The severity of AS is assessed quantitatively with Doppler echocardiography in two ways: measuring the gradient across the valve with the modified Bernoulli equation and estimating the AV area with the continuity equation. Both techniques require that the ultrasound beam be as parallel to the transvalvular blood flow as possible.
Transesophageal Echocardiographic Doppler Views for Assessing Aortic Stenosis
In AS, aligning the TEE transducer parallel to the left ventricular outflow tract (LVOT) and AV can be challenging. The deep transgastric (TG) and TG LAX views are commonly used depending on which one offers the optimal window for interrogation parallel to the stenotic jet. Advancing the probe from a TG SAX view with continued anteflexion may allow acquisition of the deep TG view from near the LV apex. Occasionally, clockwise or counterclockwise rotation of the probe and varying the angle of interrogation over a wide range may facilitate this. The TG LAX view is obtained with the probe at the mid-papillary level or slightly above and the imaging plane angle of interrogation increased from 120 to 140° where the LVOT, AV, and AoA come into view. Both techniques offer an excellent approach to AV flow dynamics; however, the patient’s
anatomy will dictate which view provides the best interrogation of transvalvular blood flow. For this reason, it is advised that both views be sought out and the highest velocities obtained are used in calculations of AS severity. In a small minority of individuals, the ME LAX view (at 120°) allows the best alignment to the transaortic flow.
Doppler Determination of the Aortic Valve Gradient: The Modified Bernoulli Equation
Once the appropriate view is obtained, the mean and peak transaortic gradients are obtained using continuous wave Doppler (CWD). The velocity profile should be a dense signal with a smooth outer edge and clear maximal velocity. The peak gradient is obtained from measuring the maximal velocity of the AV spectral Doppler profile, while the mean gradient is obtained by tracing the velocity profile and using the analysis program of the ultrasound machine (
Fig. 12.1). Care should be taken to avoid tracing or measuring faint linear signals at the peak of the velocity profile. The modified Bernoulli equation is used to calculate transaortic valve pressure gradients from aortic velocities (
Table 12.2). The modified Bernoulli equation states that the maximal pressure gradient equals four times the square of the peak jet velocity and allows calculation of the peak instantaneous gradient across any orifice. Thus, if the peak blood flow velocity across the AV is 4 m/s, the calculated peak gradient = 4 × 4
^{2} = 64 mm Hg. The mean gradient is calculated by averaging the instantaneous gradients over time. The mean velocity cannot be used to estimate the mean gradient in the Bernoulli equation. A less accurate alternative mean gradient can be calculated from the peak velocity as 2.4 (
V_{max})
^{2}.
The mean gradient, in particular, correlates well with invasively determined gradients and is most often used in evaluating the severity of AS.
Hemodynamically significant AS is generally associated with a mean gradient of 40 mm Hg or more or a maximal velocity of 4 m/s or more. The exception is in patients with a low EF or stroke volume who may not be able to generate a high gradient. These patients are described as having low-flow, low-gradient severe AS (see below) which is an increasingly recognized entity. In these patients, mean gradients as low as 20 to 30 mm Hg may be associated with critical stenosis, and the valve area by continuity equation and planimetry, as well as a dobutamine challenge in some patients should be considered to further evaluate the significance of AS. In these situations, recently published AS guidelines recommend estimation of transvalvular flow by calculation of Doppler stroke volume index (SVI) (
Table 12.3) (
7). In AS, low transvalvular flow is defined as <35 mL/m
^{2}. Systemic hypertension during echocardiographic assessment also impacts both on the apparent gradient and the left ventricular ejection fraction (LVEF) and must also be considered in evaluating the validity of the assessment of the gradient. (See below.)
The peak gradient can be influenced by the flow velocity on the ventricular side of the valve plane. Remember that the simplified Bernoulli equation ignores the impact of the LVOT blood flow velocity. However, the Bernoulli equation must factor in the LVOT blood flow velocity when it exceeds 1.5 m/s, as commonly occurs in associated aortic insufficiency and other high-output states, to avoid overestimation of the pressure gradient (see
Table 12.2). For example, if the outflow tract velocity is 1.7 m/s and the peak transvalvular velocity is 4 m/s, the actual gradient is 4 × (4
^{2} – 1.7
^{2}) = 4 × (16 – 2.89) = 52.4 mm Hg, instead of the 64 mm Hg predicted by the simplified Bernoulli equation.
Discrepancies often occur between catheterization and echocardiographic pressure gradients in AS. The peak echocardiographic gradient measures the peak instantaneous gradient between the LV and aorta. This is generally higher than the invasively determined “peak-to-peak gradient” (between the peak LV pressure and the generally later peak aortic pressure) routinely entered on cardiac catheterization reports (
Fig. 12.2). In addition, the phenomenon of pressure recovery (PR), the recovery of pressure energy from the kinetic energy of acceleration through the narrowed orifice that occurs distal to the stenotic valve, can cause an elevation in the estimated transvalvular gradient by Doppler as compared to measurements by catheterization. In general, this phenomenon only becomes a factor in patients with small aortas (sinotubular junction ≤3 cm). (See below.)
