updated sept 2022
All you need to know about the ankle-brachial index (ABI)
Measuring the ankle-brachial index ( ABI) is an essential diagnostic evaluation of arterial circulation in the lower extremities. At modern clinics today, all patients with chronic wounds on the lower extremities should get an ABI performed. The results tell us many things about the patient. They can tell us something about the blood pressure in the arteries in the leg, but the results also suggest whether the patient will tolerate compression therapy for venous disorders and what compression grade we can use. The results can also give us an indication of whether the wound can heal at all. If the patient has a chronic ulcer and an ABI of 0,5 ( critical ischemia), it is improbable that the ulcer will heal without cardiovascular intervention.
The ABI measurements also determine what further treatment the patient should receive. The standard of care is that patients with lower leg ulcers and an ABI < 0,8 always be referred to a vascular specialist. That is how easy it is- but you need to have the tools to measure the ABI.
To calculate the ABI you use the systolic blood pressure measured at ankle level and divide this with the systolic blood pressure measured in the arm. Assume that you measured a systolic presure of 120 mmHg in the arm and only 60 mmHg over the ankle - then the ABI is 60/120 = 0,5. This would be considered as a critical value, or critical ischemia. In healthy people the ABI should be somewhere between 0,9-1,1.
To measure the ABI, a handheld Doppler ultrasound device is commonly used. The term " Doppler" honors Austrian physicist Christian Doppler (1803-1853), who studied sound and light waves. A Doppler device is a very useful tool- the problem is that they are relatively expensive, and even in the western world, not all wound care personnel have access to a doppler device. Well, actually they are not so expensive. Online, you can get hold of a reasonably good doppler device for about 200 US Dollars. It is understandable ,however, that for clinics with few resources a doppler is probably not a realistic option. Did you know that you can do an ABI measurement using a cheap, battery operated digital blood pressure measuring device? These can be bought for as cheap as about 30 US Dollars and can give you equally reliable results as a hand-held doppler!
You may have heard someone ( convincingly) telling you that there is no need to measure ABI in diabetic patients because the results are unreliable. Unfortunately, these people do not know what they are talking about. In diabetic patients, the results are more challenging to interpret because, as a general rule of thumb, the ABI in people with diabetes is often measured higher than it really is. This is due to stiff, calcified arteries that require a much higher pressure in the cuff to compress the arteries. The way you have to interpret the ABI in diabetic patients is the following: Let us assume you measured an ABI of 0,8 in a diabetic foot patient. Then you can be very sure that the ABI is 0,8 or most likely even lower. In our experience, the ABI is often 0,2 lower than what you measured. If you measured ABI 0,8 in a diabetic patient, the true ABI probably is closer to 0,6. We have written more about ABI and diabetic patients in the chapter on diabetic foot ulcers.
But we are getting ahead of ourselves. Here we are talking about ABI 0,6 and 0,8, and we haven't even explained how to perform an ABI yet. If you would like to skip to the most practical part of carrying out an ABI exam, you can scroll further down the chapter. But we advise you to read the entire chapter to see the whole scope of ABI.
Wrong cuff size? The most common source of error when determining the ABI
Many healthcare workers underestimate the importance of using the correct cuff size. Since we almost never have to determine the ABI in children we will focus on correct cuff sizes for adults in this chapter. Whenever we measure bloodpressure we need to use a cuff, whether this is to check for hypertension or to determine the ABI. Choosing the correct cuff size is crucial, no matter what we want to measure! Using a too narrow/too short cuff can give you readings up to 40 mmHg higher than the real values - in other words, your readings may be completly useless. If you are working at a GP`s office and you only have one size of cuff available you may end up prescribing anti-hypertension medication to someone who doesnt even need it! Everybody should have three adult cuff sizes available when measuring blood pressure in general or measuring ABI! The American Heart Foundation recommends that the ideal cuff size should have a width that is 40% and a length that is 80% of the limbs circumference. It took us some time to understand this - how can it be that the cuff only needs to be 80% of the limbs circumference? If it is only 80% of the circumference it wont even fit around the limb- surely the length would have to be something above 100%, for example 120%?? The answer is that the sizes refer to the inflatable bladder within the cuff! We are not talking about the total length of the cuff. The inflatable part of the cuff is not in the entire length of the cuff. The remaining length of the cuff is the part for fastening it on the limb, usually with velcro type material.
To determine the correct cuff size we have to have a measuring tape to determine the limbs circumference!
