The second, or bottom, number is called the diastolic blood pressure and represents the pressure in your arteries while your heart relaxes between beats.
Both numbers are important and elevation in one, or both, are considered when making the diagnosis of hypertension.
How Is Blood Pressure Measured?
Blood pressure is measured with a sphygmomanometer, more commonly called a blood pressure cuff. The cuff is placed around the upper arm just above the elbow and inflated, then deflated while listening with a stethoscope over the artery in the arm. Home blood pressure machines may use a cuff attached to a machine but without the need of a stethoscope.
Blood pressure should be measured after a person has sat quietly for at least 5 minutes and not had caffeine, tobacco or exercised for at least 30 minutes. A proper size cuff should be used. Blood pressure can be taken on either arm and is often taken on both arms for comparison. The pressure in the dominant arm may be slightly higher. The difference between the two arms is generally 10 mm Hg or less.
The diagnosis of hypertension should be confirmed with measurements made on several days, although if severely elevated, treatment may be initiated after one reading. It is often helpful to have a special monitor that a person takes home and records blood pressure for 24 hours.