Manual blood pressure (BP) measurement uses a sphygmomanometer (pressure gauge) and a stethoscope to determine circulatory force. This technique, known as auscultatory measurement, relies on listening for specific sounds of blood flow within an artery. The standard and most accurate location for this procedure is the upper arm, where the cuff is placed over the brachial artery. While the upper arm is the preferred site, circumstances sometimes require a reliable alternative, leading to the use of the forearm method.
Situations Requiring Forearm Blood Pressure Measurement
Measuring blood pressure on the forearm is necessary when the standard upper arm site is unavailable or contraindicated due to anatomical or medical conditions. One common reason is the presence of an arteriovenous fistula or shunt, often used for dialysis access, which makes compression of the upper arm artery unsafe. Procedures like a unilateral or bilateral mastectomy, especially when accompanied by lymph node dissection, frequently lead to lymphedema, making the affected arm unsuitable for pressure application.
Other contraindications include extensive injury, a cast, or a bulky intravenous (IV) line access site in the upper extremity. For individuals with extreme obesity or a significantly tapered upper arm shape, a correctly sized cuff may not fit properly or may yield an artificially high or low reading. In these cases, the forearm provides a viable site for cuff application, but the clinician must note the alternate location for accurate interpretation.
Preparation and Locating the Radial Artery
Before beginning the measurement, gather the equipment, including a manual sphygmomanometer and a stethoscope. The patient should be seated comfortably with their forearm exposed, resting on a supportive surface. Ensure the chosen site is positioned at the level of the heart, typically the approximate level of the sternum, to prevent hydrostatic pressure errors.
Proper cuff selection is important. A standard adult cuff, often designed for the upper arm, must be sized appropriately to wrap snugly around the forearm. The technique requires locating the radial artery, which is situated on the thumb side of the wrist, where the pulse is commonly felt.
Detailed Manual Forearm Measurement Procedure
The blood pressure cuff is wrapped around the forearm, with the lower edge positioned a few centimeters above the wrist crease. The cuff bladder must be centered over the radial artery. To ensure the cuff is inflated high enough to occlude the artery and avoid missing the auscultatory gap, the systolic pressure must first be estimated using the palpation method.
While feeling the radial pulse, inflate the cuff rapidly until the pulse is no longer palpable; note this pressure on the manometer. Fully deflate the cuff, allowing the patient to rest for approximately 30 seconds before proceeding. For the final measurement, quickly re-inflate the cuff to a pressure 20 to 30 millimeters of mercury (mmHg) higher than the estimated systolic value.
Place the stethoscope lightly over the radial artery pulse point, avoiding firm pressure that could artificially alter the sounds. Release the pressure slowly and steadily, maintaining a deflation rate of about 2 to 3 mmHg per second. The first distinct, repetitive tapping sound heard (the first Korotkoff sound) marks the systolic pressure. Deflation continues until the sounds completely disappear (the fifth Korotkoff sound), which is recorded as the diastolic pressure.
Factors Affecting Forearm Reading Accuracy
Forearm blood pressure readings are less accurate than those taken at the brachial artery and are not interchangeable with upper arm values. Systolic pressure measured in the forearm may register 3 to 9 mmHg higher than the upper arm measurement. Diastolic pressure discrepancies are also common, sometimes appearing 3 to 13 mmHg higher. These differences occur due to pulse pressure amplification as blood flows farther from the heart into smaller, peripheral arteries.
Accuracy is significantly diminished if the forearm is not properly supported at the level of the heart, with errors potentially reaching 10 mmHg for both systolic and diastolic values if the arm is allowed to drop. Incorrect cuff sizing, even on the forearm, can skew the results, leading to an artificially high reading if the cuff is too small. When using the forearm site, document the location and consistently use the same arm and site for all subsequent comparative measurements.