Aliasing and angle error in a high-velocity aortic jet
A 62-year-old man with exertional syncope is referred for transthoracic echocardiography. The aortic valve is heavily calcified and the LVEF is 55%. From the apical five-chamber view, pulsed-wave Doppler in the LVOT (sample volume 5 mm, baseline shifted) shows a peak of 1.2 m/s. Across the aortic valve, PW Doppler wraps at 3.0 m/s with the Nyquist limit set at 1.5 m/s. Continuous-wave Doppler from the apex shows a peak of 4.2 m/s and a VTI of 92 cm, but the sonographer estimates the beam is 25° off the LVOT-to-aortic axis. LVOT diameter is 2.0 cm and LVOT VTI is 22 cm. Heart rate 78 bpm, BP 128/76 mmHg.
1. Why did the PW signal alias, and how should the peak aortic velocity be obtained? Calculate the true velocity and peak gradient if the 25° angle is uncorrected.
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Aliasing occurs when the peak velocity exceeds the Nyquist limit (PRF/2); here the PW scale is set at 1.5 m/s, so a 3.0 m/s signal wraps. PW cannot resolve this velocity because it range-gates. Switch to CW Doppler, which does not range-resolve and can measure high velocities, and align the beam as closely as possible with the LVOT-to-aortic axis from the apical 5- or 3-chamber view.
If the beam is 25° off axis, measured velocity = true velocity × cos 25° (0.906). Thus true peak = 4.2/0.906 = 4.63 m/s. Peak gradient = 4V²: measured 4 × 4.2² = 70.6 mmHg; corrected 4 × 4.63² = 85.8 mmHg. The uncorrected angle underestimates the peak gradient by about 15 mmHg. In practice, do not angle-correct CW; reposition the probe to maximize the envelope.
2. Calculate the aortic valve area by continuity. How does the angle error affect the severity classification?
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LVOT CSA = π × (1.0 cm)² = 3.14 cm². Stroke volume = 3.14 × 22 cm = 69.1 mL. AVA = SV / AV VTI = 69.1/92 = 0.75 cm². Using the angle-corrected AV VTI (92/0.906 = 101.5 cm), AVA = 69.1/101.5 = 0.68 cm². Dimensionless index = LVOT VTI/AV VTI = 22/92 = 0.24.
Either AVA is below 1.0 cm², so the valve is severe by area; the DVI of 0.24 is at the severe threshold (<0.25). However, the angle error matters: if the AV VTI is underestimated, AVA is overestimated, so 0.75 cm² is the less severe estimate. The LVOT VTI and diameter must also be measured correctly; if LVOT VTI is underestimated, SV and AVA fall. The combination of AVA 0.68-0.75 cm², DVI 0.24, and peak gradient around 86 mmHg supports severe AS. The key judgment is to optimize alignment and not accept the aliased PW value.
3. What instrument settings and probe maneuvers would you use to optimize the CW envelope?
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Use a dedicated continuous-wave transducer (Pedoff) or steerable CW from the apical window. Position the patient in the left lateral decubitus position, locate the apical impulse, and use a small footprint to minimize off-axis signal. Set the scale to 5-6 m/s so the envelope is not clipped, lower the baseline if needed, use a low wall filter, and adjust gain to see a dense, smooth envelope without overgain or underestimation. Use a sweep speed of 50-100 mm/s. Listen to the audio signal; it becomes crisp when aligned. Interrogate from the apex, right parasternal, and suprasternal windows and choose the highest, cleanest envelope. Measure the outer edge of the dense envelope and average 3-5 beats. Do not angle-correct CW; align the beam instead.