How To Take A Blood Pressure On The Leg

Measuring blood pressure on the leg, specifically at the ankle, is a valuable diagnostic tool used primarily when the upper arm cannot be used, or when assessing peripheral arterial disease (PAD). This comprehensive guide will walk you through the process, explain why it's done, and highlight important considerations.

Why Measure Blood Pressure on the Leg?

There are several reasons why a healthcare provider might choose to measure blood pressure on the leg instead of the arm:

Upper Arm Obstructions: Conditions like casts, burns, or surgical dressings may prevent cuff placement on the arm.
Lymphedema or Mastectomy: In patients who have had a mastectomy or have lymphedema in the arm, taking blood pressure on that limb can be harmful and should be avoided.
Dialysis Access: If a patient has an arteriovenous (AV) fistula or graft in their arm for dialysis, blood pressure should not be taken on that arm.
Peripheral Arterial Disease (PAD) Assessment: Leg blood pressure, specifically the ankle-brachial index (ABI), is crucial for diagnosing PAD, a condition where narrowed arteries reduce blood flow to the limbs.
Coarctation of the Aorta: In some cases, particularly in younger patients, leg blood pressure may be measured to help diagnose coarctation of the aorta, a congenital heart defect.

Understanding the Ankle-Brachial Index (ABI)

The ankle-brachial index (ABI) is a calculation derived from measuring blood pressure in both the arm and the ankle. It's a simple, non-invasive test used to assess the severity of PAD.

How ABI is Calculated:

Measure systolic blood pressure in both arms using a standard blood pressure cuff and Doppler ultrasound. Record the higher of the two brachial pressures.
Measure systolic blood pressure at both the dorsalis pedis and posterior tibial arteries in each leg using a Doppler ultrasound. Record the higher pressure for each leg.
Divide the higher ankle systolic pressure by the higher brachial systolic pressure for each leg.

ABI Interpretation:

1.0 to 1.4: Normal.
0.91 to 0.99: Borderline. Further evaluation may be needed.
0.41 to 0.90: Mild to moderate PAD.
0.00 to 0.40: Severe PAD.

Equipment Needed

Before you begin, gather the necessary equipment:

Blood Pressure Cuff: Choose an appropriately sized cuff. For the leg, a larger cuff may be needed compared to what's used on the arm.
Doppler Ultrasound: A handheld Doppler ultrasound device is essential for locating and amplifying the pulse in the ankle.
Ultrasound Gel: Used to improve contact between the Doppler probe and the skin.
Sphygmomanometer: This can be either an aneroid (manual) or electronic blood pressure monitor.
Stethoscope: If using an aneroid sphygmomanometer, a stethoscope is needed to listen for Korotkoff sounds.
Alcohol Swabs: For cleaning the Doppler probe.
Paper and Pen: To record the measurements accurately.

Step-by-Step Guide to Taking Blood Pressure on the Leg

Follow these steps carefully to ensure accurate measurements:

1. Patient Preparation

Explain the Procedure: Clearly explain the procedure to the patient to alleviate any anxiety. Inform them that you'll be using a Doppler ultrasound to locate the pulse in their ankle.
Positioning: Have the patient lie supine (on their back) with their legs supported and at the same level as their heart. This minimizes hydrostatic pressure effects.
Exposure: Ensure the patient's ankles are easily accessible. You may need to adjust clothing or blankets.
Rest: Allow the patient to rest for at least 5-10 minutes before taking the measurement. Physical activity can artificially elevate blood pressure.

2. Locating the Ankle Pulses

Dorsalis Pedis Artery: Locate the dorsalis pedis artery on the top of the foot, between the first and second toes. Gently palpate the area to get a sense of the artery's location.
Posterior Tibial Artery: Find the posterior tibial artery behind the medial malleolus (the bony bump on the inside of the ankle). Palpate to identify its approximate location.
Apply Ultrasound Gel: Apply a small amount of ultrasound gel to the area where you expect to find the pulse. This will help transmit the ultrasound waves.
Using the Doppler: Turn on the Doppler ultrasound device. Gently place the probe at a 45-degree angle to the skin over the dorsalis pedis artery. Slowly move the probe around until you hear the pulsatile sound of the artery.
Repeat: Repeat the process for the posterior tibial artery.
Mark the Spot: Once you've located both pulses, you may want to make a small mark with a pen to help you find them again quickly when taking the blood pressure.

3. Applying the Blood Pressure Cuff

Cuff Size: Select the appropriate cuff size. The cuff width should be approximately 40% of the limb circumference. If a standard adult cuff is too small, use a larger thigh cuff.
Placement: Position the lower edge of the cuff about 2-3 cm above the malleoli (ankle bones). Ensure the cuff is snug but not too tight. You should be able to slip two fingers comfortably under the cuff.
Tubing Placement: Position the tubing of the cuff so it aligns with the artery you will be auscultating (listening to) with the Doppler.

