Ankle Brachial Pressure Index

Doppler assessment: calculating an ankle brachial pressure index

21 October, 2003

VOL: 99, ISSUE: 42, PAGE NO: 62

Deborah Ruff, RGN, is vascular nurse specialist, The Royal Oldham Hospital, Oldham
An ankle brachial pressure index (ABPI) is a simple non-invasive method of identifying arterial insufficiency within a limb. It compares the ankle and brachial systolic blood pressures. An important factor determining the rate of healing of any wound is adequate arterial blood supply. In the management of leg ulcers, the ABPI forms a fundamental part of the assessment.

An ankle brachial pressure index (ABPI) is a simple non-invasive method of identifying arterial insufficiency within a limb. It compares the ankle and brachial systolic blood pressures. An important factor determining the rate of healing of any wound is adequate arterial blood supply. In the management of leg ulcers, the ABPI forms a fundamental part of the assessment.

It should be noted that the ABPI should not be undertaken in isolation, but should be used in conjunction with a holistic assessment, and a medical and clinical examination of the limb.

In the past, a clinical examination would have involved palpating the pulses to detect any arterial impairment. However, the evidence is that palpation of pulses is subjective and prone to considerable error (Moffatt and O'Hare, 1995).

KEY POINT - Accurate assessment is necessary to determine the correct aetiology of the ulcer and exclude those patients with arterial disease for whom compression is dangerous (Stacey et al, 2002).

How is the ABPI calculated?
Equipment The procedure requires the following equipment:

- Bed or couch;

- Hand-held Doppler ultrasound machine;

- 8mHz probe (5mHz probe if required for large or oedematous limbs);

- Ultrasound transducer gel;

- Sphygmomanometer and cuff;

- Cling film/vapour-permeable film dressing or equivalent;

- Calculator.

KEY POINT - In order to prevent cross-infection the Doppler probe should be cleaned twice with an alcohol impregnated wipe prior to its use (Kibria et al, 2002).

Preparation of the patient:

- Provide the patient with an information leaflet about the procedure prior to the appointment;

- Explain the procedure to the patient to relieve any anxieties he or she may have;

- Ensure the room temperature is comfortable;

- Ask the patient to remove any tight articles of clothing, which may cause pressure on the blood vessels proximal to the site where the blood pressure is being measured;

- Remove any dressings from the ulcers and cover with clear film to reduce risk of cross-infection (Kenny, 1997);

- Ask the patient to lie flat and rest for a minimum of 10 minutes to obtain a resting pressure (Yao, 1993).

PRACTICAL TIP - If the patient is unable to lie flat, elevate the legs to the level of the heart. By elevating the legs, the hydrostatic pressure to the legs is reduced.

Measure the brachial systolic blood pressure:

- Select a sphygmomanometer cuff of an appropriate size and wrap it around the patient's upper arm just above the elbow;

- Palpate the brachial pulse and apply ultrasound gel;

- Angle the Doppler probe at 45 degrees to the direction of the blood flow (towards the heart) and adjust the position to locate the best signal;

- The Doppler emits an audible signal. Some Doppler ultrasound machines also provide a visual wave form;

- Inflate the sphygmomanometer cuff until the signal disappears, then deflate the cuff slowly and record the pressure at which the signal returns;

- Repeat this procedure using the patient's other arm (Stubbing et al, 1996);

- Use the highest of these two readings to calculate the ABPI.

PRACTICAL TIP - Place the cuff upside down, to prevent the tubing being soiled by the gel.

Measuring the ankle systolic pressure:

- Select an appropriately sized sphygmomanometer cuff and place around the leg just above the ankle;

- Palpate the posterior tibial artery, apply ultrasound gel and locate the best signal (Fig 1);

- Inflate the sphygmomanometer cuff until the signal disappears, then deflate the cuff slowly and record the pressure at which the signal returns;

- Repeat this procedure with either the anterior tibial or peroneal artery (Fig 1);

- Use the highest of these two readings to calculate the ABPI (Vowden and Vowden, 1997).

Calculating the ankle brachial pressure index and interpreting the findings The ABPI is calculated using the following equation:

ABPI = highest ankle systolic pressure/highest brachial systolic pressure

An ABPI <0.8>0.8 does not necessarily mean that high-compression bandaging can be undertaken safely. Other factors such as diabetes, rheumatoid arthritis, peripheral neuropathy and cardiac failure need to be considered before applying compression. In these circumstances, reduced compression would be a more suitable option.

