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Using finger-toe pulse oximetry to assess arterial blood flow

VOL: 101, ISSUE: 46, PAGE NO: 47

Robin Cooper, RGN, DipDN, CPT, is specialist wound care practitioner, North Hampshire Primary Care Trust, Basingstoke

Robin Cooper, RGN, DipDN, CPT, is specialist wound care practitioner, North Hampshire Primary Care Trust, Basingstoke

Compression therapy is the mainstay of venous leg ulcer care, the degree of pressure being applied to a limb depending on the patient's ankle brachial pressure index (ABPI), which has been measured by Doppler ultrasound to identify arterial insufficiency in the limb. It is a non-invasive therapy.

Royal College of Nursing (1998) guidelines recommend that compression therapy of 40mmHg can be applied to a patient's limb if the ABPI is above 0.8 (indicating that no arterial disease is present). Stevens (2004) recommends reduced compression of various pressures for ABPIs between 0.5 and 0.8. However, a study by Field (2004) demonstrated that many patients were not receiving compression therapy for their venous leg ulcers. The main reasons identified were non-concordance on the part of patients, and nurses' lack of confidence in the results of ABPI measurements.

Pulse oximetry as a method of assessing arterial blood flow
Pulse oximeters are used for a variety of clinical conditions and in various areas, including operating theatres, doctors' surgeries and patients' homes.

A pulse oximeter has two main functions, which are as follows:

- To give an audible signal of pulsatile blood flow, similar to that given with Doppler ultrasound. (When an artery is occluded by a pressure cuff, there is a loss of signal from the oximeter);

- To measure the oxygen saturation of haemoglobin in blood or tissue.

These functions can be used for assessing which patients with leg ulcers would be suitable for compression therapy.

Development of the finger-toe (Lanarkshire) oximetry index
The index was developed by a team at Monklands Hospital, Lanarkshire (Bianchi, 2005; Zamiri et al, 2004; Bianchi and Douglas, 2002; Bianchi et al, 2000), after using a pulse oximeter as a means of assessing arterial blood flow in the lower limbs.

The members completed research studies that indicated a fair correlation in the results comparing arterial blood flow using ABPI measurements using a Doppler ultrasound and pulse oximetry. Cooper (2004) describes its use in a primary care diagnostic and treatment centre that incorporated a leg ulcer clinic. The technique involves placing the pulse oximeter transducer on a patient's digit - finger then toe - then calculating the index from the pressures attained. Accuracy is less dependent on the operator's technique than when using Doppler ultrasound to measure the ABPI, which involves the operator having to locate the patient's foot pulses.

A protocol for using finger-toe pulse oximetry (so attaining the Lanarkshire oximetry index) to determine whether a patient is suitable for compression therapy is outlined below (Bianchi, 2005).

Calculation of the Lanarkshire oximetry Index
Preliminary procedures

- Explain the procedure to the patient;

- Ensure he/she is lying comfortably in a semi-recumbent position.

Stage 1
Measuring finger pressure using the pulse oximeter

- Place an appropriate size blood-pressure cuff around the patient's upper arm;

- Place the pulse oximeter sensor on any finger (Fig 1a). When the sensor is placed on the digit, the oximeter will display two numbers: the first represents the patient's heart rate, the second the percentage of circulating oxygenated haemoglobin. Pulsatile blood flow is also displayed on the oximeter, either by a waveform or by a column of lights;

- Record a baseline reading. Inflate the sphygmomanometer cuff to 60mmHg, then inflate it in 10mmHg increments, allowing approximately 10 seconds between these increments. Once the pressure reaches 100mmHg, the incremental changes can be increased to 20mmHg;

- Record the pressure reading that is one below the point where the audible or pulsatile signal is lost on the pulse oximeter; for example, if the signal is lost at 180mmHg, record a pressure of 160mmHg;

- Do not inflate the cuff further if 180mmHg is reached before loss of the signal, and record a maximum pressure of 180mmHg;

- Repeat the measurement on the other arm, then calculate the Lanarkshire oximetry index (see below) by using the higher of the two readings.

