The measurement of four-extremity blood pressure (4E BP) is a specialized non-invasive procedure used to assess blood flow throughout the body’s largest arteries. This technique involves taking systolic blood pressure readings from both brachial arteries in the arms and the posterior tibial and dorsalis pedis arteries in the ankles. Comparing the pressure readings across all four limbs reveals pressure gradients—differences in blood pressure between different parts of the body. These gradients indicate where blockages or narrowings might be affecting blood flow, providing information that standard, single-arm blood pressure checks cannot.
Clinical Reasons for Measurement
The primary reason for performing a 4E BP measurement is to screen for Peripheral Artery Disease (PAD), which involves the narrowing of arteries, most commonly in the legs. Arterial narrowing in the lower extremities causes a measurable drop in blood pressure at the ankle compared to the arm. This pressure difference is quantified to determine the severity of blood flow restriction.
Beyond PAD, the four-limb measurement is used to identify a significant difference in pressure between the two arms. A systolic blood pressure difference of 10 to 20 millimeters of mercury (mmHg) or greater between the left and right arm can suggest a narrowing in the subclavian artery. This discrepancy indicates an upstream vascular issue affecting blood supply to the arm with the lower reading, sometimes associated with Subclavian Steal Syndrome.
The technique also helps in assessing for congenital heart defects, such as Coarctation of the Aorta, a narrowing of the large artery that delivers oxygenated blood to the body. This condition typically causes a significant difference between the upper body and lower body pressures. The arms show a noticeably higher reading than the legs. A pressure gradient greater than 20 mmHg between the arms and legs suggests this diagnosis.
Essential Preparation and Technique
Careful patient preparation ensures a true resting state. The patient should be lying flat on their back (supine position) for at least five to ten minutes before any measurements are taken. This resting period stabilizes blood pressure and prevents readings from being artificially elevated by recent exertion or anxiety. Clothing that might constrict circulation must be removed to allow the cuffs to be placed directly on the skin.
Selecting the correct cuff size is critical because an improperly sized cuff will lead to inaccurate pressure readings. For the arms, the inflatable bladder should cover approximately 80% of the patient’s arm circumference. Similarly, appropriately sized cuffs must be chosen for the lower extremities and placed just above the ankle.
For the arm pressures, the standard procedure involves placing the cuff on the upper arm and using a stethoscope over the brachial artery. The systolic pressure is marked by the first sound, known as Korotkoff Phase I. Both arms must be measured, and the higher of the two systolic readings will be used later for calculations.
The technique for measuring ankle pressures differs slightly and requires a specialized tool to ensure accuracy. A handheld Doppler ultrasound device is used to locate the pulse in the dorsalis pedis artery on the top of the foot and the posterior tibial artery near the ankle bone. Palpation alone is often insufficient for reliably finding the systolic pressure in these distal arteries.
The cuff is inflated until the Doppler signal disappears, and then slowly deflated at a rate of about two millimeters of mercury per second. The pressure reading when the Doppler signal returns is recorded as the systolic pressure for that specific artery. This process is repeated for all four arteries in the lower limbs—the dorsalis pedis and posterior tibial in both the left and right ankles.
Understanding the Results and Indices
The pressures recorded from the four limbs are analyzed by calculating specific indices. The most common is the Ankle-Brachial Index (ABI), which is a direct comparison of the blood pressure in the ankles to the blood pressure in the arms. To calculate the ABI for each leg, the highest systolic pressure from the two ankle arteries on that side is divided by the single highest systolic pressure recorded from either arm.
A normal ABI value ranges from 1.0 to 1.4, indicating that the pressure in the ankles is equal to or slightly higher than the pressure in the arms, which is expected due to the physics of blood flow. An ABI value of 0.90 or less is considered diagnostic of Peripheral Artery Disease (PAD). Values between 0.91 and 1.0 are often classified as borderline, suggesting a need for further monitoring.
The severity of PAD is categorized based on the ABI value: 0.70 to 0.90 indicates mild disease, and 0.40 to 0.70 suggests moderate disease. A low ABI value, specifically below 0.40, signifies severe restriction of blood flow. Conversely, a value greater than 1.4 suggests that the artery walls may be stiffened or calcified, which can render the ABI test unreliable in patients with advanced diabetes or kidney failure.
In addition to the ABI, the inter-arm systolic pressure difference is a significant finding that requires interpretation. A consistent difference of 10 mmHg or more between the two arms is considered clinically significant and may suggest an obstructive lesion in the subclavian artery. An inter-arm difference exceeding 20 mmHg is a stronger indicator of a significant stenosis or narrowing in the artery supplying the arm with the lower pressure.