Using Ankle Brachial Index to evaluate Cardiovascular Risk

[berkeman]

I saw this pretty interesting article today:

A meta-analysis published in the July issue of JAMA finds that a particular ratio of blood pressure measurements, called the ankle brachial index (ABI), has the potential to improve predictions of cardiovascular risk. Currently, the ABI has been used to measure risk of peripheral artery disease and atherosclerosis.

more... http://www.medicalnewstoday.com/articles/114271.php

Here is more info on how to obtain the ABI:

With the patient placed in a supine position, the brachial and ankle systolic pressure measurements are obtained. The higher systolic pressure of the anterior tibial or posterior tibial measurement for each foot is divided by the highest brachial systolic pressure to obtain an ankle brachial pressure ratio. For example, to obtain the left ABI, first measure the systolic brachial pressure in both the left and the right arm. Select the higher of these two values as the brachial artery pressure measurement. There should be a difference of less than 10 mm Hg between each brachial pressure measurement. Next, measure the left anterior tibial and posterior tibial arterial systolic pressures. Select the higher of these two values as the ankle pressure measurement. Then, divide the selected ankle pressure measurement by the previously selected brachial artery systolic pressure measurement. This will give the ABI.

ABIs as high as 1.10 are normal; abnormal values are those less than 1.0. The majority of patients with claudication have ABIs ranging from 0.3 to 0.9. Rest pain or severe occlusive disease typically occurs with an ABI lower than 0.50. Indexes lower than 0.20 are associated with ischemic or gangrenous extremities.

In patients with diabetes and heavily calcified vessels, the arteries are frequently incompressible. This results in an artifactually elevated ankle pressure, which can underestimate disease severity. In these patients, toe pressure determinations more accurately reflect perfusion.

more... http://www.guideline.gov/summary/summary.aspx?ss=15&doc_id=5393&nbr=3696#s22

I'm pretty good at taking my own brachial BP... Wonder if I can reach to take my ankle BP...

 

[Moonbear]

I was wondering how many people could take an accurate ankle BP? I wouldn't even know how to start. I'm also thinking that from a clinical perspective, one's patients might look at you a bit strangely if you suggested you were going to take their blood pressure on their ankle.

I guess what the bottom line is saying is that if you have arteries occluded enough to restrict circulation to your feet, thus lowering ankle BP, it's probably a good sign you're at risk for cardiovascular disease? If so, wouldn't other frequently reported symptoms remain good predictors, such as constantly cold feet, ankle edema, and things like that?

 

[berkeman]

I get the impression that they like the ABI because it's quantitative. It does appear to take some practice to take, though. I just took mine, and got a bit over 1.1, but I wasn't supine, obviously. Foot up on my desk at work (luckily nobody walked by while I was doing it...). I used the diaphram of my stethoscope, and the best listening position seemed to be on the dorsalis pedis artery, just proximal to the split to the lateral and medial plantar arteries. It's definitely a lot bonier place than taking a brachial BP, though. It almost seemed like it was hard to fully occlude the dorsalis pedis artery, but I have pretty lean ankles. I wonder if a little pedal adema would help to occlude the artery more easily with the cuff...

 

[Moonbear]

I used the diaphram of my stethoscope, and the best listening position seemed to be on the dorsalis pedis artery, just proximal to the split to the lateral and medial plantar arteries.

Huh? The lateral and medial plantar arteries branch off the posterior tibial artery. The dorsalis pedis artery branches off the anterior tibial artery.

Which is easier to compress with a BP cuff, the anterior or posterior tibial artery? My guess would be the posterior, because you'd be able to compress it between the cuff and bone, while the anterior tibial is in a groove that seems somewhat protected by the tibia itself. I'm not sure though. Can you get good sounds on the posterior tibial artery? That's the one running posterior to the medial malleolus (for the non-anatomists, that would be the big bump on the inside of the ankle and toward the back side of it). I can feel a pulse point there if you find a space about midway between the ankle and calcaneal tendon (achilles' tendon).

At least I can sit here poking around my ankles and feet in my office and still call it part of my job. The students are dissecting feet next week, so perfect timing.

 

[berkeman]

Huh? The lateral and medial plantar arteries branch off the posterior tibial artery. The dorsalis pedis artery branches off the anterior tibial artery.

Ack, sorry. I misread this diagram:

http://www.eorthopod.com/images/ContentImages/ankle/ankle_anatomy/ankle_anatomy_arteries02.jpg

Thanks for the correction.

I was listening just above the branch to the lateral tarsal artery. I could hear the occluded flow okay, but had trouble completely occluding the vessel.

Which is easier to compress with a BP cuff, the anterior or posterior tibial artery? My guess would be the posterior, because you'd be able to compress it between the cuff and bone, while the anterior tibial is in a groove that seems somewhat protected by the tibia itself. I'm not sure though. Can you get good sounds on the posterior tibial artery? That's the one running posterior to the medial malleolus (for the non-anatomists, that would be the big bump on the inside of the ankle and toward the back side of it). I can feel a pulse point there if you find a space about midway between the ankle and calcaneal tendon (achilles' tendon).

At least I can sit here poking around my ankles and feet in my office and still call it part of my job. The students are dissecting feet next week, so perfect timing.

I'll try getting to my posterior artery tonight at home (looks too weird with me sitting here in the office. I think you're right about the anterior artery -- it definitely was not shutting off the way I expected.

Thanks Moonbear. Looks like I'm going to have to go looking for volunteers for this one...

 

[berkeman]

I did some more googling (had to resort to google images to sort through them better) -- it looks like it may be common practice to use an ultrasound transducer to pick up the blood flow in the dorsalis pedis artery on the anterior foot...

http://www.vascularweb.org/patients/NorthPoint/Ankle_Blood_Pressure_Measurement_ABI.html

http://nursing.missouri.edu/research/venousulcers/index.htm

Or an automated cuff:

http://www.latrobe.edu.au/podiatry/vascular/fiveminreacthyper.html