Edited on June 25, 2016

Ankle-Brachial Index (ABI)

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INTRODUCTION

The anklebrachial pressure index (ABPI) or anklebrachial index (ABI) is the ratio of the blood pressure at the ankle to the higher of the brachial systolic blood pressures, which is the best estimate of central systolic blood pressure.

It is a noninvasive, simple, valid, reliable and cot effective test wich is used to detect lower extremity peripheral arterial disease (PAD), to measure the severity of atherosclerosis in the legs but is also an independent predictor of mortality, as it reflects the burden of atherosclerosis (5,16,17). However, alone it is not appropriate to detect PAD (Peripheral Arterial Disease) because of possibility of false-negative findings and does not give enough directions for revascularisation in term of localization and characterization.

Lower extremity peripheral arterial disease (PAD) is a frequent, chronic, progressive vascular disease and associated with significant morbidity and mortality (18).

Risk factors for PAD (2,12) are Advanced age (> 70yr). Smoking, past and present diabetes, dyslipidemia, hypertension, hyperhomocysteinemia, chronic renal insufficiency, family history of cardiovascular disease.

A lot of persons with APAD are undiagnosed because they are asymptomatic or have atypical symptoms.

The ABI is also used as a prognostic marker for cardiovascular events, even in the absence of symptoms of PAD.

INDICATIONS

1- Detection of PAD in all all patients who present with symptoms (pain that restrict walking, ischemic pain at rest) and signs (reduced or absent pedal pulses on palpation, skin that is cool, shiny, hairless or thin, thickening of the nails, abnormal capillary refill time, pallor of distal extremities on elevation, leg pain and tissue ulceration or necrosis) (12) suggestive of peripheral artery disease.

Measuring ABI to detect peripheral artery disease is a more sensitive and reliable test compared to palpation of a pedal pulse, especially in patients who are obese or who have significant oedema (12).

The ABI can provide also provide reliable information about the severity of the disease.

It can exclude other causes of calf pain : Spinal Stenosis, Venous, Raynaud Phenomena and popliteal artery entrapment.

2- In the absence of revascularisation, ABI can be used as a Marker of PAD Progression and clinical deterioration. It is not however correlate with clinical amelioration (5).

3- Detection of PAD in asymptomatic patients

Patients with PAD are frequently asymptomatic. Most commonly, they have atypical leg pain and a few present with typical claudication (12) .

There is currently insufficient evidence to recommend population screening for peripheral artery disease using ABPI 3in 12. However, an ABI should be conducted on patients presenting with risk factors to detect PAD, so that therapeutic interventions known to diminish their increased risk of myocardial infarction (MI), stroke, and death may be offered (2 in 2) .

Screening of PAD is recommended for patients presenting with risk factors (12):

- > 65-70 ans regardless of risk-factor status- ⩾ 50 ans + 1 FRCV ( particularly for tabaccos and diabetes)

- ⩾ 40 ans + diabètes + 1 other Risk Factor.

- All people with a Framingham risk score (age, Total CT, cigarette smoking, HDL CT, Systolic blood pressure) > 10%

- Other atherosclerosis location ( coronary a. , carotids, Renal a.…).

- Absence of Tibial posterior pulse.

4- ABI prior to surgery: Before leg or foot surgery (12) to exclude PAD that may result in vascular complications and before wound debridement to determine adequate arterial blood flow (1).

5- ABI prior to compression therapy for patients with venous disease or ulceration, to exclude peripheral artery disease wich will induce complications.

- If the ABI is less than 0.8, le degré de compression devrait être réduit et la compression contre indiquée si l’ABI is less than 0,5 (1).

6- ABPI can be used as a marker of cardiovascular risk both in the general population free of clinical CVD and in patients with established CVD (5,12).

7- ABI is useful in the setting of lower extremity traumatism with suspiction of arterial injury (10) :

An ABI less than 0.90, require CTA or Angiography or operative exploration in an unstable patient.An ABI greater than 0.90 decreases the likelihood of an arterial injury: F/U by serial ABI vs Delayed Vascular Imaging.

Contre-Indications 1- Patients who are unable to remain supine for the duration of the examination.

2- Treadmill testing may be inappropriate for people who are obese, need assistance to walk or limited by comorbidities such as Aortic aneurysm.

3- ABI measurement is also contraindicated in a patient in whom the use of an occlusive sphygmomanometer cuff may worsen the extremity injury.

4- Bilateral subclavian stenosis.

5- The use of the cuff over a distal bypass should be avoided (risk of bypass thrombosis).

ABI PROCEDURE With the Doppler Method

Courtesy of Wikipedia, the free encyclopedia (Anklebrachial pressure index).

See also Images at : http://emedicine.medscape.com/article/1839449-overview#a3

* An ABI measurement can usually be performed in less than 10 minutes. Standardisation of the technique used to measure the ABI was juged necessary because the result may vary and hence the estimate prevalence of PAD (5).

