Aortic stenosis overview

Aortic Stenosis Microchapters
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Overview
Historical Perspective
Classification
Pathophysiology
Causes
Differentiating Aortic Stenosis from other Diseases
Epidemiology and Demographics
Risk Factors
Natural History, Complications and Prognosis
Diagnosis
History and Symptoms
Physical Examination
Cardiac Stress Test
Electrocardiogram
Chest X Ray
CT
MRI
Echocardiography
Cardiac Catheterization
Aortic Valve Area
Aortic Valve Area Calculation
Treatment
General Approach
Medical Therapy
Surgery
Percutaneous Aortic Balloon Valvotomy (PABV) or Aortic Valvuloplasty
Transcatheter Aortic Valve Replacement (TAVR) TAVR vs SAVR Critical Pathway Patient Selection Imaging Evaluation Valve Types TAVR Procedure Post TAVR management AHA/ACC Guideline Recommendations
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-In-Chief: Mohammed A. Sbeih, M.D. [2]; Lakshmi Gopalakrishnan, M.B.B.S. [3] Assistant Editor-In-Chief: Kristin Feeney, B.S. [4]

Overview

The aortic valve ensures that the blood moves forward from the left ventricle into the aorta and that it does not leak backwards during diastole. When functioning appropriately, the aortic valve does not impede the flow of blood between the left ventricle and the aorta and it does not leak. Under some circumstances, the aortic valve becomes narrower than normal impeding the flow of blood. This is known as aortic valve stenosis, or aortic stenosis, often abbreviated as AS.

Classification

Aortic stenosis can be classified broadly into two main categories: acquired and congenital. Further classification can be applied based on the origin of the stenosis such as acquired rheumatic, congenital bicuspid, congenital subaortic, congenital subvalvular, and congenital supravalvular aortic stenosis.

Pathophysiology

Aortic stenosis is the progressive narrowing of the aortic valve. Calcific aortic stenosis, in particular, is an active atherosclerotic pathology where inflammation, fibrosis and calcification are involved in the progressive narrowing of the effective aortic valve area in the absence of any commissural fusion. In contrast, rheumatic aortic stenosis is due to fusion of the commissures with valvular scarring and calcification. Aortic stenosis causes an impedance to the antegrade blood flow not only at the level of the aortic valve itself, but also at the subvalvular (below the aortic valve) or supravalvular (above the aortic valve) levels. As a result, chronic pressure overload develops in the left ventricle. The left ventricle undergoes hypertrophy as an initial adaptive mechanism to overcome the increased afterload. This compensatory mechanism ends up being maladpative by causing apoptosis of the hypertrophied myocytes and subsequent heart failure. Hence, aortic stenosis is a progressive valvular disease which progression depends mainly on the degree of the narrowing of the aortic valve as well as on the maladaptive ventricular wall response.

Narrowing of the Aortic Valve

Calcific aortic stenosis has long been thought to be the result of a prolonged wear and tear mechanism and age related degeneration of the valvular cusps as opposed to cusp fusion of the aortic valve in rheumatic heart disease. However, recent studies have revealed that the underlying pathophysiology of calcific aortic stenosis is an active inflammation, fibrosis and calcification. Genetic predisposition plays a role in the rate of progression of aortic stenosis. The stages of the formation of the calcific aortic stenosis are endothelial damage, inflammation, fibrosis and calcification. Ont he other hand, Rheumatic aortic stenosis is due to the fusion of the valve commissures and subsequent scarring and calcification. Rheumatic aortic stenosis is always accompanied by mitral regurgitation.

Left Ventricular Hypertrophy

Long-standing aortic stenosis exposes the left ventricle to prolonged pressure overload which leads to concentric hypertrophy. The left ventricular wall increases in thickness (i.e. concentric hypertrophy occurs) as a result of the parallel replication of the sarcomeres.

Diastolic Dysfunction

During the initial period of concentric hypertrophy, the left ventricle is not dilated and there is preservation of left ventricular systolic function. Diastolic function, however, may be reduced due to a reduction in diastolic compliance .

