The manual method of measuring blood pressure (BP), known as auscultation, relies on a healthcare professional’s trained ear and is often considered the most accurate approach compared to automated devices. This technique, using a stethoscope and a cuff, provides a direct assessment of the pressure exerted by circulating blood against the artery walls. Blood pressure is recorded as two numbers: the systolic pressure, the higher number representing the force when the heart beats, and the diastolic pressure, the lower number reflecting the pressure when the heart is at rest between beats. The ability to precisely identify the sounds that correlate to these pressures gives the manual reading its enduring reputation for accuracy.
Essential Tools and Preparation
Manual blood pressure measurement fundamentally relies on two devices working in tandem: the sphygmomanometer and the stethoscope. The sphygmomanometer consists of an inflatable cuff, a rubber bulb for inflation, and a pressure gauge, typically an aneroid manometer, which measures the pressure in millimeters of mercury (mmHg). The stethoscope is used to listen to the sounds created by the blood flow within the artery beneath the cuff. Selecting the correct cuff size is a foundational step for accuracy; a cuff that is too small will artificially elevate the reading, while one that is too large will yield a falsely low result. The inflatable bladder inside the cuff should properly encircle 80% of the patient’s arm circumference and cover two-thirds of the upper arm.
Preparing the patient and the environment is equally important before the measurement begins. The patient should be seated comfortably with their back supported and their feet flat on the floor, not crossed, which can artificially raise the pressure. The arm used for the measurement must be bare, supported at the level of the heart, with the palm facing upward. A quiet environment is necessary so the nurse can clearly hear the subtle sounds that determine the pressure readings. The patient should also have rested quietly for at least five minutes prior to the measurement.
The Mechanics of Measurement
The physical procedure begins with the correct placement of the cuff over the patient’s upper arm, specifically positioning the cuff’s center over the brachial artery. The bottom edge of the cuff is secured approximately 2.5 centimeters above the crease of the elbow. To avoid an inaccurate reading known as the auscultatory gap, the nurse first finds the radial pulse and notes the pressure at which the pulse disappears upon initial cuff inflation.
The cuff is then inflated rapidly using the rubber bulb until the pressure gauge reads about 20 to 30 mmHg above this estimated systolic pressure. This high pressure briefly and completely occludes the blood flow through the brachial artery. The nurse then places the stethoscope’s bell or diaphragm gently over the brachial artery at the elbow crease. The controlled deflation of the cuff is a precise action, requiring the pressure to be released slowly and evenly at a rate of approximately 2 to 3 mmHg per second. This steady, controlled release allows for the accurate detection of the subtle sounds that mark the beginning and end of the blood pressure cycle.
Deciphering the Korotkoff Sounds
The sounds a nurse listens for during the slow deflation process are called Korotkoff sounds, which are generated by the turbulent flow of blood returning to the artery. When the cuff pressure exceeds systolic pressure, the artery is fully collapsed, and no sound is heard. As the pressure in the cuff drops, blood begins to surge past the constriction, creating five distinct audible phases:
- Phase I: The first appearance of faint, clear, repetitive tapping sounds, which marks the systolic pressure. This tapping occurs when the arterial pressure during the heart’s contraction exceeds the cuff pressure.
- Phase II: A brief period where the sounds soften and take on a swishing or murmuring quality as the vessel further opens.
- Phase III: Sharper, crisper sounds that often increase in intensity compared to Phase I.
- Phase IV: A distinct muffling and softening of the sounds, described as soft and blowing, which indicates a significant decrease in blood flow turbulence.
- Phase V: The point where all sounds completely disappear, which marks the standard diastolic pressure.
The disappearance of sound in Phase V signifies that the cuff pressure is no longer great enough to compress the artery, allowing for smooth, laminar blood flow, which is silent. While Phase V is the standard for diastolic pressure, Phase IV is sometimes used in specific cases where the sounds might not fully disappear.
Ensuring Accurate Readings
The reliability of a manual blood pressure reading depends heavily on the nurse avoiding common procedural errors that can skew the result. Deflating the cuff too quickly, faster than the recommended 2 to 3 mmHg per second, can cause the nurse to miss the true systolic and diastolic points. Using a cuff that is the wrong size or placing the cuff over clothing introduces measurement error, potentially leading to inaccurate diagnosis or treatment.
Physiological factors related to the patient’s state can also significantly impact the reading. The phenomenon known as “white coat hypertension,” where anxiety in a clinical setting temporarily raises blood pressure, can lead to a falsely high measurement. Similarly, the recent consumption of caffeine, nicotine, or engaging in physical activity within 30 minutes of measurement can elevate the pressure. A full bladder can also increase systolic pressure by a notable amount. To confirm the true pressure, if the first reading is elevated, nurses are advised to wait a few minutes and take at least one more measurement, often averaging the results.