Aortic Stenosis Morphology
- Steven A. Goldstein, MD
Congenital aortic stenosis
Bicuspid Aortic Valve
Congenital aortic valve malformation reflects a phenotypic continuum of unicuspid valve (severe form), bicuspid valve (moderate form), tricuspid valve (normal, but may be abnormal), and the rare quadricuspid forms. Bicuspid aortic valves (BAVs) are the result of abnormal cusp formation during the complex developmental process. In most cases, adjacent cusps fail to separate, resulting in one larger conjoined cusp and a smaller one. Therefore, BAV (or bicommissural aortic valve) has partial or complete fusion of two of the aortic valve leaflets, with or without a central raphe, resulting in partial or complete absence of a functional commissure between the fused leaflets.
The generally accepted prevalence of BAV in the general population is 1% to 2%, making it the most common congenital heart defect. Information on the prevalence of BAV comes primarily from pathology centers. Valvular aortic stenosis (AS), a chronic progressive disease, usually develops over decades. Box 94.1 lists the most common etiologies of valvular AS, as illustrated in Figure 94.1 . The majority of cases of AS are acquired and result from degenerative (calcific) changes in an anatomically normal trileaflet aortic valve that becomes gradually dysfunctional over time. Congenitally abnormal valves may be stenotic at birth but usually become dysfunctional during early adolescence or early adulthood. A congenitally bicuspid aortic valve is now the most common course of valvular AS in patients under the age of 65. Rheumatic AS is now much less common than in prior decades and is virtually always accompanied by mitral valve disease. Other forms of nonvalvular left ventricular outflow obstruction (e.g., discrete subvalve AS, hypertrophic cardiomyopathy, and supravalve AS) are discussed in other chapters.
- 1.
Congenital (unicuspid, bicuspid, quadricuspid)
- 2.
Degenerative (sclerosis of previously normal valve)
- 3.
Rheumatic
The most reliable estimate of BAV prevalence is often considered to be the 1.37% reported by Larson and Edwards. The authors have a special expertise in aortic valve disease and amassed 21,417 consecutive autopsies with 293 BAVs. An echocardiographic survey of primary school children demonstrated a BAV in 0.5% of males and 0.2% of females. A more recent study detected 0.8% BAVs in nearly 21,000 men in Italy who underwent echocardiographic screening for the military. Table 94.1 summarizes data on the prevalence of bicuspid valves. Bicuspid aortic valve is seen predominantly in males, with a 2:1 male-to-female ratio. Although BAV may occur in isolation, it may also be associated with other congenital cardiovascular malformations, including coarctation, patent ductus arteriosus, supravalve AS, atrial septal defect, ventricular septal defect, sinus of Valsalva aneurysm, and coronary artery anomalies. There are also several syndromes in which BAV is a part of left-sided obstructive lesions of left ventricular inflow and outflow obstruction, including Shone syndrome (multiple left-sided lesions of inflow and outflow obstruction), Williams syndrome (supravalvular stenosis), and Turner syndrome (coarctation).
| Author | Year | (n) | BAV Prevalence | Method | Reference |
|---|---|---|---|---|---|
| Wauchope | 1928 | 9,996 | 0.5 | Autopsy | 2 |
| Gross | 1937 | 5,000 | 0.56 | Autopsy | 3 |
| Larson and Edwards | 1984 | 21,417 | 1.37 | Autopsy | 4 |
| Datta et al | 1988 | 8,800 | 0.59 | Autopsy | 5 |
| Pauperio et al | 1999 | 2,000 | 0.65 | Autopsy | 6 |
| Basso et al | 2004 | 817 | 0.5 | 2D-echo | 8 |
| Nistri et al | 2005 | 20,946 | 0.8 | 2D-echo | 9 |
Natural History of Bicuspid Aortic Valves
Although a few patients with BAV may go undetected or without clinical consequences for a lifetime, most will develop complications. The most important clinical consequences of BAV are valve stenosis, valve regurgitation, infective endocarditis, and aortic complications such as dilatation, dissection, and rupture ( Box 94.2 ). Estimates of the prevalence of these complications and outcomes have varied depending on the era of the study, the cohort selected, and the method used to diagnose BAV (clinical exam vs. cardiac catheterization vs. echocardiography). Several large recent studies have helped to better define the unoperated clinical course in the modern era.
Valve complications
- •
Stenosis
- •
Regurgitation
- •
Infection (endocarditis)
- •
Aortic complications
- •
Dilatation
- •
Aneurysm
- •
Dissection
- •
Rupture
- •
Isolated AS is the most frequent complication of BAV, occurring in approximately 85% of all BAV cases. Bicuspid aortic valve accounts for the majority of patients aged 15 to 65 years with significant AS. The progression of the congenitally deformed valve to AS presumably reflects its propensity for premature fibrosis, stiffening, and calcium deposition in these structurally abnormal valves.
