ABPI
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The ankle brachial pressure index is a relatively simple method for quantifying the severity of arterial occlusion in the leg.

A blood pressure cuff is inflated around the lower calf muscle above the ankle joint, and a doppler ultrasound probe placed over the dorsalis pedis artery and the posterior tibial artery

  • the higher of the two pressures in the DP or PT arteries (or the peroneal artery if no audible signal is forthcoming from these two vessels) is conventionally taken as the ABPI numerator and the higher of the two brachial pressures as the denominator (2)
  • in the absence of significant stenosis or occlusion in these vessels the two values are usually within 10 mmHg of each other even in the presence of more proximal disease

The maximum cuff pressure at which the pulse can just be heard with the probe is recorded and related to the peak systolic blood pressure measured at the brachial artery.

Peripheral arterial disease (PAD) is present if the ABPI is less than 0.95. Many patients with reduced ABPI are symptom free however the following table gives some idea of the symptoms associated with various severities of peripheral arterial disease:

Clinical statusABPI
Symptom free1 or more
Intermittent claudication0.95 - 0.5
Rest pain0.5 - 0.3
Gangrene and ulceration<0.2

 

The measured ankle cuff pressure may be falsely elevated in patients with calcified arteries (particularly occurs in diabetic and renal patients). An ABPI of >1.3 has been suggested as a strong indicator of calcification (4)

  • local guidance suggests referral for further investigation if the ABPI is > 1.2 (5)

Occlusion pressures at other levels in the leg may be measured and expressed similarly.

Notes:

  • regarding calculation of ABPI (2):
    • occasionally brachial blood pressures are averaged and/or the brachial pressure is only measured in one arm; usually the right. Note though that a pressure difference between the right and left brachial arteries of at least 20 mmHg is present in 3.5% of the normal healthy population and over 20% of patients with PAD and therefore the pressure should be measured in both arms (as the higher of the two pressures will most closely reflect central aortic pressure)
    • it is possible for patients with PAD to have bilateral subclavian-axillary artery occlusive disease and in this situation both brachial pressures will be artificially low and the ABPI artificially elevated
    • if individual calf vessels are heavily diseased or occluded while a single tibial vessel is relatively preserved, the ABPI would fail to indicate the fact that part of the calf may be significantly underperfused and, therefore, more susceptible to pressure damage
      • a difference of >10 mmHg between systolic pressure readings taken from different pedal vessels should alert the clinician to this possibility
    • calcified and incompressible crural vessels may result in a spuriously elevated ABPI (as the arterial wall becomes stiffer and resists compression, giving a falsely high ankle systolic pressure)
      • with respect to diabetics, an ABPI > 1.30 has been suggested as a strong indicator of calcification (4)
      • measurement of great toe artery pressure for calculation of toe brachial index (TBI) is commonly advocated in diabetic patients because of the increased prevalence of calcification in the crural vessels
  • there is evidence from several large longitudinal studies that a low ABPI, usually taken as <0.8 or <0.9, is associated with a marked increase in cardiovascular events, recurrent events and mortality, whether lower limb symptoms are present or not (2)
  • the Edinburgh Artery Study has shown that even a near-normal ABPI (0.91-1.0) is associated with reduced 5 year survival (3)
    • a further study showed that, compared with an ABPI >or=1.1, the risk of death increased linearly in the lower ABPI categories: ABPI 0.7-0.89, hazard ratio (HR) 1.7 (1.2-2.4, P<0.001); ABPI<0.5, HR 3.6 (2.4-5.4, P<0.001) (6)
    • measurement of the ABI may improve the accuracy of cardiovascular risk prediction beyond the Framingham Risk Score (7)
  • in patients with chronic venous ulceration, it is currently recommended that the ABPI should be >0.8 if compression bandaging is to be applied safely in the community (2)

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