Blood pressure (BP) is the force exerted by circulating blood against the walls of the body’s arteries, representing a fundamental measure of cardiovascular health. Although the upper arm is the standard and most reliable location for routine blood pressure measurement, certain medical situations make this site impossible or inaccurate. In these instances, the leg becomes an alternative site for obtaining a reading, providing clinicians with important diagnostic information. This measurement requires specific techniques and equipment to account for the anatomical differences between the upper and lower limbs.
Reasons for Leg Blood Pressure Measurement
Measuring blood pressure on the leg is reserved for specific clinical circumstances where the standard arm measurement is compromised. A primary reason is the physical inability to place a cuff on the arm due to conditions like severe burns, extensive orthopedic injuries, or the presence of bilateral intravenous lines. Patients with an arteriovenous fistula, often used for dialysis access, cannot have a cuff placed on that arm, necessitating an alternative site.
Morbid obesity or certain limb deformities may prevent the proper fit of a standard arm cuff, leading to inaccurate readings. Conditions like lymphedema or the aftermath of surgeries such as a mastectomy can make one or both arms unsuitable. The leg measurement is also used to screen for conditions like coarctation of the aorta or to help diagnose Peripheral Artery Disease (PAD) by comparing the pressure between the ankle and the arm.
Preparation and Positioning for Accuracy
The reliability of a leg blood pressure reading begins with preparation and correct patient positioning. Before the measurement, the patient should rest quietly for at least five minutes, and any constricting clothing must be removed. Selecting a blood pressure cuff sized for the patient’s thigh, calf, or ankle circumference is crucial.
A correctly sized cuff ensures the inflatable bladder wraps around 80% to 100% of the limb’s circumference, and its width should be at least 40% of the circumference. The patient must be lying down in a supine or prone position, with the leg extended and relaxed. Positioning the measurement site, whether the thigh or ankle, level with the heart is necessary to minimize hydrostatic pressure errors that can skew the results.
Step-by-Step Guide to Taking Leg Blood Pressure
The most common technique involves placing a large cuff on the thigh, centered over the posterior surface. The popliteal artery, which runs behind the knee, is used for auscultation. An alternative approach uses an ankle cuff, which requires a Doppler device for accurate pulse detection over the posterior tibial or dorsalis pedis arteries, as Korotkoff sounds are often inaudible at the ankle.
Thigh Measurement Procedure
The clinician first locates the popliteal pulse behind the knee using their fingertips. While palpating the pulse, the cuff is inflated rapidly until the pulse disappears, providing an estimate of the systolic pressure. The cuff is then quickly inflated an additional 20 to 30 millimeters of mercury (mmHg) above this estimated reading.
The stethoscope is placed firmly over the popliteal artery, and the cuff is slowly deflated at a rate of approximately 2 to 3 mmHg per second. The point at which the first sound is heard marks the systolic pressure. The point where the sound completely disappears is recorded as the diastolic pressure. Taking at least two measurements and calculating an average helps ensure the reading is representative.
Interpreting Leg Blood Pressure Readings
Interpreting a leg blood pressure reading requires understanding the difference compared to an arm reading. In a healthy individual measured while lying down, the systolic blood pressure in the leg is typically slightly higher than the arm reading due to hydrostatic pressure effects. This difference averages about 17 mmHg higher at the ankle than in the arm, though the diastolic pressure remains the same.
Because of this variance, a diagnostic threshold of 155/90 mmHg may be used to define high blood pressure when only ankle measurements are available, conservatively equating to a standard arm reading of 140/90 mmHg.
If the leg systolic pressure is significantly lower than the arm pressure, it may indicate a serious underlying condition. A systolic difference of 10 to 20 mmHg lower in the leg could suggest coarctation of the aorta, especially in children and adolescents. Conversely, a drop of 33 mmHg or more is a strong indicator of Peripheral Artery Disease (PAD) in the lower extremities. The comparison between the ankle and arm systolic pressures is calculated as the Ankle-Brachial Index (ABI) to assess for arterial blockage.