The aortic valve (AV), left ventricular outflow tract (LVOT), and aortic root are positioned in the center of the heart close to the midesophagus and are separated from it by the left atrium (LA), which acts as an excellent acoustic window. Thus, 2-D and color flow Doppler imaging of this region is usually of high quality, and the information provided by transesophageal echocardiography (TEE) can have a significant impact on the management of a number of important disease states.
Conditions affecting the LVOT and AV are addressed in this chapter; conditions that specifically involve the aortic root (e.g., sinus of Valsalva aneurysm) are discussed in Chapter 11 . Prosthetic valves in the aortic position are discussed in Chapter 12 .
Anatomy
The entire LVOT comprises the following:
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The subvalvular LVOT, which is the funnel-shaped portion of the left ventricle extending from the free edges of the mitral leaflets to the aortic annulus (commonly referred to as simply the LVOT).
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The AV.
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The aortic root and the proximal ascending aorta. The aortic root is the part of the aorta between the AV annulus and the sinotubular junction.
The aortic annulus is formed by three cords, which together form the basis of the fibrous skeleton of the heart (see Figure 9-1 ). The AV is composed of three similarly sized, crescent-shaped leaflets, which are each attached to the aortic wall along a U-shaped line, with its upper attachments along the sinotubular junction.
Behind each leaflet is the respective sinus of Valsalva, a dilatation of the aortic root that functions as a reservoir for coronary blood flow in diastole and ensures separation of the leaflet from the ostium of the coronary artery in systole. Each sinus and leaflet is named according to the adjacent coronary artery (left, right, or noncoronary). The right leaflet is situated anteriorly and rightward, the left leaflet posteriorly and leftward, and the noncoronary leaflet posteriorly and rightward. Each leaflet is separated by a commissure and coapts with its adjacent leaflet along a closure (or coaptation) line. The leaflet tips usually overlap by 2 to 3 mm in systole, but in disease states that cause the aortic sinuses to dilate, the degree of coaptation is reduced, allowing regurgitation to occur. The apex or central portion of the free edge of each leaflet has a localized thickening called the nodule of Arantius ( Figure 10-1 ).
The walls of the LVOT comprise the anterior mitral leaflet, the membranous ventricular septum, and the anterior left ventricle (LV) wall (see Figure 10-1 ). The left and noncoronary leaflets are in continuity with the base of the anterior leaflet of the mitral valve (MV) and the intervalvular fibrosa (see Figure 9-1 ).
Echocardiographic examination
Four standard views are needed for a systematic examination of the AV, root, and LVOT:
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Midesophageal AV short-axis view
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Midesophageal AV long-axis view
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Deep transgastric long-axis view
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Transgastric long-axis view
Details of how to obtain the views and the important echocardiographic anatomy are provided in Chapter 3 . Summarized in the next section are details specific to the assessment of the AV and LVOT. Normal dimensions of the LVOT, aorta, and aortic root are provided in Appendix 3 .
Midesophageal aortic valve short-axis view
The midesophageal AV short-axis view usually provides excellent images of all three AV leaflets and is consequently the ideal view in which to assess leaflet morphology ( Figure 10-2 ). To improve visualization of the leaflets, it may be necessary to rotate the transducer backward or forward so that the sinuses are of equal size and to advance or withdraw the probe so that the image plane passes through the leaflet tips. Off-axis imaging may cause one leaflet to appear larger and falsely thickened or give the erroneous impression of a diastolic coaptation defect.
From the standard position, the probe may be advanced to examine the LVOT or withdrawn to visualize the origin of the coronary arteries ( Figure 10-3 ), sinuses, and sinotubular junction.
With 2-D imaging, leaflet pathology, such as bicuspid valve or calcific degeneration, may be identified. The view is useful for identifying a diastolic coaptation defect and, with color flow Doppler, the location and severity of any aortic regurgitation.
Midesophageal aortic valve long-axis view
The midesophageal AV long-axis view ( Figure 10-4 ) is used for measuring the dimensions of the AV annulus, sinuses, sinotubular junction, and proximal ascending aorta ( Figure 10-5 ). The dimensions of the annulus and sinotubular junction are of particular interest, as they are indicative of the size of prosthetic AV required. The AV annulus should be measured at the points where the leaflets insert into the aorta.
To measure these diameters accurately, it is essential that the image plane passes through the center of the aorta (see Figure 21-7 ); this may require turning the shaft of the probe to the left or right to ensure the maximum aortic diameter is visualized. A clue that the image is off axis is a thickened appearance to one of the AV cusps or one of the aortic walls. All measurements should be at the same phase of the cardiac cycle and from inner edge to inner edge.
From the standard position, withdrawing the probe and turning it to the right can aid in visualizing more of the ascending aorta; advancing the probe may improve visualization of the LVOT.
