Blood pressure measurement assesses the force exerted by circulating blood against artery walls. It is divided into systolic pressure, which reflects the pressure during a heartbeat, and diastolic pressure, which represents the pressure when the heart rests. While the upper arm is the typical location for this test, measuring pressure in other limbs, particularly the leg, offers distinct and important diagnostic insights into circulatory health. Assessing peripheral blood pressure helps medical professionals determine if blood flow is being restricted in the extremities or if there are pressure imbalances that suggest a problem closer to the heart.
The Purpose of Peripheral Blood Pressure Measurement
Measuring blood pressure away from the brachial artery in the arm, specifically at the ankle, serves to gauge the efficiency of blood flow in the body’s most distant arteries. Physiologically, the systolic blood pressure in the ankles of a healthy individual should be equal to or slightly higher than the pressure measured in the arms. This slight increase is due to the phenomenon of pulse amplification, where the pressure wave is naturally amplified as it travels down narrower arteries.
A difference in pressure readings between the upper and lower limbs can signal an obstruction or narrowing in the arteries supplying the legs. This measurement provides a non-invasive look at the patency of the peripheral vascular network. The assessment becomes particularly relevant when standard arm measurements are compromised or inaccurate for technical reasons. For instance, a patient with a dialysis access shunt in one or both arms, or a person with severe arm obesity, may require a lower limb measurement to obtain a reliable reading.
Any significant reduction in leg blood pressure compared to arm pressure suggests that the blood flow is diminished as it travels from the heart to the ankle. This reduction in pressure gradient is an indicator of vascular disease in the lower extremities. The comparison between the two sites is a powerful tool for screening for circulatory issues that may not be apparent from an arm blood pressure reading alone.
The Ankle-Brachial Index (ABI) Calculation
The Ankle-Brachial Index (ABI) is a standardized, non-invasive method that quantifies the difference between leg and arm blood pressure to screen for reduced blood flow. The test involves measuring the systolic blood pressure in both the arms and the ankles using a standard blood pressure cuff and a handheld Doppler ultrasound device. The Doppler instrument detects the pulse and determines the exact pressure at which blood flow returns as the cuff is slowly deflated.
For the arm measurement, the systolic pressure is recorded in both brachial arteries, and the higher of the two readings is used as the denominator in the final ABI calculation. At the ankle, the systolic pressure is measured over two specific arteries: the dorsalis pedis artery on the top of the foot and the posterior tibial artery along the inner ankle. The highest systolic pressure obtained from these two arteries in each leg is used for the numerator.
The ABI is then calculated as a ratio for each leg separately. The formula is the highest ankle systolic pressure for that leg divided by the highest brachial systolic pressure from either arm. This calculation converts the absolute pressure measurements into a ratio, which provides a sensitive metric for diagnosing peripheral artery disease (PAD). The calculated ABI value is typically recorded to two decimal places for precision.
Interpreting Leg Blood Pressure Results
The Ankle-Brachial Index provides a direct assessment of blood flow restriction in the lower limbs.
ABI Ranges and Interpretation
- A normal, healthy ABI ratio ranges from 1.0 to 1.4, indicating that ankle pressure is at least equal to, or slightly higher than, arm pressure.
- An ABI value that falls between 0.91 and 0.99 is considered borderline, which may suggest the early stages of reduced circulation.
- An ABI of 0.90 or lower confirms the diagnosis of peripheral artery disease (PAD). Values between 0.41 and 0.70 indicate moderate PAD, while a ratio of 0.40 or less signifies severe disease.
- Conversely, an ABI ratio above 1.4 is also considered abnormal, often described as non-compressible. This high reading suggests that the arteries are stiff or calcified, a condition called medial arterial calcification. This stiffness, frequently seen in individuals with long-standing diabetes or advanced age, prevents the cuff from fully compressing the vessel, leading to an artificially high pressure reading. In such cases, a toe-brachial index (TBI) test is often necessary to get an accurate reading of true peripheral blood flow.
Diagnosing Coarctation of the Aorta
Leg blood pressure measurement is also used to diagnose a specific congenital heart defect called Coarctation of the Aorta. This condition involves a narrowing of the aorta, which causes a significant difference in blood pressure between the upper and lower extremities. A systolic pressure difference of more than 20 mmHg where the arm pressure is markedly higher than the leg pressure strongly suggests this diagnosis. This distinct pressure gradient is an important clinical sign, particularly in children or young adults presenting with unexplained hypertension.