Left ventricular outflow tract obstruction accounts for 5% to 10% of all congenital heart defects and may be due to stenosis at a valvular, subvalvular, or supravalvular level. The obstruction can be isolated or occur at multiple levels and is often associated with other cardiac abnormalities. Valvular aortic stenosis (AS) is by far the most common type of left ventricular outflow tract obstruction and has three main causes: stenosis of a congenitally abnormal aortic valve, calcific degeneration of a tricuspid aortic valve, and rheumatic aortic valve disease. This chapter describes stenosis of a congenitally abnormal aortic valve in more detail.
Definition, Morphology, and Epidemiology
For the vast majority of adults with congenital AS, the most common morphologic finding is a bicuspid aortic valve (BAV) ( Fig. 35.1 ). Less than 5% of congenitally malformed aortic valves are unicuspid, tricuspid, or quadricuspid, all of which may result in valvular stenosis.
Unicuspid aortic valves may be (1) acommissural, characterized by a single cusp, a stenotic central orifice, and rudimentary commissures that do not divide the valve or (2) unicommissural, characterized by a single cusp with a single commissural attachment to the aortic wall and an elongated orifice. Patients with an acommissural valve typically present in the neonatal period as the pinhole opening is severely obstructive. The dysfunctional unicommissural type tends to present later in life with cardiovascular symptoms either directly related to the valve dysfunction or as a consequence of complications such as infective endocarditis or aortic dissection. Roberts and Ko examined 932 aortic valves excised predominately for AS over an 11-year period. Patients with rheumatic valvular disease were excluded. More than half of patients (54%) were found to have a congenitally malformed valve. BAVs were the most common morphologic finding and accounted for 54% of all aortic valves excised. Tricuspid aortic valves were the second most common finding (45%), and unicuspid aortic valves accounted for 5% of all valves excised. Patients with unicuspid valves typically present 10 to 20 years before bicuspid patients. In a retrospective study of patients undergoing unicuspid valve excision, the mean age of surgical intervention was 44 ± 12 years.
BAVs are the most common congenital cardiac anomaly, with an incidence of 1% to 2% in the general adult population. In an echocardiographic study of neonates, the overall prevalence of BAV was 4.6 per 1000 live births (7.1 per 1000 in male neonates and 1.9 per 1000 in female neonates). BAVs are the most common cause of isolated valvular AS in adults and the most frequent finding in aortic valve replacement (AVR) for AS up to 70 years of age.
Inherently stenotic dysplastic tricuspid aortic valves are often associated with a hypoplastic aortic annulus and usually present in infancy as a rare cause of valvular AS.
Isolated quadricuspid aortic valves ( Fig. 35.2 ) are rare. Quadricuspid aortic valve classification is dependent upon the relative size of the cusps or the position of the accessory cusp. Aortic regurgitation is the most frequent pathologic development; nevertheless, cases of severe AS have been reported. A retrospective review of 19,722 patient undergoing aortic valve surgeries identified 31 cases of dysfunctional quadricuspid aortic valves (0.0016%). The mean age at surgical intervention was 58 ± 18 years. Twenty-one patients (68% of quadricuspid valves identified) presented with isolated aortic regurgitation, five patients demonstrated AS, four patients presented with a combination of both, and one patient with a functional quadricuspid valve required excision of fibroelastoma.
Associated Lesions
In approximately 30% of individuals with a BAV, a further congenital aortic or cardiac abnormality is found. Most frequently the BAV is associated with an aortopathy, which may present with progressive dilation of the aortic root and/or the ascending aorta. Analogous to bicuspid valves, aortopathies are common in patients with unicuspid and quadricuspid aortic valves. In 149 patients with unicuspid valves undergoing aortic valve repair or replacement, 91 patients (61%) required a combined valve and aortic repair operation ; 23% of patients needing repair or replacement of a quadricuspid valve also required ascending aorta repair at surgery.
BAV disease is frequently found in association with coarctation of the aorta and less frequently with interruption of the aorta and isolated ventricular septal defect. Other associated abnormalities include a left dominant coronary artery system, subvalvular AS, parachute mitral valve, atrial septal defects, patent ductus arteriosus, bicuspid pulmonary valve, Ebstein anomaly, and hypoplastic left heart syndrome. The combined presence of multiple levels of left-sided heart obstructions (eg, subvalvular AS, BAV stenosis, aortic coarctation, parachute mitral valve, or supramitral ring) is termed Shone syndrome.