Standard adult blood pressure cuffs usually come in the following sizes:
Adult Small: Arm circumference 18-23 cm: Cuff width 10 cm
Adult Medium: Arm circumference 23-33cm: Cuff width 13 cm
Adult Large: Arm Circumference 33-45 cm: Cuff width 16 cm
The length of the cuffs is proportional to the width. If a patient’s cm measurement is overlapping between sizes, default to the larger cuff if width is appropriate. The proportions may differ slightly depending on which manufacturer you use.
Figure 1a To perform a reliable bloodpressure measurement or ABI it is essential that you use a cuff with appropriate width and length. the size is determined by the circumference of the limb - this means that you need to have a measuring tape available. copyright right image: Riester
Figure 1b Another source of error, though probably not as profound, is when the cuff is not aligned correctly on the limb. A good quality cuff should have a marking to show which area of the cuff that should be placed above the artery that is measured. Often this is marked by a dot and also the text "ART." or even "ARTERY". In the right image we have marked the brachial artery above the elbow with marker on the skin. As we draw the cuff up onto the arm we have to make sure that the arterial marking on the cuff lies close to where we expect the artery to be to get the best reading.
Figure 1c When measuring at ankle level the artery marking on the cuff should be aligned with the anterior tibial artery which is located quite at the centre of the lower leg, marked by the red dotted line in this image.
Can you measure the ABI if you do not have a doppler device?
Let us assume that you are working in an off-the-grid clinic with no resources. Here the purchase of a handheld doppler is most likely not realistic. Is there any workaround to using a doppler? Actually, there are several other ways to do this.
a) Using an automated blood pressure monitor
Battery operated blood pressure monitors are cheaply available, even in Africa. They may come as cheap as 30 US Dollars and are the easiest way to take an ABI reading if you do not have a handheld doppler. This is one of the best kept secrets about ABI - paradoxically many do not know about this "hack". Have the patient lie down flat for about 10 minutes and measure the blood pressure on both arms and around both ankles. Use the highest value from the arms. We have tested this method and compared it to special automated ABI monitors and ABI measured by doppler, and found the results to be very reliable. Incidentally, some of the special ABI monitors on the market are nothing else than this type of monitor. The only difference is that several cuffs can be attached simultaneously to the more advanced monitors.
To get reliable readings you need a correct size of cuff and the patient must rest supine for at least 10 minutes before taking the measurements.
Figure 1d A regular, cheap automated blood pressure monitor is the easiest solution to measure the ABI if you do not have access to a handheld doppler unit or to an automated ABI monitor. We have tested this method and compared it to measurements obtained with a doppler and toe pressure measurements, and found it to be reliable in most cases. Note: In this example the patient was initally not lying entirely flat on the bench- she was half sitting. This resulted in a falsely elevated pressure reading in the legs! This patient had 201/67 in the arm and 196/88 above the ankle! When we measured again with her lying completly flat for 10 minutes the reading in the ankle was 175/80. Her ABI is 175 divided by 201 =almost 0,9, so there is no indication of serious arterial disease in the leg but she certainly has a poorly managed hypertension!
Figure 1e In this example we deliberately did things wrong to illustrate how significant the reading errors will be if you do not follow protocol. This patient had quite swollen legs and we chose a cuff which was one size too small, In addition the patient is not lying completly flat but is partially sitting upright. In the arm we measured a blood pressure of 140/77. In the ankle however the recorded pressure was 192/113! The ankle reading is completly useless in this case- we cannot use it for any interpretation and have to retake the results with a wider cuff and have the patient lie completly flat prior to the examination.
b) Using a stethoscope
How do you measure the blood pressure in the arm ( not counting these fancy battery-operated, self-inflating blood pressure devices available today)? You use a stethoscope to listen to the artery in the arm. You can use the same method in the foot. You can place the cuff around the ankle as you would with any ABI measurement and try to listen to the arteria dorsalis pedis or tibialis posterior with your stethoscope. Sometimes this will be easy, at other times impossible if there is very poor arterial circulation. If you can feel the artery's pulsation with your fingers, you will most likely also be able to hear it. If you want to succeed with the stethoscope method, you need to keep out all disturbing noises around you and not press too hard on the skin with the stethoscope.
Figure1b A pilot study performed in Brazil in 2008 concluded that the stethoscope can indeed be a useful tool to evaluate the ABI and that it is a reasonably accurate method. Click on the image above to get to pub med if you want to read more about this.
