) degenerative changes in a congenitally bicuspid aortic valve, and 3) athero-calcific disease in a trileaflet aortic valve. These can be distinguished clinically by age of onset and echocardiographically by their characteristic findings. Congenital aortic stenosis usually presents in childhood, even infancy, and the echocardiographic examination will show a bicuspid or even unicuspid valve. Adults with bicuspid valves are predominantly male, often have known of a murmur for many years, and usually experience the onset of symptoms between the ages of 40 and 60 years. Bicuspid valves usually have fusion of one of the three commissures and, echocardiographically, can be distinguished by the presence of a raphe, leaflet doming, and eccentric closure. Late life calcific aortic stenosis (traditionally characterized as 'degenerative' or 'senile-type') affects trileaflet aortic valves, often in patients with other manifestations of atherosclerotic disease (Figure 1). This form of aortic stenosis progresses slowly, and patients often present between the ages of 70 and 90 years. Echocardiographic examination typically reveals various degrees of nodular thickening and calcification of the three leaflets with restricted systolic motion. Other less common etiologies of aortic stenosis include rheumatic disease, homozygous hypercholesterolemia, and systemic lupus erythematosus. |
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Valvular aortic stenosis results in chronic left ventricular pressure overloading. At any stage of life, however, the natural history of aortic stenosis largely reflects the functional integrity of the mitral valve. As long as adequate mitral valve function is maintained, the pulmonary bed is protected from the systolic pressure overloading imposed by aortic stenosis. In contrast to mitral valve disease where the pulmonary circuit is directly involved, compensatory concentric left ventricular hypertrophy allows the pressure overloaded ventricle to maintain stroke volume with modest increases in diastolic pressure, and patients can remain asymptomatic for many years. Eventually, however, left ventricular hypertrophy results in either 1) diastolic dysfunction with the onset of congestive symptoms, or 2) myocardial oxygen needs in excess of supply with the onset of angina. Some patients may also experience exertional syncope, probably reflecting the inability to increase cardiac output and maintain blood pressure in response to vasodilation. Vasodepressor syncope, however, may be an operative mechanism in a portion of these syncopal episodes. |
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Most patients with calcific aortic stenosis report knowing of a cardiac murmur for many years. The critical points in defining the cardiac history in men include athletic or military physical examinations, insurance or employment examinations. In women, pregnancy and childbearing history is important to define functional status in early adult life. Patients with typical findings of aortic stenosis should have a detailed history-taking session with inquiry into habitual activity levels and any changes in exercise tolerance. The onset of any of the classic symptoms of left ventricular outflow obstruction, namely angina, syncope, or heart failure, in a patient with valvular aortic stenosis indicates advanced valve disease and should be carefully and promptly evaluated.2 On physical examination, the harsh systolic murmur of aortic stenosis, loudest at the base of the heart and radiating to the carotids, is often, but not always, prominent. Low output states, obesity, or chronic lung disease may mask the findings. The murmur may radiate toward the cardiac apex, in which case the harsh component is lost; this finding may be mistaken for a second murmur. Other hallmarks of significant aortic valve stenosis include a single (pulmonic) component of the second heart sound and a sustained left ventricular apical impulse with a fourth heart sound. The slowly rising, low volume carotid arterial pulses of severe aortic stenosis may be noted in younger patients, but changes in arterial compliance often mask these findings in the elderly. |
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The electrocardiogram often shows changes of left ventricular hypertrophy. The chest X-ray is seldom helpful, although occasionally heavy calcification of the valve or post-stenotic ascending aortic dilation may be seen. With its widespread availability, two-dimensional and Doppler echocardiography has become the study of choice in the evaluation of patients with suspected valvular disease. Echocardiography allows assessment of the anatomy of the valve (Figure 2) as well as chamber size and ventricular function. Doppler studies permit estimation of pressure gradients, as well as aortic valve area by employing the continuity equation.3
With good quality echocardiography, cardiac catheterization is usually not required for diagnosis of patients with aortic stenosis. We generally perform pre-operative coronary angiography in men over 40 years old and women over 50. The classic catheterization laboratory studies of transvalvular gradients and cardiac output have been largely superseded by hemodynamic assessment in the echocardiographic laboratory. |
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Patients with (predominant) aortic stenosis fall into one of four categories of severity: 1. valve area > 1.2
