Lower Limb Arterial Duplex

Lower limb arterial duplex ultrasound examines the aorta and iliac arteries (where visible), the common femoral, superficial femoral, deep femoral and popliteal arteries, and the tibial and peroneal arteries, with B-mode imaging, colour Doppler and spectral Doppler. A 3 to 5 MHz curvilinear probe is used for the iliac segment and a 5 to 10 MHz linear probe for the limb.

The vessels are followed from the groin to the ankle, measuring peak systolic velocity (PSV), waveform shape and the ratio of PSV in a stenosis to the PSV in the normal segment proximal to it (PSV ratio). The examination takes 30 to 60 minutes, with no radiation or nephrotoxic contrast.

Peripheral arterial disease affects about 10 to 15% of adults over 55 years and over 20% of those over 70 years (largely asymptomatic), and is associated with a 2 to 4 times higher cardiovascular mortality. Chronic limb threatening ischaemia (rest pain, ulcers, gangrene) affects 1 to 2% of people with peripheral arterial disease each year.

Duplex ultrasound has a sensitivity of 85 to 90% and a specificity of over 95% for detecting a stenosis of 50% or more and occlusion, and is used to plan endovascular or surgical revascularisation and to monitor bypass grafts.

Indications. Intermittent claudication, rest pain, ulcers or gangrene with absent pulses or reduced ankle brachial index. Mapping of stenosis and occlusion before angioplasty, stenting or bypass. Surveillance of vein graft or prosthetic bypass (stenosis, kinks, aneurysm, occlusion), stents and angioplasty sites.

Acute limb ischaemia (diagnosis of thrombosis or embolus when angiography is not immediately available). Popliteal and femoral artery aneurysm. Pseudoaneurysm and arteriovenous fistula after femoral puncture or trauma. Popliteal entrapment syndrome, adductor canal syndrome, cystic adventitial disease. Vasculitis (Buerger disease, Takayasu). Vein mapping before bypass.

Diabetic foot (arterial supply). Blue toe syndrome (embolic source). Not for screening asymptomatic people.

Contraindications and precautions. None. The room should be warm because cold causes vasoconstriction. The patient lies supine, resting for 10 to 15 minutes, with the leg externally rotated; the popliteal and tibial arteries can be scanned with the patient prone or in the lateral position. Ankle brachial pressure index is measured first. Ulcers and wounds need sterile covering and gel. Pressure on a pseudoaneurysm or on severely diseased vessels is kept light.

Preparation and technique. Fasting for 6 hours reduces bowel gas for the iliac segment. The arteries are scanned in transverse and longitudinal views for plaque, wall calcification, aneurysm and occlusion.

Spectral Doppler samples (angle 60 degrees or less) are taken at the common femoral, proximal, mid and distal superficial femoral, popliteal and tibial arteries, and across every stenosis. The waveform, PSV, and PSV ratio are recorded. Exercise or reactive hyperaemia (calf exercise or cuff inflation) may unmask haemodynamically significant iliac disease.

Normal appearances. The artery wall is thin, with a smooth intima and no plaque. Diameters are common femoral 7 to 10 mm, superficial femoral 5 to 7 mm, popliteal 4 to 6 mm, tibial 2 to 4 mm. Spectral waveform in the resting leg is triphasic: a sharp systolic peak, early diastolic reversal, and a late forward diastolic component, with a clear spectral window.

PSV is about 100 to 120 cm each second in the common femoral artery, decreasing distally (80 to 100 in the superficial femoral and 60 to 80 in the popliteal artery). Colour Doppler fills the lumen uniformly. Ankle brachial pressure index is 1.0 to 1.4.

Interpretation.

