Blood pressure is the force of circulating blood pushing against the walls of your arteries, the vessels that carry blood from your heart to the rest of your body. It is expressed as two numbers: systolic pressure, the higher number, which measures the pressure when your heart beats, and diastolic pressure, the lower number, which indicates the pressure when your heart rests between beats. These measurements are given in millimeters of mercury (mmHg), such as 120/80 mmHg. While blood pressure is most commonly measured on the upper arm, readings taken on the forearm can appear higher.
How Blood Pressure Differs Across the Body
Blood pressure readings can vary across different parts of the body due to physiological and anatomical factors. As the pulse wave travels from the central arteries near the heart to more peripheral arteries like those in the forearm, it undergoes a phenomenon called pulse wave amplification. This amplification causes the systolic blood pressure to increase in more distant arteries, even though the mean arterial pressure and diastolic pressure remain consistent. This occurs because the pressure wave changes shape as it moves through progressively narrower and stiffer vessels.
Arterial stiffness also influences these differences, as stiffer arteries can lead to a greater increase in systolic pressure in peripheral locations. This is noticeable in older individuals or those with cardiovascular conditions. The reflected waves from the periphery contribute to this amplification, augmenting the systolic pressure more in peripheral arteries and protecting the central arteries and heart from excessive pulsatile load. Consequently, the systolic pressure measured in the forearm can be several millimeters of mercury higher than in the upper arm, by about 4 mmHg systolic and 2.3 mmHg diastolic on average.
Measurement Technique Matters
Incorrect measurement techniques can contribute to higher forearm blood pressure readings. The size of the blood pressure cuff is a factor; a cuff that is too small for the limb can lead to an overestimation of blood pressure. Forearm cuffs are smaller than those designed for the upper arm, and if not properly sized for the individual’s forearm circumference, they can yield inaccurately high results. The ideal cuff bladder length should be at least 80% of the arm’s circumference, and the width should be at least 40%.
The position of the arm relative to the heart also plays a role. If the forearm is held below heart level, gravity can increase the pressure in the blood vessels, leading to an artificially elevated reading. Studies have shown that an unsupported arm hanging at the side can overestimate systolic pressure by nearly 7 mmHg, while an arm supported in the lap can overestimate it by approximately 4 mmHg. For an accurate forearm reading, the middle of the cuff should be at heart level, and the arm should be supported to ensure the muscles are relaxed.
Why Understanding the Difference is Important
Understanding the differences in blood pressure readings between the forearm and upper arm is important for accurate health monitoring. Consistent measurement at the upper arm is the accepted standard for diagnosing and managing hypertension. This is because the upper arm provides a consistent and reproducible measurement that aligns with established guidelines for blood pressure categories. Relying solely on a forearm reading without considering these variations could lead to misdiagnosis or inappropriate treatment decisions.
Forearm readings may be used in specific situations where upper arm measurement is not feasible, such as when a patient has a large arm circumference, an injury, or medical devices that obstruct the upper arm. Even in these cases, it is important to note the reading was taken on the forearm and may be slightly higher than an upper arm reading. Consulting a healthcare professional for interpretation of all blood pressure readings is recommended, rather than self-diagnosing based on forearm measurements alone, for proper medical guidance.