Figure 2 If you are working at a low-resource clinic where the purchase of a handheld doppler is not realistic, you will have to try to measure the ABI using a stethoscope. The method is the same as measuring the blood pressure in the arm. For the ABI, the cuff is placed just above the ankle, and using the stethoscope, try to listen to the pulse of the arteria dorsalis pedis and the tibialis posterior. You will have to shut out disturbing noises and concentrate quite hard to hear the sometimes delicate pulsations. Do not be too disappointed if you cannot hear the pulses - in severe peripheral arterial disease ( PAD); they may be very hard to hear.
c) Using a pulse oximetry device
Another workaround is to use a pulse oximetry device. These are designed to measure pulse and oxygen saturation in fingers ( and toes) but can also be used to do a modified ABI examination! This method has even gotten its own name - the Lanarkshire oximetry index. For a detailed description of this method, you will find a detailed explanation in the link in figure 4.
Pulse oximetry devices have become surprisingly cheap, and we have seen them as inexpensive as 10 US Dollars on websites like Wish and AliExpress. There will be quality differences, and more expensive models will usually be more accurate and last longer, but the cheaper models may be adequate. To use this method, you need a pulsoxymeter with a digital display showing the pulse curve to see at which pressure the pulse reading disappears.
Usually, one of the first three toes is used for the measurements on the foot. Be aware that not all pulse oximeters are big enough to fit correctly on a big toe. If it does not fit properly, you will not get a correct reading and can damage the device if you force it onto the toe ( and you can, of course, hurt the patient).
According to several articles, using a pulse oximeter appears to be a relatively reliable method of assessing arterial circulation concerning sensitivity and specificity - see also the link to an overview article in figure 5. However, all methods have their drawbacks and pitfalls. Pulse oximeters work by sending a strong light beam through the finger or toe, which is registered on the other side. Many patients with chronic wounds have thickened nails, making the readings difficult as the light has difficulty passing through this. Be aware that nail varnish- even translucent varnish can interfere with the readings. It may also be challenging to get a reliable reading in patients with cold extremities or severe edema and thickened skin.
Figure 3 Pulse-oxymeters can be cheaply obtained also in Africa. We found this model, for example, on the website of PharmacyDirect in South Africa, and the price was around 19 US Dollars at the time.
Figure 4 If you are not in reach of a regular handheld doppler device and are interested in using the pulse oximetry method, we have provided you with a pdf published in woundcareadvisor.com
Figure 5 Click on the image above to link to an article published in Nursing Times in 2005 about using finger-toe pulse oximetry to assess arterial blood flow.
Figure 6 To get a reliable reading when using a pulse oximeter, the device must fit the toe well. On the left, we see a well-placed pulse oximeter. On the right, we see a situation with an oximeter that does not fit the big toe. Here you should try using a bigger oximeter if that is available or taking the reading on the second or third toes. copyright WHO.org
Another workaround method is to use a cheaper version of a doppler. The best handheld vascular dopplers have a frequency between 5-8 MHz. At this frequency, the ultrasound waves do not go too deep, and the waves are narrower, meaning that they hit their target ( the artery) more precisely. These vascular dopplers also have a probe that has about the diameter of a pencil, which makes it easier to target the arteries. However, it is possible to use a fetal doppler. These are sold very cheaply online - on eBay or Aliexpress, you can find them below 50 US Dollars. These are not the same quality as a good doppler from reputed companies but may be good enough for your needs and often better/ easier than the stethoscope method. The problem with fetal dopplers is that they usually operate at around 2 MHz. The lower the frequency, the deeper the waves travel. These fetal dopplers are designed to measure the heartbeat of a fetus in the womb and not superficial arteries. If there is a very weak arterial signal, a fetal doppler may not be able to pick this up, but otherwise, a fetal doppler can be reasonably accurate for measuring the ABI. The probes on a fetal doppler are much wider than vascular dopplers, and you may have to experiment a little with holding the probe at different angles to get a pulse signal. Also- use lots of ultrasound gelly and do not press hard on the skin with the probe as you will compress the artery and get no signal at all.
Figure 3 Vascular dopplers operate at a higher frequency ( 5-8 MHz) and are thus more accurate at locating pulsations in the superficial arteries. Fetal dopplers have lower frequencies designed to pick up fetal heartbeats at greater depths. You would think that a doppler device that can measure in the deep is more expensive to make, but it is the opposite. The high-frequency vascular dopplers are usually significantly more costly. Fetal dopplers are made for the home market- many at not very good quality but can be purchased for as low as 50 US dollars and may be a workaround if your clinic cannot afford a vascular doppler. It won't be as good as a dedicated vascular doppler, but probably good enough.