This diastolic dysfunction may in turn lead to a rise in pulmonary capillary wedge pressure and dyspnea. The cardiac output may also be reduced as a result of the diastolic dysfunction and the impaired filling of the left ventricle. Early in the course of aortic stenosis, there may be a failure to augment cardiac output during exercise resulting in dyspnea on exertion.

Systolic Dysfunction

Later in the course of aortic stenosis, left ventricular dysfunction may develop due to a variety of pathophysiological processes. Systolic dysfunction is associated with a poor prognosis, and often does not partially or fully reverse following operative repair .

Excess Hypertrophy Causes Systolic Dysfunction

The massive concentric hypertrophy, characterized by a reduced diastolic radius-to-wall thickness ratio, has shown to initially counter balance the increased systolic left ventricular pressure; nevertheless, if this process continues, an inverse relationship has been observed such that the ejection fraction eventually goes down as the left ventricular mass increases beyond a certain point..

Myocardial Ischemia

The hypertrophied left ventricle and the prolonged ejection time (the time for the heart to eject blood) result in an increase in the myocardial oxygen requirements. In addition, the elevated diastolic filling pressure reduces the gradient between the aorta and the right atrium ("the height of the waterfall") which normally drives coronary blood flow. There may be a relative reduction in the density of the capillary network. The hypertrophied ventricle may also compress the capillaries. All of the above lead to a reduction in coronary blood flow even in the absence of obstructive epicardial stenosis. This may lead to subendocardial ischemia during stress or exercise..

Myocardial Fibrosis

Myocardial scarring or fibrosis may develop with prolonged aortic stenosis possibly due to chronic subendocardial ischemia or increased wall stress.

Dyssynchronous Contraction

Another factor that may contribute to the reduced left ventricular systolic function is the dyssynchronous contraction subsequent to regional wall motion abnormalities, fibrosis or ischemia.

Atrial Fibrillation

The stiff, non-compliant left ventricle can become increasingly dependent on the left atrium for filling. The development of atrial fibrillation and the loss of atrial contractility can result in reduced left ventricular filling and reduced cardiac output.

Epidemiology and Demographics

Aortic stenosis primarily affects older adults and the majority of cases are due to calcific degeneration. Aortic stenosis tends to affect approximately 2% of patients over the age of 65, 3% of patients over the age of 75, and 4% of patients over the age 85.

Risk Factors

The most common risk factor for the subsequent development of aortic stenosis is congenital bicuspid aortic valve. Rheumatic fever is another risk factor for the subsequent development of aortic stenosis (rheumatic heart disease). Risk factors that may speed up the progression of degenerative calcific aortic stenosis include:

• Hypertension
• Diabetes mellitus
• Hyperlipoproteinemia
• Uremia
• Smoking

Causes

Aortic stenosis can be categorized as either acquired or congenital.

Valvular Aortic Stenosis:

Acquired

• Calcific degeneration
• Rheumatic fever
• Infective endocarditis
• Systemic lupus erythromatosis
• Fabry's disease
• Aortic sclerosis

Congenital

• Bicuspid aortic valve
• Hypoplastic annulus
• Three cuspid valve with commissural fusion

Subvalvular Aortic Stenosis:

• Membranous diaphragm
• Hypertrophic cardiomyopathy

Supravalvular Aortic Stenosis:

• Hypoplasia of aorta
• Hourglass constriction of aorta
• Fibromembranous aortic lesion

A complete list of causes in alphabetic order includes the following:

• Age-induced calcification of normal tricuspid aortic valve with degenerative 'wear and tear'.
• Congenital bicuspid aortic valve
• Endocarditis
• Fabry disease
• Homozygous type II hypercholesterolemia
• Ochronosis
• Paget disease
• Prosthetic aortic valve
• Radiation treatment to the chest
• Rheumatic fever (slowly progressive stenosis)
• Subvalvular aortic stenosis
• Supravalvular aortic stenosis
• Williams syndrome, autosomal dominant trait is associated with supravalvular aortic stenosis

Differential Diagnosis

Aortic stenosis must be differentiated from other cardiac or pulmonary causes of dyspnea, weakness, and dizziness. Furthermore, if there is left ventricular outflow tract obstruction, it is critical to identify whether the obstruction is subvalvular, valvular or supravalvular or due to Hypertrophic Cardiomyopathy (HOCM).

