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Cardiovascular Anesthesiology: Echo Didactics

Aortic Stenosis: Echocardiographic Diagnosis

von Homeyer, Peter MD, FASE*,†; Oxorn, Donald C. MD, CM, FRCPC, FACC*,‡

Author Information
doi: 10.1213/ANE.0b013e31825c7d9b

A 74-year-old man with coronary artery disease, hypertension, and hyperlipidemia is scheduled to undergo coronary artery bypass graft surgery. The intraoperative transesophageal echocardiogram (TEE) shows normal left ventricular function, moderate left ventricular hypertrophy, and an abnormal aortic valve (AV) with significant leaflet calcification (Video 1, see Supplemental Digital Content 1, http://links.lww.com/AA/A411).

The AV is part of the aortic root, which includes the AV annulus and leaflets, the sinuses of Valsalva, the sinotubular junction, and the proximal ascending aorta. The valve has 3 semilunar leaflets or cusps, the left, right, and the noncoronary cusps. There is no true AV annulus in the sense of an actual fibrous ring. Instead, the area of basal attachment of the AV leaflets, in the continuum of the left ventricular outflow tract (LVOT) and the ascending aorta wall, is usually referred to as the AV annulus.1

ECHOCARDIOGRAPHIC IMAGE ACQUISITION

Starting from a midesophageal (ME) 4-chamber view, the ME AV short-axis view is obtained by rotating the multiplane angle to 30 to 50 degrees. If necessary, slight advancement or withdrawal as well as turning the TEE probe to the patient's right is used to bring all 3 leaflets into view. A triangular opening can be visualized in systole, and a “Mercedes Benz” sign is seen when the valve is closed in diastole.

Rotation of the multiplane angle to 120 degrees to the ME AV long-axis view will image the LVOT and the aortic root. The AV leaflets seen on this view are the right coronary cusp anteriorly and most often the noncoronary cusp posteriorly. This view is frequently used to measure aortic root dimensions with measurements preferably performed in midsystole2 (Fig. 1).

Figure 1
Figure 1:
Normal aortic valve (midesophageal views). Midesophageal short-axis (top) and long-axis (bottom) views of a normal aortic valve in diastole (left) and systole (right). The left (LCC), right (RCC), and noncoronary cusps (NCC) are visible. Adjacent structures include the left atrium (LA) and the right ventricle (RV). The short-axis view is used to assess valvular morphology and for valve area planimetry; the long-axis view should be used to measure the LVOT diameter. Measurements should be performed in midsystole.

Transgastric (TG) TEE views are necessary for the quantitative assessment of the AV. One can use either a deep TG long-axis view with the multiplane angle at 0 to 30 degrees or a TG long-axis view with the multiplane angle rotated to approximately 120 degrees (Fig. 2). Once properly aligned, 2 separate AV leaflets of equal length should be visible in any TG TEE view. Jets of mitral regurgitation (MR) can be misinterpreted as jets of aortic stenosis (AS) especially if anteriorly directed, as the Doppler beam frequently crosses the left atrium and the proximal ascending aorta. Color Doppler can help to identify concurrent MR, and pulsed-wave (PW) Doppler is useful to determine the time of onset of the interrogated jet with MR jets generally starting earlier in systole than jets of AS.

Figure 2
Figure 2:
Normal aortic valve (AV) (transgastric views). Deep transgastric (TG) (top) and TG long-axis (bottom) views provide optimal Doppler beam alignment to interrogate transvalvular flow velocities for the calculation of gradients and valve area by continuity equation. Structures seen on this image include the AV, the left atrium (LA), the left ventricular outflow tract (LVOT), and the right ventricular outflow tract (RVOT).

QUALITATIVE ASSESSMENT

A complete 2-dimensional (2D) examination of the AV assesses appearance, cusp number and mobility, leaflet coaptation, calcification, and size of the aortic root. The 3 most common causes of valvular stenosis are calcific AS, bicuspid AV, and rheumatic valve disease (Fig. 3).