• Stenosis categories by PSV and PSV ratio: normal (PSV under 150 cm each second, triphasic), 1 to 19% (PSV under 150, ratio under 1.5, triphasic with some spectral broadening), 20 to 49% (PSV 150 to 200, ratio 1.5 to 2, loss of reverse flow component, spectral broadening), 50 to 99% (PSV over 200, ratio over 2, a monophasic waveform distal to the lesion, ratios over 4 suggest at least 75%), and occlusion (no flow in the segment, with a "thump" at the stump and collateral reconstitution distally).
• Waveform changes distal to a significant stenosis or occlusion: monophasic, damped, "tardus parvus" waveform with delayed systolic acceleration and reduced amplitude, which also signals proximal disease when the lesion is not seen (for example, iliac disease shown by damped common femoral waveform).
• Occlusion: absent colour and spectral Doppler signal, echogenic thrombus or atheroma in the lumen, and reconstitution by collaterals distally; acute occlusion shows hypoechoic thrombus and absent flow with no collaterals.
• Atherosclerosis: calcified and mixed plaques, wall irregularity, commonly in the adductor canal and superficial femoral artery; diffuse tibial calcification in diabetes and renal failure.
• Aneurysm: focal enlargement of the artery over 1.5 times the normal diameter (popliteal over 10 to 12 mm or 50% larger), often with mural thrombus, and associated risk of distal embolism, thrombosis or rupture; popliteal aneurysms are bilateral in 50% and associated with aortic aneurysm in 30 to 50% of cases.
• Pseudoaneurysm: a pulsatile sac communicating with the artery by a neck, with a "to-and-fro" waveform in the neck and the "yin yang" colour swirl in the sac; arteriovenous fistula: high velocity, low resistance turbulent flow in the feeding artery, arterialised flow in the draining vein, and perivascular colour artefact.
• Popliteal entrapment: occlusion or reduced flow in the popliteal artery with active plantar flexion or dorsiflexion of the foot; cystic adventitial disease: an anechoic cystic lesion in the arterial wall with a narrowed lumen ("scimitar" sign on angiography).
• Bypass graft surveillance: PSV over 300 cm each second, PSV ratio over 3, or PSV under 45 cm each second in the graft indicates a threatened graft; stenosis occurs at anastomoses and valve sites (vein grafts); retained valve leaflets, kinks, and aneurysmal degeneration may be seen; occluded graft shows no flow.
• Embolism: abrupt cut off with echogenic or hypoechoic embolus at a bifurcation (common femoral or popliteal), with no collaterals.
• Vasculitis: smooth, concentric wall thickening (Takayasu) or segmental occlusions with corkscrew collaterals (Buerger disease).
• Medial calcification: non-compressible vessels with falsely high ankle pressure (ankle brachial index over 1.4), common in diabetes and renal failure; toe pressures or pulse wave analysis are used.

Critical values and findings needing immediate action.

• Acute limb ischaemia (sudden pain, pallor, pulselessness, paraesthesia, paralysis, poikilothermia) with an arterial occlusion: immediate heparin and emergency vascular surgery review (embolectomy, thrombolysis, bypass), with revascularisation ideally within 6 hours; if the limb is non-viable, amputation.
• Critical limb ischaemia with tissue loss or rest pain: urgent vascular referral for revascularisation within days.
• Threatened bypass graft (PSV under 45 or over 300 cm each second, high ratio): urgent angiography or intervention before occlusion.
• Symptomatic or large popliteal aneurysm (over 20 mm) or thrombosed aneurysm: surgical or endovascular repair.
• Expanding or symptomatic pseudoaneurysm: thrombin injection under ultrasound control, compression or surgical repair; rupture is an emergency.
• Compartment syndrome after reperfusion: fasciotomy.
• Wet gangrene or infected ischaemic foot: urgent source control, antibiotics and revascularisation.
• Blue toe syndrome: search for embolic source (aorta, aneurysm), anticoagulation and vascular review.

Limitations and artefacts.

• Aortoiliac segment is poorly seen in obese patients and with bowel gas; CT angiography, MR angiography or catheter angiography are needed for planning intervention.
• Heavily calcified vessels (diabetes, renal failure) cause acoustic shadowing that hides the lumen and Doppler flow; tibial arteries are particularly difficult.
• Sequential (tandem) stenoses cause waveform damping and velocity underestimation of distal lesions; long, multilevel disease is difficult to map.
• Angle of insonation over 60 degrees overestimates velocity; low cardiac output, cold limbs, and heart failure reduce velocities; severe stenosis may show low velocity due to a trickle flow (pre-occlusive lesion).
• Collateral flow can be mistaken for the native artery, and slow flow in a pre-occlusive stenosis can look like occlusion; low flow colour settings are used.
• Operator dependent and time consuming; a complete duplex of both legs may take over an hour.
• Ankle brachial index is falsely normal or raised in calcified vessels (over 1.4) and may be normal at rest in iliac disease (exercise test needed).
• Distinguishing recent from old occlusion is difficult.
• Venous contamination and motion artefact affect the interpretation.

Special groups.

• Diabetes: distal tibial and pedal disease, medial calcification, neuropathy, infection, and gangrene; toe pressures and pedal duplex are important.
• Chronic kidney disease: avoids contrast, so duplex is the preferred first line test.
• Smokers and patients with hyperlipidaemia: early onset disease.
• Younger patients: consider popliteal entrapment, cystic adventitial disease, Buerger disease, embolic sources, and vasculitis.
• Patients after femoral puncture or catheterisation: pseudoaneurysm or arteriovenous fistula.
• Pregnancy: safe imaging.
• Elderly frail patients: duplex based planning of minimal procedures.
• Regions: late presentation with gangrene, Buerger disease, Takayasu arteritis, rheumatic heart disease and endocarditis as embolic sources, trauma, and snake bite are important; angiography may not be available, so duplex determines the level of amputation and the possibility of revascularisation.

Follow up tests. Ankle brachial pressure index with toe pressures, exercise treadmill test, CT angiography or MR angiography for planning revascularisation, catheter angiography with intervention, transcutaneous oxygen measurement for wound healing, lipid profile, glucose or HbA1c, renal function, blood pressure assessment,

echocardiography and rhythm monitoring for embolic source, aortic ultrasound, surveillance duplex of grafts and stents at 1, 3, 6 and 12 months, supervised exercise programme, antiplatelet and statin therapy, and referral to vascular surgery.