Aortic regurgitation, present in approximately 15% of patients with BAV, is usually due to dilation of the sinotubular junction of the aortic root, preventing cusp coaptation. It may also be caused by cusp prolapse, fibrotic retraction of the leaflet(s), or damage to the valve from infective endocarditis. Aortic regurgitation tends to occur in younger patients than does AS.
Why some patients with a BAV develop stenosis and others regurgitation is not clear. As mentioned, rarely, patients may not develop hemodynamics consequences. Roberts and colleagues reported three congenital BAVs in nonagenarians who underwent surgery for AS. Why some patients with a congenital BAV do not become symptomatic until they are in their 90s and why others become symptomatic in early life is also unclear.
Echocardiographic Features of Bicuspid Aortic Valves
The roles of echocardiography in the detection and evaluation are listed in Box 94.3 . The diagnosis of a BAV can usually be made by transthoracic echocardiography (TTE). When adequate images are obtained, sensitivities and specificities of up to 92% and 96%, respectively, have been reported for detecting BAV. The most reliable and useful views are the parasternal short-axis and long-axis views. The echocardiographic features and their respective views are summarized in Box 94.3 . The parasternal short-axis view (SAX) is extremely useful to examine the number and position of the commissures, the opening pattern, the presence of a raphe, and the leaflet mobility. In contrast to the normal tricuspid aortic valve (TAV), which opens in a triangular fashion with straightening of the leaflets (see Fig. 94.1 ; Fig. 94.2 , A ), the BAV opens in an elliptical (“fish-mouth” or “football”) shape with curvilinear leaflets (see Fig. 94.1 ; Figs. 94.3 and 94.4 ). There is typically a raphe, a fibrous ridge that represents the region where the cusps failed to separate. The raphe is usually distinct and generally extends from the free margins to the base of the leaflet. Calcification commonly occurs first along this raphe, ultimately hindering the motion of the conjoined cusp. Rarely, the leaflets are symmetric and there is no raphe—a “pure” bicuspid valve. Note that a false-negative diagnosis may occur when the raphe gives the appearance of a third coaptation line. In diastole, the normal trileaflet aortic valve appears like a “Y” (inverted “Mercedes-Benz” sign), with the commissures at 10, 2, and 6 o’clock (see Figs. 94.1 and 94.2 , B ). When the commissures are deviated from those clock-face position, one should suspect a BAV and evaluate carefully. An additional short-axis feature is a variable degree of leaflet redundancy. In patients with very little redundancy of the leaflet margins, the development of stenosis is likely, whereas a significantly redundant leaflet with associated prolapse is more likely to lead to regurgitation.
- •
Detection of bicuspid aortic valve
- •
Evaluation for aortic stenosis/regurgitation
- •
Careful measurements of aortic root and ascending aorta
- •
Search for coarctation
- •
Screening first-degree family members
- •
Surveillance—following valve dysfunction and aortopathy
The morphologic patterns of BAV vary according to which commissures have fused, and a number of classifications have been devised that pertain to the orientation of the leaflets ( Fig. 94.5 , Table 94.2 ). Fusion of the right and left cusps is the most common morphologic type. In an echocardiographic study by Brandenburg and colleagues, the posterior commissure was located at 4 or 5 o’clock and the anterior commissure was located at 9 or 10 o’clock when the valve is viewed in a parasternal short-axis view. The second most frequent type, fusion of the right and noncoronary cusps, has been linked to aortic arch involvement and may also be related to an increased risk of AS and regurgitation compared with the other anatomic types. The least common type is fusion of the left and noncoronary cusps. Michelena and colleagues similarly classified BAVs as typical (right-left coronary cusp fusion) if the commissures were at 4 and 10 o’clock, 5 and 11 o’clock, or 3 to 9 o’clock (anterior–posterior cusps) and atypical (right-noncoronary cusp fusion) if the commissures were at 1 and 7 o’clock or 12 and 6 o’clock.
