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.
Lowering 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.