Duplex Ultrasound of Lower Extremity Arteries: Protocol, ABI Calculation, and Ultrasound Staging of Ischemia
The Role of Duplex in the Overall Scheme: ABI First
According to current recommendations (ACC/AHA 2024, ESC 2024), the primary method for confirming peripheral artery disease (PAD) is the Ankle-Brachial Index (ABI), not duplex ultrasound. ABI is the ratio of the highest systolic pressure at the ankle (via the dorsalis pedis or posterior tibial artery) to the highest systolic pressure at the arm.
Calculation and Interpretation of ABI
| ABI Value | Interpretation |
|---|---|
| <0.90 | Diagnosis of PAD |
| 0.91–0.99 | Borderline result |
| 1.00–1.40 | Normal |
| >1.40 | Non-compressible arteries (falsely elevated result) |
In cases of non-compressibility (diabetes, CKD), a normal ABI does not exclude PAD — the Toe-Brachial Index (TBI) is performed, threshold <0.70. Segmental pressure and plethysmography (PVR) are not mandatory before duplex — these are complementary methods, especially valuable in non-compressible arteries and critical ischemia (Guidelines for Noninvasive Vascular Laboratory Testing, ASE/SVMB).
Five-Step Duplex Protocol
Research route: aorta — iliac — common femoral — popliteal — tibial. The superficial femoral artery (SFA) is documented in three sections. The choice of probe depends on the depth:
| Segment | Probe |
|---|---|
| Aorta, iliac | Convex 3.5–5 MHz |
| Thigh, popliteal, tibial | Linear 5–10 MHz |
| Deep location | Convex |
The interpretation of the spectrum is based on the principle "FLOW = FLOW = FLOW" and the normal triphasic spectrum (Blagodir B.V.). A monophasic flow in the popliteal artery is considered a sign of upstream lesions (Lower Extremity Arterial Mapping: Duplex Ultrasound).
Ultrasound Staging of Ischemia
Quantitative assessment of ischemia begins with ABI and TBI. These indices are the foundation for all severity classifications: both classic clinical stages (Fontaine, Pokrovsky) and the modern WIfI system use ABI/TBI thresholds as a measure of the ischemic component. Detailed threshold values for WIfI and stage boundaries for each scale are not provided in the given fragments [clarify].
Frequently asked questions
Can PAD diagnosis start directly with duplex?
No. According to ACC/AHA 2024 and ESC 2024, the primary method for confirming PAD is ABI, not duplex.
How is ABI calculated?
It is the ratio of the highest systolic pressure at the ankle (via the dorsalis pedis or posterior tibial artery) to the highest systolic pressure at the arm.
What to do if ABI >1.40?
These are non-compressible arteries (falsely elevated result). In patients with diabetes, CKD, the Toe-Brachial Index (TBI) is performed, threshold <0.70.
What is the sequence of artery scanning?
Five-step route: aorta — iliac — common femoral — popliteal — tibial. The SFA is documented in three sections.
What does monophasic flow in the popliteal artery indicate?
It is a sign of upstream lesions (Lower Extremity Arterial Mapping: Duplex Ultrasound).