A triphasic waveform on a lower-limb arterial duplex study is one of the most reassuring sights in vascular ultrasound. Together with a normal ankle-brachial index and an unremarkable B-mode survey, it supports the impression of a healthy high-resistance arterial bed. What it does not do is exclude significant peripheral arterial disease (PAD) on its own. Treating a triphasic trace as a standalone clearance is one of the more common interpretive traps in lower-limb arterial work.

This article unpacks what a triphasic signal genuinely represents, why its anatomical location changes its meaning, how short stenoses and collateral pathways can leave distal waveforms looking deceptively normal, and why a defensible report integrates B-mode, colour Doppler, peak systolic velocity (PSV), PSV ratio, spectral broadening and pre-, intra- and post-stenotic haemodynamics.

What a triphasic waveform actually represents

In a resting lower-limb artery, normal flow is high-resistance. The spectral trace shows a sharp systolic upstroke, a brief early-diastolic reversal as the peripheral run-off rebounds against a high-resistance arteriolar bed, and a small late-diastolic forward component. That three-part contour is what we call triphasic.

A triphasic or biphasic signal at a given sample site tells you two things: the artery is patent at that point, and the resistance milieu immediately downstream is intact. It is a statement about local physiology at the sample volume, not about the limb as a whole. It says nothing directly about a segment several centimetres upstream, and nothing about the contralateral limb.

Why the location of the waveform matters

The same trace carries different diagnostic weight depending on where it is recorded. A triphasic common femoral artery signal reflects aortoiliac inflow reasonably well, which is why the common femoral waveform has been studied specifically as a marker of proximal occlusive disease. A triphasic popliteal or tibial signal, by contrast, tells you far less about the superficial femoral or iliac segments upstream.

In practical terms, a normal trace at the ankle cannot vouch for the aortoiliac segment, and a normal trace at the groin cannot vouch for the tibial vessels. Every waveform is a local observation. The examination becomes meaningful when waveforms are sampled at multiple levels – common femoral, proximal and distal superficial femoral, popliteal, and the tibial or pedal vessels – so that a normal segment can be contrasted with an abnormal one.

How short lesions and collaterals preserve a distal waveform

Two mechanisms commonly produce a normal-looking distal trace despite genuine disease.

First, a short, focal stenosis may cause only limited haemodynamic disturbance. The velocity increase and turbulence are confined to a brief segment, and the waveform re-normalises within a few centimetres. If the sample volume is placed beyond that short abnormal segment, the trace can look entirely triphasic.

Second, collateral circulation can maintain distal perfusion pressure even across a significant or occlusive lesion. Collaterals effectively bypass the obstruction, and the distal bed may receive enough flow to preserve a triphasic contour. A triphasic tibial signal is therefore compatible with a haemodynamically important proximal stenosis.

Both situations explain why sampling only one or two sites, or relying on a single reassuring trace, can miss clinically relevant disease. The waveform is a snapshot of one location, not a summary of the arterial tree.

Interpreting the whole study, not one signal

A robust arterial duplex assessment combines several independent observations:

B-mode defines vessel calibre, plaque presence, morphology, calcification and any occlusion. Dense calcification can obscure the lumen and make velocity measurement unreliable.

Colour Doppler reveals focal aliasing, a colour bruit or a filling defect, which directs the spectral sample volume to the site of maximal narrowing.

Peak systolic velocity (PSV) measured at the point of maximal aliasing quantifies the local velocity increase. Doppler angle should be kept at 60 degrees or less to avoid over- or under-estimation.

PSV ratio compares the stenotic PSV with the immediately pre-stenotic PSV, controlling for the patient’s baseline flow state. It is more transferable between patients than an absolute value.

Spectral broadening – a widened, filled-in spectral envelope – reflects post-stenotic turbulence and supports the presence of a significant lesion.

Pre-, intra- and post-stenotic haemodynamics complete the picture: pre-stenotic velocities establish the baseline, intra-stenotic velocities define the severity, and post-stenotic damping or a tardus-parvus pattern confirms downstream energy loss.

No single one of these is decisive. The diagnosis rests on their agreement.

The 180 / 345 cm/s example

Consider a site where the pre-stenotic PSV is 180 cm/s and the stenotic PSV is 345 cm/s.

The PSV ratio is 345 divided by 180, which is approximately 1.9.