| View | |
|---|---|
| Systolic doming | PLAX |
| Eccentric valve closure | PLAX |
| Single commissural line in diastole | SAX |
| Two cusps, two commissures | SAX |
| Raphe | SAX |
| Oval opening (football-shaped; fish-mouth, elliptical; “CBS-eye”) | SAX |
| Unequal cusp size | PLAX, SAX |
The parasternal long-axis (PLAX) view typically shows systolic doming (see Fig. 94.4 , B ; and Fig. 94.6 ) due to the limited valve opening. In a normal TAV, the leaflets open parallel to the aortic walls. In diastole, one of the leaflets (the larger, conjoined cusp) may prolapse. The PLAX view with color Doppler is also useful to evaluate for aortic regurgitation (the diastolic aortic regurgitant jet is usually eccentric) and AS (turbulence in the aortic root and ascending aorta in systole). Last, the PLAX view is also important for sizing the sinus of Valsalva, sinotubular junction, and ascending aorta. With increasing age, as the leaflets become thickened, fibrotic, and calcified, systolic doming may no longer be evident and the typical short-axis appearance of the BAV may be difficult to distinguish from calcific AS of a TAV. In fact, there is an inverse association between the degree of valve stenosis and accuracy of echocardiographically determined valve structure and etiology. The elliptical systolic opening in the SAX view is not easily appreciated in a severely stenotic valve. M-mode echo of a BAV may demonstrate an eccentric closure line ( Fig. 94.7 ), but this sign is not reliable, and approximately 25% of patients with a BAV have a relatively central closure line. Moreover, occasionally TAVs can also appear to have an eccentric closure line depending on image quality and orientation of the echo beam.
If images are suboptimal or heavily fibrotic/sclerotic, then transesophageal echocardiography (TEE) may improve visualization of the leaflets and may be helpful for accurate evaluation of the aortic valve anatomy and confirmation of a BAV. In some instances, alternative cardiac imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI), may help confirm BAV anatomy. More commonly, these imaging modalities are used to visualize the thoracic aorta.
Recently, phase contrast MRI has demonstrated abnormal flow patterns in the ascending aorta in patients with a BAV, with or without stenosis or aneurysm. Even if the valve orifice is not reduced (i.e., no stenosis), it is geometrically altered in BAV, and consequently the jet flow may be abnormal in its direction. Hope and colleagues demonstrated two different flow patterns that were specific to the two most common cusp fusion types. Fusion of the right-left coronary cusps generated a right-anterior flow jet, whereas fusion of the right-noncoronary cusps generated a left-posterior flow jet.
Coarctation
Bicuspid aortic valve may occur in isolation or in association with other forms of congenital heart disease. There is a well-documented association of BAV with coarctation. An autopsy study found coexisting coarctation of the aorta in 6% of cases of BAV, and an echocardiographic study found coarctation in 10% of patients with BAV. On the other hand, as many as 30% to 70% of patients with coarctation have a BAV. *
* References , , , , .
Therefore, when a BAV is detected on an echocardiogram, coarctation of the aorta should always be sought.Infective Endocarditis
Patients with BAVs are particularly susceptible to infective endocarditis. Although the exact incidence of endocarditis remains controversial, the population risk, even in the presence of a functionally normal valve, may be as high as 3% over time. The estimated incidence is 0.16% per year in unoperated children and adolescents. In adults, the two large case series by Tzemos and Michelena and their colleagues suggest that the incidence is 0.3% and 2% per year, respectively. In a series of 128 microbiologically proven episodes of endocarditis, the commonest predisposing risk factor was BAV (16.7%). In another series of 50 patients with native valve endocarditis, 12% had BAV.
In many cases of BAV, endocarditis is the first indication of structural heart disease. This fact emphasizes the importance of either clinical or echocardiographic screening for the diagnosis of BAV. Unexplained systolic ejection sounds (clicks) should prompt echocardiographic evaluation. Surprisingly, bacterial endocarditis prevention is no longer recommended by the most recent American College of Cardiology/American Heart Association (ACC/AHA) Guideline for BAV.
Aortic Complications
Bicuspid aortic valve is associated with several additional abnormalities, including displaced coronary ostia, left coronary artery dominance, and a shortened left main coronary artery; coarctation of the aorta; aortic interruption; Williams syndrome; and, most importantly, aortic dilatation, aneurysm, and dissection. Given these collective findings, it can be suggested that BAV is the result of a developmental disorder involving the entire aortic root and arch. Although the pathogenesis is not well understood, these associated aortic malformations suggest a genetic defect.
