Measuring blood pressure in the leg is not a routine part of a standard physical examination. This measurement, taken at the ankle, is a screening tool for reduced blood flow in the lower limbs. The process involves a direct comparison between the systolic blood pressure reading from the ankle and the systolic pressure taken from the arm (brachial artery). This comparison yields a numerical ratio that provides insight into the overall condition of the peripheral vascular system.
How Leg Blood Pressure is Measured
A specialized, non-invasive procedure is used to obtain the ankle blood pressure measurement, differing slightly from a standard arm check. The patient rests in a flat, supine position for several minutes to allow blood pressures to stabilize before the test begins. Standard blood pressure cuffs are wrapped around both the upper arms and just above the ankles.
Instead of listening for blood flow with a stethoscope, the technician uses a handheld Doppler ultrasound device to locate the pulse. This Doppler probe is necessary to accurately detect the systolic pressure in the ankle’s arteries, specifically the dorsalis pedis and the posterior tibial arteries. The cuff is inflated until the Doppler signal disappears, and the pressure at which the signal returns upon deflation is recorded as the systolic pressure.
Pressures are measured in both arms and both ankles to ensure the most accurate comparison. The highest systolic pressure reading from the two ankle sites is then divided by the highest systolic pressure reading obtained from either arm. The result is a number known as the Ankle-Brachial Index (ABI), which is expressed as a ratio. This ratio indicates the efficiency of blood flow in the lower limbs compared to the upper body.
Interpreting the Ankle-Brachial Index
The resulting ABI ratio is a specific metric used to classify arterial health in the legs. A ratio between 1.0 and 1.4 is considered normal, indicating that blood flow to the ankles is adequate and not restricted. Since gravity assists blood flow downward, the systolic pressure in the ankle is normally slightly higher than the pressure in the arm.
A ratio between 0.90 and 0.99 is categorized as borderline, suggesting changes in the peripheral arteries. Any reading below 0.90 is diagnostic for Peripheral Artery Disease (PAD), a condition defined by narrowed or blocked arteries. The severity of the disease correlates directly with how low the ABI reading is.
An ABI reading above 1.4 is considered abnormal. This high value indicates that the arteries are non-compressible due to calcification and stiffening of the vessel walls. This finding is common in individuals with advanced diabetes or chronic kidney disease, and it often requires further diagnostic testing to assess the degree of arterial obstruction.
Clinical Significance of Abnormal ABI Readings
A low ABI reading signals the presence of Peripheral Artery Disease (PAD), which is caused by atherosclerosis, a buildup of fatty plaque in the arteries. This plaque causes the blood vessels to narrow and harden, restricting the delivery of oxygen-rich blood to the muscles of the legs and feet. The reduction in blood flow, or ischemia, creates the symptoms associated with the condition.
The most common symptom of PAD is intermittent claudication: muscle pain, cramping, or fatigue in the legs that occurs during physical activity like walking. This discomfort typically subsides quickly with rest, as the muscle demand for blood temporarily decreases. As the disease advances, a low ABI can be associated with non-healing wounds or ulcers on the feet and toes, known as critical limb ischemia.
Screening for PAD with the ABI is recommended for individuals over 70, or over 50 if they have a history of smoking or diabetes. Smoking, high blood pressure, and high cholesterol are risk factors that increase the risk of developing PAD. Detecting PAD is important because the disease is not limited to the legs; it serves as an indicator of widespread atherosclerosis throughout the body.
Individuals with a low ABI and diagnosed PAD have an elevated risk of having a heart attack or stroke. The same plaque buildup narrowing the leg arteries is present in the coronary and cerebral arteries. Therefore, the ABI test is a useful tool not just for diagnosing a problem in the legs, but for identifying patients who require aggressive management of their systemic cardiovascular risk.