Blood pressure is measured using an inflatable arm cuff with a pressure gauge and a device for listening to your pulse (a stethoscope). Blood pressure is measured as a pair of numbers, representing pressure in millimeters of mercury (often written as "mmHg"). Blood pressure is usually considered high if the top number (called systolic pressure) is above 140 or the bottom number (called diastolic pressure) is above 90. Blood pressure numbers are written as 140/90 and spoken as "140 over 90."
To obtain an accurate measure of your blood pressure in the doctor's office, prepare for the test:
- Don't eat or drink caffeine and don't smoke for 30 minutes before your pressure is checked
- Rest at least five minutes before your pressure is checked
- Bare your arm. (Remove a long-sleeved shirt, rather than roll it up.)
- Make sure that the right size cuff is used. (A large cuff is needed if you have a large upper arm.)
- Sit with your back supported. Your arm should be supported so that its muscles are relaxed. Your arm should rest on something that is at heart level.
The decision to start treating high blood pressure is never made after just a single blood-pressure reading in the doctor's office or hospital. (Unless your blood pressure is dangerously high, in which case urgent treatment is needed.) Usually, the doctor will measure your blood pressure on at least two or three occasions. This is because almost anyone can have a single high reading. Your doctor won't diagnose you as having high blood pressure unless your blood pressure is consistently above normal.
If your doctor thinks you are experiencing "white-coat hypertension" (meaning your blood pressure is higher at the doctor's office than at home because you are nervous), your doctor may ask you to check your blood pressure at home using a home blood-pressure machine.
