Proper PAD Assessment:Understanding the Ankle-Brachial Index (ABI)

Proper PAD Assessment:

Understanding the Ankle-Brachial Index (ABI)

Part One in a Series

Marsha M. Neumyer, BS, RVT, FSVU, FSDMS, FAIUM

 

This installment will address the cornerstone of lower extremity peripheral arterial testing, the ABI. The ABI is the most widely used indicator of peripheral arterial disease (PAD), and as such, warrants a discussion on its own: To ensure full understanding, the calculation of the ABI based on ankle pressures alone, is NOT a reimbursable procedure; however, the ABI with hard-copy waveform tracings taken at the ankle level with continuous wave (CW) Doppler or pulse volume recordings (PVR) is reimbursable under CPT code 93922.

 

Before we begin, it is important to understand

some exam fundamentals many practitioners

often neglect:

 

Patient Positioning: To assure accurate blood

pressure measurements at the arm and ankle

level, the patient should lie supine on the

examination table, with a single pillow elevating

the head.

 

Cuff size: To record an accurate blood pressure,

the pressure cuff should be 20% wider than the

diameter of the limb.

 

The “Gold Standard” for obtaining arm and ankle

pressures is using a CW Doppler probe for

determining presence or absence of blood flow.

Photoplethysmographic (PPG) sensors can also

be used, with some loss of accuracy and

specificity, but with a shorter examination time.

Arm Pressures: Apply the appropriate sized

cuffs to both biceps area.

 

If using Doppler, hold the probe at a 45-degree

angle to the patient’s arm and locate the brachial

artery. Position the probe to obtain the strongest

(loudest) signal.

 

If using PPG, place the sensor snugly on the

patient’s index or middle fingers, using clips, or 

Velcro straps, and you will see a waveform

“pulsing” on your instrument screen.

 

Inflate the Right arm cuff until the Doppler sound

ceases, or the PPG waveform “flat-lines”. Bleed

the pressure out of the cuff SLOWLY (2 – 3

mmHg per second) until the Doppler sound

returns, or the PPG tracing becomes pulsatile.

Most vascular devices will automatically inflate

and deflate the pressure cuff, and record the

pressure (systolic) at which the pulse returns.

Repeat this procedure on the Left arm. The

higher of the two arm pressures will become your

“Brachial Reference” and will be used to calculate

the ABI. The arm pressures in a normal patient

should be within 10 - 15 mmHg pressure of each

other. It is important to measure BOTH arm

pressures, as your patient may have stenosis or

occlusion of the, subclavian, axillary or brachial

artery, which would result in a lower brachial

pressure on that side.

 

When using Doppler, it is not uncommon for the

Left arm pressure to be 15 to 20 mmHg LOWER

than the right arm pressure. Most often, this is a

result of artifactual elevation of the right arm

pressure due to the patient “tensing up” when

they initially hear the Doppler sound. Should you

see such a difference, REPEAT the Right arm

pressure and you will often record a lower value.

 

Ankle Pressures: Doppler pressures (systolic

pressure measurements obtained with the use of

a Doppler probe) are considered the “Gold

Standard” in PAD assessment because you can

obtain pressures individually from the dorsalis

pedis, and posterior tibial arteries. By doing so,

you can accurately detect flow-reducing arterial

disease in specific vessels, recognize the

probability of collateral tibial arterial flow, and

better define the therapeutic options. Using a

PPG sensor at the toe to obtain ankle pressures

will give you a global ankle perfusion pressure.

Apply the appropriate size cuffs to both ankles,

taking care not to position the cuff too low on the

ankle as this could interfere with Doppler probe

position. When performing ankle pressures with

Doppler, maintain proper Doppler angles while

inflating and deflating the cuff.

 

When using the Doppler probe on the dorsalis

pedis artery, use a very LIGHT touch to avoid

compressing the artery. The posterior tibial artery

can sometimes be tortuous around the medial

malleolus, and a Doppler angle of 90 degrees to

the skin surface, will sometimes yield the best

signal.

 

When performing ankle pressures with the PPG

sensor, affix the PPG sensors to both great toes,

using clips or Velcro straps.

 

As in obtaining brachial pressures, inflate the

ankle cuff until the PPG signals disappear, bleed

off pressure slowly and note the first return of

pulse.

 

The ankle arteries can frequently be affected by

vessel calcification, especially in diabetic

patients, and it is not uncommon to inflate the

pressure cuff to 250 mmHg pressure and still

note arterial pulsation. As a rule of thumb, there

is no need to inflate the ankle cuff more than 50

mmHg pressure higher than the highest brachial

pressure.

 

Divide the ankle pressures by the higher of the

two arm pressures, and you will derive the ABI’s.

The general diagnostic values for the ABI are:

 

> 1.4 Vessel Calcification

.97 – 1.25 Normal

.75 - .96 Mild Disease

.50 - .74 Moderate Disease

< .50 Severe Disease

< .30 Critical

 

Note: If the ankle vessels are incompressible, or

yield an ABI of greater than 1.3, consider taking

digit pressures at the great toes, and calculating

a toe-brachial index (TBI), to better assess

arterial perfusion to the feet. Toe pressures and

lower limb segmental pressures will be covered

in the next vascular education installment.

As with any medical examination, the time

involved and the accuracy of the test results will

be dependent on the knowledge and experience

of the person performing the examination.

Adherence to the above nationally recognized

guidelines, will help ensure optimum results. !

 

About the Author:

 

Marsha M. Neumyer, BS, RVT, is a former assistant professor

of surgery and founding Director of the Vascular Laboratory

Section of the Penn State Vascular Institute at the

Pennsylvania State University College of Medicine. She

currently serves as CEO and International Director of Vascular

Diagnostic Educational Services and Vascular Resource

Associates, which offer international education and consulting

services in medical ultrasound. She is recognized

internationally as an educator and for her contributions to the

field of vascular technology. She was the 1997 recipient of the

Society of Diagnostic Medical Sonography’s Joan Baker

Pioneer Award and a founding member of the Intersocietal

Commission for Accreditation of Vascular Laboratories (ICAVL).

She has represented both the Society for Vascular Ultrasound

and the Society of Diagnostic Medical Sonography on the

Board of Directors of the ICAVL. She is a fellow and past

president of the Society for Vascular Ultrasound, and a fellow

and past member of the Board of Governors of the American

Institute of Ultrasound in Medicine. She is also a fellow of the

Society of Diagnostic Medical Sonography and currently serves

as a member of the Society’s Board of Directors and the

National Education Curriculum Task Force. Ms. Neumyer

serves on the editorial review boards of the major journals in

the field of medical ultrasound. She has published more than 80

juried articles, 42 book chapters, a vascular registry

examination simulation, and is the author of three textbooks on

vascular technology to be published in 2008.

Courtesy of Unetixs Vascular, Inc. (800) 486-3849

www.unetixs.com