Measurement of Blood Pressure

Blood pressure is usually measured by an instrument called a sphygmomanometer. To measure the pressure, an inflatable cuff or wide band is placed around the patient’s upper arm and a stethoscope applied to the artery just below the cuff. By listening for changes in the sound of the pulse, the individual measuring the blood pressure knows how much to inflate the cuff, in order to stop blood from flowing into the arteries of the arm. Air is slowly let out off the cuff until the blood begins flowing again. At this stage, the sphygmomanometer records what is called the systolic blood pressure. Additional air is let out of the cuff until the sounds become muffled. The instrument then indicates the diastolic pressure. The systolic pressure corresponds to the contraction of the heart muscle, and the diastolic pressure corresponds to a relaxation of the heart. The two pressures are expressed in the following way: systolic/diastolic–120/80.