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Valvular Aortic Stenosis

Aortic valve sclerosis is present when the aortic valve cusps are thickened and hyperflectant, but there is no obstruction to left ventricular (LV) outflow. Aortic stenosis isdefined as an obstruction to the LV outflow.

Historically, quantitative assessment of aortic valve disease has been based on data obtained during cardiac catheterization. However, cardiac catheterization is an expensive, invasive procedure that does not lend itself to serial follow-up studies. Over the past decade, echocardiography has gained popularity as a noninvasive method for the evaluation of aortic valve disease.

Obstruction to left ventricular outflow is localized most commonly at the aortic valve. However, obstruction may also occur above the valve (supravalvular stenosis) or below the valve (discrete subvalvular aortic stenosis) or may be caused by hypertrophic obstructive cardiomyopathy.

Anatomical Review

The normal aortic valve has three cusps usually equal in size.

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There is a thin tough endocardial layer of tissue lined on the aorta as well as the LV aspect. The cusps are thick, "U" shaped markedly concaved towards the aorta.

Aortic root opened showing the sinus of Valsalva and a coronary ostia

At the center of each cusp is a triangular shaped structure called the nodules of Artantius(meet in diastole and create a tight seal so regurgitation does not occur). The sinus of Valsalva is a bowing of the aortic cusps for a cup shaped appearance. The sinotubularjunction is the point distal and superior to the sinus of Valsalva. The aortic root is located above the sinotubular junction. The ascending aorta begins superior to the aortic root. The

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above the sinotubular junction. The ascending aorta begins superior to the aortic root. The origin of the coronary arteries is approximately 1/2 up the root.

Etiology and Pathology

Valvular aortic stenosis (AS) has three major causes: degenerative, congenital, and rheumatic. Rheumatic aortic stenosis is coexists with rheumatic mitral valve disease. Aortic stenosis without accompanying mitral valve disease is more common in men and very rarely occurs on a rheumatic basis (associated more with women) but instead is usually either congenital or degenerative in origin. The etiology of aortic stenosis is also strongly influenced by patient age. In patients older than 70 years, senile calcific degeneration of the aortic valve is the most common cause, whereas in younger adults degenerative changes would involve a bicuspid aortic valve and are more common. These two conditions (congenital and degenerative) together make aortic stenosis the most common valve lesion requiring surgery today in the United states.

Congenital Aortic Stenosis

Congenital malformations of the aortic valve may be unicuspid, bicuspid, or there may be a dome shaped diaphragm. Unicuspid valves produce severe obstruction in infancy and are the most frequent malformation found in fatal valvular aortic stenosis associated with hypoplasia of the aorta and/or left heart in children under the age of 1 year. Congenitally bicuspid valves may be stenotic with commissural fusion at birth, but more commonly they are not responsible for serious narrowing of the aortic orifice during childhood; their abnormal architecture induces turbulent flow, which wears and tears on the valve leaflets and ultimately leads to fibrosis, increased ridgity, and calcification of the leaflets and narrowing of the aortic orifice. It becomes symptomatical present between the second and fifth decade of life, and two thirds of patients have their first murmur after the age of 20. It is seen in 1% to 2% of the general population. Infective endocarditis may develop on a congenitally bicuspid valve, which then becomes regurgitant. It should be emphasized that in a majority of cases, bicuspid valve is not stenotic at birth and the changes causing stenosis resemble those occurring in senile, degenerative calcific stenosis of a tricuspid aortic valve except that in the congenitally

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New Page 1senile, degenerative calcific stenosis of a tricuspid aortic valve except that in the congenitally bicuspid valve these changes occur several decades earlier.

A raphe is the fusing of the commisure. There is usually one large fused cusp and one small cusp.

Bicuspid Aortic Valve opened Cross section Bicuspid Aortic Valve with raphe'

Bicuspid aortic valve is associated with a coarctation of the aorta and it could be associated with a patent ductus arteriosus.

