Blood pressure (BP) is recorded as two numbers: systolic pressure (the force when your heart beats) and diastolic pressure (the pressure when your heart rests between beats). Measuring BP is a standard part of any medical assessment, as it is a fundamental indicator of cardiovascular health. Current medical guidelines recommend measuring blood pressure in both arms during an initial assessment. A difference in the readings between the left and right arm is a common observation that often prompts questions about its significance.
Expected Variation and Measurement Accuracy
Physiological Variation
Small differences in systolic blood pressure between arms are a frequent and generally benign finding. A systolic difference of up to 10 millimeters of mercury (mmHg) is commonly considered the upper limit of normal due to natural physiological variation. This minor variation occurs due to slight, non-pathological differences in the anatomy of the blood vessels supplying each arm. These subtle anatomical variations can cause the blood flow and pressure to differ slightly, even in a healthy person.
Measurement Error
Many observed differences are not biological but are instead artifacts of the measurement technique itself. Using an incorrect cuff size can significantly skew a reading; a cuff that is too small usually results in an artificially high pressure. Improper positioning, such as having the arm below heart level or not resting properly before the measurement, also affects accuracy. Furthermore, when measurements are taken sequentially, a temporary drop in blood pressure can occur, making the second arm appear to have a lower pressure.
Medical Conditions Causing Large Differences
Subclavian Artery Stenosis
A persistent and larger difference in systolic blood pressure, typically 10 to 15 mmHg or more, often suggests an underlying medical issue affecting the arterial system. The most common pathological cause is Subclavian Artery Stenosis, a condition where the subclavian artery, which supplies blood to one arm, becomes narrowed due to the buildup of plaque. This narrowing reduces blood flow to the arm, causing the blood pressure reading on that side to be lower than the other.
Other Vascular Diseases
Other serious vascular diseases can also manifest as a significant inter-arm difference. Peripheral Artery Disease (PAD) can affect the arteries in the arms, causing a notable difference in pressure if one arm’s vessels are more severely blocked than the other. Conditions involving the body’s main artery, the aorta, may also be signaled by this discrepancy. These include Aortic Dissection (a tear in the aorta’s inner layer) and Aortic Coarctation (a congenital narrowing), both of which disrupt blood flow unevenly to the arteries branching toward the arms.
What a Significant Difference Indicates for Your Health
Risk Indicator
When the systolic blood pressure difference exceeds the 10-15 mmHg threshold, it becomes an important marker for future health risks. A large, persistent difference is recognized as an independent predictor of adverse cardiovascular events, including heart attack, stroke, and all-cause mortality. This risk remains elevated even after accounting for other traditional risk factors like high cholesterol or smoking. The magnitude of the difference correlates with the severity of the risk, with a difference of 15 mmHg or more being a strong indicator of vascular disease.
Management and Monitoring
If a significant disparity is detected, the first step is to confirm the reading using proper technique, typically by taking multiple measurements in both arms. The arm with the higher pressure should be used for all subsequent monitoring and for guiding hypertension treatment, as using the lower reading may lead to undertreatment. To investigate the cause, a physician may order diagnostic tests, such as an ultrasound of the arm arteries, to check for subclavian stenosis or other blockages. Addressing the underlying vascular disease and managing cardiovascular risk factors is the actionable step indicated by this finding.