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Aortic Stenosis

by Guy P. Armstrong, MD

Aortic stenosis (AS) is narrowing of the aortic valve, obstructing blood flow from the left ventricle to the ascending aorta during systole. Causes include a congenital bicuspid valve, idiopathic degenerative sclerosis with calcification, and rheumatic fever. Untreated AS progresses to become symptomatic with one or more of the classic triad of syncope, angina, and exertional dyspnea; heart failure and arrhythmias may develop. A crescendo-decrescendo ejection murmur is characteristic. Diagnosis is by physical examination and echocardiography. Asymptomatic AS in adults usually requires no treatment. Once symptoms develop, surgical or percutaneous valve replacement is required. For severe or symptomatic AS in children, balloon valvotomy is effective.

Etiology

In elderly patients, the most common precursor is aortic sclerosis, a degenerative aortic valve disease with thickening of aortic valve structures by fibrosis and calcification initially without causing significant obstruction. Over years, aortic sclerosis progresses to stenosis in as many as 15% of patients. Aortic sclerosis resembles atherosclerosis, with deposition of lipoproteins and inflammation and calcification of the valves; risk factors are similar (see Atherosclerosis : Risk Factors).

In patients < 70 yr, the most common cause is a congenital bicuspid aortic valve. Congenital AS occurs in 3 to 5/1000 live births and affects more males; it is associated with coarctation and progressive dilatation of the ascending aorta causing dissection.

In developing countries, rheumatic fever is the most common cause in all age groups.

Supravalvular AS caused by a discrete congenital membrane or hypoplastic constriction just above the sinuses of Valsalva is uncommon. A sporadic form of supravalvular AS is associated with a characteristic facial appearance (high and broad forehead, hypertelorism, strabismus, upturned nose, long philtrum, wide mouth, dental abnormalities, puffy cheeks, micrognathia, low-set ears). When associated with idiopathic hypercalcemia of infancy, this form is known as Williams syndrome. Subvalvular AS caused by a congenital membrane or fibrous ring just beneath the aortic valve is uncommon.

Pathophysiology

Aortic regurgitation may accompany AS, and about 60% of patients > 60 yr with significant AS also have mitral annular calcification, which may lead to mitral regurgitation.

The increased pressure load imposed by AS results in compensatory hypertrophy of the left ventricle (LV) without cavity enlargement (concentric hypertrophy). With time, the ventricle can no longer compensate, causing secondary LV cavity enlargement, reduced ejection fraction (EF), decreased cardiac output, and a misleadingly low gradient across the aortic valve (low-gradient severe AS). Patients with other disorders that also cause LV enlargement and reduced EF (eg, MI, intrinsic cardiomyopathy) may generate insufficient flow to fully open a sclerotic valve and have an apparently small valve area even when their AS is not particularly severe (pseudosevere AS). Pseudosevere AS must be differentiated from low-gradient severe AS because only patients with low-gradient severe AS benefit from valve replacement.

Elevated shear stress across the stenosed aortic valve degrades von Willebrand factor multimers. The resulting coagulopathy may cause GI bleeding in patients with angiodysplasia (Heyde syndrome).

Symptoms and Signs

Congenital AS is usually asymptomatic until age 10 or 20 yr, when symptoms develop insidiously. In all forms, progressive untreated AS ultimately results in exertional syncope, angina, and dyspnea (SAD triad). Other symptoms and signs may include those of heart failure and arrhythmias, including ventricular fibrillation leading to sudden death.

Exertional syncope occurs because cardiac output cannot increase enough to meet the demands of physical activity. Nonexertional syncope may result from altered baroreceptor responses or ventricular tachycardia. Exertional angina pectoris affects about two thirds of patients; about half have significant coronary artery atherosclerosis, and half have normal coronary arteries but have ischemia induced by LV hypertrophy and altered coronary flow dynamics.

There are no visible signs of AS. Palpable signs include carotid and peripheral pulses that are reduced in amplitude and slow rising (pulsus parvus, mollus et tardus) and an apical impulse that is sustained (thrusts with the 1st heart sound [S 1 ] and relaxes with the 2nd heart sound [S 2 ]) because of LV hypertrophy. The LV impulse may become displaced when systolic dysfunction develops. A palpable 4th heart sound (S 4 ), felt best at the apex, and a systolic thrill, corresponding with the murmur of AS and felt best at the left upper sternal border, are occasionally present in severe cases. Systolic BP may be high even when AS is severe but ultimately falls when the LV fails.

