Aortic valve stenosis
The aortic valve is a flap-like opening located between the left side of the heart and the aorta. The aorta is the main artery carrying blood from the heart. Blood is pumped by the left ventricle across the aortic valve into the aorta and the arteries of the body. Aortic stenosis causes restricted systolic opening of the valve leaflets, with a mean transvalvular pressure gradient of at least 5-10 mmHg. When the degree of narrowing becomes significant enough to impede the flow of blood from the left ventricle to the arteries, heart problems develop.
In the Euro Heart Survey on Valvular Heart Disease, aortic valve stenosis was the most common valve abnormality. Aortic valve sclerosis is commonly defined as a focal or diffuse thickening of the aortic cusps with calcific nodules generally at the base of leaflets and transvalvular velocity at Doppler still in the normal range (Vmax <2 m/s).
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. 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. 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.
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. Particularly, the aortic valve sclerosis (aortic valve thickening and calcification without pressure gradient) seem to affect about one fourth of adults over 65 years of age, while the aortic valve stenosis is present in 2?9% of general population over 65 years of age; an increased prevalence of both sclerosis and stenosis with aging (48% and 4% in those over 85 years) is observed.
The relative frequency of the postinflammatory disease (i.e. post-rheumatic) decreased from 30% to 18% and the relative frequency of the bicuspid aortic valve changed from 37% to 33%; in contrast, the relative frequency of degenerative-calcific aortic stenosis (an "atherosclerotic" form of disease, see below) increased from 30% to 46%. These differences were striking in subjects older than 70 years.
Aortic stenosis due to bicuspid valves affects males three times more often than females, but late-life calcific disease of a trileaflet valve involves both sexes equally.
Aortic stenosis can occur at any age (because the causes are different) but is usually asymptomatic until middle or old age. Until few years ago, aortic stenosis was considered a physiologic process related to aging without clinical relevance. However, aortic valve sclerosis is not observed in about 50% of people over 80 years old.
Causes and risk factors
A number of conditions contribute towards aortic stenosis. Three conditions that are known to cause aortic stenosis are:
1. Calcification of a bicuspid valve
Bicuspid aortic valve is the most common cause of aortic stenosis in patients under age 65. Normal aortic valves have 3 thin leaflets called cusps. About 2% of people are born with aortic valves that have only 2 cusps (bicuspid valves). Although bicuspid valves usually do not impede blood flow when the patients are young, they do not open as widely as normal valves with 3 cusps. The turbulent blood flow causes excessive wear and tear leading to calcification, scarring, and reduced mobility of the valve leaflets over time. About 10% of bicuspid valves become significantly narrowed, resulting in the symptoms and heart problems of aortic stenosis.
2. Senile calcific aortic stenosis
The most common cause of aortic stenosis in patients 65 years and over is called "senile calcific aortic stenosis." With aging, protein collagen of the valve leaflets is destroyed, and calcium is deposited on the leaflets. Once valve leaflet mobility is reduced by calcification, turbulence across the valve increases, causing scarring, thickening, and stenosis of the valve. Why this aging process progresses to cause significant aortic stenosis in some patients but not in others is not known.
3. Rheumatic fever
Rheumatic fever rarely causes isolated aortic stenosis. Rheumatic fever is a condition resulting from untreated infection by group A streptococcal bacteria. Damage to valve leaflets from rheumatic fever causes increased turbulence across the valve and more damage. The narrowing from rheumatic fever occurs from the fusion of the commissures of the valve leaflets. Rheumatic aortic stenosis usually occurs with some degree of aortic regurgitation. Under normal circumstances, the aortic valve closes to prevent blood in the aorta from flowing back into the left ventricle. In aortic regurgitation, the diseased valve allows leakage of blood back into the left ventricle as the ventricular muscles relax after pumping. These patients also have some degree of rheumatic damage to the mitral valve.
- Subvalvular aortic stenosis
- Hypertrophic obstructive cardiomyopathy
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, left ventricular hypertrophy results in either diastolic dysfunction with the onset of congestive symptoms or myocardial oxygen needs in excess of supply with the onset of angina.
Most patients with calcific aortic stenosis report knowing of a cardiac murmur for many years. Common symptoms of aortic stenosis include: coughing at night; fainting, especially with physical activity; fatigue; shortness of breath that worsens at night or with exertion; angina; and, visual impairments. 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.
On physical examination, the harsh systolic diamond shaped (crescendo-decrescendo) 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.
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. The severity of symptoms is not always related to the severity of the disease. In fact, people sometimes die suddenly from aortic stenosis without having had symptoms. Symptoms usually occur when the aortic valve area narrows to less than 1 square centimeter. Critical aortic stenosis is present when the valve area is less than 0.7 square centimeters.
The electrocardiogram often shows changes of left ventricular hypertrophy. In rare instances, electrical conduction abnormality can also been seen.
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 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.
With good quality echocardiography, cardiac catheterization is usually not required for diagnosis of patients with aortic stenosis. However, a cardiac catheterization is the gold standard in evaluating aortic stenosis. A pre-operative coronary angiography is generally performed in men over 40 years old and women over 50.
Patients with (predominant) aortic stenosis fall into one of four categories of severity:
- valve area > 1.2 cm2??????????.mild
- valve area 1.0 to 1.2 cm2????????.moderate
- valve area 0.7 to 1.0 cm2????????.severe
- valve area < 0.7 cm2??????????.critical
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.
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. Selected patients may be given modest doses of concomitant beta-blockers.
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." Stress testing is not advocated for patients with very severe left ventricular outflow obstruction.
2. Valve replacement
Symptomatic patients, ie, those with angina, syncope, dyspnea, with moderate, severe, or critical aortic stenosis should undergo valve replacement. 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. Smoking cessation and diabetic control are mandatory after the replacement. Dental care should be completed with antibiotic prophylaxis before surgery.
The advantages and drawbacks of mechanical versus bioprosthetic valves should be discussed with the patient and his or her family. 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, bioprosthetic valves are preferred in patients over the age of 60 years and mechanical valves under the age of 50. 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. If significant narrowing of the coronary arteries is found, coronary artery bypass graft surgery (CABG) can be performed during aortic valve replacement surgery.
3. Balloon valvuloplasty
Balloon valvuloplasty is a technique that lowers the pressure across the valve by slightly enlarging the opening. This is usually done when someone is not stable enough for corrective surgery.
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.
Successful replacement of the valve restores normal blood flow. The long-term outcome is usually very good. Artificial valves wear out over a period of years. Their function is monitored, and the valves are replaced as necessary. 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.
Prognosis and survival
Serious long-term effects of aortic stenosis without timely treatment include:
- congestive heart failure
- coronary heart disease
- enlargement of the left ventricle
- pulmonary edema
- sudden death (occurs in less than 1%)
With surgery, the patient can expect to live a normal life with necessary precautions as explained above.
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