Figure 3
Figure 3:
Aortic valve morphology. This cartoon shows the morphology of the normal aortic valve and common pathologic conditions during diastole (top) and systole (bottom). (Reprinted with permission from Baumgartner et al.2)

Calcific AV disease is an active inflammatory disease, most often found in older patients3 (Video 1, see Supplemental Digital Content 1, http://links.lww.com/AA/A411). Calcifications typically involve the base of the leaflets without causing commissural fusion.4 The ME AV short-axis view shows the presence of calcification and what leaflets are involved. Assessment of leaflet mobility and separation are best done in the ME AV long-axis view sometimes with the help of M-mode echocardiography across the leaflet tips. Thin leaflets with a systolic separation of >15 mm excludes severe AS. Direct measurement of valve area by planimetry requires correct imaging angle and plane. The systolic orifice is frequently found at a higher plane than the Mercedes Benz sign visualized in diastole. Advancement and withdrawal of the TEE probe and color Doppler echocardiography is needed to find the narrowest orifice, and adjusting the 2D gain settings helps to identify the true margins of the orifice for planimetry.

Bicuspid AV is a congenital condition and patients often present as young adults with AS caused by superimposed calcifications.5 The echocardiographic picture includes 2 large cusps with 2 commissures and an elliptical valvular orifice (Video 2, see Supplemental Digital Content 2, http://links.lww.com/AA/A412). A raphe, usually located on the larger cusp can make the diastolic closed valve appear like a trileaflet valve. In bicuspid AV disease, identification of the narrowest orifice can be particularly challenging and measurements of the anatomical valve area by planimetry should be interpreted with caution. ME AV long-axis views can show “doming” of the leaflets into the ascending aorta. Patients with bicuspid AV should be evaluated for associated pathologies such as aortic aneurysm, coarctation, and patent ductus arteriosus.

Rheumatic valvular disease is an uncommon cause of AS in Europe and North America, although it is still very common in developing countries. The mitral valve is primarily involved with associated AV disease occurring in approximately 40% of the cases.6 Echocardiographic findings include commissural fusion and diffuse thickening of the leaflets, especially along the edges (Video 3, see Supplemental Digital Content 3, http://links.lww.com/AA/A413). Rheumatic valve thickening does not include acoustic shadowing as typically seen in calcific disease. The ME AV short-axis view often shows a triangular orifice. The morphology can be mistaken for calcific AS, hence the presence of similar changes to the mitral valve is an important part of the diagnosis.

QUANTITATIVE ASSESSMENT

TG TEE views are necessary for parallel alignment of the Doppler beam and the blood flow through the LVOT and AV. Misalignment is a common source of error in the quantification of AS. Careful search for the highest velocity jet is imperative, sometimes with the help of color Doppler, as this is the most parallel intercept angle. Continuous-wave (CW) Doppler flow is recorded for 3 beats, in case of an irregular heart rhythm for at least 5 beats.

The shape of the CW Doppler curve can give useful information about the severity and the localization of the obstruction. Jets of severe AS are high velocity, dense, and early peaking, whereas mild AS jets peak later in systole. Jets caused by dynamic LVOT obstruction have a classic dagger shape and peak very late in systole2 (Fig. 4). Fixed LVOT obstruction may look similar to valvular AS on CW Doppler recordings. The presence of a normal appearing AV on 2D TEE suggests subvalvular obstruction. Also, PW and color Doppler can help to determine the zone of flow acceleration and the level of obstruction.

Figure 4
Figure 4:
Quantitative assessment of aortic stenosis and differential diagnosis of recorded jets. Continuous-wave (CW) Doppler echocardiography is used to interrogate flow velocities and measure pressure gradients across the left ventricular outflow tract (LVOT) and aortic valve (AV). The left side of the image shows a CW Doppler profile of severe valvular aortic stenosis indicated by the dense and early-peaking Doppler envelope and the recorded peak gradient of 111 mm Hg. The velocity envelope is traced and integrated, which yields the mean gradient and the velocity-time integral at the level of the AV (VTIAS). The dense inner envelope can be traced to generate the VTI of the LVOT (VTILVOT). In the upper right corner of the left-sided image, a transgastric long-axis view is visualized. Note the addition of color Doppler echocardiography to help with jet alignment. The shape of the flow curve is important for the differential diagnosis of the level of obstruction. The right side of the image shows a CW Doppler profile of dynamic LVOT obstruction indicated by a jet peaking in late-systole and showing the classic “dagger” shape. Note that this is severe LVOT obstruction with a peak gradient measured at 88 mm Hg.