On auscultation, S 1 is normal and S 2 is single because aortic valve closing is delayed and merges with the pulmonic (P 2 ) component of S 2 . The aortic component may also be soft. Paradoxical splitting of S 2 may be heard. A normally split S 2 is the only physical finding that reliably excludes severe AS. An S 4 may be audible. An ejection click may also be audible early after S 1 in patients with congenital bicuspid AS when valve leaflets are stiff but not completely immobile. The click does not change with dynamic maneuvers.

The hallmark finding is a crescendo-decrescendo ejection murmur, heard best with the diaphragm of the stethoscope at the right and left upper sternal border when a patient who is sitting upright leans forward. The murmur typically radiates to the right clavicle and both carotid arteries (left often louder than right) and has a harsh or grating quality. But in elderly patients, vibration of the unfused cusps of calcified aortic valve leaflets may transmit a louder, more high-pitched, “cooing” or musical sound to the cardiac apex, with softening or absence of the murmur parasternally (Gallavardin phenomenon), thereby mimicking mitral regurgitation. The murmur is soft when stenosis is less severe, grows louder as stenosis progresses, and becomes longer and peaks in volume later in systole (ie, crescendo phase becomes longer and decrescendo phase becomes shorter) as stenosis becomes more severe. As LV contractility decreases in critical AS, the murmur becomes softer and shorter. The intensity of the murmur may therefore be misleading in these circumstances.

Sound is that of a crescendo-decrescendo ejection murmur. The 6th beat in the sample is a ventricular premature beat (VPB). The 7th beat illustrates post-VPB potentiation of the murmur due to increased left ventricular filling during the post-VPB compensatory pause, which causes increased turbulent flow past the stenotic valve during the next ventricular contraction.

Aortic Stenosis Murmur (Crescendo-Decrescendo Ejection Murmur)

The murmur of AS typically increases with maneuvers that increase LV volume and contractility (eg, leg-raising, squatting, Valsalva release, after a ventricular premature beat) and decreases with maneuvers that decrease LV volume (Valsalva maneuver) or increase afterload (isometric handgrip). These dynamic maneuvers have the opposite effect on the murmur of hypertrophic cardiomyopathy, which can otherwise resemble that of AS. The murmur of mitral regurgitation due to prolapse of the posterior leaflet may also mimic AS.

Diagnosis

  • Echocardiography

Diagnosis is suspected clinically and confirmed by echocardiography. Two-dimensional transthoracic echocardiography is used to identify a stenotic aortic valve and possible causes, to quantify LV hypertrophy and degree of systolic dysfunction, and to detect coexisting valvular heart disorders (aortic regurgitation, mitral valve disorders) and complications (eg, endocarditis). Doppler echocardiography is used to quantify degree of stenosis by measuring jet velocity, transvalvular systolic pressure gradient, and aortic valve area

Severity is characterized echocardiographically as

  • Moderate: Peak aortic jet velocity 3 to 4 m/sec, mean gradient 20 to 40 mm Hg, valve area 1.0 to 1.5 cm 2

  • Severe: Peak aortic jet velocity > 4 m/sec, mean gradient > 40 mm Hg, valve area < 1.0 cm 2

  • Very severe: Peak aortic jet velocity > 5 m/sec or mean gradient > 60 mm Hg

Clinical judgment is used to resolve any discordance among these parameters (eg, moderate valve area but severe mean gradient). When LV function is normal, the valve area is the least accurate.

The gradient may be overestimated when aortic regurgitation is present. The gradient may be underestimated when the stroke volume is low, eg, in patients with LV systolic dysfunction (low-gradient AS with reduced EF) or a small, hypertrophied LV (low-gradient AS with normal EF). Sometimes LV systolic dysfunction results in low ventricular pressure that is inadequate to open nonstenotic valve leaflets, causing echocardiographic appearance of low valve area in the absence of stenosis (pseudostenosis).

Prior to intervention, cardiac catheterization is necessary to determine whether coronary artery disease (CAD) is the cause of angina and, occasionally, to resolve inconsistency between clinical and echocardiographic findings.