Although less well understood, these aortic complications of BAV disease can cause significant morbidity and mortality. As listed in Box 94.2 , BAV may be associated with progressive dilatation, aneurysmal formation, and dissection ( Tables 94.3 and 94.4 ). These vascular complications may occur independent of valvular dysfunction *
* References , , , ,
and can manifest in patients without significant stenosis or regurgitation. According to Nistri and colleagues, 50% or more of young patients with normally functioning bicuspid aortic valves have echocardiographic evidence of aortic dilatation. Therefore, the size and shape of the aortic root and dimensions should be carefully evaluated and followed serially. Aortic root dimensions should be performed at the level of the annulus, sinuses of Valsalva, sinotubular junction (STJ), and proximal ascending aorta ( Fig. 94.8 ). In BAV (unlike Marfan syndrome, where the dilation is usually more pronounced at the sinus level), the sinuses are usually normal or mildly dilated and the aortic dilation is often most pronounced in the ascending aorta distal to the STJ ( Figs. 94.9 and 94.10 ). Therefore, effort should be made to image this portion of the aorta. The midportion of the ascending aorta may not be easily imaged with echocardiography, and evaluation with CT or MRI may be required. The aortic arch and descending thoracic aorta may also become dilated. Recently, it has been reported that patients with BAV are also at increased risk for intracranial aneurysms compared with the general population.| Author(s) | Year | Frequency of Aortic Dissection in BAV | Population | Reference |
|---|---|---|---|---|
| Fenoglio | 1977 | 8/152 (5%) | Autopsy, ≥ 20 years old | 99 |
| Larsen and Edwards | 1984 | 18/293 (6%) | Autopsy, all ages | 4 |
| Roberts and Roberts | 1991 | 14/328 (4%) | Autopsy, > 15 years old | 100 |
| Michelena et al | 2011 | 2/416 (0.4%) | Echocardiography by population-based community cohort | 19 |
| Author(s) | Year | Number BAV/Dissection | Reference |
|---|---|---|---|
| Gore and Seiwert Edwards Larson and Edwards Roberts and Roberts | 1952 1978 1984 1991 | 11/85 13% 11/119 9% 18/161 11% 14/186 7.5% | 101 102 4 100 |
| Totals | __ | 54/551 = 10% | __ |
Although BAV aortopathy may share similarities with the Marfan syndrome, and aortic aneurysms are common in both conditions, a recent retrospective cohort study of 416 consecutive patients with definite BAV provides evidence that their clinical outcomes are different and that aortic dissection is more common in Marfan syndrome. The risk of aortic dissection in this BAV cohort was approximately 8 times higher than in the general population, but despite the high relative risk, the absolute incidence of aortic dissection was very low (given the BAV prevalence of 1.3% of the general population).
Surveillance (Serial Assessment of Patients with Bicuspid Aortic Valve)
Because of the risk of progressive aortic valve disease (stenosis and/or regurgitation) and aortopathy, all BAV patients should undergo annual imaging, even when asymptomatic. The 2008 focused update of the 2006 ACC/AHA guidelines recommended monitoring adolescents and young adults, older patients with AS, and patients with a BAV and dilation of the aortic root and/or ascending aorta. TTE can be used for serial imaging follow-up of the ascending aorta when the dimensions measured by TTE and CT or MRI have been confirmed. Following identification of ascending aortic enlargement in a patient with BAV, repeat imaging at 6 months is recommended. If the aorta remains stable at 6 months and is less than 45 mm in size, and if there is no family history of aortic dissection, annual imaging is recommended. Patients who do not meet these criteria should have repeat aortic imaging with TTE every 6 months. If the aortic root is poorly visualized on echocardiography, cardiac CT or MRI are excellent substitutes. TEE is generally not used for serial follow-up of BAV-related aortopathy because of its semi-invasive nature and the difficulty of comparing dimensions over time.
Family Screening of Patients with BAV
BAV appears inheritable and was present in 9.1% of first-degree relatives in one study. Although the current ACC/AHA guidelines on valve disease do not recommend screening for relatives of individuals with BAV, the ACC/AHA guidelines on congenital heart disease and thoracic aortic disease do recommended echocardiographic screening of first-degree relatives (class I; level of evidence C).
Unicuspid Aortic Valve
Other less common congenital abnormalities of the aortic valve include the unicuspid valve and quadricuspid valve. The unicuspid aortic valve (UAV) is a rare congenital malformation seen in approximately 0.002% of patients referred for echocardiography, but in as many as 4% to 6% of patients undergoing surgery for “pure” (isolated) AS. Two forms of UAV are recognized: One has no commissures or lateral attachments to the aorta at the level of the orifice (acommissural), and the second has one lateral attachment to the aorta at the level of the orifice (unicommissural). Both of these types, like the BAV, produce a dome-shaped opening in systole ( Fig. 94.11 ). The latter is the more common of the two. AS of an acommissural UAV is quite severe, presents in infancy, and is seldom, if ever, seen in adults. An acommissural type of UAV has a central round, oval, or triangular opening caused by underdevelopment of all three cusps, resulting in a “volcano-like” structure with a small, central orifice ( Fig. 94.12 , A ). Stenosis of an acommissural valve is typically very severe and occurs during infancy. In a unicommissural type of UAV, there is usually an eccentric “teardrop”-shaped opening (see Fig. 94.12 , B ). The most common position of the single commissural attachment zone in this type is posterior (Video 94.1). This configuration results in a relatively larger orifice than the acommissural type. As a result, some patients with a unicommissural UAV live into adulthood before manifesting valvular obstruction. Like BAV patients, UAV patients are more often male. Compared with patients undergoing surgery for BAV and TAV disease, unicommissural UAV patients present about 2 decades earlier than patients with BAV and 3 decades earlier than patients with TAV. Unicommissural UAV patients usually require surgery in the third decade of life.