* Before performing ABI, it is important to obtain a thorough history, symptoms and clinical signs.

* Material:

Hand-held portable Doppler device with a frequency of 8 – 10 MHz; although 5 MHz probes may be better for patients with significant ankle oedema.

Appropriately sized sphygmomanometer (blood pressure cuff) for the upper and lower extremities. The cuff width should be, at a minimum, 40 % greater than the diameter of the extremity (5).

And Ultrasonographic gel.

* Patient in supine position, with the arms and legs at the same level as the heart, relaxed for a minimum of 10 minutes before measurement.

The patient should not smoke at least 2 hours before the ABI measurement.

Ideally, the ABI must be performed in a quiet, warm environment to prevent vasconstriction of the arteries. If the room is cold, warm the patient with blankets.

The patient should stay still during the pressure measurement. If the patient is unable to not move his/her limbs (eg, tremor), other methods should be considered.

The ABI procedure may cause discomfort for patients with lower leg pain or cellulitis. If ulcers or wounds are present on the ankle then a protective barrier, e.g. a plastic wrap, shouldbe placed over the affected area before the cuff is applied.

* The ankle cuff should go on the leg between the malleolus and the calf. Enough room should be left below both cuffs (approximately five centimetres above the medial malleolus and approximately two to three centimetres above the antecubital fossa for the brachial pressure).

Make sure that cuff completely encircles lower extremity and wrapped without wrinkles and placed securely to prevent slipping and movement during the test.

* Artery is palpated by hand before Doppler device is used. Place small amount of ultrasound transmission gel at landmark where artery was located. Identify artery with Doppler device. Upon application of Doppler probe, arterial pulsations should be clearly audible before cuff is inflated. If they are not, reposition probe until appropriate sound is obtained. Typically, Doppler probe must be positioned at 45- 60 degrees, not at 90 degrees.

* The blood pressure cuff is inflated proximal to the artery in question. The inflation continues 20- 30mm above the pressure at wich the brachial pulse becomes inaudible by Doppler. The blood pressure cuff is then slowly deflated (2–3 mm Hg per heartbeat). When the artery's pulse is redetected through the Doppler probe, the pressure in the cuff at that moment indicates the systolic pressure of that artery.

The maximum inflation is 300 mm Hg. If the flow is still detected, the cuff should be deflated rapidly to avoid pain.

* 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. Obtain the anterior tibial and posterior tibial systolic pressures of the extremity in question, and select the higher of the 2 values as the ankle pressure measurement. The posterior tibial pulse is best appreciated just dorsal and inferior to the medial malleolus. The dorsalis pedis pulse is best appreciated on the dorsum of the foot between the proximal section of the first and second metatarsals, usually above the navicular bone.

The measurement of the systolic pressure of the dorsalis pedis arteries may not be possible in all patients as 12% of the general population has a congenital absence of the dorsalis pedis pulse

* Some (11) advocate to repeat each measure 2-3 times, especially for people who have little experience with the handling of Doppler probes and measuring the ABI.

However, the Scientific Statement of the AHA (5) states to wait one minute at least before reinflating the cuff because the accuracy of measurement of ABI depend on the number of measurements.

* ABI for each leg is obtained by dividing the highest ankle systolic blood pressure of dorsalis pedis or posterior tibial artery by the highest of the left and right arm brachial systolic blood pressure.

The ABI must be calculated separately for each leg.

ABI =

Higher of either the dorsalis pedis or posterior tibial pressures Higher of the brachial pressures

* Usually, the systolic pressure is first measured at the arm and ankle and subsequently at ankles. However, the AHA recommends (5), for standardization purpose, that the measurement sequence must be looped in clockwise or in

counterclockwise starting with an arm and ending by it ( e.g. Right Arm- Right Ankle- Left Ankle- Left Arm- Right Arm) to reduce the effect of "white coat" on the first measure. The average of the 2 measurements is to retain unless the difference between the two (for the 1st arm) exceeds 10mmHg which case the 2nd measure is only retained.

* For any situation, when the ABI is initially determined to be between 0.80 and 1.00, it is reasonable to repeat the measurement. The measurements should be repeated then in the reverse order of the first sequence starting with opposite arm (5).

* Postexercise ABI

This test is indicated for borderline ABI.

It usually lasts 5 to 15 minutes, sometimes less (15), depending on the importance of any discomfort but for some (8), patient should not exercise for longer than 5 minutes. Anyway, the patient must exercice only to the point of claudication.

Ankle pressures are then measured immediately after the exercise and at 3minute intervals until the pressures return to preexercise levels (8).