An ECG and chest x-ray are obtained. ECG typically shows changes of LV hypertrophy with or without an ischemic ST- and T-wave pattern. Chest x-ray findings may include calcification of the aortic cusps (seen on the lateral projection or on fluoroscopy) and evidence of HF. Heart size may be normal or only mildly enlarged.

In asymptomatic patients with severe AS, closely supervised exercise ECG testing is recommended in an attempt to elicit symptoms of angina, dyspnea, or hypotension—any of these symptoms, when due to the AS, is an indication for intervention. Failure to normally increase BP and development of ST segment depression are less predictive of adverse prognosis. Exercise testing is contraindicated in symptomatic patients. When there is LV dysfunction and the aortic valve gradient is low but the valve area is small, then low-dose dobutamine stress echocardiography distinguishes low-gradient AS from pseudostenosis.

Prognosis

AS progresses faster as severity increases, but the wide variability in progression rates requires regular surveillance, particularly in sedentary elderly patients. In such patients, flow may become significantly compromised without triggering symptoms.

Asymptomatic patients with severe AS and normal systolic function should be reevaluated every 6 mo because 3 to 6% will develop symptoms or LVEF impairment every year. The risk of surgery outweighs the survival benefit in asymptomatic patients, but with the onset of symptoms the mean survival plummets to 2 to 3 yr, and prompt valve replacement is indicated to relieve symptoms and improve survival. Risk of surgery increases for patients who require simultaneous coronary artery bypass graft (CABG) and for those with depressed systolic LV function.

In patients with severe AS, about 50% of deaths occur suddenly, and these patients should be advised to limit physical exertion.

Treatment

  • Sometimes aortic valve replacement

Nothing has yet been proved to slow the progression of AS. In randomized trials, statin therapy has been ineffective.

Drugs that can cause hypotension (eg, nitrates) should be used cautiously, although nitroprusside has been used as a temporizing measure to reduce afterload in patients with decompensated heart failure in the hours before valve replacement. Patients who develop heart failure but are too high risk for valve intervention benefit from cautious treatment with digoxin, diuretics, and ACE inhibitors.

Timing of intervention

The benefits of intervention do not outweigh the risks until patients develop symptoms and/or meet certain echocardiographic criteria. Thus, patients should have periodic clinical evaluation, including echocardiography and sometimes exercise testing, to determine the optimal time for valve replacement. Valve replacement is recommended for

Symptomatic patients (including those with symptoms or reduced effort tolerance on exercise testing) with

  • Severe AS

Asymptomatic patients with any one of the following:

  • LV EF < 50%

  • Moderate or severe AS when undergoing cardiac surgery for other reasons

  • Very severe AS (and low surgical risk)


Choice of intervention

Balloon valvotomy is used primarily in children and very young adults with congenital AS. In older patients who are not candidates for surgery, balloon valvuloplasty may be used as a bridge to valve replacement but it has a high complication rate and provides only temporary relief.

Surgical aortic valve replacement is the best choice for most patients but transcatheter (percutaneous) valve replacement (TAVR) may be chosen instead when surgical risk is high (as long as coexisting conditions do not preclude benefit from relieving the AS). TAVR affords better survival and quality of life than medical therapy and is safer than surgery in many high-risk patients. Surgery usually involves replacement with a mechanical or bioprosthetic valve but in younger patients, the patient’s own pulmonic valve can be used, providing good durability; a bioprosthesis is then used to replace the pulmonic valve (Ross procedure). Preoperative evaluation for CAD is indicated so that CABG and valve replacement, if indicated, can be done during the same procedure.


Key Points

  • The most common cause of AS in patients < 70 yr is bicuspid aortic valve; aortic sclerosis is the most common precursor in the elderly.

  • Untreated AS ultimately results in exertional syncope, angina, and dyspnea; sudden death may occur.

  • Typical heart sounds are a crescendo-decrescendo ejection murmur that increases with maneuvers that increase LV volume and contractility (eg, leg-raising, squatting, Valsalva release) and decreases with maneuvers that decrease LV volume (Valsalva maneuver) or increase afterload (isometric handgrip).

  • Nitrates may cause dangerous hypotension and should be used with caution for angina in patients with AS.

  • Replacement is indicated once symptoms begin or LV dysfunction occurs.

  • Surgical valve replacement is the best option for most patients, but transcatheter replacement is now available for patients at high surgical risk.

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