A sphygmomanometer is the instrument used to measure blood pressure, representing the pressure exerted by circulating blood against the walls of the body’s arteries. While many people are familiar with automated digital cuffs, the manual device, often called an aneroid sphygmomanometer, requires specialized reading of a gauge. This traditional instrument consists of an inflatable cuff, a rubber bulb for inflation, and a round dial gauge that displays the pressure within the system. Understanding how to interpret the needle’s movements on this gauge is fundamental to accurately determining a person’s arterial pressure.
Deciphering the Gauge Markings
The circular face of the aneroid gauge is a manometer, calibrated to measure pressure in millimeters of mercury (mmHg), the standard unit for blood pressure readings worldwide. The scale typically ranges from 0 to 300 mmHg, covering all possible physiological pressures.
The gauge features two types of markings: major lines and minor lines. The major lines are numbered and usually spaced at 10 or 20 mmHg intervals, serving as the primary reference points for the measurement.
Between these major numbered lines are smaller, unnumbered marks. On most clinical aneroid gauges, these minor lines represent increments of 2 mmHg. For example, between the 120 and 130 marks, there are typically four smaller lines, allowing the reader to pinpoint pressures like 122, 124, 126, and 128 mmHg.
Identifying Systolic and Diastolic Readings
The process of identifying the two blood pressure numbers requires the use of a stethoscope and specific procedural steps, collectively known as the auscultatory method. The patient must be seated comfortably with their arm supported at heart level. The cuff must be properly sized and placed on the bare upper arm, positioned about one inch above the elbow crease, and the stethoscope’s diaphragm is placed lightly over the brachial artery.
The first step is to rapidly inflate the cuff until the pressure gauge reads approximately 20 to 30 mmHg higher than the point where the radial pulse can no longer be felt. This ensures the artery is fully occluded, temporarily stopping blood flow. A common initial target pressure is often around 180 mmHg.
Once the cuff is fully inflated, the valve is slowly opened to allow the pressure to decrease at a controlled rate, ideally about 2 to 3 mmHg per second. This steady deflation is necessary to precisely identify the moment when blood flow is restored. The listener simultaneously tracks the needle on the gauge while listening for the Korotkoff sounds through the stethoscope.
The systolic reading is the first number recorded, marked by the appearance of the Korotkoff Phase 1 sound. This sound is characterized as a faint, repetitive tapping, indicating that the arterial pressure during the heart’s contraction phase is finally exceeding the cuff pressure. The gauge reading at this exact moment is the systolic pressure.
Continuing the slow release of pressure, the Korotkoff sounds will change in quality. The diastolic reading, the second number, is identified by the point at which the Korotkoff sounds completely disappear (Korotkoff Phase 5). This silence signifies that the artery is no longer compressed, and blood flow has returned to a smooth pattern. The number on the gauge when the last sound is heard before silence is the diastolic pressure.
Interpreting the Blood Pressure Values
Once the two numbers have been accurately recorded from the gauge, they are expressed as a fraction, with the higher systolic pressure over the lower diastolic pressure (e.g., 120/80 mmHg). Each number represents a different aspect of the heart’s cycle.
Systolic pressure measures the peak force in the arteries when the heart muscle contracts, pushing blood out into the circulatory system. Diastolic pressure, in contrast, measures the minimum pressure in the arteries when the heart is resting between beats and refilling with blood.
According to current guidelines, these numerical results are classified into distinct categories to assess cardiovascular health. A normal blood pressure reading is defined as a systolic pressure below 120 mmHg and a diastolic pressure below 80 mmHg. Readings that fall into the elevated category show a systolic pressure between 120 and 129 mmHg, with the diastolic pressure remaining below 80 mmHg.
Hypertension, or high blood pressure, is categorized into two stages. Stage 1 hypertension is diagnosed when systolic pressure is between 130 and 139 mmHg or diastolic pressure is between 80 and 89 mmHg. Stage 2 hypertension is defined by a systolic pressure of 140 mmHg or higher, or a diastolic pressure of 90 mmHg or higher. A systolic reading of 180 mmHg or higher, or a diastolic reading of 120 mmHg or higher, signifies a hypertensive crisis, requiring immediate medical attention.