Commonly cited velocity criteria associate a PSV ratio of about 2.0 or greater with a 50% or greater diameter reduction. On the ratio alone, 1.9 does not reach that threshold, so this study should not be automatically classified as a greater than 75% stenosis.

Absolute PSV thresholds are tempting because they are simple, but they are not transferable between patients or vessel segments. A high absolute number may reflect a hyperdynamic circulation, a small-calibre vessel, or an already elevated pre-stenotic velocity, rather than a tight stenosis. Conversely, a genuinely severe stenosis in a low-flow limb may produce a comparatively modest absolute PSV. The ratio, together with spectral broadening, post-stenotic turbulence and downstream damping, carries more interpretive weight than the peak number in isolation. A single absolute value, however high it looks, cannot grade a lesion on its own.

Grading criteria vary – and that is expected

There is no single universal velocity table. Thresholds differ between published criteria sets, between vascular laboratories, across vessel segments, and between native arteries, stents and grafts. International guidance from the European Society for Vascular Surgery and the Society for Vascular Surgery, and the AHA/ACC lower extremity PAD guideline, all emphasise correlation with the clinical picture and, where uncertainty persists, confirmatory imaging – rather than reliance on any one number.

The practical response is straightforward: apply your laboratory’s validated, documented criteria, state them in the report, and be explicit where the findings are borderline or technically limited.

A practical scanning checklist

• Confirm the clinical question and the limb(s) to be studied before you start.
• Follow a consistent, documented protocol so serial studies can be compared.
• Sample waveforms at multiple levels: common femoral, proximal and distal superficial femoral, popliteal, and tibial or pedal vessels.
• Obtain pre-stenotic, intra-stenotic and post-stenotic velocities at any lesion identified.
• Place the spectral sample volume at the site of maximal colour aliasing.
• Keep the Doppler angle at 60 degrees or less and document it.
• Calculate the PSV ratio against the immediately pre-stenotic segment.
• Assess spectral broadening and post-stenotic turbulence.
• Inspect the downstream waveform for damping or a tardus-parvus pattern.
• Compare with the contralateral limb and with ankle-brachial indices.
• Never sign off on a triphasic waveform alone.
• Record any technical limitations, such as dense calcification, that reduce confidence.

Reading the signal in context

A triphasic waveform is a useful, reassuring finding – but it is one data point, not a verdict. It cannot exclude significant disease elsewhere in the limb, particularly a short focal stenosis or a lesion bypassed by collaterals. Defensible reporting integrates B-mode, colour Doppler, PSV, the PSV ratio, spectral broadening and the full pre-, intra- and post-stenotic picture, and it acknowledges that grading criteria vary between laboratories and published sources.

Building that integrated interpretive habit – and applying it consistently across iliac, femoral, popliteal and tibial segments – is exactly what structured, case-based training is designed to do.

Ready to sharpen your lower-limb arterial interpretation? The ABC Vascular CME-accredited Practical Lower Limb Arterial Ultrasound course takes you from protocol to report through expert-led video lectures, real case studies and interactive quizzes – self-paced, accessible anywhere, and built to translate directly into your clinical practice.

Further reading

• Nordanstig J, et al. European Society for Vascular Surgery (ESVS) 2024 Clinical Practice Guidelines on the Management of Asymptomatic Lower Limb Peripheral Arterial Disease and Intermittent Claudication. Eur J Vasc Endovasc Surg. 2024.
• Gerhard-Herman MD, et al. 2016 AHA/ACC Guideline on the Management of Patients With Lower Extremity Peripheral Artery Disease. Circulation. 2017.
• Ranke C, Creutzig A, Alexander K. Duplex scanning of the peripheral arteries: correlation of the peak velocity ratio with angiographic diameter reduction. Ultrasound Med Biol. 1992.
• Kohler TR, et al. Duplex scanning for diagnosis of aortoiliac and femoropopliteal disease: a prospective study. Circulation. 1987.
• Jager KA, et al. Noninvasive mapping of lower limb arterial lesions. Ultrasound Med Biol. 1985.
• Spronk S, et al. Value of the duplex waveform at the common femoral artery for diagnosing obstructive aortoiliac disease. J Vasc Surg. 2005.
• Shaalan WE, et al. Reliability of common femoral artery haemodynamics in assessing the severity of aortoiliac inflow disease. J Vasc Surg. 2003.

Always follow your local laboratory’s validated criteria and recognised society guidance for grading.