In a UAV, the coronary arteries are generally in the normal position. Aortopathy similar to that seen with a BAV may be present. Unicuspid aortic valves usually have severe, diffuse calcification, and distinguishing a UAV from a BAV can be challenging (see Fig. 94.12 ). TEE is more accurate for making this distinction.
Quadricuspid Aortic Valve
Quadricuspid aortic valve (QAV) is a rare congenital cardiac abnormality with a prevalence that ranges from 0.008% to 0.043%, according to autopsy and echocardiography series ( Table 94.5 ). A much higher incidence was reported by Olson and colleagues in a review of 225 patients undergoing surgery for pure aortic regurgitation. Most cases historically were discovered incidentally at surgery or postmortem examination. However, the majority of cases are now diagnosed antemortem by echocardiography. Because of further advances in imaging, including TEE, CT, and MRI, more cases are being detected, which is likely to alter the incidence of QAV.
| Author | Year | Method | n | % | Ref. |
|---|---|---|---|---|---|
| Simonds Simonds Feldman et al Feldman et al Olson et al | 1923 1923 1990 * 1990 † 1984 | Autopsy Autopsy (literature review) 2D-echo 2D-echo Surgery for pure AR | 0/2000 2/25,666 8/60,446 6/13,805 2/225 | 0.000% 0.008% 0.013% 0.043% 1% | 64 64 65 66 66 |
Based on the relative size of the cusps and their equality, Hurwitz and Roberts delineated seven morphologic subtypes of QAV (types A through G), ranging from four cusps of equal size to four unequal cusps. The most common configuration appears to be that of four equal or nearly equal cusps ( Table 94.6 ).
| Anatomic Variation—Cusps | N |
|---|---|
| 4 equal 3 equal, 1 smaller 2 equal larger and 2 equal smaller 1 large, 2 intermediate, 1 small 3 equal and 1 larger 2 equal, 2 unequal smaller 4 unequal | 51 43 10 7 4 4 5 |
The QAV may function normally—most commonly when the cusps are relatively equal in size. In general, valve dysfunction is seldom present or minimal during childhood or adolescence. Aortic valve dysfunction is usually due to aortic regurgitation ( Table 94.7 ) and tends to occur later in life, a consequence of progressive leaflet thickening with resultant incomplete coaptation (Video 94.2). Unlike BAV, the association of ascending aortic aneurysm is extremely rare.
| Valve Function | N | % |
|---|---|---|
| AR AS + AR AS Normal | 115 13 1 25 | 75% 8% 1% 16% |
The characteristic echocardiographic finding is an “X”-shaped pattern in diastole in short-axis views (formed by the commissural lines of the closed QAV), compared with the “Y” in normal trileaflet valves ( Fig. 94.13 ). Because valve dysfunction may occur with advancing age, clinical and echocardiographic follow-up is recommended.
Although QAV is usually an isolated anomaly, various cardiac and noncardiac anomalies have been reported in association with it ( Box 94.4 ). The most prevalent cardiac malformations associated with QAV are coronary artery anomalies, which have been reported in 10% of cases.
- 1.
Patent ductus arteriosus
- 2.
Hypertrophic cardiomyopathy
- 3.
Subaortic stenosis
- 4.
Ehlers-Danlos syndrome
- 5.
Coronary ostium displacement
- 6.
Ventricular septal defect
In summary, QAV is a rare congenital disorder, usually diagnosed in adulthood, with a potential for complications—mainly aortic regurgitation. QAVs often require surgery, usually in the fifth and sixth decades, and therefore need close follow-up.
Calcific (degenerative) aortic stenosis
Calcific AS is the most common etiology of valvular AS in elderly patients. The prevalence of calcific AS increases with age. AS has a prevalence of about 5% in individuals age 65 or older and about 10% in individuals age 80 years or older. AS is the most common indication for valve replacement surgery and the second most common indication for surgery in older adults, surpassed only by coronary artery bypass grafting. Calcific AS affects men and women equally.
