Pathophysiology
P-I-22. Ankle-brachial index: definition & measurement
足関節上腕血圧比（ABI）の定義と測定
Definition
- Ankle-Brachial Index (ABI) = ratio of systolic pressures at the ankle vs. the arm
- Originally used to detect arterial stenosis in the lower limb
Interpretation
- Normal: 1.0 – 1.3
- ABI < 0.9 → possible peripheral arterial stenosis + high cardiovascular risk
How to Measure
- Equipment: BP monitor cuff + 8–10 Hz Doppler probe
- Performed in lying position, after a 10-minute rest
- Doppler measurement of systolic BP in both arms → take the higher value
- Doppler measurement of systolic BP at both ankles (over posterior + anterior tibial arteries) → take the higher value
- Divide the highest ankle systolic value by the highest arm systolic value
$$ ABI = \dfrac{\text{higher systolic value of the ankle}}{\text{higher systolic value measured on the arms}} $$
一問一答
▶What equipment is used to measure ABI?
A BP monitor cuff plus an 8–10 Hz Doppler probe.
▶What was ABI originally used to detect?
Arterial stenosis in the lower limb.
▶What is the ankle-brachial index (ABI)?
The ratio of systolic pressure at the ankle to systolic pressure at the arm.
▶What does an ABI <0.9 indicate?
Possible peripheral arterial stenosis and high cardiovascular risk.
▶What is the normal range of ABI?
1.0–1.3.
▶In what position and after what preparation is ABI measured?
In the lying position, after a 10-minute rest.
▶How is ABI calculated?
ABI = highest ankle systolic pressure / highest arm systolic pressure.
▶Which arteries are used for the ankle pressure measurement in ABI?
The posterior and anterior tibial arteries (take the higher value).
▶Why is the higher arm value used in the ABI calculation?
To account for possible inter-arm differences and avoid underestimating the reference brachial pressure.
▶Why does a low ABI normally arise from arterial stenosis?
Proximal stenosis reduces distal (ankle) systolic pressure, lowering the ankle-to-arm ratio.
▶Why is ABI a marker of generalized cardiovascular risk, not just leg disease?
Peripheral arterial disease reflects systemic atherosclerosis, so a low ABI predicts coronary and cerebrovascular events too.
▶Why is a Doppler probe used rather than a stethoscope for ABI?
At low ankle pressures Korotkoff sounds may be inaudible; Doppler detects the return of arterial flow reliably.
▶Why might an abnormally high ABI (>1.3) also be pathological?
It suggests non-compressible, calcified arteries (e.g., in diabetes/chronic kidney disease), which can mask underlying disease.
▶Why is a 10-minute rest before ABI measurement important?
It lets BP stabilize at baseline, avoiding falsely elevated readings from recent activity.
▶Why is the supine (lying) position used for ABI?
It removes the hydrostatic pressure difference between arm and ankle, allowing a valid pressure comparison.
▶Normally, how does ankle systolic pressure compare with arm systolic pressure?
Ankle pressure is normally equal to or slightly higher than arm pressure, giving an ABI of ~1.0–1.3.
▶Why measure both posterior and anterior tibial arteries at the ankle?
To capture the best-perfused vessel; the higher of the two is used as the ankle pressure.
▶Why is ABI especially useful in diabetic patients?
It screens for peripheral arterial disease, a major contributor to diabetic foot and amputation risk — though calcification can falsely raise values.
▶What does the Doppler probe detect during ABI measurement?
The frequency shift of reflected ultrasound from moving blood, signaling the systolic pressure at which flow returns under the cuff.
▶Is ABI an invasive or non-invasive test, and why is that advantageous?
Non-invasive — it uses only a cuff and Doppler, making it a cheap, repeatable screening tool for arterial disease and CV risk.