Blood pressure (BP) is the force exerted by circulating blood against artery walls and is a fundamental measure of cardiovascular health. The standard location for measurement is the upper arm, using the brachial artery. When injury, medical conditions, or structural limitations prevent the use of the arm, healthcare providers must use alternative sites. The leg is the most common secondary location for obtaining this measurement.
Reasons for Measuring Blood Pressure on the Leg
Measuring blood pressure on the leg is necessary in two scenarios: when the arm cannot be used, or when comparing arm and leg pressure is needed for diagnosis. The inability to use the arm may stem from conditions such as severe burns, intravenous lines, or recent surgery like a bilateral mastectomy. Severe obesity is another limitation, as the upper arm circumference may prevent correct application of a standard or large cuff. In these cases, the wider thigh or the smaller ankle area offers a suitable alternative site for cuff placement.
The second reason is diagnostic, used to screen for conditions affecting blood flow in the extremities or major vessels. A significant pressure difference between the upper and lower limbs suggests a vascular issue. Conditions often evaluated this way include peripheral artery disease (PAD) and coarctation of the aorta. PAD, which involves narrowing of the arteries, often results in lower blood pressure readings in the ankle compared to the arm. This comparison provides data regarding the severity and location of arterial obstruction.
Preparation and Necessary Equipment
Several preparatory steps must be taken before measuring blood pressure on the leg to ensure an accurate reading. The patient should be positioned lying down, either supine or prone, depending on the cuff location. The limb must be supported and the patient relaxed, as muscle tension can artificially elevate the pressure reading. The cuff placement site must also be positioned approximately level with the patient’s heart to minimize hydrostatic pressure effects.
Selecting an appropriately sized blood pressure cuff for the specific limb circumference is the most important equipment consideration. Since the thigh is significantly larger than the arm, a specialized, extra-large cuff is required for thigh measurements. A cuff that is too narrow for the limb results in a falsely high reading. Conversely, a standard adult arm cuff is usually sufficient for ankle measurements, provided it wraps securely around the leg just above the malleoli.
Detailed Technique for Leg Blood Pressure Measurement
The leg offers two locations for obtaining blood pressure: the thigh and the ankle. The choice depends on the clinical goal and patient comfort. For a thigh measurement, the patient is often positioned prone for easier access to the artery behind the knee. The large thigh cuff is wrapped around the mid-thigh, ensuring the bladder is centered over the femoral artery. The stethoscope is then placed in the popliteal fossa, behind the knee, to listen for sounds over the popliteal artery.
This measurement uses the traditional auscultatory method. The cuff is inflated rapidly until the pulse is obliterated and then slowly deflated. The first sound heard (Korotkoff sound) marks the systolic pressure, and the point where the sounds disappear indicates the diastolic pressure. The patient must remain still and relaxed during the procedure to prevent muscle movement from interfering with the acoustic detection of artery sounds.
When measuring at the ankle, the cuff is placed just above the ankle bones, avoiding tendons or bony prominences. The cuff must be applied directly to the skin, requiring clothing to be removed or moved out of the way. Unlike the thigh measurement, the ankle measurement targets the posterior tibial artery or the dorsalis pedis artery. The posterior tibial artery is located behind the medial malleolus, while the dorsalis pedis artery is found on the top center of the foot.
The pulse is identified at one of these two sites before inflation. A stethoscope or Doppler device is used to detect the return of blood flow. The ankle measurement is valuable when assessing for peripheral artery disease because it directly measures the pressure in the arteries supplying the foot. Both thigh and ankle measurements require careful placement and slow, controlled cuff deflation for accuracy.
Interpreting the Difference Between Arm and Leg Readings
A slight variation is expected when comparing blood pressure readings taken on the arm and the leg in a healthy individual. Due to increased vascular resistance and blood flow mechanics, the systolic pressure measured in the leg is typically 10 to 20 mmHg higher than the arm pressure. This difference is normal and reflects how pressure waves travel through the arterial system. The diastolic pressure often remains similar between the two sites.
Deviation from this expected pattern provides the diagnostic power of the leg measurement. If the systolic pressure in the ankle or thigh is significantly lower than the arm pressure, it strongly indicates an underlying issue. A drop of more than 20 mmHg in leg pressure compared to arm pressure raises suspicion for conditions like peripheral artery disease. This pressure drop suggests a blockage or narrowing is impeding blood flow to the lower extremity.
Conversely, a much higher-than-expected leg pressure reading, sometimes seen in younger patients, may suggest coarctation of the aorta (a narrowing of the aorta itself). The comparison between upper and lower extremity pressures is often calculated as a ratio, such as the ankle-brachial index (ABI), which quantifies the severity of potential vascular disease. Therefore, the leg measurement is less about the absolute number and more about its relationship to the standard arm reading.