Blood pressure is the measurement of the force the blood exerts against the artery walls as the heart pumps. Blood pressure is measured in two numbers; the top number, or systolic pressure, is the force exerted when the heart beats. The lower number, the diastolic measurement, measures the pressure in the artery when the heart is between beats. Several tools are needed to measure blood pressure.
Blood Pressure Cuff
A blood pressure cuff consists of a bladder, or inflatable tube, inside of a or cloth sleeve. The cuff is part of the unit called the sphygmomanometer. The cuff wraps around the patient's upper arm, above the elbow, and has a Velcro edge so it can be held in place on the arm. The bladder is connected to a bulb that when squeezed causes the bladder to inflate. A valve on the bulb is closed during measurement so air won't escape from the bladder. The practitioner pumps the cuff up to 180 milligrams of mercury (mm Hg), or until no blood flow is heard in the artery, and then slowly deflates it by opening the valve a little at a time until the first sound of blood pumping though the artery is heard.
Cuffs must be accurately sized to the person's arm. A cuff that's too small may give an artificially high reading, the Mayo Clinic says, and one that's too large may give a slightly low reading. People whose arm measures 13 inches or more around need a large cuff, according to Vincent Maloney, M.D.; some people with an arm that's 11 to 12 inches may also need a large cuff, depending on the shape of the arm.
Sphygmomanometer Gauge
The sphygmomanometer gauge is attached to the blood pressure cuff by a tube. The gauge measures the pressure the cuff exerts to constrict the artery to the point that no blood flow is heard. As the pressure in the cuff is released, the blood is heard traveling through the artery; the point at which the sound is first heard is the systolic pressure. Blood pressure is measured as millimeters of mercury, even though many gauges no longer use mercury. They may instead have a dial that looks like a watch face. These are called aneroid sphygmomanometers.
Stethoscope
A stethoscope is a tool that magnifies sound within the body. It consists of two long slender tubes whose ends are placed in the practitioner's ears. The two tubes meet and converge into a single tube that leads to a flat metal piece called the diaphragm. This is the portion that amplifies sound during the measurement. The stethoscope is placed on the brachial artery in the crook of the elbow.
As pressure is released, the practitioner notes when the first sound is heard as the systolic pressure. The blood is heard pumping in a steady rhythm that gradually becomes fainter as more pressure is let out of the cuff, and then disappears. The point at which sound disappears is the diastolic pressure.