Doppler Estimation of the Aortic Valve Area: The Continuity Equation
The continuity equation is based on the law of conservation of mass and states that the volume of blood that enters the stenotic aortic orifice through the LVOT is equal to the volume of blood that exits it (
Fig. 12.3). If we can calculate the volume of flow entering a stenotic AV through the LVOT and measure the velocity at which it exits the stenotic valve, then the equation can be rearranged to solve for the area of the stenotic valve (see
Table 12.3). The AV area derived from the continuity equation is the effective area, the area of the orifice at the vena contracta which is immediately distal to the anatomic area, which is derived from planimetry of the valve. The effective area has been clinically validated even though it is slightly smaller than the anatomic area as the primary predictor of clinical outcome (
7,
9). The area of the normal AV is between 3 cm
^{2} and 4 cm
^{2} (
Fig. 12.4). Guidelines viewing the disease as a continuum based on hemodynamic and
natural history data define severe stenosis as an AV area <1 cm
^{2}, a mean gradient >40 mm Hg, or peak jet velocity >4 m/s (see
Table 12.1) (
7).
When determining AV area by continuity equation, one first calculates the cross-sectional area of the LVOT. In the ME AV LAX view, the LVOT or annular diameter is obtained by measuring the inner dimension (endocardium to endocardium) of the LVOT generally at the insertion point of the AV leaflets in midsystole with the electronic calipers (
Fig. 12.5,
top). However, measurement of the outflow tract should ideally be performed where the optimal outflow tract velocity profile is obtained by pulsed-wave Doppler (PWD) in
the apical (by transthoracic echo) or TG (by transesophageal echo) view (see below). This is generally either at, or within a centimeter of, the aortic leaflet insertions into the valve annulus. The diameter of the LVOT is generally 2.0 ± 0.2 cm and varies somewhat in proportion to body size. Inaccuracies in measurement of the outflow tract can account for much of the error in this technique because the radius is squared in the continuity equation. The most common discrepancies occur during the imaging of elderly women, who often have a smaller outflow tract (and body surface area) than average, and large men, who often have a larger outflow tract (and body surface area). Another source of error occurs in the presence of upper septal hypertrophy which results in tapering of the proximal LVOT; in these situations, the annular dimension can be utilized as it is less affected by this phenomenon (
10).
Lastly, in the continuity equation, we assume that the LVOT exhibits circular geometry with the cross-sectional area calculated utilizing the formula π
r^{2} (or π[
D/2]
^{2}). However, recent observations of LVOT geometry by computed tomography (CT) angiography and 3D echocardiography (
11,
12,
13), have shown that it is often not circular but elliptical (
Fig. 12.5,
bottom) and that its true size is best determined by direct planimetry. While the assumption of a circular LVOT by 2D imaging can result in up to a 20% underestimation of the true LVOT area compared to planimetry (
13), routine planimetry of the LVOT area with 3D imaging for the continuity equation-derived AVA is not recommended, as doing so results in potentially much larger AVA values which can potentially lead to erroneous assessment of disease severity (
9). Three-dimensional planimetry of the outflow tract, on the other hand, is very important for appropriate transcatheter valve sizing, while the continuity equation utilizing the circular assumption has held up well for clinical assessment of severity and cutoffs based on this technique have shown to be strong predictors of outcomes (
7,
14). Nevertheless, 3D multiplanar images of the LVOT are still useful to ensure that the maximal anteroposterior LVOT and annular diameter are measured for the continuity equation (see
Fig. 12.5,
bottom).
Secondly, the LVOT time-velocity integral (TVI) is then determined with PWD. The sample volume is placed just proximal to the AV cusps and then gradually moved away from the AV in the LVOT until a smooth velocity profile with a narrow range of velocities at each time point is obtained (
Fig. 12.6). PWD is essential for this flow measurement because the profile should be obtained at the level of the outflow tract where the LVOT dimension was measured. If the profile is obtained just below the aortic leaflet insertion point, then the LVOT dimension should be remeasured at the same place. The internal calculation package available on all echocardiographic machines calculates the TVI after the LVOT velocity profile is traced. A second alternative method, which is less well validated, uses CWD interrogation through the AV. If the alignment is correct, a more intense lower velocity inner envelope representing the lower-velocity LVOT flow is imaged within the higher velocity aortic jet envelope. This inner profile
can be traced as previously described to calculate the LVOT TVI. However, the inner envelope peak can be erroneously high because the subaortic jet accelerates into the stenotic orifice to form a proximal isovelocity surface area as it narrows to fit into the orifice which can result in an overestimation of the AV area.