Because the prevalence of AS increases with age and because calcification occurs in regions of mechanical stress, AS was previously thought to be a degenerative disorder caused by passive “wear and tear.” However, the view that aortic valve calcification is a passive consequence of cellular aging has been challenged. AS is now considered to be an active process with some similarities to atherosclerosis, including inflammation, lipid infiltration, and dystrophic calcification. Therefore, the term calcific AS seems more appropriate than degenerative AS. Currently, the pathology of calcific aortic valve disease is an area of active research.
Calcific AS results from slowly progressing fibrosis and calcification, which occurs over several decades, leading to variable degrees of thickening and rigidity of the aortic valve cusps. This process begins with aortic valve sclerosis that does not limit flow through the aortic orifice. The morphologic hallmark is the formation of calcified masses along the aortic side of the cups. The earliest deposits occur at the cusp attachments and along the line of cusp coaptation—the sites of greatest bending and unbending during valve opening and closing. Irregular leaflet thickening and focal increased echogenicity (calcifications) are the echocardiographic hallmarks of calcific AS. These focal areas of thickening are typically seen in the center of the valve cusps. The degree of calcification is best assessed in the parasternal short-axis view. The degree of calcification can be qualitatively classified as mild (small isolated spots or nodules), moderate (multiple larger nodules), and severe (extensive thickening and calcification of all of the cusps).
The degree of leaflet calcification is a marker of disease progression and should be reported. As the leaflets become more sclerotic, they become progressively more rigid and less mobile and begin to obstruct flow. Increases in aortic transvalvular flow velocity mark the progression from aortic sclerosis to AS. In the most severe cases, the aortic root appears to be filled with dense, amorphous echoes that have little or no motion. In some patients, one of the leaflets may become immobile while the others move freely. When only one leaflet is immobile, there is usually only a mild increase in transaortic velocity (mild AS). Unlike rheumatic AS, commissural fusion is usually absent or only minimal in calcific AS. The valve orifice tends to be triradiate—three slitlike openings in systole ( Figs. 94.14 , E and 94.15 ). Calcification often extends onto the base of the anterior mitral leaflet. Calcification may also extend from the valve cusps into the ventricular septum and may induce conduction abnormalities.
Rheumatic aortic stenosis
Rheumatic AS has become uncommon in the developed world, although it remains a significant cause of AS worldwide. In adults undergoing aortic valve replacement for symptomatic AS in the United States, calcific tricuspid AS accounts for 5% of cases, bicuspid AS for 36%, and rheumatic AS for 9%. Aortic rheumatic valve disease is never isolated, but is virtually always associated with rheumatic mitral valve disease. Rheumatic valvular dysfunction may affect not only an anatomically normal TAV, but also a congenital BAV.
Similar to rheumatic mitral valve disease, rheumatic aortic valve deformities are characterized by diffuse cuspal thickening that extends to their free edges and by commissural fusion. These features contrast with the morphologic features of degenerative (calcific) AS, which manifests basal calcific nodules, minimal or no involvement of the free edges, and no commissural fusion. The acquired commissural fusion in rheumatic AS may affect one, two, or all three commissures and is usually distinguishable from the commissural fusion of congenital valve abnormalities. The commissural fusion, which begins at the annulus and progresses toward the center, often affects each commissure equally, producing a small, central, circular or triangular orifice (see Fig. 94.1; Fig. 94.16 ). Subsequent calcium deposition occurs secondarily. Commissural fusion is the primary lesion of AS, as opposed to fibrosis/sclerosis, shortening, and retraction of the cusps, which produce rheumatic aortic regurgitation. Interestingly, the sole pathognomonic feature of rheumatic valve disease, the Aschoff granuloma, is virtually never found in aortic valve tissue.
Quantification of Aortic Stenosis Severity
- Steven A. Goldstein, MD
Aortic stenosis (AS) is the most common cardiac valve lesion in developed countries, including North America and Europe, with an incidence of 2% to 9% in elderly patients older than age 65 years. Moreover, the incidence is increasing as the population ages. Aortic sclerosis, the precursor of AS, is present in nearly one third of patients older than age 65 years.
AS is suspected clinically when a harsh systolic ejection murmur is heard, a delayed carotid upstroke is palpated, or when typical symptoms (angina pectoris, exertional dyspnea, or exertional syncope) occur. However, the clinical diagnosis of AS can be challenging. Clinical signs and symptoms are limited for distinguishing critical AS from noncritical AS, and these signs have reduced sensitivity and specificity in the elderly. Cardiac catheterization, once considered the gold standard for quantitation of AS, is invasive, and the frequency of complications increases with age. Omran et al. demonstrated evidence of acute, focal embolic events on magnetic resonance imaging in 22% of 152 patients who underwent retrograde catheterization.
