Sphygmomanometer: An instrument for measuring blood pressure, particularly in arteries. The two types of sphygmomanometers are a mercury column and a gauge with a dial face. The sphygmomanometer in most frequent use today consists of a gauge attached to a rubber cuff which is wrapped around the upper arm and is inflated to constrict the arteries.
A blood pressure reading consists of two numbers: systolic and diastolic. Systolic refers to systole, the phase when the heart pumps blood out into the aorta. Diastolic refers to diastole, the resting period when the heart refills with blood. At each heartbeat, blood pressure is raised to the systolic level, and, between beats, it drops to the diastolic level.
With the cuff inflated with air, a stethoscope is placed over an artery (the brachial artery) in the crook of the arm. As the air in the cuff is released, the first sound heard through the stethoscope marks the systolic pressure. As the release of air from the cuff continues, a point is reached when the sound diminishes and then is no longer heard. The point where the sound disappears marks the diastolic pressure. The blood pressure reading might show the systolic and diastolic pressures to be, for example, 120 and 78mm of mercury (Hg) respectively -- written 120/78 and said to be "120 over 78."
A typical blood pressure reading for an adult might, in fact, be 120/78. Readings vary depending on age and many other factors. Children and adults with smaller or larger than average-sized arms may need special-sized pressure cuffs.
The sphygmomanometer was introduced in 1896 by the Italian physician Scipione Riva-Rocci (1863-1937). The American physiologist Joseph Erlanger (1874-1965) studied the principles of sphygmomanometry and devised a recording sphygmomanometer.