Measuring the ABI with vascular dopplers
Figure 4 The ankle-brachial index can be measured using modern, fully automatic devices like the one from MESI (left) or by the conventional manual method using a handheld doppler device. The fully automatic devices are slowly making their way onto the market, but the standard method with a handheld doppler is still considered the gold standard today.
Automatic devices to measure ABI.
Even though it is easy to carry out an ABI reading, it is often not performed by wound care providers. Some say that they do not have the time to perform this test; others are unsure how to do it. Fully automated devices are therefore an interesting alternative as they simplify the process. Often it only takes a few minutes to check the ABI with these devices. The most known and documented device available today is probably the MESI ABPI device. It measures blood pressure by processes called oscillometry and volume plethysmography ( for those interested in physics). The blood pressure in the upper and lower extremities is measured simultaneously.
All that needs to be done is to place the colored cuffs on the right extremities and press the start button. In the case of the MESI ABPI device, the red cuff is placed on one of the upper arms, the green cuff above the right ankle, and the yellow one above the left ankle. Once you have started the device, it only takes about a minute before the ABI is measured. The device has a rechargeable battery, so you are not dependent on electricity other than charging the battery.
When performing a conventional ABI measurement with a handheld doppler, the patient should rest in a flat position for about 10 minutes. This is because the blood pressures in the lower extremities are higher when we are standing, and it can take up to 10 minutes for the pressure to stabilize once we lay down. The producers of the automatic devices state that this resting phase in a flat position is not necessary when you use these modern devices. We have to admit that we do not understand the reasoning behind this statement. Even if the devices have some algorithm that compensates for higher pressures due to the patient not lying flat for at least 10 minutes, it must still be an advantage to have the patient rest for a while to get even more accurate readings? We will investigate this more for you and post an update here as soon as we know more.
How accurate are the automatic devices? Several studies have been published by now that show that these devices provide accurate results. As with a conventional ABI measurement using a handheld doppler, there are some pitfalls also here. Still, in our opinion, the automatic devices have no disadvantage over a handheld doppler - on the contrary - they are easier to use. Another advantage is that the digital display shows the pulse as a wave, giving us additional information about the arteries- for example, whether the patient has stiff arteries. The only disadvantage of these automatic devices is the price tag.
We have tested the MESI ABPI for several weeks and have compared the results with our handheld doppler: the results were very similar. We conclude that automatic devices deliver quick and reliable ABI measurements. We are convinced that far more patients would get their arterial blood circulation checked if such devices were readily available at most workplaces. As we mentioned a little earlier, these devices are still quite expensive and cost about 3000 US Dollars. This investment will undoubtedly be justified at institutions/ clinics with a high volume of patients with chronic leg ulcers.
Figure 2 MESI ABPI is an automatic device to measure the ABI quickly and reliably. The entire process usually takes less than two minutes to perform. The digital display is user-friendly, and pathological results are marked in red. The device has a chargeable battery.
Figure 3 A demonstration of the MESI ABPI at the EWMA conference 2018 EWMA (copyright medi UK).
Video 1 A film explaining how to get started using the MESI ABPI ( Copyright MESI)
Another fully automatic ABI device is the WatchBP Office ABI device from Microlife. We have not tested this yet and have no experience with this device. According to an instructional video, this manufacturer recommends that the patient rests in a flat position for 5 minutes before taking the measurement.
Figure 4 The WatchBP Office ABI device is another example of a fully automated device to measure the ABI quickly. We have recently (2022) tested it and found it to measure accurately. In reality this type of device contains the same technology as a simple battery operated blood pressure monitor. However, it allows you to connect two cuffs simultaneously and calculates the results automatically. They are quite expensive - if you cannot afford this type of ABI monitor you can use a regular blood presure monitor, as described earlier in the chapter.
Measuring the ABI using a handheld doppler
Measuring the ABI using a handheld doppler device is still the international gold standard. Health personnel treating patients with chronic leg wounds should have access to this tool and basic knowledge of using it.
Figure 5 Measuring the ABI is considered obligatory for patients with chronic leg wounds. Healthcare personnel dealing with this patient group must have access to this tool and train how to use it and interpret the results.
What equipment do you need to measure the ABI with a handheld doppler?