Measuring toe pressure using the pulse oximeter

- Place an appropriate size cuff around the ankle immediately above the malleoli. It is important to protect any fragile skin or ulcers beforehand; for example, with a sterile towel or clingfilm;

- Place the oximetry sensor on one of the first three toes (Fig 1b);

- Inflate the cuff as outlined in Stage 1, and record the pressure at which the signal is lost (or 180mmHg if this is reached without loss of signal);

- Repeat the measurement on the other leg, then calculate the Lanarkshire oximetry index by using the higher of the two readings.

Stage 2
Calculating the toe/finger (Lanarkshire) oximetry index

The Lanarkshire oximetry index for each leg is calculated by dividing toe pressure by finger pressure and expressing the figure gained as a decimal. For example:

Toe pressure = 140mmHg

Finger pressure = 120mmHg

Lanarkshire oximetry index = toe pressure/finger pressure = 140/120 = 1.17.

Patients with an index of 0.8 or above can have full compression applied.

Stage 3
Reassessment following the application of compression

- Apply an appropriate graduated compression bandage or stocking to the leg, then place the sensor on one of the first three toes. Check the signal with the leg horizontal (Fig 1c), then after it has been elevated for approximately 30 seconds (Fig 1d). This checks that blood flow is still present and not adversely affected by compression or the position of the leg.

Advantages of using pulse oximetry

A pulse oximeter indicates that blood flow is present or absent in the toes, which are more vulnerable to ischaemic damage than the mid-foot that is used in the measurement of ABPI. It is also useful for patients with oedematous limbs or for those with lymphoedema, both conditions making it difficult to locate foot pulses.

A study by Field (2004) investigating why nurses did not always apply compression on patients with leg ulcers revealed that many were fearful of causing compression damage and felt unsure of the accuracy of the Doppler ultrasound result. However, applying the probe of the pulse oximeter after compression has been applied offers an extremely useful indicator that blood flow has not been interrupted.

In addition, using the pulse oximeter allays the fear that many patients have that the compression bandages will cut off their circulation. This helps with treatment concordance and also gives reassurance to carers.

[H2] Limitations
Although pulse oximetry has many benefits over Doppler ultrasound, its use is limited in that it does not identify the physiological state of individual arteries.

Listening to or viewing actual waveforms using Doppler ultrasound provides valuable information to the experienced operator. In certain situations - where there is severe peripheral arterial disease or critical limb ischaemia, for example - assessment using Doppler ultrasound or measurement of toe wave forms using photoplethysmography (Carter and Tate, 1996) may be of more benefit than pulse oximetry. (Photoplethysmography is a non-invasive investigation used to measure venous refilling time for both acute and chronic venous insufficiency.) However, further studies are being undertaken in this area (Bianchi, 2005).

Another limitation of pulse oximetry is that the signal may be difficult to detect in patients with awkward shaped toes or dystrophic nails, and in those with extreme cyanosis or with conditions where there is peripheral vasoconstriction (Bianchi, 2005).

Costs in nursing time
Measuring ABPI using Doppler ultrasound can be time-consuming: the patient has to rest for 20 minutes beforehand and it is my estimation that the actual procedure can take 15-20 minutes, depending on the nurse's skill and ability to locate the pulses.

French (2005) completed a research study on community nurses' practices in using Doppler ultrasound, which concluded that 91 per cent of nurses completed this procedure in pairs and took between 51 and 90 minutes. Pulse oximetry, on the other hand, takes between seven and 10 minutes, which is a considerable saving in nursing time and cost.

Training for nurses
Ray et al (1994) reported that if accurate Doppler ultrasound results are to be achieved, training and extensive supervision over six weeks is needed to improve the accuracy of ABPI measurements. Many practitioners, however, do not have a large number of patients on whom to practise, and therefore find it difficult to retain their skills. Pulse oximetry is easier to learn and is less operator-dependent.

Conclusion
It is important to remember that whether Doppler ultrasound or pulse oximetry is used to assess arterial supply to the lower limb of patients, either procedure is only part of an holistic assessment.

In my experience, the greatest benefit of using pulse oximetry to identify arterial insufficiency is that it indicates whether or not it is possible to compress safely at pressures which, previously, I would have had concerns about. Yet another benefit is that it allays patients' fears about compression bandaging, and this has had the effect of improving concordance with treatment.

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