The midesophageal AV long-axis view is also useful for assessing for diastolic leaflet prolapse (suggesting regurgitation) or impaired systolic opening (suggesting stenosis). The LVOT may be examined for evidence of fixed or dynamic obstruction. Color flow Doppler can be used to look for turbulent flow (suggesting obstruction or stenosis) in the LVOT or proximal ascending aorta and to assess aortic regurgitation.
Transgastric views
Both the deep transgastric and the transgastric long-axis views (see Figures 3-21 and 3-22 ) provide windows with which to image the LVOT and AV from the ventricular aspect. In patients with prosthetic aortic or mitral valves, imaging the LVOT from the midesophageal windows may be impossible because of echo dropout and far-field shadowing. In this circumstance, the transgastric views may provide some details of this region.
The main utility of these views is the Doppler interrogation of the LVOT and AV, as they provide the only images of the AV in which reasonable alignment with a spectral Doppler signal may be obtained using TEE.
Unfortunately, both views are difficult to obtain in some patients, and even if a view can be obtained, adequate alignment with the Doppler signal is not always possible. Furthermore, the depth of the LVOT may be beyond the maximum depth that can be unambiguously resolved with pulse wave (PW) Doppler, particularly if the velocity in the LVOT is increased (see Chapter 2 ).
Aortic stenosis
Aortic stenosis may occur as a consequence of calcific degeneration of a normal trileaflet valve, a congenital valve abnormality, (particularly bicuspid AV), or rheumatic aortic disease.
Calcific aortic stenosis
Calcific aortic stenosis is the most common form of aortic stenosis, and it tends to occur in patients older than 70 years. It is characterized by fibrocalcific changes in the body of the leaflets, resulting in a marked increase in echogenicity and restriction of leaflet motion (Figures 10-6 and 10-7 ). The leaflets may be affected asymmetrically. Commissural fusion is relatively uncommon (in contrast to rheumatic aortic stenosis). The degree of calcification predicts severity, can be graded, and is a marker of prognosis. Calcification may extend down into the LVOT (teardrop calcification) and onto the base of the anterior mitral leaflet.
Bicuspid aortic valve
A congenital valve abnormality of the bicuspid AV is the most common congenital cardiac malformation. It occurs in approximately 2% of the normal population, may be familial, and accounts for approximately 50% of all valve replacements for aortic stenosis. Secondary calcification occurs in a significant number of cases and usually leads to the onset of symptoms in the fourth to sixth decades of life. The characteristic echocardiographic sign is that of two leaflets opening in systole, resulting in an elliptical rather than a star-shaped orifice. A raphe or fibrous ridge is usually present on one of the leaflets, perpendicular to the orifice, and is typically more sclerotic than the leaflets. A calcified raphe may have the appearance of a commissure (or closure line) in diastole, so the diagnosis of a bicuspid AV can be made reliably only in systole ( Figure 10-8 ).
The most common form of congenitally bicuspid AV is that of a larger anterior cusp and a smaller posterior cusp. A raphe is usually noted at the point of fusion of the left and right cusps. The two coronary arteries arise from the sinus of Valsalva adjacent to the anterior cusp. Less commonly, in approximately 20% of cases, there is a large rightward leaflet and a smaller leftward leaflet. A raphe may be seen on the rightward leaflet, giving the appearance of fused right and noncoronary cusps. A severely calcified bicuspid valve may be indistinguishable from a trileaflet valve. When examining an AV that appears to be bicuspid, the following points should be considered:
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Off-axis imaging in the AV short-axis view can be misleading and give the appearance of a trileaflet valve.
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A doming appearance to the valve leaflets in diastole in the AV long-axis view may be due to eccentric closure of the leaflets and is not necessarily due to prolapse.
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If a bicuspid AV is detected, it is important to rule out associated abnormalities, including coarctation of the aorta, subaortic obstruction, and dilatation of the aortic root.
Rarely, an AV may be congenitally unicuspid ( Figure 10-9 ) or quadracuspid. Unicuspid valves almost always become stenotic in childhood or early adulthood.
Rheumatic aortic stenosis
Rheumatic valvulitis tends to produce thickening of the free edges of the AV leaflets and fusion of the commissures, causing a circular or triangular, rather than a star-shaped, orifice in the midesophageal AV short-axis view. In the midesophageal AV long-axis view, only the AV leaflet tips may appear to open during systole ( Figure 10-10 ). Rheumatic aortic stenosis is usually associated with a degree of aortic regurgitation due to retraction of the aortic leaflets and is almost always associated with significant rheumatic changes of the mitral leaflets and chordae tendineae.