Examination bench which can be adjusted to be completely flat
A blood pressure measuring device with cuff, preferably with a manual pump*
Measuring tape to check the circumference of the limb if you are unsure if you have the right cuff size whose width should be at least 40% of the circumference of the limb
Handheld doppler device with a vascular probe ( ideally 5-8 MHz frequency)
Plastic wrap/Clingfilm (Clingwrap)
Paper to dry the patient's skin after using the ultrasound gel
*Note that the width of the cuff should be at least 40% of the circumference of the leg area where you will place the cuff. When we measure the ABI, we usually place the cuff right above the ankle malleoli ( the " knuckles of the ankle") - a standard arm cuff usually fits well here, but if you have a very obese patient, you may have to use a wider cuff to get a correct reading.
Figure 6 You will need these six items to carry out an ABI measurement appropriately. Use plastic cling wrap around the skin to avoid unnecessary contamination of the blood pressure cuff.
Figure7 There is a wide choice of handheld dopplers for vascular examinations available. Note that these dopplers should have a so-called vascular probe with a frequency between 5 - 8 MHz to perform optimally. The cheapest models have no digital display - only a speaker to transmit the sound of the pulse. More advanced models have a digital display where you can see the pulse as a curve. The model furthest to the right from Arjo can also be used to measure the toe- pressure with a special cuff designed for toes. Vascular doppler devices are quite expensive - the simpler models cost about 150 US Dollars while the high-end models can cost over 1000 US dollars. We had a very simple model at our clinic for many years- without a display- it was a very rugged model that served us well for almost 15 years. For most caregivers, the simple models without display will do a perfect job.
How is the ABI measurement with doppler carried out?
1. Have the patient lie down flat on the examination bench for at least 5-10 minutes before taking the measurement. As with all blood pressure measurements, try to create a calm atmosphere as any stress will cause the blood pressure of the patient to be higher than otherwise.
2. As a routine, you should always measure the pressure in the arms first. The reason for this is that the blood pressure in the arm will give you some indication of what pressure you should expect at the ankle level. If you, for example, measure the brachial ( arm) blood pressure to be 160 mmHg, then there is no reason for you to insufflate the cuff at the ankle to 200 mmHg. In this way, you can save the patient from unnecessary discomfort. To get the measurements as accurate as possible, use the same equipment on the arms as on the lower extremities. Therefore, the blood pressure on the arm should also be measured using the doppler device instead of a stethoscope. We have also seen some people who apply an automated blood pressure device on the arms while measuring the pressure in the ankle with the handheld doppler - this is also not a correct method.
Note that the midpoint of the inflatable cuff should be at about the heart level. Your doppler probe can be placed at the arteria brachialis or arteria radialis (figure 8). It doesn't matter which one you use- remember the blood pressure you are measuring is actually the pressure at the cuff level- not where you have placed the doppler probe!
Measure the blood pressure in both arms and use the highest measurement for the ABI calculation. Many caregivers only measure the blood pressure in one arm - but if you want to perform the ABI perfectly, measure the blood pressure in both arms. Many people have higher blood pressure in their right arm, and a variation of 10- 20 mmHg between the left and right arm is not very unusual. If you find a higher deviation between the two arms this may indicate peripheral arterial disease in the arms, diabetes, heart defect, dissection of the aorta, or renal disease, amongst others. This is another reason to always check the blood pressure in both arms - you may detect another condition in the patient who may have to be checked by a doctor.
Figure 8 Always start with measuring the blood pressure in both arms. Use the same equipment as you use to measure the blood pressure at ankle level- that is- use the handheld doppler to detect the pulse in the arm and not a stethoscope as you would do when measuring normal blood pressure. You can use either the brachial artery or the radial artery for this measurement. Remember- the blood pressure you measure is the pressure at the cuff level - not where you have placed the doppler probe!
3. To measure the blood pressure at ankle level, place the cuffs just proximal to the malleoli ( the bony prominences of the ankle). Many place the cuffs a little too high above the ankle where there is more muscle. This may lead to false high pressures as the cuff may need to be inflated more to be able to compress the arteries since muscle is in the way. It may not always make a huge difference, but we are here to teach you the correct method.
For hygienic reasons: always use clingwrap around the skin where you will place the cuff so that it is not contaminated more than necessary. This is especially true if there is a wound close to where you are placing the cuff. The same cuff is, after all, used on many other patients. Wipe off the cuff with an appropriate disinfectant after each patient.