Unicuspid valves have been reported in association with patent ductus arteriosus, coarctation of the aorta, coronary artery anomalies, and great vessel anomalies.
Anomalous origins of the coronary arteries are associated with quadricuspid aortic valves.
Genetics and Molecular Biology
The genetic and molecular bases of bicuspid valves are much better understood than the rarer quadricuspid and unicuspid valves; however, a considerable number of questions remain unanswered. Studies have demonstrated substantial genetic heterogeneity in the pathogenesis of the bicuspid valve and many types of genetic variations exists from common single nucleotide polymorphisms to rare copy number variants and chromosomal abnormalities. The bicuspid valve and its associations are likely to represent the final common pathway for a wide variety of altered molecular events, genetic defects, and environmental modifiers. No single gene model clearly explains bicuspid valve inheritance.
Inheritances in familial clusters and sporadic cases have both been described. Studies of familial clusters are consistent with autosomal dominant inheritance with variable penetrance. Screening studies of index cases with BAV demonstrate an 8% to 10% prevalence of BAVs in asymptomatic first-degree relatives. First-degree relatives of patients with various types of left ventricular outflow tract obstruction are also at an increased risk of BAV. Turner syndrome demonstrates the highest syndromic penetration of bicuspid valve; 30% of affected individuals have a bicuspid valve. A bicuspid valve is also found in approximately 20% of patients with Loeys-Dietz syndrome. In addition to genetic syndromes, the BAV is associated with some congenital heart disorders, such as hypoplastic left heart syndrome, although again, the genetic basis remains unclear.
Investigators have uncovered multiple genetic loci for BAV through linkage analysis of BAV pedigrees. The most significant genetic loci identified are on chromosomes 3p22, 5q15-21, 9q22.33, 9q34-35 (NOTCH1), 10q23.3 (ACTA2), 13q33qter, 15q25-q26.1, 17q24 and 18q. Mutations in the signaling and transcriptional regulator NOTCH1 result in developmental aortic valve abnormalities and severe valve calcification in affected families. NOTCH1 remains the only gene identified through linkage analysis and positional cloning in isolated bicuspid valve. Mutations in the smooth muscle ACTA2 gene, which encodes vascular smooth muscle cells α-actin, are associated with familial thoracic aortic aneurysms and BAVs alongside premature coronary artery disease and cerebrovascular disease. Other potential candidate genes include endothelium-derived nitric oxide and the ubiquitin fusion degradation 1-like gene.
A genetic basis for the unicuspid valve may be extrapolated from the association with Turner syndrome, discovery in siblings and the finding of altered gene expression in patients with unicuspid valves.
The BAV arises from abnormal valvulogenesis and cusp formation, resulting in adjacent cusp fusion. Formations of two cusps of unequal size are found in the vast majority, and often the fibrous raphe is visible between the conjoined cusps. These valves tend to have three identifiable sinus of Valsalva, and the most common morphologic finding is fusion of the right and left coronary cusps. Truly bicuspid valves are rare and are identified as an absence of raphe associated with two identifiable sinuses of Valsalva.
The aortopathy associated with BAV disease is, in all probability, due to a combination of genetic inheritance resulting in intrinsic structural abnormalities of the aortic wall, abnormal postvalve hemodynamics, and the influence of traditional cardiovascular risk factors. The associated aortopathy can be markedly heterogenic with phenotypic expression encompassing, alternatively, the entire ascending aorta, the sinuses of Valsalva, or the tubular aorta as described in the literature.
The inherent aortic wall defects in patients with BAVs (including those valves which function normally by echocardiographic definition) may be mediated by coexisting defects in the aortic media. Potential defects identified include fragmentation of elastin, loss of smooth muscle cells, and an increase in collagen. These abnormalities may in part be due to increased release of matrix metalloproteinase-2 from microfibrils secondary to deficient expression of matrix protein fibrillin-1 (inhibitor). The increased activity of the matrix metalloproteinase leads to apoptosis and degeneration of the aortic wall with the eventual progression of aortic dilation. Inadequate fibrillin-1 has also been linked to deficiencies in valvulogenesis which may result in a BAV.