cm2………………………….mild Asymptomatic patients with mild to moderate aortic stenosis should have medical follow-up with regular inquiry as to changes in exercise tolerance or other symptoms. Serial echocardiographic examination should be based on an understanding of the natural history of the lesion, as outlined below. Patients should avoid strenuous activity, and particularly avoid post-prandial exertion. Infective endocarditis precautions following American Heart Association guidelines must be emphasized at each visit.4 Current evidence indicates that calcific aortic stenosis progresses, on the average, at a rate of about 0.1 cm2 per year decline in valve area. To date, no medical therapy exists for the treatment of degenerative aortic stenosis. The possible impact of 'secondary prevention' measures, particularly lipid lowering with HMG-CoA reductase inhibitors ("statins"), on the progression of aortic stenosis is under investigation.5 Hypertension occurs in about 20% to 30% of patients with mild to moderate aortic stenosis and should be managed with angiotensin-converting enzyme inhibitors or angiotensin receptor blockers titrated slowly. We do not hesitate to use modest doses of concomitant beta-blockers in selected patients. A supervised exercise tolerance test may provide helpful objective assessment in patients with echocardiographic evidence of moderate aortic stenosis who report atypical symptoms, who minimize complaints, or who are sedentary and therefore might not experience exercise intolerance. Functional limitation with inability to exercise to levels greater than 6 metabolic equivalents (METs) may, in some cases, be viewed as a "symptom." Nevertheless, we rarely encounter truly asymptomatic individuals with critical aortic stenosis, and do not advocate stress testing for patients with very severe left ventricular outflow obstruction. Symptomatic patients, ie, those with angina, syncope, dyspnea, with moderate, severe, or critical aortic stenosis should undergo valve replacement, as the natural history of symptomatic moderate aortic stenosis is the same as that of symptomatic severe aortic stenosis. Additional indications for aortic valve surgery include moderate aortic stenosis in patients requiring coronary bypass grafting and/or any other cardiac surgery, exercise-induced hypotension, and asymptomatic severe aortic stenosis with evidence of left ventricular dysfunction. The preoperative evaluation should address any major co-morbid conditions and optimize their management. Carotid duplex examination should be done, since distinguishing carotid bruit from a radiating murmur clinically is extremely difficult. Coronary angiography is indicated to evaluate the need for concomitant direct surgical revascularization. Patients scheduled for valve surgery should not have percutaneous angioplasty if the preoperative catheterization shows coronary disease, since surgical revascularization with arterial grafts adds little to the risk of the valve replacement procedure. Smoking cessation is mandatory, and diabetic control to HgbA1c levels close to 6 may reduce infection risk. Thyroid disease and the need for thyroid hormone replacement should be assessed. Dental care should be completed with antibiotic prophylaxis before surgery. The consulting cardiologist and/or the cardiac surgeon should discuss the advantages and drawbacks of mechanical versus bioprosthetic valves with the patient and his or her family during the pre-surgical evaluation. Often the choice of prosthesis is straightforward, but younger patients in particular may have special needs, which should be addressed. Bioprosthetic valves offer the advantage of not requiring long-term oral anticoagulation, but have the drawback of relatively limited durability. In contrast, mechanical valves offer long-term durability, but require lifelong warfarin therapy. The generally accepted risk of serious bleeding with warfarin is on the order of 3% per year. Childbearing in women and vigorous sports activities in men are contra-indications to chronic oral anticoagulation with warfarin, and may figure importantly in the choice of valves. In general, we favor bioprosthetic valves in patients over the age of 60 years and mechanical valves under the age of 50. For male patients in their 50s, clinical outcomes with bioprosthetic valves are quite good; in this group, current estimates place the likelihood of reoperation for late (after 10 years) deterioration of a bioprosthesis at about one chance in 10, using "competing outcomes" analysis. In contrast, healthy women in their 50s should probably receive mechanical valves, since many can expect another 30 years of life. Homograft aortic valve replacement with a cryopreserved cadaveric valve may offer specific advantages in patients with infective endocarditis or with disease of the aortic root. Patients and physicians should bear in mind that valve replacement is palliative, not curative. A prosthetic heart valve commits a patient to continued infective endocarditis prophylaxis, regular cardiac follow-up, and often to continued medical therapy, including anticoagulation with warfarin for those with mechanical prostheses. Re-operation may be required for malfunction of the prosthetic valve. In addition, a small but not insignificant subset of patients may require implantation of a permanent pacemaker after valve surgery. Patients should clearly indicate willingness to accept the limitations that valve replacement imposes before surgery. In addition, patients must understand that surgical risks include wound infection and stroke as well as perioperative mortality. |
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| Aortic regurgitation is defined by incompetence of the aortic valve, in which a portion of the left ventricular forward stroke volume returns to the chamber during diastole. The etiology of the regurgitation as with aortic stenosis, can be further defined based on the anatomy of the valve and aortic root, and the disease process affecting the valve. | ||||||||
