Measuring blood pressure in the arteries is a simple medical procedure that almost all people have experienced. A constricting cuff is wrapped around the upper arm. A stethoscope is used to listen for the flow of blood in the brachial artery near the pit of the elbow. The bulb is used to force air into the cuff, and the cuff pressure is measured using a gauge. The bulb is pumped until a pressure of 180 mm of mercury is attained. For most people, blood will not flow through the arm when the cuff is inflated to this pressure. The pressure is slowly released using a valve at the top of the bulb. During this time, the clinician listens through the stethoscope for the flow of blood. The first knocking sound, called a Korotkoff sound, occurs when the cuff pressure is just below the peak pressure of the blood.
This is known as the subject's systolic pressure. At this pressure range the blood only flows intermittently. As the pressure in the cuff continues to drop, eventually the knocking sound will disappear. This is an indication that blood in continuously flowing and the cuff pressure is below the lower range of the person’s blood pressure. This is known as the diastolic pressure. Blood pressure is reported as the systolic pressure over the diastolic pressure, 120/80, for example. Note: 120/80 is considered to be normal arterial blood pressure.