4. Adjust the examination bench such that the midpoint of the cuff at the ankle is in height with the patient's heart. You may have to elevate the ankle with a pillow to achieve this.
5. Always make it a habit to try to palpate the pulse with your finger first - it may help you as a guide where to start searching for the artery with the doppler probe.
6. Apply ultrasound gel on the skin over the artery.
7. Measure the blood pressure in the two largest superficial arteries of the foot: the arteria dorsalis pedis (ADP) and the artery tibialis posterior (ATP). The first is located just on the lateral of the extensor tendon of the toe, the latter behind the medial malleolus. (see figure 4). Sometimes it may be difficult to locate the ADP because of poor circulation. A helpful tip is that you sometimes get a better signal if you check for the artery just proximal to the webspace of the first and second toe. We see that some colleagues only bother to measure the pressure in one of the arteries in the foot. The correct method is to measure the pressure in both arteries and use the highest value for the ABI calculation.
Many make the mistake of holding the probe at a 90-degree angle to the skin - the ideal angle for the best signal is actually to hold it at a 40-60 degree angle to the skin. However, no rules without exceptions- sometimes you may actually get a better signal if you hold the probe at 90 degrees. The point is- if you have a poor signal, try holding the probe at a different angle. Also - the probe should rest feather-light on the skin - if you use too much force, you will compress the artery ( especially if there is poor circulation) and receive a poor signal or no signal.
Figure 9 Use the signal from both the arteria dorsalis pedis (ADP), which lies parallel to the extensor tendon of the great toe, and the arteria tibialis posterior (ADP) behind the medial malleolus to measure the blood pressure in the foot. Use the highest value for the final ABI calculation. Remember that the pressure you are measuring refers to the blood pressure in the ankle and not the foot itself.
Figure 10 If you cannot find a good signal for the arteria dorsalis pedis over the back of the foot, a good tip is to search just proximal to the webspace between the first and second toes ( see red arrow in the image above) - sometimes you can find a very good signal there. Another good tip is not to hold the probe as shown above ( basically everybody holds it there, of course) but to hold it by the cable just behind the probe. This will prevent you from pressing too hard on the skin
8. While keeping the doppler probe steady over the artery, inflate the cuff until the point where the arterial signal can no longer be heard. If the signal can still be heard even though you have inflated the cuff beyond 200 mmHg, something is not right. This may indicate that the patient has arteries with calcified walls which the cuff cannot compress ( very common in diabetic patients), edema in the lower extremities, a cuff that isn't wide enough, or that the patient has very high blood pressure. Do not inflate the cuffs to more than 220 mmHg because this is very uncomfortable for the patient and may even cause tissue damage.
As we mentioned earlier - always take the blood pressure in the arms first. This will give you an indication of how much you need to inflate the cuffs on the legs. For example- if you found a blood pressure of about 140 on both arms, you would expect about the same value in the legs in healthy patients. As you inflate the cuff on the leg, go slowly once you pass 140 mmHg to avoid unnecessary pressure on the tissue. We often observe caregivers inflating the cuff to 250 mmHg or more out of habit. That is a bad habit, and now you know better.
If you inflate up to 220 mmHg and still have a pulse signal on the doppler, you have to stop, which means that you cannot determine the ABI in that particular patient. However- this information is not useless because it is a good indicator that the patient has a vascular disease and should be referred to a vascular specialist. Again: if you cannot compress the artery using 220mmHg or more, stop the examination and refer the patient to a vascular specialist if that is an option that is available to the patient.
NB: If the patient has had previous vascular surgery in the leg ( bypass /stent), then it is contraindicated to do an ABI examination on the leg as the pressure from the cuff may damage the graft/stent! In these cases, it is advantageous to do a toe pressure measurement. We have written a separate chapter on toe pressures.
9. It is essential to release the pressure from the cuff slowly - as a rule of thumb, about 1mmHg/second. This requires skill, especially if you work alone and have to hold the doppler probe in one hand and deflate the cuff with the other. Many release the pressure all too quickly and miss the exact point where the signal comes back - they end up having to inflate again and starting from the beginning. It is easier to do this if you work in pairs. One person focuses on holding the doppler probe still, and the other deflates the cuff slowly.
If you deflate slowly, you will hear the pulse signal gradually returning. The pressure measurement is done at the point where you hear the pulse signal again loud and clear. Some mistakenly think that the pressure is read when you start hearing the signal. Once you start hearing the pulse signal returning, very slowly deflate the cuff a little more until you get a strong signal again- that is the correct blood pressure!
