Normally, the blood pressure is measured at the brachial artery, located in the upper arm, and is representative of the overall blood pressure in the body. However, in case of arterial blockage in the peripheral arteries, the blood pressure may vary in body parts that are away from the heart.
Therefore, if the blood pressure in the ankle is significantly different than that measured at brachial artery, it is considered to be an indication of peripheral artery disease (PAD). PAD refers to the build up of plaque in the peripheral arteries, which supply blood to the head and limbs.
Ankle Brachial Index or Ankle Brachial Pressure Index (ABPI), simply refers to the ratio between the blood pressure measured at the ankles to that measured at the arms. A ratio in the range of 0.9 to 1.3, under resting conditions, is considered normal; whereas a value lower than that implies comparatively low blood pressure in the ankles, and is indicative of PAD.
Ankle Brachial Pressure Index Test
The apparatus used may be a Doppler ultrasound blood flow detector (doppler wand or doppler probe), or a sphygmomanometer (standard blood pressure cuffs). Using this, the pressure at both the arms, or brachial arteries, is measured and recorded. Then, the blood pressure at posterior tibial artery and dorsalis pedis artery near each ankle is measured and recorded.
The value for ABPI is obtained by dividing the highest pressure recorded at the ankles by the highest pressure recorded at the brachial artery.
ABPI = (Highest pressure recorded at ankle) / (Highest pressure recorded at brachial artery)
For example, consider the following hypothetical blood pressure measurements for an individual.
Artery Blood Pressure
(mm of Hg)
Left brachial 110
Right brachial 120
Left posterior tibial 130
Left dorsalis pedis 95
Right posterior tibial 120
Right dorsalis pedis 90
Highest pressure at ankles = 130
Highest pressure at brachial artery = 120
Therefore, ABPI = 130/120 = 1.09
In certain cases, the left and right ABPI are calculated separately, and the lower of the two is considered to be the overall ABPI. According to this, the ABPI measurements for the above blood pressure values are:
ABPIleft = 130/110 = 1.18
ABPIright = 120/120 = 1.00
Thus, the value 1.00 is considered to be the overall ABPI of the individual.
These blood pressure measurements are performed under resting conditions. In some cases, the individual may be asked to exercise on a treadmill for 5-10 minutes, and then blood pressure may be measured again to obtain the ABPI value after exercise. This is then compared to the resting ABPI to assess PAD.
Interpretation of the Ankle Brachial Test
An ABPI greater than 0.9 is considered to be normal and holds no risk of peripheral artery disease. But, if the ABPI value is more than 1.2, it is an indicator of severe calcification of the arterial walls, which may be observed in individuals with diabetes or chronic kidney diseases.
A detailed interpretation of the ABPI values are as follows:
Below 0.5 Severe arterial disease
0.5 to 0.8 Moderate arterial disease
0.8 to 0.9 Mild arterial disease
0.9 to 1.0 Acceptable
1.0 to 1.2 Normal
Above 1.2 Abnormal hardening of arteries
If the resting ABPI value for an individual falls in the normal range, but drops after exercising, it may indicate the presence of PAD.
ABI or ABPI test is a quick and noninvasive test to determine the presence of blockage in peripheral arteries. It may be advised in individuals complaining of pain and cramping in thighs or calves, while walking, climbing, or exercising. These are considered to be the most common signs of PAD. In case of abnormal results, additional tests may advised to determine the precise location and degree of blockage.