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Department
of Podiatry - Vascular Assessment
Ankle
Brachial Pressure Index (ABPI)
The
ankle brachial pressure index (ABPI), also known as the ankle to arm systolic
pressure index is the most common diagnostic test used in the non-invasive
physical examination of the suspect lower extremity. The arterial pressure
measurement in the lower limb represents the single most important index
for diagnosing peripheral arterial disease (PAD) (McGee, 1998). The index
is calculated through measuring the maximum systolic pressure at the ankle
(taking the highest of either dorsalis pedis
or posterior tibial) with a handheld Doppler
then dividing it by the systolic pressure in the brachial artery. In healthy
individuals the ABPI of between 0.97 and 1.1 indicates the absence of
organic disease. Values less than 0.97 identifies patients with angiographically
proven occlusions or stenoses with a sensitivity of 96% and specificity
of 94 – 100% (McGee, 1998). Most patients with symptoms of claudication
will have an ABPI between 0.5 and 0.8 and those with symptoms of rest
pain will generally have values less than 0.5 (McGee, 1998). It must
however be stressed that these values are guidelines and that it is impossible
and irresponsible to interpret the meaning of the ABPI, and the blood
flow solely by looking at the number.
There are two main reasons that the ABPI can be misleading. The ABPI is
a measure of the pulse pressure taken at the level of the ankle and although
it may reflect adequate blood flow at that point, it does not account
for the possibility of an occlusion distal to that point. The presence
of distal emboli, micro-emboli and atherosclerotic plaques for example
can lead to tissue breakdown, ulceration and amputation. The second point
to consider is that of Monckeberg’s Sclerosis (aka medial wall calcification),
a condition more commonly seen in the diabetic population and leads to
a falsely elevated ABPI (Bowker, 2001). As a result of the deposition
of calcium in the tunica media of the muscular arteries of individuals
with atherosclerosis, particularly those with diabetes, the arterial walls
offer greater resistance to compression. The ABPI therefore reflects the
ability of the vessel wall to resist compression rather than being a true
indicator of the blood flow through, and the pressure within the vessel.
In order to properly interpret the significance of the pedal pulse pressure,
the clinician must question if the recorded blood pressure correlates
well with the pitch and waveform morphology recorded for that artery with
the Doppler unit, and for that matter whether the ABPI is an accurate
reflection of what is seen throughout the rest of the examination.
HOW TO PERFORM AN ABPI.
Instrumentation: Continuous wave Doppler unit.
Pressure Manometer, the same cuff should be used for both ankle and brachial
measurements.
Method: in an attempt to address issues associated with inter and intra-tester
reliability, when taking pressure measurements the same protocol should
be used both between patients and between clinicians.
1. In order to ensure that any pressure changes that may have occurred
while the patient has been moving about prior to the examination, have
the patient rest quietly in the supine position for a minimum of five
minutes.
2. The Doppler probe should be held at an angle of 50Ž to the artery to
ensure the best quality of signal is obtained. Care must be taken to approximate
the loudest signal by moving the probe side-to-side, back and forth over
the artery in order to record the highest possible velocity.
3. The pressure should be measured in both arms, the higher pressure of
the two being taken and recorded.
4. The pressure must be taken at the point at which the first Doppler
signal appears during deflation of the cuff.
5. The cuff should be inflated to at least 30mmHg above arm systolic pressures,
thus ensuring complete collapse of the dorsalis pedis and posterior tibial
arteries.
6. Cuff deflation must proceed slowly, no greater than 2mmHg per second
is recommended in order to accurately obtain the pressure at which blood
flow returns.
7. To determine the arm pressure, the Doppler probe may be held at the
brachial artery just distal to the cuff or at the radial artery at the
wrist.
NOTE: When cuff inflation greater than 260mmHg cannot obliterate arterial
signals at the ankle, it should be concluded that medial wall calcification
is present. When ankle pressures are greater than 75mmHg above the brachial
or the index is greater than 1.3, partial medial calcification should
be concluded, giving rise to a falsely elevated ABPI (Orchard, 1992).
WHEN SHOULD AN ABPI BE PERFORMED?
The following recommendations have been adopted from a report sponsored
by the American Diabetes Association and the American Heart Association,
its justification based on the low sensitivity of clinical history taking
and examination to detect PAD and the high morbidity and mortality of
patients with the condition. The report focused on the need for a standard
approach to measurement of PAD in diabetes to ensure appropriate care
and reproducibility of results across studies and over time.
It
recommended ABPI measurement be performed in the following clinical situations:
• Any patient with diabetes especially if they have decreased pulses,
foot ulcers and femoral bruits.
• Any patient with diabetes with leg pain of unknown etiology.
• All type 1 diabetes mellitus patients aged over 35 years, or with
20 or more years’ duration of diabetes undergoing baseline examination.
• All patients with type 2 diabetes mellitus aged 40 years or older
undergoing baseline examination.
• Ideally pressures should be taken bilaterally, and the same protocol
be used between patients and over time.
• Any patient with a history of organic occlusion, ulceration, or
ischaemic pain.
RECOMMENDED ACTION TO TAKE AFTER ABPI TESTING
• ABPI<0.50 in any vessel. This patient will almost certainly
have severe PAD and is recommended to see a specialist vascular laboratory
immediately (Orchard, 1992).
• ABPI>0.50 and <0.80 in any vessel tested. The ABPI should
be repeated in 3 months as this patient most likely has moderate PAD.
If in 3 months the repeat ABPI is <0.90, initiate intensive risk factor
modification. If repeat ABPI is > 0.90 repeat ABPI every 2 to 3 years
(Orchard, 1992).
• Any incompressible ankle artery or ankle pressure greater than
75mmHg over the brachial pressure. This patient is likely to have medial
wall calcification and should therefore be re-assessed in 3 months to
confirm this diagnosis. This patient should then be referred for a specialist
vascular consult as they are at greater risk for both macro and micro
vascular complications. This patient should also undergo intensive risk
factor modification (Orchard, 1992).
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