The patient must be placed supine, without the head or any extremities dangling over the edge of the table. Measurement of ankle blood pressures in a seated position will grossly overestimate the ABI (by approximately 0.3).
A Doppler ultrasound blood flow detector, commonly called Doppler Wand or Doppler probe, and a sphygmomanometer (blood pressure cuff) are usually needed. The blood pressure cuff is inflated proximal to the artery in question. Measured by the Doppler wand, the inflation continues until the pulse in the artery ceases. The blood pressure cuff is then slowly deflated. When the artery's pulse is re-detected through the Doppler probe the pressure in the cuff at that moment indicates the systolic pressure of that artery.
Where PLeg is the systolic blood pressure of dorsalis pedis or posterior tibial arteries
and PArm is the highest of the left and right arm brachial systolic blood pressure
The ABPI test is a popular tool for the non-invasive assessment of PVD. Studies have shown the sensitivity of ABPI is 90% with a corresponding 98% specificity for detecting hemodynamically significant (Serious) stenosis >50% in major leg arteries, defined by angiogram.
Resting ABPI is insensitive to mild PAD. Treadmill tests (6 minute) are sometimes used to increase ABPI sensitivity, but this is unsuitable for patients who are obese or have co-morbidities such as Aortic aneurysm, and increases assessment duration.
Lack of protocol standardisation, which reduces intra-observer reliability.
Skilled operators are required for consistent, accurate results.
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 due to the perceived difficulties. Technology is emerging that allows for the oscillometric calculation of ABI, in which simultaneous readings of blood pressure at the levels of the ankle and upper arm are taken using specially calibrated oscillometric machines.
Interpretation of results
In a normal subject the pressure at the ankle is slightly higher than at the elbow (there is reflection of the pulse pressure from the vascular bed of the feet, whereas at the elbow the artery continues on some distance to the wrist).
The ABPI is the ratio of the highest ankle to brachial artery pressure. An ABPI between 0.9 and 1.2 considered normal (free from significant PAD), while a lesser than 0.9 indicates arterial disease. An ABPI value greater than 1.3 is also considered abnormal, and suggests calcification of the walls of the arteries and incompressible vessels, reflecting severe peripheral vascular disease.
Provided that there are no other significant conditions affecting the arteries of the leg, the following ABPI ratios can be used to predict the severity of PAD as well as assess the nature and best management of various types of leg ulcers:
^ abVowden P, Vowden K (March 2001). "Doppler assessment and ABPI: Interpretation in the management of leg ulceration". Worldwide Wounds. - describes ABPI procedure, interpretation of results, and notes the somewhat arbitrary selection of "ABPI of 0.8 has become the accepted endpoint for high compression therapy, the trigger for referral for a vascular surgical opinion and the defining upper marker for an ulcer of mixed aetiology"
^McDermott MM, Criqui MH, Liu K, Guralnik JM, Greenland P, Martin GJ, Pearce W (December 2000). "Lower ankle/brachial index, as calculated by averaging the dorsalis pedis and posterior tibial arterial pressures, and association with leg functioning in peripheral arterial disease". JJ Vasc Surg.32 (6): 1164–71. doi:10.1067/mva.2000.108640. PMID11107089.
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^Aboyans V, Ho E, Denenberg JO, Ho LA, Natarajan L, Criqui MH (November 2008). "The association between elevated ankle systolic pressures and peripheral occlusive arterial disease in diabetic and nondiabetic subjects". J Vasc Surg.48 (5): 1197–203. doi:10.1016/j.jvs.2008.06.005. PMID18692981.
^Stein R, Hriljac I, Halperin JL, Gustavson SM, Teodorescu V, Olin JW (February 2006). "Limitation of the resting ankle-brachial index in symptomatic patients with peripheral arterial disease". J Vasc Med.11 (1): 29–33. doi:10.1191/1358863x06vm663oa. PMID16669410.
^Montgomery PS, Gardner AW, (June 1998). "The clinical utility of a six-minute walk test in peripheral arterial occlusive disease patients". J Am Geriatr Soc46 (6): 706–11. PMID9625185.
^Jeelani NU, Braithwaite BD, Tomlin C, MacSweeney ST (July 2000). "Variation of method for measurement of brachial artery pressure significantly affects ankle-brachial pressure index values". Eur J Vasc Endovasc Surg.20 (1): 25–8. doi:10.1053/ejvs.2000.1141. PMID10906293.
^Caruana MF, Bradbury AW, Adam DJ (May 2005). "The validity, reliability, reproducibility and extended utility of ankle to brachial pressure index in current vascular surgical practice". Eur J Vasc Endovasc Surg.29 (5): 443–51. doi:10.1016/j.ejvs.2005.01.015. PMID15966081.
^Kaiser V, Kester AD, Stoffers HE, Kitslaar PJ, Knottnerus JA (July 1999). "The influence of experience on the reproducibility of the ankle-brachial systolic pressure ratio in peripheral arterial occlusive disease". Eur J Vasc Endovasc Surg.18 (1): 25–9. doi:10.1053/ejvs.1999.0843. PMID10388635.
^Feringa HH, Bax JJ, van Waning VH, et al. (March 2006). "The long-term prognostic value of the resting and postexercise ankle-brachial index". Arch. Intern. Med.166 (5): 529–35. doi:10.1001/archinte.166.5.529. PMID16534039.