Is systolic blood pressure in the ankle equivalent to systolic blood pressure in the arm?

From the Guidelines

No, systolic ankle pressure is not the same as arm pressure. Systolic ankle pressure refers to the blood pressure measured at the ankle, while arm pressure (brachial pressure) is measured at the upper arm. These measurements are often compared to calculate the ankle-brachial index (ABI), which is the ratio of ankle systolic pressure to arm systolic pressure.

Key Points to Consider

• In healthy individuals, ankle pressure is typically equal to or slightly higher than arm pressure, resulting in an ABI of 1.0-1.4 1.
• Lower ABI values may indicate peripheral artery disease, where blood flow to the legs is reduced due to arterial narrowing.
• The difference exists because blood pressure naturally varies throughout the body based on distance from the heart, vessel elasticity, and local vascular resistance.
• Measuring both ankle and arm pressures provides valuable diagnostic information about circulation that a single measurement cannot provide.
• According to the American Heart Association, patients should be lying flat for an accurate ABI measurement, with the head and heels fully supported 1.
• The ABI is calculated as the systolic blood pressure obtained at the ankle divided by the systolic blood pressure obtained at the brachial artery while the patient is lying down, and a ratio of less than 1 (typically defined as <0.9) is considered abnormal and is commonly used to define PAD 1.

Considerations for Measurement

• The effect of the duration of the rest period on the reliability of ABI measurement is unknown, but most studies use a 10-minute rest period 1.
• Smoking cigarettes may affect the ABI, with a significant decrease in ABI measured after smoking compared to after 12 hours of smoking abstinence 1.
• The ankle–brachial blood pressure index should normally be ≥0.9, and an index <0.5 or an ankle pressure <50 mm Hg is indicative of severely impaired circulation to the foot 1.

From the Research

Systolic Ankle and Arm Blood Pressure Comparison

• The systolic ankle blood pressure is not the same as the arm blood pressure, as they are measured at different locations in the body 2, 3.
• The ankle-brachial index (ABI) is a measure that compares the systolic blood pressure at the ankle with the systolic blood pressure at the arm, and is used to diagnose peripheral artery disease (PAD) 3, 4.
• A low ABI value (≤0.9) indicates PAD, while a high ABI value (≥1.40) indicates poorly compressible arteries 4.
• Studies have shown that there is a significant correlation between inter-arm pressure difference and percentage ischemic myocardium, as well as ABI 2.
• The US Preventive Services Task Force (USPSTF) has concluded that the current evidence is insufficient to assess the balance of benefits and harms of screening for PAD with the ABI in asymptomatic adults 5, 6.

Key Findings

• The ABI is an underutilized and easy-to-perform test to diagnose atherosclerotic lower-limb arterial occlusive disease 3.
• Measurement of ABI is recommended as the initial diagnostic test for patients with exertional leg pain, to assess the healing potential of foot lesions, and as part of the routine health assessment of patients with diabetes and PAD 3.
• Community-dwelling patients with severe PAD or poorly compressible arteries have high rates of revascularization or limb loss, respectively 4.
• Guideline-recommended management strategies for secondary risk prevention are underused in the community 4.