Increased tensile and shear stresses have also been shown to play a role in the pathogenesis of valvular disease and aortopathy. Distensibility of the aortic root permits a normal closed tricuspid aortic valve to open as a triangle and then a circle without causing flexion deformity of cuspal tissue. In contrast, even a “normally functioning” BAV is characterized by abnormal folding and creasing throughout the cardiac cycle, extended areas of valve contact, turbulent flow, and subclinical stenosis. The outlined stresses lead to valve damage, scarring, and calcification, possibly through activation of molecular pathways. Imaging studies have confirmed these “functionally normal” bicuspid valves (based on traditional echocardiographic criteria) demonstrate abnormal flow patterns and elevated aortic wall shear stress.
Turbulent flow into the ascending aorta, when added to the aforementioned intrinsic medial abnormalities, contributes to progressive dilation and increases the likelihood of rupture or dissection. Dilation of the root, ascending, and, occasionally, descending aorta can be found in association with the congenitally abnormal aortic valve although aortic arch involvement is rare. Traditional risk factors for atherosclerosis are also likely to play a role. The pathogenesis of progressive BAV disease and the associated aortopathy is complex and further work is required to fully understand the genetics and molecular development of the disease process.
Pathophysiology and Clinical Course (Without Intervention)
Only 1 in 50 children born with bicuspid or unicuspid aortic valves will develop significant obstruction or regurgitation by adolescence. BAV disease is usually a slowly progressive disorder which can present with either stenosis or regurgitation, although patients may present acutely with associated complications such as aortic dissection or infective endocarditis. Echocardiographic surveillance studies have demonstrated similar life expectancy to the general population at up to 20 years of follow-up in bicuspid patients with absent or minimal hemodynamic abnormality. However, cardiovascular events (medical and surgical) were more frequent affecting approximately 4 in 10 patients over 20 years. Furthermore, sudden death has been shown to be more common in patients with bicuspid valve after AVR compared with AVR for tricuspid AS.
Evidence of echocardiographic valve thickening may be seen as early as the second decade, and calcification is often evident by the fourth decade. Concomitant aortic regurgitation can also accelerate progression of valvular AS.
Asymptomatic patients with AS have similar outcomes to age-matched normal adults; however, disease progression with symptom onset is common. In a longitudinal study of the long-term outcomes of 622 adults with asymptomatic but hemodynamically severe AS at study inception, most developed symptoms within 5 years. Sudden death in patients with AS is an uncommon event, estimated at less than 1% per year when patients with known AS are observed prospectively ( Fig. 35.3 ).
There is marked individual variability in the rate of hemodynamic progression and a wide variability in the degree of outflow obstruction that causes symptoms depending in part on patient size and the level of physical activity. For this reason regular clinical follow-up is mandatory in all patients with asymptomatic mild-to-moderate AS.
Eventually, symptoms of angina, syncope, or heart failure develop after a long latent period, and the outlook changes dramatically. After the onset of symptoms, average survival is 2 to 3 years, with a high risk of sudden death. It is important to emphasize that symptoms may be subtle and unless specifically and carefully sought may not be elicited when taking a routine clinical history.
In most patients with severe AS, impaired platelet function and decreased levels of von Willebrand factor can be demonstrated. The severity of the coagulation abnormality correlates with the severity of AS and resolves after valve replacement, except when the prosthetic valve area is small for patient size (<0.8 cm 2 /m 2 ). This acquired von Willebrand syndrome is associated with clinical bleeding, most often epistaxis or ecchymoses, in approximately 20% of patients.
The prevalence of ascending aortic dilation associated with BAV varies from 35% to 80%, subject to the criteria for aortic dilation and screening methods used. The rate of dilation of the aorta is higher in the tubular ascending aorta than the sinus of Valsalva. Prevalence increases with age, beginning in childhood and continuing throughout life. A study of the prevalence of aortic dilation in association with bicuspid valve, by age quintile, demonstrated aortic dilation in 56% of those aged younger than 30 years old and up to 88% of those aged more than 60 years old. However, ongoing progression of the dilated aorta should not always be presumed. In one study, 43% of BAV patients with aortopathy did not show further dilation over a follow-up period of 3.6 ± 1.2 years.
Aortic dissection and rupture have been described in association with BAV disease. Aortic diameter has previously been shown to be a significant predictor of dissection and rupture in a cohort of mixed etiology. The lifetime risk of aortic dissection in patients with bicuspid valves is estimated at eight times that of the general population, but the overall absolute risk remains low and considerably lower than age-matched Marfan patients. Identified risk factors for aortic dissection in this patient group include greater aortic stiffness, male gender, hypertension, aortic size, concurrent aortic valve disease, Turner syndrome, family history of aortic disease, and prior coarctation.
