Blood pressure is expressed in millimeters of mercury, or 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 technician wearing a stethoscope listens carefully as air is slowly let out of the cuff. He or she notes the systolic and diastolic pressures. Systolic pressure is the blood pressure at the time the heart is contracted. Diastolic pressure is the blood pressure at the time the heart is relaxed.