Blood pressure is a fundamental measure of cardiovascular health, typically checked in the arm to assess the force of blood against artery walls. While the arm reading provides a systemic view, medical professionals sometimes need a more specific insight into circulation in the lower limbs. Measuring pressure in the ankle offers a non-invasive way to check if blood is flowing freely to the feet and legs. This comparison helps to identify potential blockages or narrowing in the peripheral arteries that would not be apparent from a standard arm measurement alone. The difference in pressure between these two points serves as an indicator of blood flow efficiency in the extremities.
Why Ankle Pressure is Measured: The Ankle-Brachial Index
The specific assessment tool that relies on measuring ankle pressure is called the Ankle-Brachial Index, or ABI. The ABI is a simple diagnostic ratio calculated by dividing the systolic blood pressure measured at the ankle by the systolic blood pressure measured in the arm. This comparison is a highly effective way to screen for Peripheral Artery Disease (PAD), a common circulatory problem where narrowed arteries reduce blood flow to the limbs.
If the arteries in the legs are narrowed or obstructed by plaque buildup, the systolic blood pressure recorded at the ankle will be lower than the pressure recorded in the arm. This difference indicates that the blood flow is restricted as it travels from the heart to the lower limbs. Because PAD often presents without noticeable symptoms in its early stages, the ABI test is a valuable, non-invasive method for early detection. Identifying reduced blood flow in the legs is also significant because it signals a higher likelihood of widespread atherosclerosis, which carries increased risks for heart attack and stroke.
How the ABI Test is Performed
The ABI test is a straightforward, non-invasive procedure often performed in a doctor’s office, requiring the patient to lie flat on their back for the duration of the measurement. The equipment used is similar to that for a standard blood pressure check, involving inflatable cuffs and a handheld Doppler ultrasound device. This Doppler probe emits sound waves to detect the arterial blood flow and helps the technician determine the precise systolic pressure.
To begin, a cuff is placed on both arms to measure the brachial artery pressure, and the highest reading from either arm is recorded. Cuffs are then positioned just above the ankles to measure the systolic pressure in both the posterior tibial and dorsalis pedis arteries. The highest pressure obtained from the two ankle arteries in each leg is used for the calculation. The final ABI ratio is then determined by dividing the highest ankle systolic pressure by the highest brachial systolic pressure.
Interpreting the ABI Ratio: Normal Ranges and Diagnosis
The resulting ABI ratio provides a numerical gauge of blood flow and circulatory health in the lower extremities. A normal ABI ratio falls between 1.0 and 1.4, indicating that the blood pressure in the ankle is equal to or slightly higher than the pressure in the arm, which is expected for healthy circulation. An ABI in this range suggests that there is no significant restriction of blood flow to the legs.
A borderline result is an ABI between 0.91 and 0.99, which may suggest a mild degree of arterial disease or signal a person is at risk for developing PAD. When the calculated ratio drops to 0.9 or lower, it is considered diagnostic for Peripheral Artery Disease.
A ratio between 0.70 and 0.90 typically indicates mild PAD, while a ratio between 0.40 and 0.70 suggests moderate disease severity. An ABI falling below 0.40 or 0.50 signifies severe PAD, often associated with critical limb ischemia.
As the ratio decreases, the severity of the blockage increases, demonstrating impaired circulation. Patients with such low scores may experience pain even at rest and face an elevated risk of non-healing wounds, which can ultimately lead to the need for amputation if not addressed. A low ABI score, even without leg symptoms, is independently linked to a higher prevalence of coronary artery disease and increased risk of cardiovascular death.
Limitations of the ABI Test
While the ABI test is a highly effective screening tool, its accuracy can be compromised in certain patient populations. The primary limitation involves a falsely elevated or unreliably high ratio, specifically a result greater than 1.4. This high value does not mean circulation is too robust; rather, it often points to arterial calcification or stiffness.
Calcified arteries become non-compressible when the blood pressure cuff is inflated, leading to an artificially high systolic pressure reading in the ankle. This issue is particularly common in individuals with long-standing Type 1 or Type 2 diabetes, chronic kidney disease, or advanced age, where calcium deposits harden the arterial walls. In these cases, the ABI test may fail to detect PAD despite the presence of actual blockages. When an incompressible artery is suspected, alternative diagnostic methods, such as a Toe-Brachial Index (TBI), may be used to obtain a more accurate assessment of blood flow to the limb.