A device for measuring
blood pressure. The most common instrument consists of an upright glass column
filled with mercury and calibrated in millimeters. To this is attached, by a length of plastic
or rubber tubing, an inflatable cuff designed to be wrapped around the patient's upper arm. Inflation
of the cuff by compression of a rubber
bulb applies pressure to the arm and the pressure in the cuff is recorded by the height to which the mercury rises in the calibrated glass tube (manometer).
The stethoscope is applied over the main artery of the arm just below the cuff. The cuff then is inflated to about 180 mm., as shown on the manometer. The pressure
in the cuff now exceeds the highest pressure
generated in the arteries
with each heartbeat. No blood will pass through
the artery and no sound will be heard. Lowering the pressure slowly, a point is reached
at which the blood
begins to flow through
the vessel with each heartbeat but the vessel
collapses between beats. The coming together of the vessel walls results in a sharp tapping noise, heard by the examiner. This point is described
as the systolic pressure
---the highest pressure generated by each heartbeat.
the pressure still further, a point is reached
where the blood flow becomes continuous and no sound can be heard. This point is described
as the diastolic pressure
. It represents the steady
pressure remaining in the arteries between heartbeats. The final result of the measurement is recorded in millimeters of mercury pressure---often expressed, for example, as 120/80 (indicating a systolic
pressure of 120 millimeters
of mercury and a diastolic pressure of 80).
Modern variants of the instrument use direct
reading manometers of the pressure may be electrically
recorded, but the underlying
principle remains the same.