Blood pressure (BP) measures the force exerted by circulating blood against artery walls. The reading includes two numbers: systolic pressure (the higher number), which occurs when the heart beats, and diastolic pressure (the lower number), which reflects pressure when the heart rests. Health professionals commonly use the left arm as the default location because anatomical differences allow this reading to most accurately reflect the body’s central circulatory pressure.
The Anatomical Rationale for the Left Arm
The preference for the left arm stems from the unique pathway of blood vessels branching directly from the aorta, the body’s largest artery. After oxygenated blood exits the heart’s left ventricle, it enters the aortic arch, a curve at the top of the aorta. Three major vessels branch off this arch to supply the upper body and head.
The left subclavian artery, which carries blood to the left arm, branches directly off the aortic arch. This direct connection provides the shortest and most unimpeded route from the central circulation to the brachial artery, where the blood pressure cuff is placed. This proximity means the reading taken in the left arm is the most accurate representation of the pressure within the aorta itself.
The pathway to the right arm involves an extra branch point. The first major vessel to split off the aortic arch is the brachiocephalic trunk, which then divides into the right common carotid artery and the right subclavian artery, supplying the right arm.
Because the right subclavian artery is further removed from the aorta by this extra branching, the blood flow to the right arm is subjected to a slightly longer and more indirect route. This difference can result in a minor reduction in pressure compared to the left arm. Modern guidelines recommend an initial bilateral screening to identify the arm with the higher pressure, which should then be used for all subsequent monitoring.
Factors Influencing BP Reading Accuracy
Accurate blood pressure measurement relies heavily on proper preparation and technique, as minor deviations can significantly skew the final reading. Before any measurement, the patient should sit quietly for at least five minutes with their back fully supported and their feet flat on the floor, without crossing their legs. Activities like exercise, caffeine consumption, or smoking should be avoided for at least 30 minutes prior to the measurement, as these can temporarily elevate blood pressure.
Correct positioning of the arm is important for obtaining a true reading that is not artificially inflated or deflated. The arm must be supported, and the blood pressure cuff should be positioned at the level of the heart, which typically aligns with the middle of the sternum. If the arm is allowed to hang down, the reading may be falsely elevated, and if the arm is held too high, the reading may be falsely low.
Using a correctly sized cuff is a significant factor, as an improperly fitted cuff can introduce errors of up to 30 mmHg. The inflatable bladder of the cuff should cover at least 80% of the upper arm’s circumference and be placed directly on bare skin, approximately one inch above the elbow crease. The cuff should be snug but not overly tight, and the patient should remain silent and still during the inflation and deflation process, as talking or moving can increase the systolic pressure.
Interpreting Arm-to-Arm Differences
While a small difference in blood pressure between the two arms is common and generally considered normal, a persistent and significant disparity can carry clinical meaning. Clinicians will often perform an initial bilateral screening, measuring both arms, to determine if a disparity exists. A systolic blood pressure difference of 10 to 15 mmHg or more between the arms is considered a significant finding that warrants further investigation.
This persistent difference may be an indication of an underlying circulatory issue that is impeding blood flow to one of the limbs. One potential cause is subclavian artery stenosis, which is a narrowing of the blood vessel supplying the arm, often caused by atherosclerosis or plaque buildup. The arm with the lower pressure is the one whose blood supply is compromised.
A consistent inter-arm difference is also associated with an increased long-term risk of cardiovascular events, including heart attack and stroke. Identifying this difference allows healthcare providers to use the arm with the higher reading for all future measurements, ensuring that any diagnosis of hypertension is based on the most accurate and representative pressure. Relying only on the lower reading could lead to a misdiagnosis or undertreatment of high blood pressure.