To take blood pressure using the lower leg and lower arm, the tester needs to measure systolic pressure at the ankle, then divide it by systolic pressure at the arm. This is known as the "Ankle Brachial Index," or ABI.
The Ankle Brachial Index is used to determine cardiovascular conditions, mortality and for the diagnosis of Peripheral Arterial Disease (PAD), as stated by Stanford School of Medicine.
- Measure the brachial (lower arm) blood pressure Measure the brachial blood pressure by asking the patient to lie flat for 10 minutes. Once they have done this, apply the cuff over the brachial pulse and inflate it to 20 mmHg above his or her expected blood pressure. Deflate the cuff by 1 mmHg per second until the Doppler signal reappears. Make a note of the pressure at this point, and repeat the process on the opposite arm.
- Measure the ankle blood pressure Place the ankle cuff over the dorsalis pedis artery. Adjust the Doppler until the strongest signal appears. Inflate the cuff to 20 mmHg above the expected blood pressure, or until the signal disappears. Deflate the cuff by 1 mmHg per second until the signal reappears, then make a note of the blood pressure number. Repeat this process for the posterior tibial artery, and then repeat on the opposite leg.
- Calculate the ABI
Take the higher pressure of the two arteries at the ankle and divide by the brachial pressure from both arms. To calculate the pressure on the right side, this means taking the higher right ankle value and dividing by both brachial pressures. The same applies to the left side.