A manual blood pressure cuff, technically known as a sphygmomanometer, is used alongside a stethoscope to measure the force of blood moving through the arteries. This auscultatory method is often considered the most accurate way to measure blood pressure. The device consists of an inflatable cuff, a pressure gauge (manometer), and an inflation bulb with a release valve. Blood pressure itself is a two-part measurement, expressed in millimeters of mercury (mmHg), which quantifies the pressure exerted by circulating blood against the vessel walls.
Preparing for an Accurate Reading
Achieving a reliable blood pressure reading begins with meticulous preparation, as errors in setup can significantly skew the results. The individual should be seated quietly for at least five minutes with their back fully supported and their feet flat on the floor, not crossed. This resting period is crucial because physical exertion or anxiety can temporarily elevate blood pressure.
The arm chosen for the reading must be bare, as placing the cuff over clothing can erroneously increase the reading by 5 to 50 mmHg. The arm should be supported on a table or surface so that the middle of the cuff is positioned at the level of the heart, specifically corresponding to the right atrium. Failure to support the arm at heart level can lead to an artificially high reading, sometimes by as much as 10 mmHg.
Selecting the correct cuff size is another factor directly affecting accuracy. A cuff that is too small results in an artificially high reading, while one that is too large can produce a false low. The inflatable bladder should cover approximately 80% of the arm’s circumference. The cuff is applied snugly to the bare upper arm, positioned about one inch (2.5 cm) above the elbow crease.
The Step-by-Step Measurement Procedure
The measurement process begins by locating the brachial artery, which is the major vessel in the upper arm. This artery is found on the inside of the arm, just above the elbow crease. Once the pulse is located by palpation, the stethoscope head is placed directly over this spot.
Before inflation, estimate the systolic pressure to avoid over-inflating the cuff and causing discomfort. This is done by palpating the radial pulse while rapidly inflating the cuff, noting the pressure where the pulse disappears. The cuff is then fully inflated to a pressure 20 to 30 mmHg higher than this estimated systolic pressure.
Next, the air is slowly released from the cuff using the valve, ensuring a controlled deflation rate of approximately 2 to 3 mmHg per second. As the pressure drops, turbulent blood flow creates tapping sounds known as Korotkoff sounds. The pressure reading on the manometer when the very first clear, repetitive tapping sound is heard marks the systolic pressure (Korotkoff Phase I).
The slow deflation continues as the sounds go through various phases of muffling. The diastolic pressure is recorded at the point where the sounds completely disappear (Korotkoff Phase V). Once the diastolic reading is noted, the remaining air in the cuff should be released quickly to restore normal blood flow.
Understanding and Interpreting the Results
The result is documented as two numbers, written as a fraction, such as 120/80 mmHg. The first, higher number is the systolic pressure, reflecting the maximum force exerted on artery walls when the heart contracts. The second, lower number is the diastolic pressure, representing the lowest force exerted when the heart rests between beats.
These two numbers are categorized to provide context for cardiovascular health. A reading is considered Normal if systolic pressure is less than 120 mmHg and diastolic pressure is less than 80 mmHg. An Elevated reading is 120–129 mmHg systolic and less than 80 mmHg diastolic.
Hypertension begins with Stage 1, defined as 130–139 mmHg systolic or 80–89 mmHg diastolic. Stage 2 Hypertension is 140 mmHg or higher systolic or 90 mmHg or higher diastolic. A diagnosis is typically based on the average of two or more readings taken on separate occasions.