How is the ankle/arm index calculated?

The ankle/arm index (AAI) is a crucial measurement used to evaluate peripheral arterial disease (PAD). It is determined by comparing blood pressure readings in the ankle and the arm, which gives insight into the circulatory efficiency in the lower extremities compared to the upper body.

Calculating the AAI involves using the higher of the two brachial pressures, as this value is more representative of the optimal arterial supply to the arm. This higher value is then divided by the ankle pressure. The reasoning behind this calculation process is that it helps in identifying the most efficient blood flow to the ankle by benchmarking it against the best possible arterial pressure from the arms. A suitably high brachial pressure indicates that there isn't significant obstruction in the upper limb, making it a reliable reference point.

Using the lower of the two brachial pressures or an average would not accurately reflect the optimal perfusion available for comparison, as these alternatives may underestimate the blood flow capability of the upper body. Therefore, the calculation emphasizes the higher brachial pressure, and dividing it by the ankle pressure provides the necessary index to assess lower extremity blood flow sufficiency effectively.