In contrast, echocardiography provides noninvasive assessment of both valve morphology and hemodynamics. Because of its versatility, noninvasiveness, reproducibility, and accuracy, current guidelines endorse echocardiography as the diagnostic method of choice for the assessment and management of AS. Cardiac catheterization is no longer recommended and is only performed in a limited subset of patients in whom echocardiography is nondiagnostic or discrepant with clinical parameters. In most situations, transthoracic echocardiography (TTE) is sufficient, and it is the current standard procedure for assessing both severity and serial evaluations of AS. Moreover, the prediction of clinical outcomes of patients with AS has been studied mainly using TTE.
Precise assessment of AS severity is necessary for clinical decision-making. The primary hemodynamic parameters recommended for the quantitation of AS severity are peak jet velocity, transaortic gradients, and aortic valve area (AVA) calculated by the continuity equation. Box 95.1 lists the echocardiographic and Doppler parameters that should be evaluated in patients with valvular AS. These are subsequently discussed in the following.
- 1.
Two-dimensional (2D) measurement of the left ventricular outlet tract (LVOT) diameter and aortic annulus
- 2.
LVOT velocity (V1)—by pulsed wave Doppler
- 3.
Velocity across the aortic valve (V2 or Vmax) by continuous wave Doppler (from apex, right parasternal view, suprasternal notch, subxiphoid view)
- 4.
Calculation of peak instantaneous gradient and mean gradient
- 5.
Calculation of aortic valve area by the continuity equation
- 6.
Dimensionless index
- 7.
M-mode/2D measurements of left ventricular size
- 8.
Calculation of LV mass
- 9.
Assessment of aortic insufficiency
- 10.
Assessment of other cardiac defects
Normal aortic valve
Two-Dimensional Echocardiography
The normal aortic valve is composed of three leaflets or cusps (the left, right, and noncoronary cusps [NCCs]) of equal or nearly equal size. Two-dimensional (2D) TTE of the normal aortic valve in the parasternal long-axis (PLAX) view shows two leaflets: (1) the right coronary cusp, which is the most anterior cusp; and (2) either the noncoronary cusp [NCC] (most commonly) or the left coronary cusp. Normal aortic valve cusps appear thin and delicate. In the PLAX view, the cusps open rapidly in systole and appear as parallel lines close to the aortic walls ( Fig. 95.1 ). In diastole, the leaflets come together and appear as a linear density in the center of the aortic root, parallel to the aortic walls. The aortic leaflets are seldom seen during the opening and closing because their motion is very rapid relative to the frame rate of the 2D ultrasound system. In the short-axis (SAX) view, the three thin leaflets open in systole to form a triangular or circular orifice ( Fig. 95.2 ). During diastole, the closure lines of the three leaflets form a Y shape (an inverted Mercedes Benz sign). Sometimes, there is a slight thickening of the mid-portion of each closure line formed by nodules known as the nodules of Arantius . In the SAX view, the NCC is located posteromedially. The atrial septum always points to the NCC. The left coronary cusp is located posterolaterally.
M-Mode Echocardiography
M-mode echocardiography of the aortic valve is formed by directing the M-mode echo beam through the aortic leaflets. This can be done from both the PLAX and SAX views. At the onset of systole, the leaflets open rapidly and become parallel to, and nearly oppose, the walls of the aortic root ( Fig. 95.3 ). They remain open throughout systole and rapidly close again at end-systole, forming a box or parallelogram. Normally, these leaflets show fine, regular vibrations during systole. These fine vibrations actually indicate that the leaflets are thin, and are able to luff, like a sail, due to the rapid flow through them on one side (their ventricular surface) and eddy currents swirling behind the leaflets on the aortic side, resulting in opposing forces that cause these vibrations. During diastole, the coapted leaflets form a single (or sometimes multiple parallel) central closure line(s) midway between the aortic walls (see Fig. 95.3 ). The left ventricular ejection time can be measured from the point of the cusp opening to the point of the cusp closing.
A rough estimate of the severity of AS can be obtained by noting the maximal degree of separation of the leaflets at the onset of systole. In patients with valvular AS, the thickened leaflets (due to fibrosis and/or calcium) appear as dense echoes in both systole and diastole. In systole, the thickened rigid leaflets fail to open widely. The distance between the anterior cusp (right coronary cusp) and the posterior cusp (usually the NCC; sometimes, the left coronary cusp) is reduced or not even visible, which suggests moderate or severe AS ( Fig. 95.4 ). In the absence of a bicuspid valve, a maximal opening of the leaflets of at least 1.5 cm virtually excludes significant valvular AS. When any of the three leaflets opens normally and/or maximally, regardless of the degree of limitation of the other two, the degree of AS is not more than mild.
