How to capture duplex velocities and calculate percentages of obstruction
A benefit of duplex ultrasound is that the velocities are obtained in segments throughout the leg using the Doppler. Let’s go over the steps that are used to capture duplex velocities and how they can be used to calculate the degree of obstruction within a vessel.
Four steps for capturing duplex velocities
There are four basic steps for capturing duplex velocities:
- Angle the cursor in the direction of blood flow in the middle of the vessel.
- Adjust the Doppler angle line so it is parallel to the vessel walls.
- Capture the Doppler waveform.
- Press the measure / caliper button with the cursor at the tallest peak.
Let’s go over those steps again in a little more detail.
Step 1: Angle the cursor in the direction of blood flow in the middle of the vessel
To capture duplex velocities, start by angling the cursor (known as steering) in the direction of the blood flow for the vessel being examined. The center of the cursor contains a sample volume and Doppler angle. Place the sample Doppler in the middle of the vessel that you want to examine.
Step 2: Adjust the Doppler angle line so it is parallel to the vessel walls
While keeping the Doppler cursor angle at 60°, move the probe’s angle to get the Doppler angle line as parallel to the vessel walls as possible. If the vessel is too steep, you can adjust the Doppler angle between 0–60° and still be accurate. Angles greater than 60° overestimate the velocity and the degree of stenosis.
Step 3: Capture the Doppler waveform
Next, obtain the Doppler waveform and freeze it by pressing the freeze button on the keyboard.
Step 4: Press the measure / caliper button with the cursor at the tallest peak
Press the measure / caliper button so that the cursor appears, and then place the cursor at the tallest peak of the waveform to record the peak systolic velocity (PSV).
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How to calculate the percentage of obstruction caused by stenosis
Remember, the ultrasound machine does not calculate or compare the obstruction percentage. Instead, you must do manual calculations using the PSV values.
The patient’s age and natural hemodynamic state can affect their vasculature. Thus, velocities in the peripheral arteries will vary. To calculate the percentage of obstruction, the differences in velocities from different arterial segments are compared.
This categorizes the degree of obstruction into four ranges:
- 0–50% obstructed
- 50–75% obstructed
- 75–99% obstructed
- 100% obstructed (e.g., occluded)
To calculate the obstruction percentage, measure the velocity proximal to stenosis and the velocity within stenosis. Then, confirm that there are monophasic waveforms with diminished flow distal to stenosis. Next, compare the proximal PSV to the PSV within the stenotic artery:
- A PSV less than two times the proximal PSV suggests that the vessel is less than 50% blocked.
- A PSV two times the proximal PSV indicates that the vessel is approximately 50% obstructed.
- A PSV four or more times the proximal PSV indicates that the vessel is at least 75% blocked.
In our sample patient, the area of concern is the proximal superficial femoral artery (SFA). Take the velocity from the segment just proximal to the SFA, the common femoral artery (CFA), and then compare it to the velocity in the SFA:
- CFA PSV is 125 cm / s
- SFA PSV is 398 cm / s
The PSV of the SFA is double, but does not increase over four times the CFA velocity. So, the range, in this case, is between 50–75% of lumen diameter reduction.
What if you can’t obtain a velocity?
If you are unable to obtain a PSV value in a vessel, it may be occluded. The occlusion is usually well-visualized with color flow duplex ultrasound, but it may be obscured by calcific shadowing. In this case, you can rely on the ankle-brachial index (ABI) and the duplex waveforms distal to the occlusion to determine the presence and extent of the disease.
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