Duplex ultrasound waveforms are obtained in segments throughout the leg and compared for degradation from triphasic to biphasic or monophasic waveforms. The waveforms are then correlated with velocities to confirm stenosis.
The concept of ankle-brachial index (ABI) Doppler phasicity, or peaks and pits, holds true for duplex ultrasound waveforms as well. For example, triphasic waveforms look and sound triphasic on duplex ultrasound.
However, duplex ultrasound is different from ABI because if there is stiffening and degradation of phasicity, you can image the area and identify if the degradation is due to plaque build-up or wall calcification. With duplex ultrasound, you can see the vessel and confirm if it is widely patent. Duplex ultrasound also records the waveforms on a velocity scale, which helps grade the percentage of stenosis.
The stages of waveform degradation on duplex ultrasound
A loss of phasicity occurs in stages; each stage represents different severities of disease. These stages of waveform degradation are clearly seen on duplex and fall into one of three categories:
Identifying stages of waveform degradation on duplex ultrasound
When performing duplex ultrasound, the technologist will begin at the common femoral artery (CFA) and move distally down the leg to thoroughly evaluate all the main arteries. Similar to ABIs, multiphasic waveforms are considered normal. If there is a blockage in the leg from significant stenosis, triphasic waveforms will only be obtained proximal to the blockage.
Let’s take a look at a patient case to demonstrate how to interpret duplex waveforms and identify stages of degradation.
On ultrasound, the patient’s CFA waveforms are triphasic which indicate normal aortoiliac inflow since the CFA is distal to the aorta and iliac arteries. As mentioned, arteries distal to significant stenosis will not be triphasic.
Further down the leg at the mid-superficial femoral artery (SFA), the patient’s waveforms are still multiphasic which indicate normal blood flow. However, the waveforms at the popliteal artery show spectral broadening (e.g., snowy filling of the waveform window due to turbulence produced by stenosis). This is categorized as an intra-stenosis popliteal artery waveform.
With this patient, you can also see an increase in velocity at the popliteal artery in comparison to the velocity at the SFA (Fig. 4). This happens because the lumen diameter decreases from stenosis which causes the blood velocity to increase.
Arterial waveforms distal to significant stenosis are usually monophasic. Keep in mind that waveforms can sometimes occur below the baseline simply because of phasic vein Doppler interference—which is blood flow in the veins.
Comparing ABI and duplex ultrasound findings
For the patient case featuring popliteal artery stenosis, compare the patient’s ABI test to their duplex ultrasound findings. The dampened monophasic ankle waveforms and abnormal ratios suggest stenosis. But, the findings don’t give any information about the location. The duplex ultrasound confirmed that the blockage location was in the popliteal artery.
Recognizing a Doppler bruit on duplex ultrasound
Occasionally, a Doppler bruit will occur on a duplex ultrasound. This is from the vibration of a severely stenotic vessel. If this is seen, but the velocities are not elevated, keep searching nearby since the vibration is usually coming from a stenotic artery.
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