Natural History, Complications & Prognosis

Degenerative Calcific Aortic Stenosis

When aortic stenosis is due to the degeneration of a calcified aortic valve, there is a prolonged latent period during which symptoms may be minimal or even lacking . This form of aortic stenosis presents later in life, usually after the age of 75 . The average rate of progression in valvular aortic stenosis, once moderate stenosis is present and symptomatic, is a decrease in valve area of 0.1 cm² per year . Also on average, there is an increase in the jet velocity of 0.3 m / second per year and an increase in the mean pressure gradient of 7 mm Hg per year . There is tremendous individual variability in the rate of progression of aortic stenosis. Risk factors for atherosclerosis (such as age, smoking, hypertension, obesity and diabetes, lipid abnormalities, chronic renal failure and dialysis) and atherosclerotic disease itself (as in the case of a concomitant coronary artery disease) are associated with more rapid rates of aortic stenosis progression.

Aortic Stenosis Due to Rheumatic Heart Disease

These patients generally become symptomatic after the sixth decade.

Bicuspid Aortic Valve Disease

Bicuspid aortic valve stenosis presents one to two decades earlier than regular aortic valve stenosis. The rate of progression of degenerative aortic stenosis can be faster than in those with congenital or rheumatic disease . Bicuspid aortic valve functions without any significant pressure gradient during childhood. However, the thickening and calcification of the valves may be detectable pathologically and on echocardiography in the second decade. Bicuspid aortic valve progresses to aortic stenosis requiring operative correction in approximately 75% of cases.

Bicuspid aortic stenosis progressively leads to heart failure, arrythmias, angina and other symptoms. The manifestations of aortic stenosis occur between 40 to 60 years of age which is relatively younger than the age at which the symptoms of tricuspid aortic stenosis manifests. However, children who develop early pathologic changes in bicuspid aortic valve are more likely to develop aortic insufficiency than stenosis.

Aortic Sclerosis

Aortic sclerosis (defined as aortic valve thickening without obstruction to ventricular outflow) may progress to narrowing of the aortic valve or aortic stenosis. If the pulse pressure or upstroke of the pulse diminishes in the patient with aortic sclerosis, this can be a sign of progression to aortic stenosis.

Complications

Degenerative Calcific Aortic Stenosis

If left untreated, aortic stenosis may lead to complications such as angina, syncope, or heart failure. A complete list of complications of aortic stenosis includes the following:

• Angina
• Arrhythmias
• Atrial fibrillation
• Bleeding: Impaired platelet function and coagulation abnormalities, as decreased levels of Von Willebrand factor, can be seen in most patients with severe AS. This resolves after valve replacement procedure. 20% of patients have clinical bleeding, most often epistaxis or ecchymoses Aortic stenosis may result in a form of von Willebrand disease due to an increased turbulence around the stenosed aortic valve which subsequently triggers a break down of coagulation factor VIII-associated antigen, (also called von Willebrand factor) and results in a variant of von Willebrand disease.
• Congestive heart failure, particularly left-sided heart failure or systolic dysfunction
• Endocarditis
• Fainting or syncope: Since the stenosed aortic valve may limit the heart's output, people with aortic stenosis are at risk of syncope and dangerously low blood pressure with the use of some common medications. Ironically, these same medicines are used to treat a variety of cardiovascular diseases, many of which may co-exist with aortic stenosis. Examples include nitroglycerin, nitrates, ACE inhibitors, terazosin (Hytrin), and hydralazine. Note that all of these substances lead to peripheral vasodilation. Normally, however, in the absence of aortic stenosis, the heart is able to increase its output and thereby offset the effect of the dilated blood vessels. However, some cases of aortic stenosis can be associated with outflow blood obstruction which can prevent an increase of the cardiac output. Hence, low blood pressure or syncope may ensue.
• Left ventricular hypertrophy
• Myocardial infarction

Bicuspid Aortic Valve Disease

Bicuspid aortic valve disease is associated with the following complications:

• Aortic stenosis in the majority of patients (75%).
• Aortic insufficiency
• Endocarditis
• Aortic aneurysm
• Aortic dissection
• Sudden death can occur in children during and immediately after exertion especially among those with pressure gradient > 50 mmHg across the aortic valve.

