Blood pressure is the force exerted by circulating blood against the walls of the body’s arteries as the heart pumps it through the circulatory system. This measurement, expressed in millimeters of mercury (mmHg), is a standard vital sign used to evaluate a person’s overall health status. Consistently elevated blood pressure, known as hypertension, is a major risk factor for serious health complications like stroke, heart disease, and kidney failure. Monitoring blood pressure regularly provides an early indication of potential cardiovascular issues, allowing for timely intervention. The reading almost always refers to the pressure found within the brachial artery in the upper arm.
Why the Brachial Artery is the Standard Site
The brachial artery is the standard location for non-invasive blood pressure measurement due to physiological and practical factors. This large vessel runs along the inner side of the upper arm, making it easily accessible. Its location allows a compression cuff to be placed around the limb, where the artery can be reliably compressed against the humerus bone.
Measuring pressure in the brachial artery provides a close, reliable estimate of the central aortic pressure, which is the pressure experienced by major organs like the brain, heart, and kidneys. Although the systolic pressure measured here can be slightly higher than the central aortic pressure, the diastolic and mean pressures remain relatively constant throughout the arterial tree. This consistency makes the brachial reading a clinically validated surrogate for systemic blood pressure, and the ease of applying a cuff at the elbow crease solidifies its role as the standard.
The Procedure for Measuring Brachial Blood Pressure
The manual measurement of brachial blood pressure, known as the auscultatory method, uses a blood pressure cuff (sphygmomanometer) and a stethoscope. Before the procedure, the patient should be seated quietly with their back supported and their bare arm resting on a surface at the level of the heart for at least five minutes. Proper cuff selection is necessary, as the inflatable bladder must cover at least 80% of the arm’s circumference.
The cuff is wrapped snugly around the upper arm, with the bladder centered over the brachial artery near the elbow crease. The cuff is rapidly inflated to a pressure high enough to completely stop blood flow, usually 30 mmHg above the point where the pulse disappears. The practitioner then places the stethoscope bell over the artery and slowly releases the pressure at a rate of 2 to 3 mmHg per second.
As the cuff pressure drops, the practitioner listens for Korotkoff sounds, which are caused by turbulent blood flow returning to the artery. The first distinct tapping sound marks the systolic pressure reading (Phase I). As the cuff continues to deflate, the sounds become muffled, and the point at which they disappear completely is recorded as the diastolic pressure (Phase V). Automated devices, which are common today, use an oscillometric method that detects pressure waves in the artery wall instead of listening for these sounds.
Understanding Systolic and Diastolic Readings
A blood pressure measurement is always expressed as two numbers, such as 120/80 mmHg. The first and higher number is the systolic pressure, which represents the maximum force exerted on the artery walls when the heart contracts and pushes blood out. This peak pressure reflects the heart’s pumping effort.
The second, lower number is the diastolic pressure, which measures the residual pressure in the arteries when the heart is at rest and refilling between beats. This pressure indicates the constant force sustained by the blood vessels throughout the cardiac cycle. Both numbers are necessary for a complete assessment, though high systolic pressure is often a strong predictor of cardiovascular risk, particularly in older adults.
Health organizations classify blood pressure readings into specific categories to guide diagnosis and treatment. A normal reading is defined as a systolic pressure of less than 120 mmHg and a diastolic pressure of less than 80 mmHg. Readings between 120–129 mmHg systolic and less than 80 mmHg diastolic are categorized as elevated. Hypertension is diagnosed when the systolic pressure is 130 mmHg or higher, or the diastolic pressure is 80 mmHg or higher, which falls into Stage 1, with higher sustained readings moving into Stage 2 hypertension.
Factors Affecting Brachial Artery Measurement Accuracy
Several variables can influence the accuracy of a brachial blood pressure reading, potentially leading to misdiagnosis. Patient preparation requires avoiding caffeine, exercise, and smoking for at least 30 minutes before the measurement. The patient must also be calm and have an empty bladder, as a full bladder can artificially increase the reading.
The size of the blood pressure cuff is a common source of error. A cuff that is too small for the arm circumference will produce a falsely high reading, while an oversized cuff can result in an inaccurately low reading. Additionally, the arm position matters; if the cuff is not supported at the level of the heart, the measurement will be skewed by hydrostatic forces.
Current guidelines recommend measuring blood pressure in both arms at the initial visit to detect any inter-arm difference. A consistent difference in systolic pressure of 10 to 15 mmHg or more between the arms may indicate a vascular issue, such as a narrowing of the artery in the arm with the lower reading, which is associated with increased cardiovascular risk.