Ankle-brachial index (ABI) is a noninvasive test that provides assessment of the presence of peripheral artery disease and can be more reliable and specific than the history and physical examination. The ABI is calculated by dividing ankle blood pressure (in mm Hg) by brachial blood pressure. Many laboratories perform a resting ABI followed by an exercise ABI.

Segmental pressures provide physiologic information that can confirm a vascular cause for ischemic rest pain and claudication. It is performed by using a four-cuff technique; pneumatic cuffs are applied to the upper thigh, the lower thigh, the upper calf, and the area just above the ankle. Additionally, cuffs are applied to the upper arms to determine brachial pressures.

1. Tell the patient that the total examination time (for pressures only) is generally <15 minutes. If an exercise/stress ABI is ordered, the at-rest study will be followed by exercise. After walking for 5 minutes on a treadmill, the ABI procedure is repeated.

2. Ask the patient to lie on the table with the extremity extended.

3. Place pneumatic cuffs (usually four) at intervals along the extremity.

4. Place a flow-sensing device (often a continuous-wave 5- to 10-MHz Doppler device) distal to a cuff. Inflate the cuff (this is often done automatically) to suprasystolic values and then slowly deflate until flow resumes. Record the pressure at which flow resumes.

5. Repeat this technique, distal to each cuff, until the entire extremity has been evaluated. Measure the brachial pressures as well.

6. Examine both extremities.

7. Generally, the highest measurements are used in the calculation.

8. See Chapter 1 guidelines for intratest care.

1. Asymmetry in brachial pressure >10 mm Hg (>10 torr or >1.35 kPa) is suspicious for arterial disease.

2. ABI <1.0 is suspicious for disease. The lower the numeric value for this index, the more severe the disease may be (e.g., ABI <0, associated with impending tissue loss).

3. Pressure gradients between successive segments on the same extremity should vary by <20 mm Hg (<20 torr <2.7 kPa). Variations that exceed this value suggest significant disease (occlusion or stenosis).

4. A difference of >20 mm Hg (>20 torr or >2.7 kPa) between similar segments on opposite sides may suggest obstructive vascular disease.

Pretest Patient Care

1. Explain the test purpose, benefits, and procedure. Instruct the patient to refrain from smoking or consuming caffeine for at least 2 hours before the study. Assure the patient that no radiation is employed, typically no contrast medium is injected, and no pain is involved. Warn that some discomfort may be experienced from lying with the extremity extended or when pneumatic cuffs are inflated.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Severe obesity compromises examination quality.

2. Cardiac arrhythmias and disease may cause changes in hemodynamic patterns.

Normal

• ABI 0.9-1.3 (when a normal multiphasic waveform is present)

• ABI >1.3 indicates poor compressibility

• ABI 0.7-0.9 indicates mild obstruction

• ABI 0.4-0.7 indicates moderate obstruction

• ABI <0.4 indicates severe obstruction

• A difference of 20 mm Hg (or 20 torr or 2.7 kPa) between the right and left brachial pressures may indicate proximal arterial obstruction on the side with reduced pressure.

• The gradual pressure drop, as measured from upper thigh or arm to ankle or wrist, should not exceed 20 mm Hg (or 20 torr or 2.7 kPa) between any two segments.