Prognosis

Asymptomatic Patients

The prognosis of patients with aortic stenosis who do not have symptoms is quite good . The annual mortality rate is < 1% per year in asymptomatic patients. Only 4% of sudden cardiac deaths that occur in patients with aortic stenosis occur in those patients who are asymptomatic.

Symptomatic Patients

Medical treatment of newly diagnosed moderate to severe symptomatic aortic stenosis is associated with a 25% mortality at one year, and a 50% mortality at two years. Half of the deaths are due to sudden cardiac death .

When aortic stenosis is left untreated, the average survival is 5 years after the onset of angina, 3 years after the onset of syncope, and 1 year after the onset of congestive heart failure .

Diagnosis

History and Symptoms

The main symptoms of aortic stenosis include angina, syncope and congestive heart failure. Left untreated, the average survival is 5 years after the onset of angina, 3 years after the onset of syncope, and 1 year after the onset of congestive heart failure . Other symptoms include dyspnea on exertion, orthopnea and paroxysmal nocturnal dyspnea.

Physical Examination

Aortic stenosis is most often diagnosed when it is asymptomatic and can sometimes be detected during routine examination of the heart and circulatory system. The major signs include pulsus parvus et tardus (a slow-rising, small volume carotid pulse), a lag time between apical and carotid impulses, and a distinct systolic ejection murmur.

Electrocardiogram

The electrocardiogram in the patient with moderate to severe aortic stenosis may reveal left ventricular hypertrophy and heart block.

Chest X-ray

Chest x ray may be used as a diagnostic tool in the evaluation of aortic stenosis. Findings associated with aortic stenosis include left ventricular hypertrophy and calcification of the aortic valve.

MRI

Magnetic resonance imaging is rarely used in the diagnosis of aortic stenosis, except in rare cases where the echocardiographic findings are inconclusive. There is a signal void where the high velocity jet exits the aortic valve.

CT

Computed tomography can be helpful as a diagnostic tool in conditions where the echocardiographic findings are inconclusive.

Echocardiography

Echocardiography is the best non-invasive test to evaluate the aortic valve anatomy and function. Echocardiography can be used to estimate the gradient across the aortic valve using the modified Bernoulli equation (gradient = 4 X velocity²). The flow must be constant, so as the velocity increases, the valve area decreases proportionally. Echocardiography can also be used to assess the severity of left ventricular hypertrophy.

Cardiac Catheterization

Left and right heart catheterization as well as angiography may be useful in the assessment of the patient prior to aortic valve replacement surgery.

Aortic Valve Area

The aortic valve area is the size of the orifice for blood to flow from the left ventricle to the aorta. The aortic valve area is reduced in aortic stenosis, and the aortic valve area is the metric that is used to gauge the need for aortic valve replacement surgery. The pressure gradient across a narrowed aortic valve cannot be used to gauge the need for valve replacement as the gradient may be low in patients with impaired left ventricular function.

Aortic Valve Area Calculation

The calculation of the aortic valve area is an indirect method used to determine the area of the aortic valve. The calculated aortic valve orifice area is currently one of the measures for evaluating the severity of aortic stenosis. An aortic valve having an area less than 0.8 cm² is considered to be severe aortic stenosis.

There are many ways to calculate the aortic valve area. The most commonly used methods involve measurements taken during echocardiography. For interpretation of these values, the aortic valve area is generally divided by the body surface area.

