How is the ankle-brachial index (ABI) calculated?

The ankle-brachial index (ABI) is a simple, non-invasive test used to assess the presence of peripheral artery disease (PAD). The calculation involves measuring the systolic blood pressure in the ankle and in the brachial artery (located in the arm) and then using these measurements to calculate the index.

To calculate the ABI, the systolic blood pressure recorded at the ankle is divided by the systolic blood pressure recorded at the brachial artery. This ratio effectively compares blood flow in the legs to that in the arms, which helps evaluate the severity of arterial occlusion in the lower extremities. A normal ABI value ranges from 1.0 to 1.4, while lower values indicate varying degrees of arterial narrowing.

The correct answer reflects this established method for determining the ABI. The other options do not adhere to the standard ABI calculation: dividing by diastolic pressures is not part of the process, taking averages is not the method used, and subtracting pressures does not yield the ABI value. Understanding this calculation is crucial in critical care settings, as it helps identify patients at risk for cardiovascular complications related to poor lower extremity perfusion.