Quantitative diagnosis of aortic stenosis
With the development of acquired AS, the cusps became thickened, and their motion is restricted. The degree of thickening and restriction progresses as the severity of AS increases. In severe AS, the leaflets become markedly thickened and calcified, and there is nearly a total lack of mobility. Identification of individual cusps is often difficult or impossible. Moreover, attempts to planimeter the aortic valve orifice by TTE have been largely unsuccessful. Nevertheless, a qualitative estimation (gestalt) of AS severity should be attempted and correlated with quantitative methods. If leaflet separation is at least 15 mm or if at least one cusp moves normally, critical AS is highly unlikely. As will be discussed later, planimetry is, however, possible in the majority of patients by using TEE.
Quantitative doppler assessment of severity of aortic stenosis
The previously mentioned 2D and M-mode features are useful for detecting AS, but they are unreliable for quantitating AS. The severity of AS is determined by a combination of 2D and Doppler echocardiography. As the aortic valve becomes stenotic, and obstruction to blood flow occurs, a pressure gradient develops across the valve. This obstruction is associated with an increase in transaortic jet velocity. The primary routine parameters used to quantitate AS include the peak aortic jet velocity, the mean pressure gradient, and the AVA.
Transaortic Velocities
Transaortic jet velocities are directly obtained using a continuous wave (CW) Doppler probe. To obtain the highest velocity, the angle of interrogation should be as parallel to flow as possible. Therefore, multiple transducer windows should be used to obtain the Doppler signal that is aligned most parallel to the direction of the stenotic jet. These windows include the apical 3- and 5-chamber views, the right sternal border, the suprasternal notch (SSN), and subxiphoid views. A careful, thorough, meticulous manipulation of the transducer is necessary to achieve optimal alignment and to determine the highest velocity possible ( Fig. 95.5 ). The highest velocity obtained from any window is used in the calculation of the gradient and the aortic valve area. Lower values from the other windows are ignored. Using a nonimaging CW Doppler probe (so-called Pedoff probe or pencil probe ) is recommended because it is smaller, easier to manipulate between the ribs and the SSN, and has a higher signal-to-noise ratio.
Pressure Gradients
The highest transaortic jet velocity (Vmax) measured by Doppler reflects the pressure gradient according to the Bernoulli equation. The maximum pressure gradient (▵ Pmax) across the stenotic aortic valve can be calculated by using the simplified Bernoulli equation that ignores viscous losses and the effects of flow acceleration. These can be neglected in the usual clinical setting:
However, when the proximal or left ventricular outflow tract (LVOT) velocity (V LVOT ) exceeds 1.5 m/sec, the modified Bernoulli ejection should be used:
The mean pressure gradient is obtained by a manual tracing of the Doppler velocity envelope. The ultrasound machine’s software integrates the instantaneous velocities throughout systole and provides a mean value. Both peak and mean gradients should be reported. A mean gradient more than 40 to 50 mm Hg is consistent with severe AS ( Table 95.1 ). However, because calculated pressure gradients depend not only on the degree of stenosis, but also on (flow stroke volume and/or cardiac) output, higher gradients than those outlined in Box 95.1 may occur in patients with altered volume flow rates. Examples of increased flow rates occur in aortic regurgitation, anemia, and pregnancy. In these situations, relatively high-pressure gradients may be present, although the degree of AS may only be mild. In contrast, patients with significant left ventricular systolic dysfunction, small left ventricles, high systemic vascular resistance, or mitral regurgitation may have relatively low gradients despite severe AS. The accuracy of Doppler-derived peak instantaneous maximal and mean pressure gradients has been validated with simultaneous cardiac catheterization data ( Figs. 95.6 and 95.7 ). It is important to recognize that the peak instantaneous systolic pressure gradient measured by Doppler is higher than the peak-to-peak gradient obtained during cardiac catheterization ( Fig. 95.8 ). Potential sources of error in Doppler assessment of transaortic gradients are listed in Box 95.2 .
| Characteristic | Mild | Moderate | Severe |
|---|---|---|---|
| Aortic jet velocity (m/sec) | 2.6–2.9 | 3.0–4.0 | > 4.0 |
| Mean gradient * (mm Hg) | < 20 | 20–40 | > 40 |
| Mean gradient † (mm Hg) | < 30 | 30–50 | > 50 |
| Aortic valve area (cm 2 ) | > 1.5 | 1.0–1.5 | < 1.0 |
| Dimensionless index | – | – | < 0.25 |
* According to American Heart Association/American College of Cardiology guidelines.