Coarctation of the aorta with bicuspid aortic valve

It must be appreciated that the true incidence of the malformation of the valve of the aorta is probably grossly underestimated because the congenital bicuspid aortic valve may be undetected in early life and becomes stenotic and of clinical significance only in adult life at a time when it may be indistinguishable from the acquired form of aortic stenosis (degenerative

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or rheumatic). Congenital bicuspid aorta is the most common adult congenital abnormality.

Degenerative Aortic Valve Stenosis

Degenerative aortic valve stenosis is now the most frequent cause of aortic stenosis requiring valve replacement. Sclerosis and calcifications are present at the bases and margins of the cusps and may extend to the tip portions, but no commissural fusion is present. The condition typically is not accompanied by significant aortic regurgitation. Degenerative "wear and tear" appears to be the most likely cause of this form of AS, which commonly accompanied by calcifications of the mital annulus and coronary arteries. This may be also prompt hemolytic anemia or hemolysis, which occurs because of damaged red blood cells due to turbulence and "wear and tear".

A low morbidity and mortality usually are the case. According to Roldan and Abrams (2002), the average rate of change in aortic valve area in patients with aortic valve stenosis is 0.12cm2 per year. However, it is impossible with the methods currently available to predict the rate of progression in the individual patient. More than half of patients will show little or no progression over a 3 to 9 year period, whereas others will have rapid progression with increases in pressure gradients of 15 to 19 mm Hg per year and a decrease of aortic valve area of 0.1 to 0.3 cm2 per year. For these reasons, careful clinical and echocardiographicfollow-up is mandatory in patients with moderate to severe aortic valve stenosis. Several studies have indicated the relationship of atherosclerotic risk factors such as diabetes mellitusand hypercholesterolemia with aortic valve stenosis. Clinically significant obstruction tends to occur at an age >65 years.

Rheumatic Aortic Valve Stenosis

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Rheumatic valve stenosis is characterized by adhesion and fusion of all the commissuresproducing a central triangular aperture, thickening and predominantly at the tip portions of the cusps, and retraction and stiffening of the free boarders of the cusps lead to aortic stenosis, consequently, concomitant aortic regurgitation and stenosis often is present.

The diagram below summarized the three etiologies of aortic stenosis

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Pathophysiology

Left ventricular output is maintained by the presence of left ventricular hypertrophy, which may sustain a large pressure gradient across the aortic valve for many years without a reduction in cardiac output, left ventricular dilatation, or the development of symptoms. The obstruction to the left ventricular outflow imposes a pressure load on the left ventricle that increases the tension in the left ventricular wall during systole. The physiological response to this increase in wall tension is concentric left ventricular hypertrophy and increased left ventricular mass and thickness which, in turn, tends to reduce wall tension and preserve systolic function. The heart weight can be as great as 1000 grams compared to a normal heart weight of 300 grams. When the left ventricle can no longer compensate for the increased pressure load, systolic function starts to decline and the ventricle begins to dilate.

Mixed forms of aortic valve stenosis also occur. Patients with bicuspid valve may develop rheumatic disease resulting in commissural fusion or infective endocarditis which usually produces aortic regurgitation. Degenerative calcification may also occur in mildly affected rheumatic aortic valves.

All forms of aortic valve stenosis cause obstruction to left ventricular outflow and therefore produce left ventricular hypertrophy characterized by a normal to small cavity and thick wall. In endconsiderable subendocardial fibrosis may occur leading to papillary muscle dysfunction and dilated left ventricular cavity.

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For a given level of systolic function, the systolic pressure gradient across a stenotic valve is inversely related to the effective valve orifice area. Conversely, for a given effective orifice area, the systolic pressure gradient is directly related to transaortic flow. When left ventricular function is normal, a peak instantaneous gradient > 80 mm Hg or a mean gradient > 50 mm Hg is generally consistent with severe aortic stenosis. However, if

the patient is in a low cardiac output state due to poor systolic function, the peak and mean gradient across a severely stenotic valve may be much lower.

