The aneroid sphygmomanometer is a non-electronic device that measures blood pressure, serving as a reliable tool for healthcare providers and individuals monitoring their health at home. The name “aneroid” means “without liquid,” distinguishing it from older sphygmomanometers that relied on a column of mercury. By avoiding mercury, the aneroid device is safer for the environment and more portable. This instrument requires manual inflation and the use of a stethoscope, offering a traditional method for determining both systolic and diastolic blood pressure readings.
Defining the Aneroid Device
The aneroid sphygmomanometer consists of several integrated physical parts. The inflatable cuff is a fabric band that wraps around the upper arm, containing an air-filled bladder that compresses the brachial artery when inflated. A rubber inflation bulb is attached to the cuff by tubing and is manually squeezed to pump air into the bladder. The bulb also incorporates an air release valve, which the user controls to slowly deflate the cuff and release the pressure.
The circular gauge, known as the manometer, displays the pressure in millimeters of mercury (mmHg). This gauge is connected to the cuff via tubing. Unlike digital devices, the aneroid manometer is purely mechanical, relying on an internal gear-driven mechanism to move the needle.
The Mechanism of Measurement
The core principle of the aneroid device involves temporarily stopping and then controlling the flow of blood through the brachial artery. When the cuff is manually inflated, the pressure exceeds the pressure inside the artery, which stops the blood flow. The air pressure from the cuff is simultaneously transmitted through the tubing to the manometer, causing the needle to move up the calibrated dial.
Inside the manometer, the air pressure pushes against a thin, corrugated metal bellows or capsule. This mechanical expansion is translated by a system of levers and gears to the needle on the face of the gauge. As the air is slowly released, the pressure in the cuff drops, and the needle descends. During this controlled deflation, the clinician uses a stethoscope placed over the artery to listen for specific sounds, known as Korotkoff sounds, which signal the moments of systolic and diastolic pressure.
Operating the Device Correctly
Taking an accurate blood pressure reading requires the user to master a coordinated technique. Proper preparation involves seating the subject with their arm resting at heart level. A correctly sized cuff is wrapped snugly around the upper arm, about one inch above the elbow crease. The stethoscope’s head is then placed lightly over the brachial artery just below the cuff’s edge.
The user closes the air release valve on the bulb and rapidly inflates the cuff to a pressure typically around 180 mmHg, or 20 to 30 mmHg above the point where the pulse is no longer felt. Next, the air release valve is opened slightly to allow the pressure to drop at a slow, controlled rate of about 2 to 3 mmHg per second. This slow deflation is critical for accurately identifying the distinct sounds.
The first faint, rhythmic thumping sound heard through the stethoscope marks the systolic pressure, which is the higher number, and the corresponding reading on the gauge should be noted immediately. As the pressure continues to drop, the sounds will change in character and then eventually disappear completely. The point on the gauge where the sounds vanish is the diastolic pressure, or the lower number, representing the pressure when the heart is at rest between beats.
Maintaining Accuracy
Unlike electronic blood pressure monitors, the mechanical nature of the aneroid sphygmomanometer means its accuracy can degrade over time due to wear on its internal gears and springs. Regular calibration is therefore necessary to ensure the readings remain reliable. Professionals often check the device’s accuracy every six months, or at least annually, by comparing its readings against a known, accurate reference device.
A simple check involves ensuring the needle rests exactly on the zero mark when the cuff is fully deflated. If the pointer is off-zero, the internal mechanism has likely been stressed and requires adjustment or replacement. The user should also protect the device from physical damage, as dropping the gauge can easily disrupt the delicate internal gear system. Storing the sphygmomanometer away from extreme temperatures and regularly inspecting the rubber tubing and cuff for cracks or air leaks will also help maintain its longevity and reliability.