Doppler echocardiography allows measurement of various hemodynamic parameters that are important for the evaluation of CAVD severity (Table 11.1). By measuring backscatter ultrasound frequency changes due to movement of red blood cells, blood flow velocity can be accurately measured using the Doppler equation [1]. Normal aortic transvalvular blood flow is laminar and unobstructed with a peak aortic jet velocity (V_max) between 1 and 2 m/s. In CAVD, as the aortic valve orifice narrows, the peak aortic velocity increases. Measuring V_max requires a continuous-wave (CW) Doppler probe that is parallel to the direction of blood flow. Recorded velocities are displayed as a velocity curve with velocity on the vertical axis over time on the horizontal axis. In CAVD, irregular opening of the valve can lead to an unpredictable direction of the maximum blood flow velocity; thus, a thorough TTE Doppler evaluation requires searching for the highest velocity in multiple acoustic windows from the apical, parasternal, subcostal, and suprasternal transducer positions. TEE probe positions are constrained, and thus measuring aortic velocities can be frequently underestimated from misalignment.

Under certain assumptions that are generally met in the clinical setting, instantaneous transvalvular pressure gradients (∆P) can be estimated from measured aortic jet velocities (v) using the simplified Bernoulli equation, ΔP = 4v². A mean gradient can be obtained easily with commercial echo packages by averaging the instantaneous gradients over the duration of the systolic ejection period. Mean gradients calculated by Doppler measurements have been shown to have strong correlation with mean gradients obtained during catheterization [2, 3].

Transaortic valve velocity and pressure gradients can determine the presence of hemodynamically important aortic stenosis and categorize the severity into mild, moderate, or severe [4]. Although these parameters are easily measured with good reproducibility, there are potential pitfalls with measurement and interpretation that need to be avoided. First, optimal Doppler measurements require the intercept angle of the ultrasound beam to be as parallel as possible to the direction of blood flow. Nonparallel alignment will lead to underestimation of the velocity and pressure gradient. Measurements should be averaged over at least 3 beats in sinus rhythm and more if in an irregular rhythm; care should be taken to avoid measurement of post-extrasystolic beats. Assumptions of the simplified Bernoulli equation may not be met if there are very high velocities present in the left ventricular outflow tract (>1.5 m/s) or abnormalities of blood viscosity from profound anemia or polycythemia. Finally, velocity and pressure gradients are heavily flow-dependent measures. To avoid erroneous CAVD severity assessment, a high or low transaortic flow state and aortic valve area (AVA) need to be assessed.

The AVA can be estimated using the continuity equation which states that stroke volume through the left ventricular outflow tract (LVOT) is the same as the stroke volume through the aortic valve, SV_LVOT = SV_AV. Proximal to the stenotic valve, blood flow is laminar which allows calculation of stroke volume as SV = CSA × VTI, where CSA is the cross-sectional area of the flow and VTI is the velocity-time integral (area under the velocity curve). Substituting and rearranging the above equations provides the formula AVA = (CSA_LVOT × VTI_LVOT)/VTI_Ao. A simplified continuity equation substituting peak velocity (V) for VTI can be applied, AVA = (CSA_LVOT × V_LVOT)/V_max, because the shape and timing of the outflow tract and aortic jet velocity curves are similar. The cross-sectional area of the LVOT can be approximated from a zoomed in 2D TTE parasternal long-axis view. The diameter (D) of the LVOT is measured during mid-systole from the inner edge of the septal myocardium to the inner edge of the anterior mitral leaflet at the aortic cusp insertion points (Fig. 11.3a). By assuming a circular LVOT, the CSA_LVOT = π(D/2)². This is an adequate assumption for stroke volume calculation with numerous studies validating continuity equation valve area with catheter-based valve area [5–7]. However, the actual LVOT is frequently more oval than round, which is important to account for in transcatheter aortic valve replacement (TAVR) which relies on 3D imaging measurements, either by computed tomography (CT) or TEE, of area and eccentricity to minimize complications [8].

Measurement of the peak velocities and VTI requires that the Doppler beam have a parallel alignment with the blood flow. This is achieved with the transducer in the apical position. To obtain the velocity curve at a specific location such as the LVOT, a pulsed-wave (PW) Doppler signal is needed. The PW Doppler allows placement of a sample volume on the ventricular side of the aortic valve at the same location where the LVOT diameter is measured. Velocities are recorded over time at the location of the sample volume, and velocity curves are produced on the spectral Doppler display. The V_LVOT and VTI_LVOT are measured by tracing the modal velocity (brightest or most dense) portion of the spectral tracing (Fig. 11.3b) [1]. The V_max and VTI_Ao through the aortic valve are measured from tracing the outer edge of the velocity curve from the maximum aortic jet obtained from the CW Doppler (Fig. 11.3c). Gain should be adjusted to produce a dark envelope around the spectral velocity curve with careful attention to avoid measuring the faint signal broadening produced from the transit-time effect of the ultrasound signal.

