Blood pressure (BP) measurement is a routine part of any medical evaluation. While the brachial artery measurement provides a systemic pressure reading, comparing this value to the pressure in the ankle offers specialized diagnostic information. This focused technique is used to evaluate circulation in the lower limbs. Measuring pressure in the leg, specifically at the ankle, helps healthcare providers determine if blood flow is restricted further down the body. This non-invasive assessment serves as a powerful indicator of vascular health beyond the central circulation.
Understanding Pressure Differences Between Limbs
In a healthy circulatory system, the systolic blood pressure measured at the ankle is normally equal to or slightly higher than the pressure measured at the arm’s brachial artery. The ankle pressure is expected to be about 10-15 mmHg greater than the arm pressure. This slight increase is a normal physiological phenomenon known as pressure amplification. It occurs because the pulse wave quickens and reflects off the narrowed, more rigid artery walls as it travels distally away from the heart, causing the systolic pressure to be mildly elevated compared to the central pressure. The entire measurement process is standardized with the patient lying flat (supine) to eliminate the effect of hydrostatic pressure. Deviations from this expected pressure relationship signal a potential issue with blood flow in the lower limbs, which is the basis for the diagnostic test.
Measuring Blood Pressure Using the Ankle-Brachial Index Technique
The specialized procedure used to compare limb pressures is called the Ankle-Brachial Index (ABI) test. This technique uses a standard blood pressure cuff along with a handheld Doppler ultrasound device to precisely detect the systolic pressure. The patient rests for at least ten minutes before any measurements are taken to allow their blood pressure to stabilize.
The highest systolic pressure is first obtained in both arms from the brachial artery, with the higher of the two arm pressures serving as the denominator for the final calculation. Next, the cuff is moved to the lower leg, just above the ankle, to measure the systolic pressure in two specific arteries: the posterior tibial artery and the dorsalis pedis artery. The Doppler probe is placed over these arteries to listen for the return of the blood flow signal as the cuff is slowly deflated. The ABI is calculated for each leg separately by dividing the highest systolic pressure recorded at that ankle by the highest systolic pressure recorded in either arm. The resulting ratio is an objective measure of arterial flow efficiency to the lower extremities.
Interpreting the Ankle-Brachial Index Score
The numerical ABI ratio provides a direct interpretation of the arterial condition in the leg compared to the arm. A score ranging from 1.0 to 1.4 is considered normal. A borderline score falls between 0.91 and 0.99, suggesting a small reduction in blood flow may be present. An ABI score of 0.90 or lower is diagnostic for reduced blood flow, with the severity increasing as the number decreases.
A ratio between 0.41 and 0.90 indicates mild to moderate arterial blockage. A score below 0.40 signifies severe arterial compromise, which often requires urgent attention. A score above 1.4 is also considered abnormal. This high reading suggests that the arteries in the leg are stiff and non-compressible, a condition often seen in patients with long-standing diabetes or advanced age. In these cases, the high reading is unreliable for diagnosing blockages because the calcified artery walls artificially elevate the pressure, necessitating further testing.
Identifying Peripheral Artery Disease
The ABI test is the primary, non-invasive method for diagnosing Peripheral Artery Disease (PAD). PAD is a common circulatory problem where narrowed arteries reduce blood flow to the limbs. This occurs when fatty deposits (plaque) build up inside the artery walls, a process called atherosclerosis. A low ABI score directly indicates a flow-limiting blockage because the blood pressure at the ankle is significantly lower than the arm pressure.
Common risk factors for developing PAD include a history of smoking, which is the greatest risk factor, and having diabetes, high blood pressure, or high cholesterol. While many people with PAD experience no symptoms, a classic sign is intermittent claudication, which is muscle pain or cramping in the legs that begins with exercise and resolves with rest. As the disease progresses, symptoms can include numbness or weakness, coldness in the lower leg, and sores or ulcers on the feet that are slow to heal. The ABI test provides an objective measurement of this systemic circulatory issue, linking the abnormal leg pressure reading to the underlying health condition. Early diagnosis through the ABI allows for timely intervention, reducing the risk of complications such as non-healing wounds, infection, and potential amputation.