Aortic regurgitation may occur due to leaflet pathology or aortic root disease. As a dominant isolated lesion, aortic regurgitation usually occurs due to a bicuspid aortic valve (Figure 3) often resulting from leaflet prolapse. Infective endocarditis involving a bicuspid valve may cause aortic regurgitation from loss of coaptation or perforation of a cusp. However, any pathologic process that results in aortic root dilatation and loss of leaflet coaptation can also result in significant aortic regurgitation. Examples include diseases of the aortic root, particularly annuloaortic ectasia, occurring in long-standing hypertension and hereditable diseases of connective tissue such as Marfan syndrome and cystic medial necrosis. Additionally, ascending aortic dissections and congenital diseases such as ventricular septal defects as seen in Tetralogy of Fallot can lead to aortic regurgitation. Other less common problems include radiation-induced valvular disease and sterile inflammatory aortic valvulitis due to autoimmune disease. Regardless of etiology, chronic aortic regurgitation causes predominantly volume overloading of the left ventricle, and in contrast to mitral regurgitation, causes a component of pressure overload as well. The volume overload is usually well tolerated for long periods of time, possibly even decades. The pathoanatomic sequelae of aortic regurgitation reflect the severity of the aortic valve diastolic leak, and include left ventricular dilatation and hypertrophy with remodeling of the left ventricle to a more spherical shape. Ejection fraction is usually preserved until the late stages of the disease. Since patients may tolerate severe aortic regurgitation with minimal symptoms, management should include careful monitoring of left ventricular dimensions and systolic function. In addition, as aortic root and proximal ascending aortic dilatation may be concomitantly present, careful monitoring of aortic enlargement is warranted in these patients. Surgical intervention is indicated even in asymptomatic individuals when left ventricular dilatation reaches critical dimensions (see below). |
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Patients with chronic aortic regurgitation due to bicuspid valves or long-standing hypertension often have little clinical history other than a known cardiac murmur noted on routine auscultation. In contrast, patients with aortic regurgitation due to infective endocarditis or aortic root disease may recount rather dramatic illness or systemic complaints. Nonetheless, cardiac complaints are unusual until the later stages of volume over-loading when effort intolerance becomes a problem. Symptoms with aortic regurgitant lesions often begin with nonspecific fatigue. Patients may relate that their ability to get through a day's work is maintained, but that they are "exhausted" on returning home. "Palpitation" or awareness of a forceful heartbeat is also an early complaint, sometimes noted by spouses! If allowed to progress, typical heart failure symptoms will follow. Angina pectoris and syncope are much less common with aortic regurgitation than with aortic stenosis. In contrast, "palpitations" and ventricular premature beats are much more frequent, and non-sustained ventricular tachycardia has often been reported. Overt heart failure and cardiac chest pain are infrequent in today's clinical practice and, if present, may reflect a more acute process. Careful physical examination will yield a host of eponymous signs (ie, Hill's sign, Corrigan's pulse), almost all of which reflect a high stroke volume and wide pulse pressure. The wide pulse pressure, bounding arterial pulses, and hyperdynamic circulation of chronic moderately severe aortic regurgitation are easily noted. In contrast, the soft blowing diastolic murmur may be subtle, requiring careful auscultation with the patient sitting forward in fully held expiration to appreciate. The murmur is almost always best heard using the diaphragm of the stethoscope applied firmly to the upper right parasternal area of the anterior chest. If the skin does not blanch under the pressure of the diaphragm, the murmur will not be optimally heard! A systolic murmur may be audible due to increased stroke volume. The duration of the diastolic murmur should be noted, since the duration of the murmur reflects the severity of the leak until the very late stages of disease, when left ventricular diastolic pressure rises and shortens the diastolic murmur. An "Austin-Flint" apical diastolic murmur may also be present. |
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The electrocardiogram of patients with aortic regurgitation commonly demonstrates voltage consistent with left ventricular hypertrophy, but often without the ST-segment depressions and T-inversion of the "strain pattern." The generous voltage and upright T-waves in the lateral chest leads has been referred to as "volume overload left ventricular hypertrophy (LVH)." In addition, premature ventricular contractions may be present. Echocardiography will, in almost all cases, define the functional anatomy of the valve and the aortic root, and Doppler imaging will help assess the severity of the diastolic leak (Figure 4). In addition, the echocardiogram will document the left ventricular diastolic dimension, ejection fraction, and wall thickness. If transthoracic echo imaging is not adequate to define the pathoanatomy, transesophageal echocardiography should be performed. The anatomic consequences of aortic regurgitation include, as mentioned above, both left ventricular hypertrophy and dilatation. Serial echocardiographic measurements of left ventricular systolic function and end-diastolic dimensions provide excellent objective parameters for long-term follow-up of asymptomatic patients.6 Echocardiography combined with exercise stress testing can document the functional response to increase demand for cardiac output. A fall in ejection fraction with exercise indicates poor functional reserve, and may be considered an indication for surgical intervention.
