Why Is Brachial Artery Blood Pressure the Standard?

Blood pressure is the force of blood pushing against the walls of blood vessels as the heart pumps. This measurement is used to evaluate a person’s general health. When blood pressure is measured, it is the pressure within the brachial artery in the upper arm that is assessed. This site has become the standard because it provides a consistent and reliable indicator of cardiovascular function.

The Brachial Artery as the Standard Site

The brachial artery is a major blood vessel in the upper arm. Its selection as the standard for blood pressure measurement is due to its convenient location and its anatomical relationship to the heart. The artery is easily accessible, allowing a blood pressure cuff to be wrapped smoothly and securely around the upper arm.

Physiologically, the brachial artery’s proximity to the aorta, the main artery leaving the heart, is an advantage. Pressure measured in the brachial artery provides a reasonable estimate of the central aortic pressure, which is the pressure experienced by the body’s major organs. While not identical, the brachial pressure is a well-established surrogate that has been used in large-scale studies to determine cardiovascular risk. This long history has solidified its role in clinical practice for diagnosing and managing conditions like hypertension.

The Measurement Process

Blood pressure measurement is performed using two primary techniques: the auscultatory method and the oscillometric method. The auscultatory method is the traditional technique, requiring a stethoscope and a sphygmomanometer, which consists of an inflatable cuff and a pressure gauge. A healthcare provider inflates the cuff to stop blood flow in the brachial artery and then slowly releases the pressure while listening for specific sounds, known as Korotkoff sounds.

The oscillometric method is used by most automated digital devices. These devices automatically inflate and deflate the cuff, detecting pressure oscillations in the arterial wall to calculate blood pressure. This method reduces the potential for human error associated with the manual technique and has become widely used in both clinical and home settings.

For an accurate reading with either method, proper patient preparation and positioning are necessary. The individual should be seated quietly for at least five minutes with their back supported and feet flat on the floor. The arm should be bare and supported, with the cuff placed so its midpoint is at the level of the heart. Using a cuff that is the wrong size for the person’s arm can lead to inaccurate results; a cuff that is too small may give a falsely high reading.

Interpreting the Readings

Blood pressure results are given as two numbers, representing systolic and diastolic pressures, measured in millimeters of mercury (mmHg). The top number, systolic pressure, measures the force the heart exerts on the walls of the arteries each time it beats. The bottom number, diastolic pressure, measures the pressure in the arteries when the heart is at rest between beats.

According to the American Heart Association, a normal reading is less than 120/80 mmHg. Readings with a systolic pressure between 120–129 mmHg and a diastolic pressure less than 80 mmHg are categorized as elevated.

Stage 1 hypertension is defined by a systolic pressure of 130–139 mmHg or a diastolic pressure of 80–89 mmHg. Stage 2 hypertension involves readings where the systolic pressure is 140 mmHg or higher, or the diastolic pressure is 90 mmHg or higher. A hypertensive crisis is a severely high reading above 180/120 mmHg that requires immediate medical attention.

Comparison with Other Measurement Locations

While the brachial artery is the standard, blood pressure can be measured at other locations, most commonly the wrist’s radial artery. Wrist cuffs are often used for convenience or when an upper arm cuff cannot be used, such as in cases of injury, the presence of intravenous lines, or for individuals with very large arms. However, these devices are considered less accurate than upper arm monitors.

The radial artery is narrower and not as deep under the skin as the brachial artery, which can make obtaining a reliable reading more difficult. Pressure naturally increases in arteries that are farther from the heart, meaning systolic readings at the wrist are often higher than those taken at the upper arm. On average, radial systolic pressure can be about 5.5 mmHg higher than brachial systolic pressure.

To achieve the best accuracy with a wrist monitor, correct positioning is important. The wrist must be held at heart level during the measurement to avoid errors caused by gravity. For this reason, healthcare providers recommend validating a wrist monitor’s readings against a brachial measurement taken in a clinical setting. In specific medical situations, pressure may also be measured at the ankle, as part of the ankle-brachial index test to screen for peripheral artery disease.