4. Taking the Measurement

Position the Doppler: Reapply a small amount of ultrasound gel and position the Doppler probe over the artery you've chosen to measure.
Inflation: If using an aneroid sphygmomanometer, close the valve on the bulb and inflate the cuff until the Doppler signal disappears. Continue inflating another 20-30 mmHg above that point to ensure you are above the systolic pressure. If using an electronic device, follow the device's instructions for inflation.
Deflation: Slowly release the pressure in the cuff at a rate of 2-3 mmHg per second.
Systolic Pressure: Note the pressure reading on the sphygmomanometer when you hear the first clear pulsatile sound return through the Doppler. This is the systolic blood pressure.
Diastolic Pressure (Optional): While the ABI only requires the systolic pressure, you can also record the diastolic pressure. Continue to slowly deflate the cuff, noting the pressure when the pulsatile sound changes or disappears. This is the diastolic blood pressure. However, diastolic pressure measurements using Doppler in the legs can be less reliable.
Repeat on the Other Leg: Repeat the entire process on the other leg.

5. Recording and Interpreting Results

Record Accurately: Record the systolic blood pressure for both the dorsalis pedis and posterior tibial arteries in each leg. Note which artery provided the higher pressure.
Calculate ABI: Calculate the ABI for each leg by dividing the higher ankle systolic pressure by the higher brachial systolic pressure (obtained from the arm measurement).
Interpret the Results: Use the ABI ranges mentioned earlier to interpret the results and assess the likelihood and severity of PAD.
Communicate with Healthcare Provider: Share the results with the patient's healthcare provider for further evaluation and management.

Special Considerations and Troubleshooting

Calcified Arteries: In some individuals, especially those with diabetes or chronic kidney disease, the arteries can become calcified and less compressible. This can lead to falsely elevated blood pressure readings and an ABI > 1.4. In these cases, other diagnostic tests may be necessary.
Technique Consistency: Ensure that the technique used for measuring blood pressure is consistent each time. This includes cuff size, patient positioning, and Doppler probe placement.
Proper Doppler Use: Practice using the Doppler ultrasound to become proficient at locating and identifying arterial pulses.
Troubleshooting Weak Signals: If you have difficulty locating a pulse with the Doppler, try repositioning the probe, applying more gel, or gently pressing harder on the skin.
Patient Comfort: Be mindful of the patient's comfort throughout the procedure. If they experience any pain or discomfort, stop and reassess.
Environmental Noise: Minimize environmental noise to ensure accurate auscultation with the Doppler.

Understanding Potential Errors

Several factors can contribute to inaccurate blood pressure readings when measuring on the leg:

Incorrect Cuff Size: Using a cuff that is too small will result in falsely high readings, while a cuff that is too large will result in falsely low readings.
Patient Positioning: The leg should be at heart level. If the leg is below heart level, the blood pressure reading may be falsely elevated due to hydrostatic pressure.
Movement: Any movement during the measurement can affect the accuracy of the reading.
Improper Doppler Technique: Inconsistent probe placement or excessive pressure can alter the blood flow and affect the accuracy of the reading.
Aortic Stenosis: If the patient has severe aortic stenosis (narrowing of the aortic valve), the ABI may be falsely normal.
External Compression: External compression from tight clothing, stockings, or bandages can affect blood flow and alter the readings.

Clinical Significance and Follow-Up

Measuring blood pressure on the leg and calculating the ABI are important tools in diagnosing and managing peripheral arterial disease. Early detection and treatment of PAD can help prevent serious complications such as:

Critical Limb Ischemia: Severe blockage of arteries in the legs, leading to pain, ulcers, and potential limb loss.
Amputation: In severe cases of PAD, amputation may be necessary to remove infected or necrotic tissue.
Increased Risk of Cardiovascular Events: PAD is often associated with other cardiovascular diseases, such as coronary artery disease and stroke.

If PAD is suspected or diagnosed, the healthcare provider may recommend further evaluation and management, including:

Lifestyle Modifications: Encouraging smoking cessation, regular exercise, and a healthy diet.
Medications: Prescribing medications to manage blood pressure, cholesterol, and blood clot formation.
Interventional Procedures: Performing procedures such as angioplasty or bypass surgery to improve blood flow to the legs.

Conclusion

Measuring blood pressure on the leg, particularly when calculating the Ankle-Brachial Index (ABI), is a valuable technique for assessing peripheral arterial disease and other conditions where arm measurements are not feasible. By following the proper steps, using the appropriate equipment, and being aware of potential sources of error, healthcare professionals can obtain accurate and reliable measurements to guide patient care. Remember that ABI results should always be interpreted in conjunction with a thorough clinical evaluation.