The walk is usually undertaken with a small incline (10%). The speed is usually from 3km per H. and can vary from 2 to 5 km / h. This elevation and speed simulate the circulatory response induced by normal ambulation (8). It is important that the same speed and elevation are consistent for each patient on followup (8).

Treadmill testing may be inappropriate for people who need assistance to walk or who are limited by other medical conditions.

In addition to evaluating the effect of exercise on the ankle level blood pressure, treadmill exercise testing also offers a means to characterize the functional impact of claudication symptoms. The distance walked before the onset of symptoms (painfree walking distance) and the maximum distance that can be walked can be measured using a standardized speed and grade on the treadmill. These values establish a baseline for comparison, allowing objective assessment of change in walking performance with medical therapy or interventions.

An alternative method simple method wich doesn’t need special equipment, the active pedal plantar flexion technique, has been assessed for office purpose. It consists of heel raising while standing. Excellent correlation of this method have been obtained in correlation with treadmill test (5).

* Toe -Brachial Pressure Index (8, 13)

Compares the toe pressure to the arm pressure and is derived by dividing the toe systolic pressure by the higher of the right and left arm’s systolic pressures.

Continuous wave Dopplers are not reliable to measure toe pressures due to the small size of digital arteries and vasospasms if toes are cold. Toe pressures, when indicated ( see below) are commonly measured in the vascular laboratory by vascular technicians using standard laboratory photoplethysmography (PPG) equipment. Toe pressures can be also measured by clinicians using a portable PPG if the clinician is educated/skilled in the use of the equipment and it is available.

RESULTS INTERPRETATION.

* ABI------------------------------------------------------- Perfusion Status

> 1.3 -----------------------------------------------Elevated, incompressible vessels

> 1.0 -----------------------------------------------------------Normal

0.99- 0.91 ------------------------------------------------------ Borderline values

< 0.9----------------------------------------------------- PAD= Lower extremity arterial disease

- An ABI <0.9 suggests significant narrowing of one or more blood vessels in the leg.

- The majority of patients with claudication have ABIs ranging from 0.3 to 0.9.

- IPS entre 0.7 et 0.9 : Low intensity well compensated arterial disease. Stenosis> 50% at this stage. Cardiovascular risk would be the same in a limping and in an asymptomatic patient with IPS <0.9 (6).

- IPS entre 0.5 et 0.7 : Moderately Compensated Arterial Disease (6).

- Rest pain or severe occlusive disease typically occurs with an ABI <0.5. This is a critical ischemia. Patients have pretty high likelihood of developing ischemic leg pain, to have ulcers that do not heal and large rate of death and amputation (6).

- ABIs <0.2 are associated with ischemic or gangrenous extremities. - IPS > 1.3 : Médiacalcosis. The arteries are incompressible. It’s another cardiovascular risk marker. Often in the setting of diabetes, advanced age and renal failure. Referral to a vascular laboratory shoulds be regarded, as this result is Clinically inconclusive.

* ABI in Case of Clinical Presentation of PAD

- The ABI test approaches 95% accuracy in detecting PAD. However, a normal ABI value does not absolutely rule out the possibility of PAD. So, when the ABI is >0.90 but there is clinical suspicion of PAD, postexercise ABI or other noninvasive tests, which may include imaging, should be considered (5).

* Postexercise ABIAn ABI of 0.91-0.99 is considered borderline. The patient may be asymptomatic at rest but may experience symptoms when ambulating. When there is an additional reasons to suspect peripheral artery disease, e.g. symptoms and risk factors, further investigations, such Treadmill testing wich rises the sensitivity of the ABI to detect PAD, may be recommended (10).

In Healthy patients: There is Mild decrease in the ABI (average of 5%) when measured immediately after exercise cessation. The ankle pressure then increases rapidly and reaches the

pre-exercise values within 1 to 2 minutes. A recovery of at least 90% of the ABI to baseline value within the first 3 minutes after exercise was found to have a high specificity to rule out PAD (5).

In the case of even moderate occlusive PAD (typically in the proximal vessels), the ankle pressure decreases of more than 30 mm Hg during exercice or a postexercise, ABI decreases of >20% and the recovery time to the pre-exercise value after exercise cessation is prolonged, proportional to the severity of PAD (5). Critical ischemia and vascular claudication cause a dramatic decrease in the postexercise ankle pressure to 60 mmHg or less (8).

When postexercise ankle pressures initially decrease but return to baseline values within 3- 5 minutes, a single segment lesion is most often indicated (8).

Reconstitution of distal vessels is significantly delayed when multisegmental disease is present. In such cases, ankle pressures return to baseline values within 10-12 minutes dependent on the extent of collateral compensatory flow (8).

* ABI as a Marker of PAD Progression.

In the absence of revascularization, an ABI decrease is correlated with clinical deterioration. Clinical improvement in terms of an increased walking distance, however, is not correlated with an ABI increase

Clinical prognosis of the limb is better predicted by ankle pressure rather than the ABI. An ankle pressure < 50 mm Hg has been reported to be associated with higher risk for amputation (19).