The maximum pressure gradient (ΔPmax) across the AV is calculated using the simplified Bernoulli equation with VAV being the peak velocity:

However, when the proximal or LVOT velocity (VLVOT) exceeds 1.5 m/s, the modified Bernoulli equation should be used:

The mean pressure gradient is obtained by tracing the velocity envelope and integrated using the echocardiography system's software. A mean gradient above 40 to 50 mm Hg is consistent with severe AS (Table 1).

Calculated pressure gradients are dependent on flow through the valve and diagnostic error can occur in patients with altered volume flow rates. Examples for an increased volume flow rate are aortic regurgitation, anemia, pregnancy, and low systemic vascular resistance, where high-pressure gradients may be recorded, although the degree of stenosis is only mild. In contrast, patients with significant left ventricular systolic dysfunction, high systemic vascular resistance, or MR can have low gradients despite severe AS.

Flow convergence at the narrowest point of the stenosis causes conversion of potential to kinetic energy resulting in a reduction of pressure. After the narrowest point, there is some reconversion of kinetic into potential energy, a phenomenon called pressure recovery.7 CW Doppler measures velocities at the narrowest point of the jet, which can result in an overestimation of pressure gradients and a subsequent underestimation of calculated valve area, especially in the presence of a narrow aorta.

Assuming that stroke volumes (SV) in the LVOT and at the level of the stenotic AV (AS) are equal, the continuity equation is used to calculate AV area (AVA):

SV [cm3] is defined as the product of the cross-sectional area (CSA) [cm2] and the velocity-time integral (VTI) [cm]:

The CSA at the level of the AV is equal to the AVA. With the exception of the AVA, all variables can be measured using 2D (CSALVOT), PW Doppler (VTILVOT), and CW Doppler (VTIAS) echocardiography. To solve for the AVA, the equation is:

With TEE, the CSALVOT is best measured in the ME AV long-axis view just proximal to the AV in an inner-edge-to-inner-edge fashion. This is the anatomical level where the LVOT is most likely to be circular, whereas further upstream it is more elliptical. The LVOT diameter is measured and then used to calculate the CSALVOT using the formula:

Because of the squared relationship, measurement error has substantial consequences for the correct calculation of AVA. The AVA cutoff for severe AS is 1.0 cm2 (Table 1).

The VTILVOT is measured using PW Doppler. The sample volume is first placed into the jet next to the stenotic valve, which is often indicated by an audible valve-closing click. It is then moved apically and until a smooth velocity curve is seen indicating a position proximal to the flow acceleration zone. It is important to make CSA and VTI measurements at the same location in the LVOT.

Instead of measuring the velocities nonsimultaneously at the level of the valve and the LVOT, the double-envelope technique has been described and found to be equivalently accurate.8 Using the CW Doppler-derived flow curve, the VTI of both, the high-velocity outer envelope (VTIAS) and the more intense low-velocity inner envelope (VTILVOT), is traced and measured (Fig. 4). The dimensionless index (DI) or velocity ratio is a simplified parameter independent of cardiac output and LVOT diameter, which is a Doppler-only method and calculated using the formula:

A DI of ≤0.25 is consistent with severe AS (Table 1).

Table 1
Table 1:
Grading Severity of Aortic Stenosis
Table
Table:
No title available.

A way to simplify the continuity equation is to use peak velocities instead of VTI, which has the theoretical advantage of avoiding LVOT measurements and errors related to this.

DISCLOSURES

Name: Peter von Homeyer, MD, FASE.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Peter von Homeyer approved the final manuscript.

Name: Donald C. Oxorn, MD, CM, FRCPC, FACC.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Donald C. Oxorn approved the final manuscript.

This manuscript was handled by: Martin J. London, MD.

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