A blood pressure reading, which is measured in millimeters of mercury, consists of two numbers read as a ratio (for example, 120 over 80). A diagnosis of high blood pressure is made on the average of a number of readings over a period of months.

Systolic and Diastolic Numbers

The top number of the blood pressure reading (or systolic number) measures the pressure in the arteries when the heart beats, and the heart muscle is contracted. The bottom number of the blood pressure reading (or diastolic number) measures the pressure in the arteries between beats, when the heart is resting between contractions.

Normal Blood Pressure

The American Heart Association defines a systolic reading of less than 120 and a diastolic reading of less than 80 as normal blood pressure (any unusually low readings should be evaluated by a doctor).

Prehypertension

A systolic reading of 120-139 or a diastolic reading of 80-89 indicates prehypertension. MayoClinic.com recommends that anyone with prehypertension maintain or adopt a healthy lifestyle.

Stage 1 Hypertension

A systolic reading of 140-159 or a diastolic reading of 90-99 means a patient has stage 1 hypertension. If readings do not change after several months, he should talk to his doctor about medications to control blood pressure.

Stage 2 Hypertension

A systolic reading of 160 or higher, or a diastolic reading of 100 or higher, means a patient has stage 2 hypertension. One or more medications may be required to keep the blood pressure controlled.

Hypertensive Crisis

A systolic reading of greater than 180 or a diastolic reading of greater than 110 indicate a medical emergency and can lead to stroke or heart attack. Emergency care should be sought immediately.

Source:

American Heart Association: Understanding Blood Pressure Readings

MayoClinic.com: Blood Pressure Chart: What Your Reading Means