Alternative echocardiographic approaches can be used to estimate valve area when the continuity equation AVA appears discrepant to clinical findings or the above parameters cannot be obtained due to poor image quality. Planimetry in a short-axis view to measure the anatomic (geometric) orifice area can be performed [9, 10]. This can be limited if using 2D TTE due to difficulty obtaining an accurate planar image of an irregular 3D valve orifice. Additionally, poor image resolution and calcification artifacts can lead to inaccurate tracing of the valve area. When TTE quality is suboptimal, a TEE can be used to measure the LVOT diameter from a high esophageal probe position angled between 120 and 150°. The high-resolution and multiplane imaging capability of TEE also allows for improved planimetry of the valve area [11] but can still suffer from similar difficult visualization problems in advanced CAVD [12]. The anatomic valve area when correctly measured from planimetry is expected to be slightly larger than the effective orifice area calculated from the continuity equation due to contraction of the flow stream through a narrowing orifice [13].

New advances in 3D echocardiography may improve valve area estimation by overcoming some inherent limitations of 2D imaging. Improved planimetry has been demonstrated by allowing accurate determination of the narrowest valve plane to measure and has shown good correlation with Doppler-derived and catheter-derived valve areas (Fig. 11.4) [14]. 3D echocardiography may also produce a more accurate estimation of stroke volume from either directly measuring LVOT area [15], using real-time 3D ventricular volumes [16], or calculating color Doppler flow-volume curves [17]. Despite initial studies showing promise of 3D techniques improving AVA estimation, 2D Doppler-based continuity equation remains the echo gold standard until further studies demonstrate that 3D techniques are clinically reproducible and are associated with outcomes.

Traditional markers of hemodynamic severity (peak aortic jet velocity, mean gradient, and aortic valve area) may not always correlate with each other, clinical symptoms, left ventricular function, or outcomes. To resolve discrepancies, additional severity indices have been suggested that may help improve CAVD classification. A simple measure is the velocity ratio (or dimensionless index) defined as V_LVOT/V_max, where V_LVOT is the peak velocity obtained in the LVOT from the PW Doppler and V_max is the peak aortic jet velocity as described in the previous sections. The ratio avoids errors in the LVOT area estimation which may be significant due to the LVOT diameter measurement being squared in the area equation.

In small individuals, the indexed AVA may be appropriate to use, AVA/body surface area (BSA), as a person with a small body type (height <135 cm, BSA<1.5 m², body mass index (BMI) <22) may have a small but normal LVOT, AVA, and aorta. Indexing the valve area may reclassify smaller individuals from severe to moderate aortic stenosis (AS), but indexing all patients, particularly with extreme obesity, can lead to substantial overestimation of severity and should not be relied upon [18].

Another issue common to small body types and aortas is the concept of pressure recovery, the reconversion of kinetic energy into potential energy leading to an increase of pressure downstream from a stenosis. Doppler-derived pressure gradients represent the difference between left ventricular pressure and pressure in the vena contracta (lowest pressure in a stenosis), but do not measure the pressure recovery phenomena captured by a catheterization-based gradient (net pressure gradient). Pressure recovery accounts for differences between catheter- and Doppler-derived data [19, 20]. The net pressure gradient better reflects the hemodynamic significance and the amount of energy loss by the stenosis. To account for pressure recovery using echocardiography, Garcia and colleagues developed and validated the energy loss index (ELI) = [EOA × A_a/(A_a–EOA)]/BSA, where EOA is the effective orifice area using the Doppler-based continuity equation valve area and A_a represents the cross-sectional area of the aorta measured at 1 cm downstream of the sinotubular junction [21]. The ELI can be considered in situations when pressure recovery may be significant, such as individuals with aortic diameters <3 cm. The ELI has been shown to reclassify CAVD severity and provide independent and prognostic information above traditional severity measures [22, 23].

Hypertension is common in CAVD and can adversely affect hemodynamics, symptom burden, and traditional echo measure assessment [24–26]. To account for the “double load” of hypertension and aortic stenosis on the left ventricle, calculation of the valvuloarterial impedance (Z_va) can be obtained using the equation Z_va = (SBP + MG_net)/SVi, where SBP is the systolic blood pressure, MG_net is the mean net pressure gradient (taking into account pressure recovery), and SVi is the stroke volume indexed for BSA [27]. This global measure of vascular, valvular, and ventricular interplay may improve risk stratification in patients with asymptomatic moderate to severe CAVD [28].