If the ulcer is classified as having a mixed aetiology, and the ABPI is 0.5-0.8, it is important to refer the patient to a specialist team with immediate access to vascular services. Following a holistic assessment, the patient may be treated with reduced compression at 15-25mmHg.

For patients with an ABPI <0.5, compression therapy is contraindicated and urgent referral to a vascular specialist is recommended.

Factors creating an erroneous ankle brachial pressure index:

- Inadequate preparation, for example uncomfortable room temperature. Extremes of temperature can make the patient restless and extreme cold can make the signal difficult to hear due to vasoconstriction;

- Patient anxiety;

- Incorrect positioning of the patient;

- Inappropriate gel;

- Incorrect size of sphygmomanometer cuff;

- Incorrect size of Doppler probe;

- Excessive pressure on the blood vessel during the procedure;

- Releasing the sphygmomanometer cuff from the patient too quickly;

- Prolonged inflation of the sphygmomanometer cuff or repeated inflation;

- Moving Doppler probe during the procedure;

- An inexperienced practitioner.

[H2] Contraindications
There is controversy about the circumstances in which an ABPI should not be performed, so more research is required. It has been suggested that an ABPI should not be performed under the following circumstances:

- If the patient has a suspected deep vein thrombosis, because there is a risk of emboli;

- If the patient has cellulitis, because the procedure would be too painful;

- If the patient has severe ischaemia, because there is a risk of further tissue damage.

How often should the ABPI be performed?
It is recommended that the APBI should be repeated every 12 weeks (Simon et al, 1994). However, if the patient's condition changes during that time, for example if he or she experiences an increase in pain or deterioration in the condition of the ulcer, the procedure should be repeated. If the ulcer reoccurs, repeat the leg ulcer assessment. Never presume the ulcer is of the same origin.

How reliable is the ABPI?
For most patients, the ABPI is a reliable method of detecting arterial insufficiency. However, in some cases:

- The result may be falsely elevated in patients with diabetes due to calcification of the medial layer of the artery, preventing the arteries from being compressed (Williams et al, 1993);

- An exercise test may be more accurate for patients with suspected arterial disease who have a normal resting ABPI, but where there is doubt about the diagnosis. Following exercise, a fall in the ABPI is indicative of arterial disease (Laing et al, 1983).

For these patients, Doppler waveforms and toe pressures are a more reliable method of assessment (European Leg Ulcer Advisory Board, 2002).

Waveform analysis
As the practitioner becomes more experienced with the procedure, he or she may also be able to detect a difference in the quality of the audible signal transmitted from the Doppler machine.

Triphasic waveform - In the lower limb, the Doppler signal detected from the arterial flow has three phases and is therefore described as 'triphasic' (Fig 2).

Phase one - During systole (as the heart contracts), blood flow accelerates in a forward direction within the blood vessel.

Phase two - A drop in peak systolic pressure leads to a reverse flow of blood within the blood vessel.

Phase Three - Elastic recoil of the vessel at the end of diastole (when the heart muscle relaxes) leads to a further forward flow of blood in the vessel.

Biphasic waveform - Vessels naturally lose their elasticity as part of the ageing process, making the signal biphasic. Fig 3 illustrates the loss of the reverse blood flow component (phase three).

Monophasic waveform - A monophasic signal produces only one sound and usually denotes vessel disease. The sound heard is usually lower in pitch. Fig 4 illustrates the loss of phases two and three and reduced blood flow during systole (phase one).

Conclusion
Provided that the ABPI is performed correctly and the factors affecting the results are considered, the literature suggests it is a simple, reliable method of assessing arterial insufficiency within a limb (Vowden and Vowden, 1996). Calculating the ABPI is a vital part of the assessment process when examining a patient with leg ulceration, but the results should always be used in conjunction with a holistic assessment and never in isolation. This reduces the risk that compression bandaging will be applied inappropriately.

FURTHER INFORMATION
European Wound Management Association (2003) Understanding Compression Therapy. London: Medical Education Partnership.

This useful document is divided into four sections:

- Understanding the pathophysiological effects of compression;

- Compression bandages: principles and definitions;

- Cost effectiveness of compression therapy;

- Compression therapy: a guide to safe practice.

It contains a treatment pathway developed by the Leg Ulcer Advisory Board for the use of compression therapy in venous leg ulcers. It is sponsored by a educational grant from Smith and Nephew and can be downloaded at: www.proguide.net
Have your say