The ankle-brachial index (ABI) is a non-invasive tool for the assessment of vascular status. It consists of the ratio between the systolic blood pressure of the lower extremity, specifically the ankle, and the upper extremity. This ratio is a comparison of the resistance of the blood vessels with one of the primary factors being the diameter of the vessels. This diameter is narrowed either from internal factors (plaque, intimal tear) or external factors such as compression by soft tissues.
The ankle-brachial index is measured using three arteries: the brachial artery for the upper extremity, and the dorsalis pedis and or posterior tibial artery at the ankle.
Dorsalis pedis artery: The dorsalis pedis artery arises primarily from the anterior tibial artery and starts higher in the anterior compartment of the leg between the tibialis anterior and extensor hallucis longus (EHL). In the majority of cases, the vessel above the ankle passes under the EHL to reside between EHL and the extensor digitorum longus (EDL). Other variations include later crossing sites either at the ankle or distal. Ranging the great toe can help identify the EHL for guidance about where to find the dorsalis pedis pulse, especially in a patient with faint pulses. Additionally, the artery can be traced proximally from the first dorsal metatarsal artery extending into the great toe web space or distally from the supplying artery.
Posterior tibial artery: The posterior tibial artery passes posterior to medial malleolus at the ankle between the tibialis posterior and flexor digitorum longus tendons.
Brachial artery: The brachial artery is palpable medial to biceps tendon in the antecubital fossa.
The ankle-brachial index has uses in screening, diagnosis, treatment, guidance, and prognosis. See the clinical relevance section for further information.
Deep vein thrombosis (DVT): The American Heart Association guidelines recommend avoiding compression of the extremity with known or suspected DVT due to concern for breaking and embolizing a thrombus.
Severe leg pain: Performing ABI measurement requires significant pressure to be applied to the leg. Whether related to leg ischemia, fracture/swelling, or wounds, this procedure can cause significant pain for the patient.
Compression device: Blood pressure cuff (sphygmomanometer). The appropriate size is a “width at least 40% of the limb circumference”.
Pulse detection device: doppler or stethoscope
Oscillometer: Use regular cuff for calf diameter less than 35cm and large cuff for over 35 cm. Take note; there are limits to the use of an automated device as the device has difficulty reading low blood pressures. Also, there is a concern the value is less accurate as the device detects the point of maximum impulse, which is more consistent with the mean arterial pressure.
An ankle-brachial index is a tool that can be used by many medical providers, including physicians, nurse practitioners/physician extenders, and nurses. One study showed in general practice setting that nurses were the highest utilizer of this diagnostic tool.
This tool plays a valuable clinical role across many specialties, including:
Cuff Application Sites
Ankle: Above level of malleoli
For the manual method, apply stethoscope or doppler to sites of brachial, dorsalis pedis, and posterior arteries as described in the anatomy section.
Inflate cuff 20 to 30mm Hg above the last heard sound then slowly release pressure. The first heart sound is the pressure that should be recorded as the systolic pressure at that site.
Order: The AHA suggests performing the order of measurements as follows: First arm, same side ankle (both PT and DP), opposite leg and opposite arm.
If there is a 10m Hg difference in the arm, it is recommended to recheck the initial arm to address the “white coat effect” of blood pressure measurement.
Calculation: The calculation is the ratio of the systolic pressure at the ankle over the systolic pressure at the arm. The appropriate value to use for the systolic ankle pressure has been a subject of debate. The most commonly used is the highest value of the ankle values. One study looked at alternative methods including averaging values, dorsalis pedis only, posterior tibial only, and lowest value. This study suggests all have prognostic value. The difference is the “threshold” for abnormal values and varies slightly (0.87 to 0.95).
Studies demonstrate value variability based on experience as well as the same patient different days and between offices/ specialties. One study suggests repeatability is the best using high ankle/highest arm. A suggested minimal significant difference between recorded ABI values is 0.15. Also, consider training. About one-third of providers who reported performing procedure demonstrated “correct” use per study.
Special considerations for the pediatric population: In early infancy, ABI values average lower values than adults (58% below 0.9); however, ABI reaches normal values after 12 months. Studies support use in the pediatric population to reduce radiation exposure of more invasive tests with good reliability.
