Doppler Ultrasound of the LVOT and Aortic Valve:
©Walter Rasmussen, R.D.C.S.
After imaging the left ventricular outflow tract (LVOT) and aortic valve, activate color Doppler and size the box so that the LVOT and Aortic valve are sampled. Remember to narrow the sector width and to use the smallest practical color box in order to keep the Color Doppler frame rate and resolution as high as possible. If aortic insufficiency is present, size the box so that it is long enough to record the jet as far is it extends into the left ventricle. Always check that the standard Color Doppler scale setting of 50 to 60 cm/sec is set, otherwise the jet area may be overestimated.Apical 5 chamber view with color Doppler, Diastole.
After recording Color Doppler of the AI jet, use CW Doppler to record its velocity profile (see text below). After evaluating aortic insufficiency, angle the transducer a color Doppler image of forward flow, from the LVOT into the aorta is obtained. This will allow placement of cursors for making spectral Doppler recordings of the LVOT and the aortic valve.
PW Doppler of the LVOT:
LVOT color Doppler demonstrating layers of acceleration (blue to red) as blood approaches the aortic valve. Color Doppler turns red due to aliasing (exceeding the Nyquist Limit).
While in the apical five-chamber view, position the color Doppler box over the left ventricular outflow tract and aortic valve. Adjust the color box size so that it covers only the area of interest (LVOT, valve annulus, and ascending aorta), in order to preserve color Doppler image resolution and adequate frame rate.
Place the PW Doppler sample volume on the ventricular side of the valve and move it within the brightest blue of the Color Doppler display. Advance the sample volume toward the valve until a closing sound is heard, with the corresponding spike on spectral Doppler, seen at end-systole. The closing spike indicates that the valve is hitting against the sample volume, thus assuring that it is in good position. If an opening spike is recorded and heard while using PW Doppler at the aortic valve, the sample volume is too close and should be retreated from the valve until the opening spike disappears. If a high velocity outflow tract obstruction is present, the typical LVOT recording may be difficult or impossible to obtain. In this case, record several points, documenting the LVOT flow characteristics.
Typical PW spectral Doppler of the LVOT
Ideally, the position of the sample volume should correspond to where the LVOT diameter measurement will be taken for the calculation of aortic valve area. After recording the PW Doppler velocity, CW Doppler is used to record the velocity profile and maximal velocity across the prosthetic valve.
If a prosthetic valve is present in the aortic position, pre-valvular acceleration may produce higher LVOT velocities than those found in patients with a native valve. Observe and record velocities from several locations within the LVOT so that the reader can choose which profile to use for diagnostic purposes.
CW Doppler of the Aortic Valve:
In the Apical 5-Chamber view, trans-valvular assessment of the velocity and pressure gradient across the aortic valve is performed using CW Doppler rather than PW Doppler because it is able to record the very high velocities encountered in aortic stenosis, sub-aortic stenosis, mitral insufficiency, tricuspid insufficiency, and shunts.
When aortic velocities are high, the LVOT velocity profile is often visible within the CW Doppler signal of the aortic valve, (as shown in the above illustration on right). This illustrates the instantaneous pressure gradient across the aortic valve.
While imaging the apical five-chamber view, use Color Doppler to locate the systolic blood flow through the valve and then position the CW Doppler cursor across the valve. Activate the CW Doppler mode. Because aortic systolic and regurgitant diastolic jets are not typically aligned in the same axis, it is usually necessary to concentrate on them individually and make separate recordings for each, demonstrating their highest velocities and sharpest velocity profiles. More accurate measurements are possible if the scale and baseline are adjusted so that the velocity profile is occupying at least two-thirds of the strip chart.