Remember: the pressure you have registered is the systolic pressure in the arteries at the ankle level ( where the cuff is placed) and not where you have placed the doppler probe. If you have poor blood pressure at the ankle level, the circulation in the foot below is also poor. On the other hand - if you have a good pressure reading at the ankle level, the patient still may have very poor arterial circulation further down the foot. Keep this in mind when you interpret the results. For this reason, it is also advantageous to have a toe pressure measuring device available.
10. After registering the pulse in the arteria dorsalis pedis - deflate the cuff, then inflate it again and measure the pressure in the arteria tibialis posterior.
The standard method is to use the HIGHEST reading of the two arteries to calculate the ABI. Some people are confused by this. Are we not "cheating" if we use the highest value - should we not use the lowest value to show how bad the circulation really is? Again- remember: the pressure reading you have reflects on how the blood circulation is at the ankle level- not where you have placed the doppler probe. Using the lowest reading may erroneously give the impression that the circulation at the ankle level is more impaired than it is.
However, others argue that if we have a patient with a foot ulcer, we should use the pressure from the artery supplying blood to the area with the ulcer. For example- if we have an ulcer on the heel, then the blood running through the arteria dorsalis pedis is not so relevant. The arteria tibialis posterior supplies blood to the heel, and in this case, it may be better to use the reading from that artery? Incidentally- a fancy name for the area of tissue served by a particular artery is called the angiosome of that artery.
11. The ABI is calculated by the following formula
Systolic blood pressure in the arm
ABI = ________________________
Systolic bloodpressure at ankle level
An example: We measured 115mmHg in the left arm and 120 mmHg in the right arm. At ankle level, we measured 100 mmHg in the dorsalis pedis artery and 108 in the posterior tibialis artery. Earlier in this chapter, we learned that we should use the highest measured values from both the arms and the arteries in the foot. The ABI for this patient then is:
ABI = ________________________ = 0,9
How do we interpret the ABI values?
1. ABI 0,91-1,3: normal.
2. ABI >1,3: This indicates an incompressible blood vessel due to calcification in the blood vessels walls. If the patient has an ulcer on the lower extremity or signs of ischemia, refer to a vascular specialist.
3. ABI 0,81 - 0,9: borderline peripheral arterial disease. If the patient has a leg ulcer that shows signs of healing and has no ischemic pain, you can wait for a referral to a vascular specialist. The patient should receive information about the importance of daily exercises to promote better circulation and the importance of quitting all nicotine products, especially smoking. If a patient with ABI 0,8- 0,9 has a foot ulcer that does not heal despite adequate treatment or has signs of ischemic pain, refer to a vascular specialist.
Note: we advise using special guidelines for diabetic patients. Since the blood vessels in diabetic patients are often stiffer than in other patient groups, the blood pressure readings in the foot are often higher than they really are. If you, for example, measure an ABI of 0,8 in a diabetic patient, you can be sure that the actual blood pressure is at best 0,8 but maybe even lower. It can be a lot lower - the actual ABI may even be 0,5 for all we know. That is why some caregivers think there is no use to measure ABI. They are wrong! You should also measure the ABI in diabetic patients. From our own experiences, we have found that the actual ABI in diabetic patients is about 0,2 units below the measured ABI in many cases. If you, for example, measure an ABI of 0,8, the actual ABI is probably closer to 0,6. If you have a diabetic patient with a foot ulcer and an ABI of 0,8 or lower, you should always refer them to a vascular specialist where this is available. Many vascular surgeons advise referring all patients with diabetic foot ulcers to a vascular specialist!
4. ABI 0,5-0,8: moderate vascular disease. Always refer these to a vascular specialist where this is available. Note: Be careful when using compression bandages in patients with ABI < 0,8. You must have experience and follow the patient closely when you start them on compression. Ideally, you should only use compression therapies that are documented safe for this patient group.
5. ABI < 0,5 : severe vascular disease. Always refer these to a vascular specialist where this is available. These patients should be prioritized in the system.
The recommendations above apply to a healthcare system where these resources are available. In many areas of Africa, patients will never get the chance to be referred to a vascular center or to get an angiography performed. These recommendations are the standard of care guidelines, but you will have to improvise in low resource areas. Unfortunately, there are very few good alternatives to improving arterial circulation other than invasive procedures that have to be performed at a specialized clinic or hospital. Daily exercises of the legs can improve blood circulation if carried out several times daily over many months. This is best documented in younger patients. However, most patients with critical limb circulation are elderly with comorbidities, and daily leg exercises are not a very realistic treatment option. Sadly in many regions in Africa, for a patient with critical limb ischemia, the only treatment option may be to do nothing or a major amputation.
