A sphygmomanometer is the medical device used for the manual measurement of blood pressure, relying on an inflatable cuff, a rubber bulb, and a pressure gauge (manometer) to display the reading in millimeters of mercury (mmHg). This manual, auscultatory method, often using a stethoscope, remains the preferred technique in many clinical environments. When performed correctly, this approach is highly accurate because it directly measures the pressure points where the turbulent sounds of blood flow begin and cease, providing a reliable physiological assessment.
Ensuring Patient Readiness and Proper Setup
Achieving an accurate blood pressure reading requires careful attention to patient preparation. The patient should be seated comfortably with their back fully supported and feet flat on the floor, avoiding crossed legs. They must rest quietly for a minimum of five minutes prior to the measurement and refrain from speaking or moving during the process.
The arm selected for measurement must be bare, as clothing can compress the artery and lead to an inaccurate reading. The arm should be fully supported on a flat surface, positioned so the middle of the upper arm is level with the heart. If the arm hangs lower than heart level, the reading can be artificially increased by as much as 10 mmHg due to hydrostatic pressure.
Selecting the correct cuff size directly influences measurement accuracy, as errors of up to 30 mmHg are possible if the wrong size is used. The inflatable bladder should cover approximately 80% of the arm circumference and 40% of the mid-upper arm circumference in width. A cuff that is too small yields a falsely high reading, while one that is too large results in a falsely low reading.
To place the cuff correctly, the practitioner must first locate the brachial artery pulse point on the inner side of the arm. The bottom edge of the cuff should be positioned about one inch (two to three centimeters) above the elbow crease. An arrow or marker on the cuff must be aligned precisely over the brachial artery, and the cuff should be wrapped snugly.
Step-by-Step Manual Measurement Technique
The core of the manual measurement technique is the auscultatory method, which relies on using a stethoscope to hear specific sounds. The diaphragm or bell of the stethoscope is placed lightly over the brachial artery, just below the cuff. The bell is often preferred because it better transmits the low-pitched sounds created by blood flow turbulence.
Before the final measurement, it is standard practice to estimate the systolic pressure by palpation to avoid missing the auscultatory gap. This involves inflating the cuff while feeling the radial pulse until it disappears, noting the pressure, and then deflating the cuff completely. This estimated pressure determines the necessary maximum inflation for the actual reading.
The cuff is then rapidly inflated to a pressure 20 to 30 mmHg higher than the estimated systolic pressure, ensuring the artery is fully occluded. Once the target is reached, the valve is opened to begin a slow, controlled deflation. The pressure must be released at a steady rate of about two to three mmHg per second.
The slow deflation rate allows the practitioner to accurately pinpoint the exact pressure at which the turbulent sounds of blood flow, known as Korotkoff sounds, begin and end. The first distinct, repetitive tapping sound heard through the stethoscope is recorded as the systolic pressure, corresponding to Korotkoff Phase I. At this pressure, the blood is just beginning to push through the constricted artery. As the cuff pressure continues to drop, the sounds will become louder, then muffled, and finally disappear entirely. The point at which the sounds disappear is recorded as the diastolic pressure, corresponding to Korotkoff Phase V. This marks the moment when blood flow through the artery becomes smooth again because the cuff no longer constricts the vessel. Once both readings are noted, the cuff should be fully and quickly deflated.
Decoding Systolic and Diastolic Values
The two numbers recorded from the manual measurement reflect the pressure exerted on the artery walls at two distinct points in the cardiac cycle. The higher number, the systolic pressure, represents the maximum force of blood against the vessel walls when the heart’s ventricles contract. This is the peak pressure achieved during the heartbeat cycle. The lower number, the diastolic pressure, represents the minimum pressure in the arteries when the heart relaxes and refills with blood between beats. Both readings are expressed in millimeters of mercury (mmHg) and are written as a fraction, such as 120/80 mmHg.
The recorded values are used to classify a person’s blood pressure status, providing a general indication of cardiovascular health. A reading under 120/80 mmHg is generally considered normal for adults. Elevated blood pressure is categorized as a systolic pressure between 120 and 129 mmHg with a diastolic pressure less than 80 mmHg. Hypertension is categorized into stages, with Stage 1 starting at 130–139 mmHg systolic or 80–89 mmHg diastolic.