The ankle brachial pressure index (ABPI) calculation is based on the Doppler principle, which states that the frequency of a sound emitted or reflected from a moving object varies with the velocity of the object.
The ratio of arm and ankle systolic pressure (the ABPI), which eliminates systolic pressure variation, is used to assess and monitor peripheral arterial disease.
The technique, widely practised by vascular surgeons, is now integrated into the assessment process of patients with lower limb ulceration. It is used to define the level of compression applicable to patients with venous leg ulceration1 and forms part of the regular foot assessment process of diabetic patients.
A reduced ABPI has also been shown to be an excellent predictor of risk associated with cardiovascular disease.2 Doppler ABPI measurement can therefore be a useful investigation in patients with exercise-induced lower limb pain, possible rest pain or lower limb ulceration and should form part of the assessment process for patients undergoing any form of lower limb compression therapy.
Doppler ABPI method
The method for Doppler ABPI calculation is outlined (see box below). The basic technique compares the 'best' estimate of non-invasive measured central systolic BP, that is the highest of the two brachial systolic pressures as measured by Doppler, with the highest pressure recorded from the vessels at the ankle.
Some variation between arm pressures is acceptable but differences of greater than 15mmHg may indicate upper limb arterial disease and in symptomatic patients may justify referral for further assessment. It is important that the Doppler probe method is used to measure both the upper limb and lower limb BPs otherwise errors can occur when calculating the ABPI.3
The method used to derive the ABPI is both accurate and reliable with good reproducibility both over time and between individual observers. The highest ankle pressure is selected to calculate the ABPI but it is vital to record the pressure in each vessel as they may differ; a pressure difference of 10-15mmHg can be taken as indicating proximal disease in that vessel. Changes in ABPI over time can also indicate the presence of arterial disease, a variation of >0.15 usually implying a significant pathological change.
Although most management decisions are based on ABPI calculations, some Doppler equipment provides additional information, such as details on the pulse waveform. This information, along with the sound characteristics of the Doppler output, can provide additional information in relation to the status of the vessel under examination4 and may be useful when vessels are difficult to compress or pain in relation to an ulcer and the position of the BP cuff prevents accurate assessment of the ABPI.
Other than the necessary rest period before the investigation is undertaken, the test itself should take no more than 10 minutes. The patient may leave the surgery immediately after or when any wound dressing is completed.
Doppler equipment is portable and the test can be carried out in any environment in which the patient can lie flat. Prices start at under £250 for a basic unit with an 8MHz probe.
Causes of error in ABPI calculations
Errors may be made when obtaining the systolic BP. Cuff size is important, using too small a cuff size at the ankle will result in a spuriously elevated systolic pressure reading and therefore lead to an overestimate of the ABPI.
Repeatedly inflating and deflating the cuff over a long period will also result in an inaccurate pressure reading. Cuff position is also important, as the pressure recorded is the pressure at the level of the cuff and not the pressure at the site of the Doppler probe.
It is important to select the correct probe when assessing superficial blood vessels. For most patients an 8MHz probe is best as this offers the best combination of depth of tissue penetration and focus. In an obese patient or one with oedematous legs it may be necessary to use a 5MHz probe. To obtain the best signal strength use adequate ultrasound contact gel and position the probe at 45 degrees to the skin surface.
Systolic BP in the lower limb varies according to the level at which the reading is taken.
Pressures recorded in the thigh are usually 10-15mmHg higher than those at the ankle. If the cuff is placed above the ankle then a higher reading will be obtained.
The effect of hydrostatic pressure is also important. Readings should all be taken at heart level with the patient lying flat. A dependent limb will give an abnormally raised systolic pressure reading.
An irregular pulse or too rapid a deflation of the cuff can result in an inappropriately low pressure being recorded and an irregular pulse can make it difficult to record the true systolic pressure. Vessel calcification, as can occur in diabetes and renal failure, 'large' oedematous legs or dependent lower limb oedema can result in falsely elevated lower limb systolic pressures.
Abnormally high or low central systolic pressure can also affect the ABPI calculation.5 These problems are common to all techniques that rely on a sphygmomanometer cuff to occlude blood vessels. Resting patients for an inadequate period will give a lower ankle systolic BP. The period a patient should rest for is dependent on the severity of their arterial disease symptoms but may be as much as 25 minutes.
The same principles that apply to the detection of red cell movement in the arterial system can be applied to the venous system to detect valvular incompetence and reflux.
The technique can be used to establish the likely cause of varicose veins and can be used to select patients for venous surgery, although additional information obtained from colour-flow duplex ultrasound may also be required.6,7
- Professor Vowden is a consultant vascular surgeon at the Bradford Royal Infirmary
|Doppler ABPI methodology|
Explain the procedure and reassure the patient. Ensure the patient is lying flat and is comfortable, relaxed and rested with no pressure on the proximal vessels.
To measure the brachial systolic BP:
Measure the ankle systolic BP:
Calculate the ABPI for each leg using the formula below:
ABPI(r) = Pa(r)/Pb
ABPI(l) = Pa(l)/Pb
where (r) = right and (l) = left or look up the ABPI using a reference chart.
The following broad categories can be defined based on the ABPI:
1. Royal College of Nursing. Clinical Practice Guidelines: The management of patients with venous leg ulcers. London: RCN Institute; 1998.
2. Hooi JD, Kester AD, Stoffers HE, et al. Asymptomatic peripheral arterial occlusive disease predicted cardiovascular morbidity and mortality in a 7-year follow-up study. J Clin Epidemiol 2004; 57 (3): 294-300.
3. Jeelani NU, Braithwaite BD, Tomlin C, MacSweeney ST. Variation of method for measurement of brachial artery pressure significantly affects ankle-brachial pressure index values. Eur J Vasc Endovasc Surg 2000; 20(1): 25-8.
4. Vowden KR, Goulding V, Vowden P. Hand-held Doppler assessment for peripheral arterial disease. J Wound Care 1996; 5(3): 125-8.
5. Carser DG. Do we need to reappraise our method of interpreting the ankle brachial pressure index? J Wound Care 2001; 10(3): 59-62.
6. Campbell WB, Niblett PG, Ridler BM, et al. Hand-held Doppler as a screening test in primary varicose veins. Br J Surgery 1997; 84(11): 1541-3.
7. Mercer KG, Scott DJA, Berridge DC. Preoperative duplex imaging is required before all operations for primary varicose veins. Br J Surgery 1998; 85(11): 1495-7.