Echocardiographic assessment of aortic stenosis
The severity of aortic stenosis may be graded on the basis of transvalvular velocities and transvalvular pressure gradients and by estimating the valve area ( Table 10-1 ). The two main methods for evaluating AV area are planimetry and the continuity equation.
| V max (m/sec) | P max (mm Hg) | P mean (mm Hg) | AV AREA (***) | AV Area Indexed (cm 2 /m 2 ) | Velocity Ratio * | |
|---|---|---|---|---|---|---|
| Normal | <2.5 | <16 | 25 | >2.5 | ||
| Mild | 2.6-2.9 | <36 | <20 (<30 † ) | >1.5 | >0.85 | >0.5 |
| Moderate | 3.0-4.0 | 36-64 | 20-40 ‡ (30-50 † ) | 1.0-1.5 | 0.60-0.85 | 0.25-0.5 |
| Severe | >4.0 | >64 | >40 ‡ (>50 † ) | <1.0 | <0.6 | <0.25 |
* Velocity ratio = LVOT max / V max .
† European Society of Cardiology guidelines.
‡ American Heart Association/American College of Cardiology guidelines.
Two-dimensional imaging
A detailed two-dimensional (2-D) examination of the valve is an important first step. If the valve leaflets are thin and mobile, then significant valvular stenosis can be excluded. The absence of valvular calcification makes significant stenosis unlikely, but the presence of calcification does not necessarily imply severe stenosis. An increased velocity across the valve detected with continuous wave (CW) Doppler, in the absence of significant AV pathology on 2-D imaging, should alert the echocardiographer to the possibility of LVOT obstruction (as described later and in Chapter 7 ).
With TTE, in the parasternal long-axis view, a maximal cusp separation of less than 8 mm on 2-D imaging is strongly suggestive of severe stenosis, and a maximal cusp separation greater than 12 mm indicates that stenosis is no more than mild. With TEE, similar measurements can be made in the midesophageal AV long-axis view.
An additional feature that may be present is poststenotic dilatation of the aortic root and proximal ascending aorta.
Direct planimetry of the AV orifice may be helpful if Doppler interrogation (as described later) is suboptimal. However, there are multiple possible sources of inaccuracy with this method:
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When there is severe thickening or calcification of the aortic leaflets or root, particularly posteriorly, causing shadowing of the valve orifice
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When a true short-axis view of the AV is difficult to obtain because of distortion or marked dilatation of the aortic root
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When there is accentuated cardiac motion (e.g., in high-cardiac-output states), causing the valve to move in and out of the image plane so that the smallest orifice is not visualized
Planimetry yields higher valve areas compared to those obtained by applying the continuity equation, as anatomic rather than effective orifice area is measured. Planimetry by 2-D TEE cannot be used to differentiate moderate from severe aortic stenosis. Therefore, information obtained from planimetry is adjunctive to other data. Three-dimensional (3-D) TEE may be a more accurate method of directly assessing aortic stenosis severity than 2-D TEE.
Color flow Doppler imaging
While color flow Doppler cannot be used to quantify the severity of aortic stenosis directly, the presence of intense aliasing in the proximal aorta (see Figures 10-7 and 10-10 ) implies turbulent, high-velocity flow in this region, consistent with a poststenotic flow pattern.
Spectral Doppler: velocity and pressure gradients
Transvalvular velocities and pressure gradients can be estimated from a CW Doppler signal directed through the AV ( Figure 10-11 ) and can provide a rapid estimate of the severity of aortic stenosis. However, it is important to recognize the limitations of this approach.
First, there is no universally accepted numeric grading system, but the values given in Table 10-1 reflect the consensus of the major societies of echocardiography. Second, it can be difficult to orientate the Doppler beam parallel to aortic flow with TEE, which can result in an underestimation of true velocity and pressure gradients (as described later). Third, the relationship between valve area and velocity (and with pressure gradient) is predicated on flow. A high transvalvular flow (such as occurs with elevated cardiac output or aortic regurgitation) increases the transvalvular velocity for a given valve area. Conversely, low cardiac output reduces the transvalvular velocity. Fourth, the simplified Bernoulli equation assumes a low velocity (<1 m/sec) proximal to the orifice (in this case, the LVOT); this is not always the case (e.g., in the presence of a subaortic membrane or dynamic LVOT obstruction). If the LVOT velocity is greater than 1.5 m/sec, this should be taken into consideration in the Bernoulli equation to calculate maximum gradients by using ▵ P = 4( V 2 max − V 2 LVOT ) (see Chapter 21 ). The velocity ratio can be used to incorporate variability in LVOT velocity and give a guide to the severity of aortic stenosis (see Table 10-1 ). Finally, peak velocity measured with echocardiography is peak instantaneous velocity, whereas peak velocity obtained from catheter-based measurements is peak-to-peak velocity, which is numerically less than peak instantaneous velocity ( Figure 10-12 ).
Spectral Doppler: aortic valve area by the continuity equation
AV area may be estimated by application of the continuity equation, which overcomes some limitations of grading aortic stenosis based on transvalvular pressure gradients. Estimating AV area by the continuity equation has been well validated for TTE.
The continuity principle states that forward stroke volume (VTI × area) in one part of the heart equals forward flow in another (see Chapter 21 ), which for the LVOT and AV can be formulated as
VT I L V O T × C S A L V O T = V T I A V × C S A A V
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