This scenario leads itself to errors in valve orifice are estimation, whether data are acquired in the echocardiography or cardiac catheterization environments. In the first place, the rigid valve leaflets may not open as well at low flows as at higher flows, this could result in spurious systematic underestimation of the true valve orifice area. Second, at low flows small errors in gradient or flow measurements will produce large errors in orifice area estimation. To circumvent this problem, pharmacologic interventions such as incremental dobutamineinfusion can be employed to increase transaortic flow and pressure gradient.

Left ventricular end-diastolic function is present because of delayed relaxation due to increase muscle mass and increased chamber stiffness; later on, muscle stiffness may also increase. Delayed relaxation is a common expression of hypertrophy and pressure overload. As a consequence, left ventricular end-diastolic pressure frequently is elevated, despite a normal systolic function.

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Signs and Symptoms

In adults with aortic valve stenosis, the outflow obstruction increases gradually over a prolonged period. Cardiac output is maintained by the development of LV hypertrophy, which may sustain cardiac performance for many years without any symptoms despite a large pressure gradient across the aortic valve. The cardinal manifestations of severe aortic valve stenosis are angina pectoris, syncope, dyspnea, and heart failure. Without relief of the obstruction, the time from onset of symptoms to death is approximately 5 years for angina, 3 years for syncope, and 2 years for heart failure. Up to two thirds of patients with critical aortic valve stenosis have classic angina pectoris, only half of whom have obstructive coronary artery disease. The angina is commonly precipitated by exertion and relieved by rest. In patients without coronary artery disease it results from the combination of increased oxygen needs by the hypertrophied myocardium and reduction of oxygen delivery secondary to the excessive compression of coronary vessels. In patients with coronary artery disease, angina is caused by a combination of epicardial coronary artery obstruction and the above-described oxygen imbalance characteristic of AS.

Syncope usually is exertional and related to reduced cerebral blood flow due to a fixed cardiac output and systemic vasodilation. Warning symptoms may occur, and exertionaldizzy spells and near fainting are common. syncope at rest can be due to ventricular fibrillation, atrial fibrillation, or atrioventricular block due to calcific extension into the conduction system. Loss of atrial contraction, as occurs in atrial fibrillation or atrioventriculardisassociation, may result wit severe aortic stenosis.

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Left heart failure symptoms and manifestation of low cardiac output usually are late symptoms. Sudden death usually occurs in patients who have manifested symptoms related to aortic stenosis.

Physical Examination

In the absence of hypertension or associated aortic regurgitation, systolic blood pressure is normal in aortic valve stenosis. With advanced degrees of outflow obstruction, the pulse pressure narrows as systolic arterial pressure decreased. Even in severe aortic stenosis, only a minority of patients will have a systolic blood pressure < 100 mm Hg.

Contrary to popular belief, systemic arterial hypertension can be associated with significant aortic valve stenosis, particularly in the older population. When substantial aortic regurgitation also is present, systolic arterial pressure will increase and diastolic pressure decreases.

Heart Sounds

First and Second Heart Sound

In detecting aortic stenosis, the focus of the examination of the arterial system should be on the carotid arteries and not on the peripheral pulses. The hallmark of aortic stenosis is the typical slow rising, small-volume carotid arterial pulse, also labeled pulsus parvus et tardus (slow and late). There can be an associated systolic thrill or shudder on the upstroke of the carotid pulse

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The first heart sound (S1) usually is unaltered in aortic valve stenosis. It may be decreased in intensity, but never accentuated. A loud "S1" in a patient with suspected or proven aortic stenosis suggests the presence of an aortic ejection sound or associated mitral stenosis. Because A2 commonly is soft or absent in aortic stenosis, a normal or increased pulmonary valve component (P2) can be mistaken for A2. Hemodynamically significant aortic stenosiscauses abnormal splitting of S2. If LV ejection time is substantially delayed (pressure gradient > 75 mm Hg), reversed or paradoxic splitting of S2 occurs.

