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Blood pressure is expressed in millimeters of mercury, abbreviated as mm Hg.

The systolic blood pressure is the top number of a blood pressure reading. This shows the maximum pressure in the blood vessels as the heart contracts and circulates blood throughout the body.

The diastolic blood pressure is the bottom number of a blood pressure reading. It shows the lowest pressure in the blood vessels between heartbeats, when the heart is at rest.

How is the test performed?

A person is seated with his or her bare arm resting on a table. The arm is bent at the elbow so that it sits at the same level as the heart. A blood pressure cuff is placed on the upper arm and inflated. A healthcare professional wearing a stethoscope listens carefully as air is slowly let out of the cuff.

When the cuff pressure falls below the individual's systolic blood pressure, the artery will begin to snap open and shut with each heart beat. The examiner notes the reading when this happens, and records it as the systolic pressure. When the cuff pressure falls below the diastolic pressure, the artery will stay open and the snaps will stop. The examiner then notes this reading and records it as well.

Occasionally, blood pressure is taken at other body sites, such as the leg. Blood pressure can also be monitored with a cuff that inflates automatically, or in an intensive care unit, with a device in an arterial catheter.

Blood pressure monitors that read the blood pressure at the wrist or the fingers are generally not as reliable.

What is involved in preparation for the test?

No preparation is needed for a blood pressure reading.

What do the test results mean?

High blood pressure (also called hypertension) is defined as a systolic blood pressure reading of 140 mm Hg or higher or a diastolic blood pressure reading of 90 mm Hg or higher. The National Institutes of Health has further defined high blood pressure. These categories are for people 18 years and over, who are not taking medication for high blood pressure and who do not have a short-term serious illness.

These categories are:

• Normal blood pressure is a systolic pressure under 120 and a diastolic pressure under 80.
• Prehypertension is a systolic pressure of 120 to 139 or a diastolic pressure of 80 to 89.
• Stage 1 high blood pressure is a systolic pressure of 140 to 159 or a diastolic pressure of 90 to 99.
• Stage 2 high blood pressure is a systolic pressure of 160 to 179 or a diastolic pressure of 100 to 109.
• Stage 3 high blood pressure is a systolic pressure of 180 or above, or a diastolic pressure of 110 or above.

Before making a diagnosis of prehypertension or high blood pressure, the readings should be taken on two different occasions. Healthcare professionals should be aware that sometimes, anxiety surrounding the clinical encounter can make an individual's blood pressure rise temporarily.