Ankle Brachial Index (ABI) For PAD Risk Calculator

How does this Ankle Brachial Index (ABI) for PAD risk calculator work?

This is a health tool used in the diagnosis of peripheral arterial disease (PAD) and also in the risk stratification for cardiac events, all dependent of the systolic pressure values.

Subsequent studies have proved that the ABI test is a specific and sensitive measure in the diagnosis of PAD.

The application of this blood pressure test is advised by medical groups for all patients over 70 years of age and all patients over 50 if smoking or diabetic.

The Ankle Brachial Index (ABI) calculator is basically defined by the ABI formula, which is the systolic pressure at the ankle divided by the systolic pressure at the arm.

It is used as diagnostic mean with a sensitivity of 90% and a specificity of 98% in detecting stenosis but also to monitor medical treatments or recovery after angioplasty.

One of the criticisms received by the model is referred to its lack of accuracy, with false negatives, in the cases of patients with hardened, incompressible arteries.

ABI technique

This test is non invasive and places the patient in a supine position. The pressure cuff is used subsequently for every arm and the systolic pressures are registered.

The upper arm procedure differs from classic pressure measuring because it also involves the use of ultrasound gel in the antecubital fossa over the brachial pulse.

By using the transducer of the handheld Doppler on that area, an effect of intensity maximization is obtained. The cuff then needs to be inflated at above 20 mmHg higher than the expected blood pressure or up until the signal on the Doppler disappears. While deflating the cuff (the recommendation is of approximately 11 mmHg/s), the Doppler is monitored in order to register the brachial systolic pressure accurately.

The measuring of the ankle pressure for both feet is made by placing the cuff proximal to the malleoli. The ultrasound gel in applied to the skin overlying the dorsalis pedis (DP) and posterior tibial (PT) arteries. Similar to the arm procedure the cuff is inflated until the Doppler signal disappears and deflated slowly to register the systolic pressure. The same repeats to the other artery and then to the other arteries of the other foot.

Since resting ABI result might at times be insensitive to mild PAD, some tests are performed twice, between them being added a treadmill test of around 6 minutes. However, such effort test cannot be performed on all patients.

ABI calculation

In total there are 6 pressure measurements taken, 2 for arms and 4 for feet. There are certain considerations regarding the norms in which these blood pressures should be such as this one below.

Inter-arm difference in the systolic brachial should be less than 10 mmHg. Bigger differences, higher than 10 mmHg or even 20 mmHg are indicative of subclavian or axillary arterial stenosis, a specific sign of atherosclerosis.

In the calculation of the ankle brachial index the highest of the brachial pressures is taken into account while for each feet, the highest from dorsalis pedis (DP) and posterior tibial (PT) is considered.

The two formulas used are the following:

Right ABI = Highest pressure in right foot / Highest pressure in both arms

Left ABI = Highest pressure in left foot / Highest pressure in both arms

ABI result interpretation

The ABI test with the above formula provides the gradient between the ankle and brachial arteries with a precision of two decimals and then several medical considerations are applied.

■ Normal range: 1.0 – 1.4 because ankle pressure tends to be higher than the brachial one. This indicates there is no narrowing of blockage along the vessels. In case the patient has most of the risk factor, periodic monitoring is advised.

■ Values above normal range: >1.4 indicate a non-compressible calcified vessel. This is often due to the fibrosis or calcification in elderly patients or diabetics. This sometimes leads to an artifactually elevated BP value because the vessel is resistant.

■ Values below the normal range: <0.9 are indicative of PAD while values well below normal range i.e. <0.5 suggest a highly severe PAD that might require revascularization in some wound cases. The range between 0.9 and 1.0 is considered borderline abnormal, meaning a higher change or narrowing in the arteries, however, not yet to put a diagnosis.

Table source: Stanford Medicine 25

Peripheral vascular disease guidelines

PAD is diagnosed through a series of test results, medical history and physical examinations, often only after some symptoms have been reported. Quick action is recommended due to its higher risk of coronary heart disease (CHD), heart attack, transient ischemic attack TIA or stroke.

Amongst the PAD symptoms there are:

■ Pain with walking (claudication) – specific symptom but it appears that in conducted studies, only 10% of patients with ABI results lower than 0.9 have reported claudication.

■ Lack of palpable pulse of DP and/or PT.

■ Pallor of distal extremities due to the lack of blood flow.

The latter stages with severe ischemia exhibit paralysis and paraesthesia.

The American College of Cardiology (ACC) and the American Heart Association (AHA) has developed PAD management guidelines which regulate the use of ABI tests.

Other diagnostic functional tests include magnetic resonance angiogram (MRA), arteriograms and blood tests for PAD risk factors such as high cholesterol or diabetes.

References

1) Stone NJ, Robinson JG, Lichtenstein AH, Bairey Merz CN, American College of Cardiology/American Heart Association Task Force on Practice Guidelines et al. (2014) 2013 ACC/AHA guideline on the treatment of blood cholesterol to reduce atherosclerotic cardiovascular risk in adults: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol; 63(25 Pt B):2889-934.

2) Aboyans V, Criqui MH, Abraham P, Allison MA, American Heart Association Council on Peripheral Vascular Disease; Council on Epidemiology and Prevention; Council on Clinical Cardiology; Council on Cardiovascular Nursing; Council on Cardiovascular Radiology and Intervention, and Council on Cardiovascular Surgery and Anesthesia et al. (2012) Measurement and interpretation of the ankle-brachial index: a scientific statement from the American Heart Association. Circulation; 126(24):2890-909.

3) Allison MA, Hiatt WR, Hirsch AT, Coll JR, Criqui MH. (2008) A high ankle-brachial index is associated with increased cardiovascular disease morbidity and lower quality of life. J Am Coll Cardiol; 51(13):1292-8.

4) McDermott MM, Criqui MH, Liu K, Guralnik JM, Greenland P, Martin GJ, Pearce W. (2000) Lower ankle/brachial index, as calculated by averaging the dorsalis pedis and posterior tibial arterial pressures, and association with leg functioning in peripheral arterial disease. J Vasc Surg; 32(6):1164-71.

5) Caruana MF, Bradbury AW, Adam DJ. (2005) The validity, reliability, reproducibility and extended utility of ankle to brachial pressure index in current vascular surgical practice. Eur J Vasc Endovasc Surg; 29(5):443-51.