In general, the variation in measurements is less in young, healthy individuals.
Peripheral arterial disease (PAD)
The AHA suggests considering evaluation in patients of age over 50 and smoker, patients with diabetes, or over age 70 for peripheral arterial disease. In one screening study involving asymptomatic 50 years old patients; 20% were identified to have PAD. One study suggested that providers in the general medical community use the information to initiate cardiovascular risk reduction interventions. Peripheral arterial disease results suggest that ABI is better at detecting greater than 50% stenosis and proximal lesions better than distal lesions. Both low and high ABI values have an independent association of cardiovascular events. Also, abnormal ABI correlates with the risk of chronic kidney disease.
Several studies have used ABI for prediction of additional risk due to low invasiveness and relative ease of performance to the test:
Traditionally ABI was thought to have a role in wound healing. One study evaluating heel wounds showed ABI was not reliable for heel pressure sores. Another study suggested that ABI is predictive of further need for amputation in diabetic wounds, but not in wound healing itself. An ABI under 0.5 increased the likelihood of amputation by 40%.
ABI is useful in the evaluation of critical limb ischemia. One management protocol suggests that in the setting of critical limb ischemia, a normal or inability to perform ABI warrants more invasive evaluation. If there is abnormality to the ABI, promote surveillance if no tissue loss/gangrene and revascularization if the case of tissue loss or gangrene.
After revascularization, there is a correlation between change in ABI value and ulcer wound healing (ABI delta greater than or equal to 0.23, HR 1.87 for wound healing).
For post-surgical healing
Surgical healing prediction:
ABI has a role for initial evaluation and management of blunt trauma and knee dislocations as a method of assessing for vascular injury. The widely used criteria is an ABI less than 0.9 as an abnormal value for knee dislocations. For low values (less than 0.9), further evaluation is warranted, including methods such as ultrasound, MRI, CT, and angiography. For values greater than 0.9, serial monitoring is the recommendation (48 to 72 hours). ABI can provide correlation with physical exam monitoring, exhibiting good reliability (sensitivity ABI 49.5 % vs. ABI + exam 100%); however, if the two do not agree, further evaluation may be warranted.
Occult PAD in 20% of patients referred to orthopedic surgery for leg pain in patients over 50 years old.
One study found that the value of the ABI was not necessarily predictive of current functional status, but there is prognostic value with a greater likelihood to have decreased in functional status over time with lower baseline values.
Also, there are conditions associated with claudication unrelated to arterial disease. These conditions include functional popliteal syndrome, and exercise-induced leg pain. In these individuals, muscle hypertrophy of the soft tissues can lead to vessel occlusion, which is important to note/consider when evaluating a patient for exertional compartment syndrome as these patients would not be expected to improve after fasciotomy.
The use of ABI for patient care requires knowledgeable individuals who are trained to do so. Training is necessary for the full team of providers from the physicians, physician extenders, and nurses. One randomized control trial evaluated the optimal way to teach the ABI procedure to medical students. The study compared didactic versus experiential learning. Results showed that experiential learning significantly improved the ability to perform the test correctly accurately. (Level 2 evidence - small randomized control trial.)
Performance, application, and interpretation of ABI require an interprofessional team approach, including physicians, specialists, and specialty-trained nurses, all collaborating across disciplines to achieve optimal patient results. [Level V]
Performing ABI assessment does take time; estimated 15 minutes. As such, it will often fall on the duty of the nurse to perform the evaluation to assist in clinical flow.
For wound care providers, including nursing:
The key to nursing monitoring is establishing protocols for ABI.
An appropriate protocol for nursing monitoring should address the following three questions: Which values should I use? What periods to perform the test? When should I notify someone? Routine trauma monitoring for injuries such as knee dislocations has been suggested at intervals of every 2 hours. This approach should consist of clinical assessment and determination if ABI is necessary at the same time with every clinical evaluation. Other options include ABI at specific periods (i.e., a clinical exam every 2 hours and ABI every 4 hours) or only if there is a change in the clinical exam.
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