Ankle brachial pressure index

The Ankle Brachial Pressure Index (ABPI), known more commonly as an ABI, is the ratio of the blood pressure in the lower legs to the blood pressure in the arms. Compared to the arm, lower blood pressure in the leg is an indication of blocked arteries (peripheral vascular disease). The ABI is calculated by dividing the higher systolic blood pressure in either the dorsalis pedis or posterior tibial arteries by the higher of the two systolic blood pressures in the arms.

Method

A Doppler ultrasound blood flow detector, commonly called Doppler Wand or Doppler probe, and a sphygmomanometer (blood pressure cuff) are usually needed. The blood pressure cuff is inflated proximal to the artery in question. Measured by the doppler wand, the inflation continues until the pulse in the artery ceases. The blood pressure cuff is then slowly deflated. When the artery's pulse is re-detected through the doppler probe the pressure in the cuff at that moment indicates the systolic pressure of that artery.

The higher systolic reading of the left and right arm brachial artery is generally used in the assessment. The pressures in each foot's posterior tibial artery and dorsalis pedis artery are measured with the higher of the two values used as the ABI for that leg.

<math>ABPI_{Leg} = \frac { P_{Leg} }{ P_{Arm} }</math>

Where P_Leg is the systolic blood pressure of dorsalis pedis or posterior tibial arteries

and P_Arm is the highest of the left and right arm brachial systolic blood pressure

The ABPI test is a popular tool for the non-invasive assessment of PVD. Studies have shown the sensitivity of ABPI is 90% with a corresponding 98% specificity for detecting hemodynamically significant (Serious) stenosis >50% in major leg arteries, defined by angiogram.

However, ABPI has known issues:

• ABPI is known to be unreliable on patients with arterial calcification (hardening of the arteries) which results in less or incompressible arteries, as the stiff arteries produce falsely elevated ankle pressure, giving false negatives). This is often found in patients with diabetes melitus (41% of PAD patients have diabetes), renal failure or heavy smokers. ABPI values < 0.9 & >1.3 should be investigated further regardless.
• Performing ABPI is time consuming.
• Resting ABPI is insensitive to mild PAD. Treadmill tests (6 minute) are sometimes used to increase ABPI sensitivity, but this is unsuitable for patients who are obese or have co-morbidities such as Aortic aneurysm, and increases assessment duration.
• Lack of protocol standardisation, which reduces intra-observer reliability.
• Skilled Operators are required for consistent, accurate results.

However, technology is emerging that allows for the oscillometric calculation of ABI. This is achieved by doing simultaneous readings of blood pressure at the levels of the ankle and upper arm using specially calibrated oscillometric modules. This idea has been pioneered by Uwe DIEGEL and Roland ASMAR in France. The main interest in the introduction of an oscillometric system to measure ABI is that it standardizes the method and makes the measurement of ABI accessible to all doctors.

Interpretation of results

In a normal subject the pressure at the ankle is slightly higher than at the elbow (there is reflection of the pulse pressure from the vascular bed of the feet, whereas at the elbow the artery continues on some distance to the wrist). The ABPI is the ratio of the highest ankle to brachial artery pressure and an ABPI of greater than 0.9 is considered normal (Free from significant PAD).

However, an ABPI value greater than 1.3 is considered abnormal, and suggests calcification of the walls of the arteries and incompressible vessels, reflecting severe peripheral vascular disease.

Provided that there are no other significant conditions affecting the arteries of the leg, the following ABPI ratios can be used to predict the severity of PAD as well as assess the nature and best management of various types of leg ulcers:

ABPI value	Interpretation	Action	Nature of ulcers, if present
above 1.2	Abnormal Vessel hardening from PVD	Refer routinely	Venous ulcer use full compression bandaging
1.0 - 1.2	Normal range	None
0.9 - 1.0	Acceptable
0.8 - 0.9	Some arterial disease	Manage risk factors
0.5 - 0.8	Moderate arterial disease	Routine specialist referral	Mixed ulcers use reduced compression bandaging. Claudication may be present at values less than 0.6.
under 0.5	Severe arterial disease	Urgent specialist referral	Arterial ulcers no compression bandaging used. Pain at rest may be present at values less than 0.25.

Predictor of atherosclerosis mortality

Studies in 2006 suggests that an abnormal ABPI may be an independent predictor of mortality, as it reflects the burden of atherosclerosis.

See also

• Peripheral vascular disease
• Peripheral vascular examination
• Intermittent claudication

References

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