We no longer employ cardiac catheterization as the primary diagnostic imaging modality for aortic regurgitation. Angiographic assessment of regurgitant valve lesion severity is subjective and highly dependent on technical factors such as catheter position and the rate and volume of contrast injection. Diagnostic coronary angiography should be performed as part of the pre-surgical evaluation when valve repair or replacement is planned. |
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In theory, patients with aortic regurgitation should benefit from long-term administration of a direct-acting vasodilator. Angiotensin-converting enzyme inhibition or angiotensin receptor blockers may not be effective until the renin-angiotensin system is activated, so that there may be a role for long-acting dihydropyridine calcium channel antagonists such as amlodipine in the attempt to reduce the regurgitant fraction.7 In addition, although beta-blockers in theory prolong diastole, many clinicians employ modest doses of beta-blockers because of the known association between aortic regurgitation and aneurysmal diseases of the aorta. In addition, patients with aortic regurgitation should have detailed counseling about physical activity. Isometric exercise, weight lifting and heavy exertional activities, which involve strenuous arm work, should be specifically prohibited because of the reflex increase in peripheral vascular resistance that occurs with arm exercise. In contrast, rhythmic low-resistance large muscle group exercise such as bicycling reduces peripheral resistance, and should be encouraged for fitness and a sense of well-being. Infective endocarditis prophylaxis for gastrointestinal or genitourinary procedures must be emphasized. Patients should have regular dental examinations with appropriate prophylaxis every 6 months. Alluded to earlier, most patients with chronic aortic regurgitation have a protracted clinical course, despite evidence of severe regurgitation. Nevertheless, long-term care of the asymptomatic individual with aortic regurgitation consists of carefully monitoring for the onset of either symptoms or, more often, left ventricular dysfunction/dilatation. Asymptomatic patients with severe chronic aortic regurgitation and normal left ventricular function should be assessed clinically and echocardiographically approximately every 6 months. The current guidelines suggest surgical intervention at symptom onset or in asymptomatic individuals when one of the following are present: 1) left ventricular end-systolic dimension > 55 mm, 2) left ventricular end-diastolic dimension greater > 75 mm, or 3) left ventricular ejection fraction < 55%. Many experienced clinicians would recommend surgery at 50 mm end-systolic or 70 mm end-diastolic diameter if serial echocardiograms have confirmed continued dilatation on appropriate therapy and the surgical mortality at a given institution is low. When concerns arise about the physiologic significance of aortic regurgitation and possible indications for surgery, maximal cardiopulmonary exercise testing and stress echocardiography may be useful. Patients who can achieve high levels of activity, ie, over 12 METs or a maximal oxygen consumption over 25 ml O2/kg/min, with evidence of good contractile reserve can generally be managed conservatively. Impaired functional capacity or evidence of deteriorating left ventricular systolic function under stress should prompt consideration of valve surgery. The issues involved in the choice of a prosthetic valve and in post-surgical care are much the same for patients with aortic regurgitation as those described above for aortic stenosis. A notable distinction is the select group of patients with pliable bicuspid valves and aortic regurgitation, in which valve repair may be a viable option. However, clinicians should remember the association between aortic root diseases (ie, aneurysm formation and dissection) and patients with bicuspid aortic valves or connective tissue diseases. We favor maintaining these patients who undergo aortic valve repair or replacement on long-term beta-blockade. |
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