Third and Fourth Heart Sound

A third heart sound (S3) is not a normal or expected finding in adults with AS. Its presence suggests significant LV dysfunction or decompensated heart failure. A fourth heart sound (S4) is a valuable clue to the presence of LV hypertrophy and decreased LV compliance associated with severe aortic stenosis.

Aortic Ejection Click

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The ejection click is a high-frequency, crisp sound. It occurs just after S1(longer interval than that separating the closure

of the mitral valve and tricuspid valve). A typical click can be mistaken for S1, particularly at the base of the heart. The detection of an aortic ejection click or sound is an important observation. It confirms the diagnosis of structural heart disease in a patient with a systolic ejection murmur.

Heart Murmurs

Aortic stenosis produces the classic systolic ejection murmur: a crescendo decrescendo murmur that begins after S1 and ends before S2. The murmur typically is harsh, rough, or grunting and maximal at the second right intercostal space, the so called aortic area. The length of the murmur is proportional to the severity of the valvular obstruction in the absence of other factors that modify stroke volume or the rate of ejection. A murmur that peaks within the first third or half of systole suggests mild aortic stenosis. A murmur that peaks within the second half of systole indicates severe disease. When the left ventricle fails and the cardiac output falls in AS, the murmur becomes softer or disappears altogether.

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Laboratory Examination

Electrocardiography

The principal electrocardiographic change is left ventricular hypertrophy, which is found approximately 85 per cent of patients with severe AS.

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Chest X-ray

The heart is usually of normal size or slightly enlarged, with a rounding of the left ventricular boarder and apex. Poststenotic dilatation of the ascending aorta is a common finding. This is due to the high degree of turbulence after the stenosis. Calcification of the aortic valve is found in almost all adults with hemodynamically significant AS. The left atrium may be slightly enlarged, and there may be radiological signs of pulmonary venous hypertension.

Calcified Aortic Valve

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Cardiac Catheterization

In valvular aortic stenosis, there is a systolic pressure difference across the valve.

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Left ventricular hypertrophy and post-stenotic aorta may be seen by left ventriculography. In valvular aortic stenosis the typical change is the thickening of the cusps, with doming in systole, and a central ejection jet may be seen. In patients with severe AS, large a waves, usually appear in the left atrial pressure pulse because of the combination of enhanced contraction (associated with S4) of a hypertrophied left atrium and diminished left ventricular compliance.

Peak to peak and mean gradients are important. LV pressure wave form may display a more rounded flattened summit over time due to slow ejection. Aortic valve area should be calculated using the Gorlin Formula. In low flow states, the Gorlin equation may be misleading. Normal valve area is 2.5 to 4.5 cm2. LV angiography may be performed to evaluate the size and thickness of the LV and LV wall.

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Treatment

Asymptomatic patients have an excellent prognosis. Sudden death, like syncope, in patients with severe AS may be due to cerebral hypoperfusion followed by arrhythmia. Patients with known severe AS who are asymptomatic should be advised to report promptly to the physicians when symptoms begin to occur.

The indication for surgery as well as the techniques and results of operations depends on the patient's age and nature of the valvular deformity. The aortic valve should, in general, be replaced in patients who have hemodynamic evidence of severe obstruction (aortic valve area < 0.8cm2) and symptoms believed to result from AS. Surgical treatment should also be carried out in asymptomatic patients with progressive left ventricular dysfunction and or significant ventricular ectopic activity at rest or an abnormal hemodynamic response to exercise. The findings that the strongest predictor of post-operative left ventricular dysfunction is preoperative dysfunction suggests that patients should, if possible, be operated on before left ventricular function becomes seriously impaired.

Balloon Aortic Valvuloplasty represents an increasingly attractive alternative to aortic valvotomy in children, adolescents, and young adults.

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