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Measurement of Arterial Blood Pressure


Systemic arterial blood pressure usually is measured using an instrument called a sphygmomanometer. This device consists of an inflatable rubber cuff connected by tubing to a compressible bulb and a glass tube containing a column of mercury. The bulb is used to pump air into the cuff, and a rise in the mercury column indicates the pressure produced. Thus, the pressure in the cuff can be expressed in millimeters of mercury. A pressure of 100 mm Hg, for example, would be enough to force the mercury column upward for a distance of 100 mm.

To measure arterial blood pressure, the cuff of the sphygmomanometer is usually wrapped around the arm so that it surrounds the brachial artery. Air is pumped into the cuff until the cuff pressure exceeds the pressure in that artery. As a result, the vessel is squeezed closed, and its blood flow stopped. At this moment, if the diaphragm of a stethoscope is placed over the brachial artery at the distal border off the cuff, no sounds can be heard from the vessel because the blood flow is interrupted. As air is slowly released from the cuff, the air pressure inside it decreases. When the cuff pressure is approximately equal to the systolic blood pressure within the brachial artery, the artery opens enough for a small amount of blood to spurt through. This movement produces a sharp sound (Korotkoff's sound) that can be heard through the stethoscope. The height of the mercury column when this first tapping sound is heard represents the arterial systolic pressure (SP).

As the cuff pressure continues to drop, a series of increasingly louder sounds can be heard. Then, when the cuff pressure is approximately equal to that within the fully opened artery, the sounds become abruptly muffled and disappear. The height of the mercury column when this happens represents the arterial diastolic pressure (DP).

The results of a blood pressure measurement are reported as a fraction, such as 120/80. In this notation, the upper number indicates the systolic pressure in mm Hg (SP), and the lower number indicates the diastolic pressure in mm Hg (DP).

The difference between the systolic and diastolic pressure (SP-DP), which is called the pulse pressure (PP), is generally about 40 mm Hg.

The average pressure in the arterial system is also of interest because it represents the force that is effective throughout the cardiac cycle for driving blood to the tissues. This force, called the mean arterial pressure, is approximated by adding the diastolic pressure and one-third of the pulse pressure (DP + 1/3PP).

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