* ABI is a Marker of Cardiovascular Risk and Atherosclerosis

Un ABI <0.90 or >1.40 are considered at increased risk of cardiovascular events and mortality independently of the presence of symptoms of PAD and other cardiovascular risk factors (5).

The ABS improves Framingham score's ability to predict CV complications for patients classified at "low risk or intermediate" (11).

* After performing a vascular examination, criteria that would indicate an increased urgency of referral to a vascular surgeon include: - An ABPI < 0.5

- Known peripheral artery disease presenting with a new ulcer or area of necrotic tissue - An ulcer that is not responding to treatment - Intermittent claudication when walking for less than 200 m - Young and otherwise healthy patients with claudication to rule-out rare causes, e.g. popliteal artery entrapment * Discussion with a vascular surgeon should also be considered when:There is doubt concerning the patient’s diagnosisThere is uncertainty around the significance of an ABPI resultThere is doubt about the need for treatment or what treatment options are available

METHOD LIMITATIONS

* 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 mellitus, renal failure, rheumatoid arthritis or heavy smokers. Vascular Calcifications doesn’t mean that there is underlying stenotic or occlusion lesion but stenosis is frequently present and can’t be excluded by normal ABI but ABI values above 1.3 should be investigated further. This may be obvious (ABI above 1.3) but when the arteries are partially calcified it can simulate normal ABI but actually the values are decreased . So, Toe pressures/ brachial index are recommended if the ABI is > 1.3 because the digital arteries are generally less affected by calcifications than the ankle arteries and Comparison with pedal artery velocity waveform shape is prudent.

* False negative can be induced by large collateral circulation supplying downstream arterial stenosis or occlusion. ABI can be normal while patient experience claudication with activity. Further vascular evaluation is then needed (Treadmill test, Doppler).

* Resting ABI is insensitive to mild PAD. Treadmill tests is then indicated to increase ABI sensitivity, but this is unsuitable for patients who are obese or have comorbidities such as Aortic aneurysm.

* ABI correlates poorly with result after revascularisation so it is not reliable method alone of surveillance.

* The exact location of the stenosis or occlusion cannot be determined by ABI alone.

* Lack of complete protocol standardisation reduces intraobserver reliability.

* Skilled operators are required for consistent, accurate results. An incorrectly performed test may lead to a false negative or a false positive result and thereby delay the diagnosis or prompt unnecessary further testing.

* When performed in an accredited lab, the ABI is a fast, accurate, and painless exam, however these issues have rendered ABI unpopular in primary care offices and symptomatic patients are often referred to specialty clinics (13) due to the perceived difficulties.

CONCLUSIONABI It is a noninvasive, cost effective and reliable test used to detect lower extremity peripheral arterial disease (PAD), to measure the severity of atherosclerosis in the legs but is also an independent predictor of cardiovascular events and mortality. However, alone this test is not appropriate to investigate PAD because of possibility of false-negative findings and does not give enough directions for revascularisation in term of localization and characterization.

Few contreIndications must be considered, especially in the setting of distal bypass.

Standardization of the technic is recommended as in AHA Scientific Statement (5).

Because of several limitations, Complementary tests are sometimes necessary to detect PAD as Post Exercice ABI ( if borderline ABI values), Toe-Brachial Pression Index (for incompressible arteries) and Doppler in inconclusive pressions Tests.

EXTERNAL LINK: 1- Stanford Medicine 25: Ankle Brachial Indexhttps://www.youtube.com/watch?v=KnJDrmfIXGw

2-Ankle--Brachial Index for Assessment of Peripheral Arterial Disease. SECEI ESCS.

https://www.youtube.com/watch?v=8q4Cz-a6zkQ

3- Victor Aboyans et co, Measurement and Interpretation of the Ankle-Brachial Index. A Scientific Statement From the American Heart Association.

http://circ.ahajournals.org/content/126/24/2890.full

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3-Stuart J. Hutchison, Katherine C. Holmes, Lower Extremity Arterial Disease. PRINCIPLES OF VASCULAR AND INTRAVASCULAR ULTRASOUND, chap 5 ISBN 978-1-4377-0404-4 Copyright © 2012 by Saunders, an imprint of Elsevier Inc..

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5- Victor Aboyans et co, Measurement and Interpretation of the Ankle-Brachial Index. A Scientific Statement From the American Heart Association.(Circulation. 2012;126:2890-2909.)© 2012 American Heart Association, Inc. DOI: 10.1161/CIR.0b013e318276fbcb

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http://emedicine.medscape.com/article/1839449-overview#showall

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http://www.deh-inc.com/documents/How%20to%20Perform%20a%20TBI2.pdf

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