What do to in a situation where you have a painful ulcer at ankle level where you would place the cuff
1. Chronic wounds are often located on the lower end of the leg, and not rarely are they located close to the ankle area where we place the cuff when we measure the ABI. If the patient has little or no pain in the wound area, it may be possible to place the cuff here. Remember to work hygienically - always wrap a few rounds of clingwrap around this part of the leg before placing the cuff to avoid unnecessary contamination of the cuff.
However, if the patient complains of chronic pain in the wound, we most likely cannot place the cuff there and look for an alternative. After all, we usually pump up the cuff to about 200 mmHg, which can lead to significant pain in the wound area, which is unacceptable. We cannot ask the patient to bite their teeth together to do the ABI examination.
In these cases, the most elegant solution is to do a toe pressure reading if you have this equipment available to you ( most of you will not have access to this equipment). We have written a separate chapter on toe pressure measurement- please refer to this for further information.
As a workaround solution, you can measure the pressure higher on the leg - that is, to place the cuff a little higher than usual (figure 10) while you register the pulse signal in the arteries of the foot as usual. In this area, it is even more critical not to inflate the cuff to more than 220 mmHg as it usually is more painful higher up on the leg than at ankle level. If the patient has very large ulcers that stretch over large leg areas, it may even be necessary to place the cuff above the knee! In that case, you will need a wider cuff than usual. Remember, the pressure you measure refers to the blood pressure where the cuff is placed, not where you have the doppler probe. If you have placed the cuff on the middle of the leg, the pressure you are measuring refers to how the blood circulation is in the middle of the leg. If you have the cuff above the knee, the ABI refers to how the blood circulation is above the knee. The higher the cuff is placed, the less we know about the circulation in the lower leg, where it is often more critical. That is why we ideally should place the cuff in the ankle area.
Figure 11 If the patient has a painful ulcer in the ankle area, then it will not be possible to place the cuff over the ulcer because it will cause the patient unacceptable pain. We have to place the cuff a little higher on the leg in this case. Depending on the patient's body size, you may need a wider cuff than usual here. It is usually more uncomfortable to inflate the cuff where there is more muscle in the leg. Do not inflate the cuff more than necessary and take care not to inflate more than 220 mmHg here.
Advanced use of ABI
In most parts of Africa, radiological examinations like angiography will not be available for most patients. Did you know that you can use the ABI method to determine where the vascular occlusions in the extremity are situated with a fair amount of accuracy? In many cases, the arteries in the thigh area may function quite well but start to get more and more occluded the further down the leg you go. Sometimes the occlusion may begin high up in the thigh area.
With a bit of patience, you can do a sequential ABI examination at several locations on the extremity to determine where the occlusions start. A simple method is to take a reading above the knee, at the middle of the leg, and lastly at the ankle level (figure 11). In most cases, you will need at least two cuff sizes - a larger cuff on the thigh area to get an accurate reading. Remember - the width of the cuff should be at least 40% of the circumference of the extremity. If you measure a circumference of 38 cm in the lower thigh area, you will need a cuff about 15 cm wide. The formula for the cuff size is 0,4 x cm circumference.
This method has been somewhat forgotten in western countries as the vascular examination is often performed by radiological contrasting procedures like an angiography. But in low-resource areas, it can be useful to understand how severe the peripheral arterial disease is. Also- it can be a valuable tool to determine the level of a major amputation: will it be possible to amputate at the leg level, or do we need to amputate above the knee? As a rule of thumb: if you measure an ABI of <0,5 at the middle of the leg, a below-knee amputation will likely not heal.
Figure 12 Using a sequential ABI measurement to determine where the occlusions begin to be severe. Place a cuff- one at a time- above the knee, at the middle of the leg, and ankle level and use the doppler as usual over the pulses of the foot to get the readings. You will probably need a wider cuff above the knee.
Figure A shows a reduced ABI above the knee ( 0,7) and only slightly less in the leg readings (0,6). This tells us that the occlusions of the blood vessels are in the thigh area or even higher.
In figure B, the ABI is measured as normal above the knee, but there are very poor ABI values from the middle of the leg. This tells us that the occlusions start somewhere below the knee area.