Manual blood pressure measurement, known as auscultation, uses a sphygmomanometer and a stethoscope to determine the force exerted by circulating blood against artery walls. This method requires a trained ear to detect subtle changes in blood flow sounds corresponding to the pressure readings. The process involves temporarily stopping blood flow in a major artery and then slowly releasing the pressure while listening for the specific noises that indicate the systolic and diastolic pressures. Correctly positioning the stethoscope is fundamental to hearing these low-frequency sounds accurately and obtaining a reliable reading.
Identifying the Brachial Artery Pulse Point
The precise location for stethoscope placement is directly over the brachial artery, the main blood vessel in the upper arm. This artery runs down the inner side of the arm and is most accessible in the antecubital fossa, the triangular area at the front of the elbow. To prepare for the measurement, a healthcare provider first uses their fingertips to gently palpate (feel) for the brachial pulse on the medial side of the arm, slightly above the elbow crease.
Finding the strongest pulse point by touch is a necessary preliminary step to ensure the stethoscope head is positioned exactly over the artery’s path. This manual confirmation guarantees optimal sound transmission when the cuff is inflated and deflated. The artery lies relatively close to the surface here, making it an ideal site for listening to the turbulent blood flow created during the measurement. Once the pulse is identified, the stethoscope head is positioned directly over this spot on the bare skin.
Preparing the Arm and Positioning the Cuff
Before measurement, the person should be seated comfortably with their back supported and their feet flat on the floor, ideally resting for five minutes. The arm chosen should be bare, free of restrictive clothing, and supported at the level of the heart. Positioning the arm at heart level is important because deviations can cause hydrostatic pressure errors, leading to falsely high or low readings.
The blood pressure cuff must be the correct size for the individual’s arm circumference to ensure an accurate reading. The cuff is snugly wrapped around the upper arm, ensuring the artery marker is centered directly over the palpable brachial artery. The bottom edge of the cuff should be positioned approximately one to two inches (2.5 to 5 cm) above the antecubital crease, leaving space for the stethoscope head. This placement ensures the cuff’s bladder is correctly situated to compress the artery when inflated.
Stethoscope Placement Technique and Pressure
Once the cuff is correctly positioned and the brachial pulse identified, the stethoscope’s chestpiece is placed over that pulse point, just below the lower edge of the cuff. Although the bell is sometimes suggested for low-frequency sounds, the diaphragm is also commonly used and provides comparable results. Either side must be placed directly on the skin without intervening clothing to prevent sound distortion from friction or muffling.
The pressure applied with the stethoscope head must be light yet firm enough to create an acoustic seal against the skin. Pressing too hard is a common error that can artificially compress the brachial artery, which may create faint sounds or distort the natural sounds of blood flow. Excessive pressure can lead to an inaccurate or falsely low reading, particularly affecting the diastolic measurement. The tubing should hang freely, avoiding contact with the cuff or other objects that might introduce extraneous noise.
Hearing the Sounds of Blood Flow
The sounds heard through the stethoscope during cuff deflation are called Korotkoff sounds, generated by the turbulent flow of blood through the partially compressed artery. Initially, the cuff is inflated to a pressure high enough to completely occlude the artery, typically 20 to 30 millimeters of mercury (mmHg) above the point where the pulse disappears. At this maximum pressure, no sound is heard because blood flow has stopped.
As the cuff pressure is slowly and steadily released at a rate of about 2 to 3 mmHg per second, the first Korotkoff sound appears. This clear, repetitive tapping sound, known as Phase I, corresponds to the systolic blood pressure reading. The sound occurs when the pressure in the artery during the heart’s contraction (systole) is finally able to force blood past the cuff.
The sounds then progress through several phases, becoming swishing, crisper, and eventually muffled. The final event is the complete disappearance of the sound, which marks the transition back to smooth, laminar blood flow through the fully open artery. This point, known as Phase V, is recorded as the diastolic blood pressure. Noting the pressure at the first sound and the final disappearance of sound provides the two numbers that constitute the blood pressure reading.