Physical Examination
AS is typically first suspected on finding a systolic ejection murmur on cardiac auscultation. The classic finding is of a loud (grade 4/6), late-peaking systolic murmur that radiates to the carotids. The intensity of the murmur does not correspond to the severity of the AS. The Gallavardin phenomenon occurs when the murmur is heard best at the left ventricular apex. A single or paradoxically split second heart sound and a delayed and diminished carotid upstroke confirm the presence of severe AS. The only physical examination finding that is reliable in excluding the possibility of severe AS is a normally split second heart sound ( Box 35.1 ). A fourth heart sound may be palpable and audible due to vigorous left atrial contraction. The apical impulse is not usually displaced but is often sustained due to secondary left ventricular hypertrophy.
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The classic findings of a loud (grade 4/6), late-peaking systolic murmur that radiates to the carotids, a single or paradoxically split second heart sound, and a delayed and diminished carotid upstroke confirm the presence of severe aortic stenosis.
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Physical examination findings are specific but not sensitive for the diagnosis of aortic stenosis severity.
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An aortic ejection click may be heard after the first heart sound early in aortic stenosis with a congenital bicuspid valve, when the cusps are stiff but still somewhat compliant and mobile. Vigorous left atrial contraction can commonly lead to a fourth heart sound.
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The only physical examination finding that is reliable in excluding the possibility of severe aortic stenosis is a normally split second heart sound.
An aortic ejection click may be heard after the first heart sound early in AS with a congenital bicuspid valve, when the cusps are stiff but still somewhat compliant and mobile. This tends to diminish as the valvular disease progresses. Unicuspid and quadricuspid aortic valves do not demonstrate the same ejection click and cannot be reliably diagnosed by physical examination only.
Investigations
Brain Natriuretic Peptide
Plasma brain natriuretic peptide (BNP) and N-terminal pro-BNP (NT-ProBNP) concentrations are higher in symptomatic than asymptomatic severe AS patients, come down after AVR, and among asymptomatic patients are independently predictive of symptom-free survival.
The predictive value was illustrated in a study of 130 patients with severe AS (mean aortic valve area [AVA] of 0.64 cm 2 and mean transvalvular gradient of 64 mm Hg) who were observed for 1 year. Patients with a plasma NT-ProBNP concentration less than 80 pmol/L had a significantly higher rate of symptom-free survival when compared with those with values greater than 80 pmol/L at 3 (100% vs. 92%), 6 (88% vs. 58%), 9 (88% vs. 35%), and 12 months (69% vs. 18%) ( p <.001).
BNP has also been found to be significantly higher in truly severe rather than pseudosevere low-flow/low-gradient AS and predicted survival in those patients who had valve replacement surgery.
At present, routine measurement of plasma BNP or NT-ProBNP in asymptomatic patients with AS is not a guideline recommendation. However, in a patient with equivocal symptoms and severe valve obstruction, a markedly elevated value suggests the need for close follow-up or intervention.
Electrocardiography
The primary electrocardiography (ECG) findings are related to the presence of left ventricular hypertrophy and are not specific. The voltage of the QRS complex is often markedly increased, often with associated ST-T wave changes reflecting chronic subendocardial ischemia. There may be left atrial enlargement with a broad bifid p wave in lead II. Left ventricular hypertrophy is found in 85% of patients with severe AS; however, severe AS is not excluded by the absence of hypertrophy on ECG.
Intraventricular or atrioventricular conduction abnormalities are common and may be due to severe hypertrophy, extension of calcium from the valve and valve ring into the interventricular septum, or concomitant heart disease. Ventricular and supraventricular arrhythmias are uncommon and usually reflect underlying progressive left ventricular dysfunction. Atrial fibrillation is typically a late arrhythmia.
Chest Radiography
The routine chest radiograph is frequently normal when AS is mild to moderate. Findings that may be seen include rounding of the left ventricular apex due to left ventricular hypertrophy, calcification of the aortic cusps and root, and dilation of the ascending aorta. As the disease progresses, cardiomegaly may be seen alongside the characteristic x-ray changes of pulmonary edema.
The chest radiograph may also be helpful in determining other associated congenital lesions, such as the presence of aortic coarctation. In these cases the x-ray features may include rib notching and convexity of the proximal descending aorta.
