High blood pressure is signalled by a systolic reading of 140 or higher or a diastolic reading of 90 or higher, according to the American Heart Association. These numbers indicate the pressure in the arteries when the heart contracts or beats and when it relaxes between beats, respectively.
The systolic number is the higher of the two numbers in a reading, and the diastolic number is lower one of the two, according to the American Heart Association. Normal blood pressure readings are a systolic number below 120 and a diastolic number below 80. When the systolic reading is between 120 and 139 or the diastolic reading is between 80 and 89, a patient may be considered to be in the pre-hypertension stage, and proper treatment and behavioral changes may be recommended to prevent the development of hypertension, which is the medical term for high blood pressure.
Systolic readings above 180 and diastolic readings above 110 can signal a hypertensive crisis, and emergency care is recommended, according to the American Heart Association. Doctors do not make diagnoses for high blood pressure from a single reading or often from a single visit because many extenuating circumstances can affect a single reading. Even with an initial reading that signals a hypertensive crisis, a medical professional takes a second reading after a certain period of time to make sure.