Patients with peripheral vascular disease are very familiar with the procedure of lower extremity artery examination. Lie down, wear several cuff to measure blood pressure, measure ABI, and be ready in a few minutes. Some patients wonder, what exactly is an ABI exam?

1. What is an ABI?

The ABI, or ankle-brachial index, is also known as the ankle-brachial ratio. It's the ratio of the systolic blood pressure of the ankle artery to the systolic blood pressure of the brachial artery. The left ABI refers to the ratio of systolic blood pressure at the left ankle to the systolic blood pressure at the higher end of both upper limbs. The right ABI refers to the ratio of systolic blood pressure at the right ankle to the systolic blood pressure at the higher end of both upper limbs.

The picture shows: doppler artery blood waveform parameters interpretation

A: peak blood flow velocity

B: peak velocity of reflux blood flow

D: blood flow velocity at the end of diastole (an important indicator of vascular resistance, intuitively understood as the blood flow velocity in the lumen during the interval of beating. Usually with the vascular cavity. 

As the diameter decreases, the D value will gradually decrease, and the small blood vessel at the end of the limb will even be 0, which means the blood flow in the lumen stops during the interval of beating.

Mean: Mean blood flow rate.

For example, if the systolic blood pressure of the left ankle is 80mmHg, the systolic blood pressure of the left upper limb is 110mmHg, and the systolic blood pressure of the right upper limb is 120mmHg, then the ABI value of the patient should be 0.67, which means the systolic blood pressure of the left ankle is 80, and the systolic blood pressure of the upper limb is 120, which is the ratio of the systolic blood pressure of the right upper limb, namely 80/120.

2. What is the numerical meaning of ABI?

Normal ABI values are between 0.9 and 1.3.

ABI>1.3, suggesting calcification of artery wall;

ABI<0.9 indicates the possibility of ischemia, such as arterial stenosis or occlusion.

ABI<0.5, indicating severe arterial stenosis or occlusion;

ABI<0.3 indicates severe limb ischemia and the risk of amputation.

Special attention:

Some diabetic patients with suspected lower extremity vascular disease can be detected through basic consultation and physical examination, and ABI examination is required for further diagnosis. Small and medium-sized arteries in patients with diabetes, because sometimes accompanied by severe calcification, some patients may appear when measuring ankle pressure after pressure artery can't shut down, leading to higher measured pressure (pseudo pressure), appear even normal ABI values, in this case, the measure of ABI does not reflect the real lesions.

3. Clinical significance of ABI examination

Currently, ABI can be an important indicator of systemic atherosclerosis, which is closely related to risk factors of atherosclerosis and the prevalence of CVD in other blood vessels. Decreased ABI is associated with many cardiovascular risk factors, including hypertension, diabetes, dyslipidemia, smoking, and novel cardiovascular risk factors, such as c-reactive protein procystin-6, homocysteine, and chronic kidney disease.

Generally, an increase or decrease of 0.15 ABI value can be regarded as an improvement or aggravation of the disease. For example, the ABI before treatment was 0.70, while the ABIafter treatment was 1.17, suggesting that the symptoms of limb ischemia were significantly improved and the arteries were unobstructed after treatment.

4. ABI checking advantage

The measurement of ABI value is an effective and reliable method for the diagnosis of ischemic diseases of lower limbs, and it has a high specificity and sensitivity, which can be used in the perioperative judgment and follow-up of ischemic diseases of lower limbs. In addition, this examination is non-invasive and fast, so it has been one of the common examinations for lower extremity arterial examination and clinical application.

· Patients should rest in the supine position for 5 to 10min in a room with an appropriate temperature (19 to 22 °C) and with head and heels supported, and should not smoke for at least 2 h before the ABI.

· Appropriate cuff should be selected according to the limb size, with a width of at least 40% of the measured limb circumference.

The cuff should not be wrapped around a limb bypass (risk of thrombosis) or ulcer. Any open wound with a potential contamination risk should be covered with a non-permeable dressing. The patient should remain stationary during the pressure measurement. Other methods should be considered if the patient cannot control the limbs to remain stationary (such as tremors).

· Similar to measuring brachial arterial blood pressure, when measuring ankle blood pressure, the cuff should be wrapped around the ankle with parallel winding method. The lower edge of the cuff should be placed 2cm above the medial malleolus and an 8~10MHz doppler probe should be used.

Ultrasonic gel shall be applied on the surface of the probe. When the doppler device is turned on, the probe shall be placed in the pulse pulsing area and be 45°~ 60° from the skin surface. The probe should be moved until the clearest signal is heard.

· The cuff should be gradually inflated until 20mmHg(1mmHg= 0.133kpa) when the blood signal disappears, and then slowly deflated to detect the blood level when the blood signal reappears. The highest inflation pressure is 300 mm Hg; If blood flow signals can still be detected, the cuff should be deflated quickly to avoid pain.

· Doppler should also be used to detect brachial artery blood flow when measuring arm blood pressure. Blood pressure of limbs should be measured in the same order. Clinicians at the same medical center should use the same sequence of measurements. During the measurement process, the first measured limb should be repeated after the completion of a measurement sequence. The average of the 2 results is taken to offset the white coat effect of the first measurement, unless the difference between the 2 measurements of the first measured limb is more than 10mm Hg. In this case, the first measurement results should be abandoned and only the second measurement results should be considered.

For example, when counterclockwise measurements are taken, i.e., right arm, right PT, right DP, left PT, left DP, and left arm, the right arm should be repeated at the end of this measurement sequence, and the average of the 2 measurements taken with the right arm is then taken, unless the 2 measurements differ by more than 10mm Hg. In this case, only the second measurement of the right arm should be considered

· If repeated measurement of limb pressure is required, the order of measurement shall be reversed from the first measurement (for example, the first measurement shall be counterclockwise, i.e., right arm, right PT, right DP, left PT, left DP, left arm, right arm), that is, the measurement shall be changed to a clockwise direction starting and ending with the left arm. ABI: ankle-brachial index; PT: posterior tibial artery; DP: dorsal foot artery.