Echocardiography
Transthoracic echocardiography is the key investigation used in the assessment of patients across the spectrum of AS. A complete echocardiographic assessment of AS should make use of all echocardiographic modalities ( Figs. 35.4 and 35.5 ). A summary of the comprehensive echocardiographic evaluation of a patient with congenital AS is presented in Table 35.1 .
| Assessment | |
|---|---|
| Left ventricle | Size, wall thickness, systolic and diastolic function |
| Aortic valve | Morphology, mobility, and degree of calcification |
| Degree of aortic stenosis | Peak echocardiographic aortic velocity, mean gradient, calculation of aortic valve area, valve area indexed to body surface area |
| Aortic regurgitation | Severity and etiology |
| Thoracic aorta | Aortic root dimensions, aortic arch and descending thoracic aorta |
| Concomitant lesions | Congenital (eg, aortic coarctation or other valve) |
In most patients with preserved left ventricular systolic function the severity of the stenotic lesion can be defined and categorized by echocardiography, as shown in Table 35.2 . Serial echocardiograms assess changes in stenosis severity, left ventricular hypertrophy, and left ventricular function.
| Stage | Definition | Valve Anatomy | Valve Hemodynamics |
|---|---|---|---|
| A | At risk of AS | Bicuspid aortic valve | Aortic V max <2 m/s |
| B | Progressive AS | Mild-to-moderate calcification with some reduction in systolic motion | Mild AS ; Aortic V max ≤2.9 m/s or mean gradient ≤20 mm Hg Moderate AS ; Aortic V max ≤3.9 m/s or mean gradient ≤39 mm Hg |
| C1 | Asymptomatic severe AS | Severe leaflet calcification or congenital stenosis with severely reduced leaflet opening | Aortic V max ≥4 m/s or Mean gradient ≥40 mm Hg AVA typically ≤1 cm 2 (or AVAi ≤0.6 cm 2 /m 2 ) |
| C2 | Asymptomatic severe AS with LV dysfunction | Severe leaflet calcification or congenital stenosis with severely reduced leaflet opening | Aortic V max ≥4 m/s or mean gradient ≥40 mm Hg AVA typically ≤1 cm 2 (or AVAi ≤0.6 cm 2 /m 2 ) |
| D1 | Symptomatic severe high gradient AS | Severe leaflet calcification or congenital stenosis with severely reduced leaflet opening | Aortic V max ≥4 m/s or mean gradient ≥40 mm Hg AVA typically ≤1 cm 2 (or AVAi ≤0.6 cm 2 /m 2 ) but may be larger with mixed AS/AR |
| D2 | Symptomatic severe low-flow/low-gradient AS with reduced LVEF | Severe leaflet calcification with severely reduced leaflet motion |
|
| D3 | Symptomatic severe low gradient AS with normal LVEF or paradoxical low-flow severe AS | Severe leaflet calcification with severely reduced leaflet motion |
|
Further additional imaging can be useful as transthoracic echocardiography does not always clearly delineate the valvular morphology. In a study of 100 patients, interpretation of transthoracic echocardiographic images was only successful in correctly identifying 57% of valvular morphologies subsequently determined by examination of the operatively excised stenotic aortic valve. This cohort included unicuspid, bicuspid, and tricuspid valves.
Transesophageal echocardiography (TEE) can add incremental value in the assessment of valvular morphology and function in patients with nondiagnostic transthoracic echocardiography. It provides excellent visualization of the short-axis view of the aortic valve ( Fig. 35.6 ), and three-dimensional (3D) planimetry of the systolic orifice can be performed. This area correlates well to the area derived by the standard continuity equation as applied to Doppler transthoracic echocardiography. TEE can be used effectively to evaluate the ascending aorta ( Fig. 35.7 ) and is useful in patients with suspected or confirmed aortic valve endocarditis. TEE is also a highly accurate method for diagnosing aortic dissections.
Dobutamine stress echocardiogram is increasingly used in the diagnosis of “low-flow/low gradient” AS and is discussed further in Stress Testing.
Computed Tomography and Magnetic Resonance Imaging
Computed tomography (CT) and magnetic resonance imaging (MRI) are valuable for assessing the anatomy of the entire aorta. International guidelines recommend the use of either CT or MRI when the morphology of the ascending aorta cannot be clearly delineated on echocardiography. Aortic dimensions of greater than 40 mm require serial evaluations, and once greater than 45 mm, evaluations should be performed on a yearly basis. Both are particularly helpful in evaluating associated lesions, such as aortic coarctation, and also in serial assessment of a dilated aortic root and ascending aorta. For serial assessment of young adults, MRI has the benefit of freedom from ionizing radiation.