Treatment

Once a patient becomes symptomatic with aortic stenosis, aortic valve replacement should be performed as long as the patient can tolerate surgery and has no co-morbidities. Patients undergoing aortic valve replacement and having co-existing multi-vessel coronary artery disease should have concomitantly CABG.If a patient has extensive co-morbidities, transcatheter aortic valve implantation can be considered. Aortic valvuloplasty can be considered in those patients who are too sick for surgery or transcatheter aortic valve implantation. If severe left ventricular dysfunction is present in the setting of aortic stenosis, it is of utmost importance to differentiate between true severe aortic stenosis and pseudo-severe aortic stenosis as these two entities have different pathophysiologies and different outcomes after aortic valve replacement. Medical therapy reduces symptoms but does not prolong life. Asymptomatic patients should be followed up yearly for any new onset of symptoms. No treatment has been proven to delay the progression of aortic stenosis. Follow up with an echocardiography should be done every 1, 3, 5 years in severe, moderate and mild aortic stenosis respectively.

Shown below is an algorithm showing the management of symptomatic and asymptomatic patients with aortic stenosis.

Management of patients with aortic stenosis

Shown below is an algorithm showing a more detailed approach to patients with low flow, low gradient aortic stenosis.

Approach to patients with low flow, low gradient aortic stenosis

Medical Therapy

While medical therapy may improve the symptoms of patients with aortic stenosis, medical therapy does not prolong life expectancy. Aortic valve replacement remains the definitive treatment for symptomatic aortic stenosis and it improves both the symptoms and life expectancy of the patients. When pharmacological therapies are used, extreme caution must be taken in the administration of vasodilators as an excess in vasodilation may lead to hypotension, a reduction in perfusion pressure to the heart, a further decline in cardiac output and further hypotension. This vicious circle can be fatal and must be avoided at all costs.

Surgery

Aortic stenosis requires aortic valve replacement once the patient becomes symptomatic.

Percutaneous Aortic Balloon Valvotomy (PABV) or Aortic Valvuloplasty

Surgical aortic valve replacement is the mainstay of the treatment of aortic stenosis as it improves both symptoms and life expectancy. However, some patients may not be surgical candidates due to coexisting comorbidities. Hence, minimally invasive treatment such as percutaneous aortic balloon valvotomy (PABV) maybe an alternative to surgery as a palliative strategy. PABV is a procedure during which one or more balloons are placed across a stenotic valve and then inflated in order to cause a decrease the severity of aortic stenosis. This is to be distinguished from transcatheter aortic valve implantation (TAVI) which is a different method that involves replacement of the valve percutaneously.

Transcatheter Aortic Valve Implantation

Until recently, aortic valve replacement (AVR) was the only effective treatment for severe symptomatic aortic stenosis. However, over the past decade percutaneous treatment of aortic valve disease with the implantation of a stent-based valve prosthesis has been introduced as a new treatment in patients considered inoperable because of severe co-morbidities. In Transcatheter Aortic Valve Implantation (TAVI) also known as Percutaneous Aortic Valve Replacement (PAVR), a synthetic valve is advanced to the heart through a small hole made in the groin. This procedure is similar in its mechanism to the insertion of a stent, or performing balloon angioplasty albeit with much larger equipment. Traditional aortic valve replacement is an invasive surgical procedure, with considerable mortality and morbidity, especially in more fragile patients. In the newly developed TAVI procedure, the dysfunctional aortic valve is replaced percutaneously, which obviates the need for open heart surgery.

Prevention

Aortic stenosis associated with rheumatic heart disease can be minimized with antibiotic therapy in patients with documented streptococcal pharyngitis (strep throat). Bicuspid aortic valve disease is a congenital variant and cannot be prevented. Calcific degeneration of the valve can potentially be minimized by rosouvistatin and other measures targeting prevention of atherosclerosis.

Precautions and Prophylaxis

People with aortic stenosis of any etiology are at risk for the development of infection of their stenosed valve, i.e. infective endocarditis and antibiotic prophylaxis should be considered. Patients with severe aortic stenosis should avoid strenuous exercise and any exercise that greatly increases afterload such as weight lifting.