# How to capture duplex velocities and calculate percentages of obstruction

Master the four steps for capturing and interpreting blood flow velocity with duplex ultrasound.
Last update26th Feb 2021

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:

1. Angle the cursor in the direction of blood flow in the middle of the vessel.
2. Adjust the Doppler angle line so it is parallel to the vessel walls.
3. Capture the Doppler waveform.
4. 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.

Figure 1. When capturing duplex velocities, begin by angling the cursor in the direction of the blood flow and placing the cursor 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.

Figure 2. When capturing duplex velocities, move the Doppler cursor angle parallel to the vessel walls without exceeding 60° for accurate readings.

### Step 3: Capture the Doppler waveform

Next, obtain the Doppler waveform and freeze it by pressing the freeze button on the keyboard.

Figure 3. When capturing duplex velocities, obtain the Doppler waveform 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).

Figure 4. When capturing duplex velocities, press the measure / caliper button and place the cursor at the tallest waveform peak to record the peak systolic velocity (PSV).

## 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:

1. 0­–50% obstructed
2. 50–75% obstructed
3. 75–99% obstructed
4. 100% obstructed (e.g., occluded)
Figure 5. The degree of obstruction of a vessel is categorized into four ranges, 0–50% obstructed, 50–75% obstructed, 75–99% obstructed, and 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.
Table 1. Calculating the degree of obstruction in a vessel by comparing peak systolic velocities (PSV) within stenosis and proximal to stenosis.

### Sample calculation

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.

Figure 6. A patient has a peak systolic velocity (PSV) of 125 cm / s in the common femoral artery and a PSV of 398 cm / s in the superficial femoral artery (SFA). The percentage of SFA obstruction is between 50–75% since the PSV is between two to four times greater than the proximal segment.

## 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.

Figure 7. An occlusion can usually be well-visualized with color flow duplex ultrasound.

That’s it for now. If you want to improve your understanding of key concepts in medicine, and improve your clinical skills, make sure to register for a free trial account, which will give you access to free videos and downloads. We’ll help you make the right decisions for yourself and your patients.

• Aboyans, V, Criqui, MH, Abraham, P, et al. 2012. Measurement and interpretation of the ankle-brachial index: a scientific statement from the American Heart Association. Circulation126: 2890–2909. PMID: 23159553
• Cervin, A, Wanhainen, A, and Björck, M. 2020. Popliteal aneurysms are common among men with screening detected abdominal aortic aneurysms, and prevalence correlates with the diameters of the common iliac arteries. Eur J Vasc Endovasc Surg59: 67–72. PMID: 31757587
• Cleveland Clinic. 2021. Leg and foot ulcers. Cleveland Clinichttps://my.clevelandclinic.org
• Cleveland Clinic. 2021. Marfan syndrome. Cleveland Clinichttps://my.clevelandclinic.org
• Cleveland Clinic. 2021. Popliteal artery entrapment syndrome (PAES). Cleveland Clinichttps://my.clevelandclinic.org
• Cleveland Clinic. 2021. Statin medications & heart disease. Cleveland Clinichttps://my.clevelandclinic.org
• Collins, L and Seraj, S. 2010. Diagnosis and treatment of venous ulcers. Am Fam Physician81: 989–996. PMID: 20387775
• Høyer, C, Sandermann, J, and Peterson, LJ. 2013. The toe-brachial index in the diagnosis of peripheral arterial disease. J Vasc Surg58: 231–238. PMID: 23688630
• Jaoude, WA. 2010. Management of popliteal artery aneurysms. SUNY Downstate Department of Surgeryhttp://www.downstatesurgery.org
• Johns Hopkins Medicine. 2021. Aneurysm. Johns Hopkins Medicinehttps://www.hopkinsmedicine.org
• Kassem, MM and Gonzalez, L. 2020. “Popliteal artery aneurysm”. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov
• Moxon, JV, Parr, A, Emeto, TI, et al. 2010. Diagnosis and monitoring of abdominal aortic aneurysm: current status and future prospects. Curr Probl Cardiol35: 512–548. PMID: 20932435
• Richert, DL. 2016. Gundersen/Lutheran Ultrasound Department Policy and Procedure Manual. Gundersen Health Systemhttps://www.gundersenhealth.org
• Rivera, PA and Dattilo, JB. 2020. “Pseudoaneurysm”. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov
• Stanford Medicine 25. 2021. Measuring and understanding the ankle brachial index (ABI). Stanford Medicine 25https://stanfordmedicine25.stanford.edu/
• Teo, KK. 2019. Acute peripheral arterial occlusion. Merck Manuals Professional Editionhttps://www.merckmanuals.com
• The Regents of the University of California. 2020. Diabetic foot ulcers. UCSF Department of Surgeryhttps://surgery.ucsf.edu
• Zwiebel, WJ and Pellerito, JS. 2005. Introduction to Vascular Ultrasonography. 5th edition. Philadelphia: Elsevier Saunders. (Zwiebel and Pellerito 2005, 254–259)