Interpreting ankle-brachial index (ABI) waveforms

In this video, we'll explore both audible and analog Doppler waveforms, and learn why knowing the difference is crucial in circumstances where ABI numbers are inaccurate. 

Elizabeth Tenny, BS RVT RDCS
Elizabeth Tenny, BS RVT RDCS
2nd Feb 2021 • 4m read

In this video, from our Ultrasound Masterclass: Arteries of the Legs course, we'll explore both audible and analog Doppler waveforms, and learn why knowing the difference is crucial in circumstances where ABI numbers are inaccurate.

Join our Ultrasound Masterclass: Arteries of the Legs course now!

From traumatic, acute limb ischemia to chronic toe ulcers, arterial diseases pop up in many forms and stages. In this course, you’ll learn how to recognize the symptoms and severity of peripheral arterial diseases, master the basic ankle-brachial index (ABI), and interpret findings to quickly determine if revascularization is needed. We’ll teach you how to diagnose arterial diseases so that you can save some limbs!

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Video transcript

In the previous lesson, we learned how the ABI indices are calculated and why they are important within the report. The waveforms are also an important component of the report. Understanding both the audible and analog waveforms of Dopplers is especially important in circumstances where the ABI numbers are inaccurate and irrelevant.

For example, some vessels are so calcified, they resist compression and falsely elevate the ABI. The pulse sounds, the audible waveforms, and the analog waveforms are qualitative data produced by the Doppler probe. The sound is crucial but it's not recorded. The analog waveforms are recorded. Their quality can be user dependent.

It is easy to make a healthy vessel look diseased, but not a diseased vessel look healthy. In other words, it may overestimate presence of disease if technique is poor. Good technique comes with practice and experience. Ideally, the pen is held at 45 degrees or less to the vessel or skin pointing toward the heart. The more perpendicular you are to the vessel, the more you will overestimate the disease.

The waveforms are named by the number of times they cross the baseline. Another way to understand it is by the number of peaks and pits around the baseline. It is important to note that ideally, you have a triphasic waveform with two peaks and one pit. Artifact can affect waveforms and aren't true peaks and pits.

These multiple tiny peaks and pits are artifacts. Compared to the waveform before it, which more clearly has just one peak, the artifact makes this particular waveform difficult to classify. If the user has a shaky hand or if there is venous flow interference, it can cause a lot of artifact. If there is venous interference press gently on the vein with the probe.

The vein has a lower pressure, thus it will collapse. The flow will pause and allow for a clear arterial Doppler without artifacts. There are five basic stages of waveform degradation that occur with worsening PAD. In general two peaks and one pit is ideal and considered normal. As the PAD worsens from mild, to moderate, to severe, the waveform changes.

The waveforms begin to decrease the number of peaks and pits and develop a rounded upstroke or humpback peaks, which are widen peaks without pits. And then they eventually lose all peaks and pits and become absent. It's important to note that dampened monophasic waveforms are also commonly called tardus parvus waveforms.

The healthiest waveforms are called triphasic, meaning they have two peaks and one pit and are both audible and visible. The triphasic waveform indicates shift of directions for blood flow which reflect normal vessel flexibility. Notice the sharp incline to the tallest peak. As an artery wall becomes diseased they lose elasticity reflected in loss of humps.

Listen to the triphasic waveforms. Notice three distinct sounds representing the peak, pit, peak, one pit and two peaks. Biphasic waveforms have two sounds, they have one peak and one pit. Notice it still has the sharp upstroke to the tallest peak. The loss of that third hump can sometimes be due to asymptomatic loss of elasticity.

This is due to wall calcification. Recall that that naturally happens with age, but can become severe at younger ages in patients with diabetes. The upstroke represents blood flow acceleration to peak systole and the pit is early diastole which is a reversal of flow. The second peak is late diastole, which is forward flow.

These waveforms represent flow that is essentially normal at rest. These patients usually present with symptoms of intermittent claudication. However, it can also be found in mild to moderate arterial insufficiency. And correlating the waveforms with the ABI indices becomes very helpful in this situation.

For example, if the ABI is 1.0 and the waveform is biphasic, there could be a mild calcification of the vessels, but no significant atherosclerosis. Biphasic waveforms obtained with an ABI of 0.7 indicates mild to moderate PAD. Listen to these biphasic waveforms and notice only two distinct sounds.

As long as it is multiphasic meaning biphasic or triphasic as opposed to monophasic, it is likely not an immediate arterial surgical concern. Waveforms with a sharp upstroke and shallow pits are called a weak biphasic. When they have no pits but still have one peak with a sharp stroke, they're called high monophasic.

These both represent moderate arterial insufficiency and can be difficult to classify because it involves a subjective component of the reviewer. This is dependent on the experience of the operator to appreciate the sharp upstroke as opposed to the dampened waveforms that is seen with a more severe disease. Listen to the high monophasic waveform, noting only one hump and one sound.

Dampened humpback monophasic waveforms indicates severe PAD. There is a slow rising peak above the baseline and the upstroke is rounded, not sharp. This can be seen with non-healing ischemic ulcers. Listen to the dampened monophasic waveform with one peak and continuous flow throughout the cardiac cycles. Absent ABI waveforms are due to no peripheral pulses. These are found in patients with rest pain and critical limb ischemia.