CT also plays an important role of in assessing annular size and eccentricity in patients undergoing transcatheter aortic valve implantation (TAVI) and has the potential advantage of simultaneously evaluating the coronary artery system.
Cardiac magnetic resonance (CMR) is increasingly used qualitatively and quantitatively to assess valvular lesions, including BAV disease. CMR imaging studies can image the entire aorta and have also been used to evaluate systolic flow in the ascending aorta and systolic wall sheer stress. As mentioned earlier, turbulent flow into the ascending aorta is likely to contribute to the aortic dilation associated with bicuspid valve disease and has indeed been visualized in four-dimensional (4D) flow assessment CMR studies. In the largest 4D CMR study to date, bicuspid valve patients had predominantly abnormal right-handed helical flow in the ascending aorta, with the vast majority of atypical flow normalizing in the proximal descending aorta. All types of abnormal flow were associated with greater aortic dimensions when compared with normal flow.
Cardiac Catheterization
Cardiac catheterization is used selectively for hemodynamic measurements when noninvasive tests are inconclusive or when there is a discrepancy between noninvasive tests and clinical findings regarding the severity of AS ( Fig. 35.8 ).
Coronary angiography is recommended before aortic valve surgery in patients with AS at risk for coronary artery disease and before a Ross procedure if noninvasive imaging of the proximal coronary arteries is inadequate.
CT coronary angiography may be an acceptable alternative to coronary angiography for many of these patients.
Stress Testing
Exercise testing should not be performed in symptomatic patients owing to a high risk of complications. However, in asymptomatic patients, exercise testing is relatively safe when performed under the supervision of an experienced physician and may provide information that is not uncovered during the initial clinical evaluation.
In asymptomatic adults younger than 30 years of age, exercise stress testing may be useful to determine exercise capability, symptoms, and blood pressure response. Exercise stress testing is considered reasonable for patients with a mean Doppler gradient greater than 30 mm Hg or peak Doppler gradient greater than 50 mm Hg if the patient is interested in athletic participation or if clinical findings differ from those of noninvasive measurements. It is also of value for the evaluation of asymptomatic young adults with a mean Doppler gradient greater than 40 mm Hg or a peak Doppler gradient greater than 64 mm Hg or when the patient anticipates athletic participation or pregnancy.
Low-flow/low gradient severe AS occurs when the mean pressure gradient across the valve is less than 40 mm Hg, while the AVA by continuity equation is typically less than 1.0 cm 2 in the setting of reduced left ventricular ejection fraction (LVEF). The hypothesis for this phenomenon is that a reduction in left ventricular systolic function results in reduced transvalvular flow and a low resting mean gradient despite significant stenosis of the valve. Exercise or low-dose pharmacologic (ie, dobutamine infusion) stress can be used to determine whether there are any significant changes in Doppler measurements during stress. For the test to have meaning there must be a 20% increase in stroke volume, otherwise the severity of AS cannot be accurately assessed. Patients with true severe AS will demonstrate an increase in mean gradient (>40 mm Hg) secondary to the inotropic effect without an increase in the fixed stenotic valve area. These patients are likely to respond favorably to surgery. Patients who do not have true anatomically severe stenosis will exhibit an increase in the valve area, as the improved left ventricular function leads to improved valvular excursion, and little change in gradient during an increase in stroke volume.
Although patients with low-output severe AS have a poor prognosis overall, in those with contractile reserve outcomes are improved with AVR when compared with medical therapy. Patients who fail to show an increase in stroke volume with dobutamine (a lack of “contractile reserve”) have a poor prognosis with medical therapy and a high operative mortality.
A further group who may benefit from stress testing are the symptomatic severe low-gradient AS with normal LVEF or what is increasingly known as paradoxic low-flow severe AS. Comparable to the previously described low-flow/low-gradient severe AS subset, echocardiography demonstrates a mean gradient less than 40 mm Hg with an AVA of less than 1 cm ; however, the LVEF in this group is within normal limits. The proposed mechanism for a low mean gradient is a reduction in transvalvular flow secondary to a small hypertrophied left ventricle. One small study has suggested a role for dobutamine stress echocardiography in identifying true AS in this group, but this remains controversial at present.
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