From the Guidelines
Ankle systolic blood pressure is typically slightly higher than arm (brachial) blood pressure in healthy individuals, with a normal ankle-brachial index (ABI) ranging from 1.0 to 1.4. This difference occurs because of pressure amplification as the pulse wave travels down the arterial tree, with the arterial walls becoming more rigid in the lower extremities, causing the systolic pressure to increase while maintaining similar mean arterial pressure throughout the body 1. However, in patients with peripheral arterial disease (PAD), ankle pressure may be lower than arm pressure, resulting in an ABI below 0.9, which is diagnostic for PAD. Various factors can affect this relationship, including age, arterial stiffness, and cardiovascular conditions. When measuring blood pressure in different limbs, it's essential to have the patient in a supine position for at least 5-10 minutes before taking measurements to ensure accurate comparisons, as the patient's body position and knee or hip flexion can influence the ABI measurement 1. Some key points to consider when measuring ABI include:
- The patient should be lying flat for an accurate ABI measurement, with the head and heels fully supported 1
- The ABI should be measured in both legs in all new patients with PAD of any severity to confirm the diagnosis of lower extremity PAD and establish a baseline 1
- ABI results should be uniformly reported with noncompressible values defined as greater than 1.40, normal values 1.00 to 1.40, borderline 0.91 to 0.99, and abnormal 0.90 or less 1
- The toe-brachial index should be used to establish the lower extremity PAD diagnosis in patients in whom lower extremity PAD is clinically suspected but in whom the ABI test is not reliable due to noncompressible vessels 1.
From the Research
Ankle Systolic Blood Pressure Comparison to Arm Blood Pressure
- The study 2 found that there was a clinically and statistically significant difference between arm and ankle systolic blood pressure (SBP) measurements, with an average difference of -13 mmHg.
- Only 37% of SBP measurements were within an error range of 10 mmHg, indicating poor precision and making ankle blood pressure unreliable as a substitute for arm blood pressure 2.
- Another study 3 compared blood pressure measurements at the arm, ankle, and calf, and found a poor agreement between the different sites with respect to systolic blood pressure, with the ankle measurement being 8 mmHg higher than the arm on average.
- The studies 4, 5, and 6 do not provide direct evidence on the comparison of ankle systolic blood pressure to arm blood pressure, as they focus on peripheral artery disease treatment and management rather than blood pressure measurement.
- Overall, the available evidence suggests that ankle systolic blood pressure is not equivalent to arm blood pressure, and should not be used as a substitute for arm blood pressure measurements 2, 3.