Blood pressure is measured with a device called a sphygmomanometer, which typically consists of a bulb, inflatable compression cuff, and gauge or digital readout. The doctor, nurse, or other practitioner wraps the cuff around your upper arm and squeezes the bulb to inflate the cuff. Then the air in the cuff is slowly released, which gradually deflates the cuff and lowers the pressure constricting your arm. Meanwhile, the doctor or nurse listens with a stethoscope, just below the cuff, for sounds from the artery in your arm.
Systolic pressure is indicated by the first sound heard through the stethoscope as the cuff deflates; diastolic pressure is the point at which the sound disappears. Many offices now use automated devices.
Normal blood pressure is less than 120/80. You have hypertension when your average systolic pressure (the first number) is 140 or higher and/or your average diastolic pressure (second number) is 90 or higher on at least two separate occasions. If your systolic number falls between 120 and 140 and/or your diastolic number is between 80 and 90, you have prehypertension, which puts you at elevated risk for eventually developing full-blown hypertension.